Chapter 21 of Naval Ordnance and Gunnery, Volume 2 — Fire Control covers battery alignment: the process of aligning all guns, directors, and instruments of a fire control system to a common system of reference points, lines, and planes. The chapter works through gun-sight alignment and boresighting, train and elevation alignment in drydock (including the gunner's quadrant and roller-path data), the procedures for realigning a battery afloat, and the firing-stop cutout cams that prevent a ship from firing into her own structure.
A. Alignment of Gun Sights
21A1. Introduction
No matter how well the elements of a gun battery and fire control system function individually, they cannot function as a system unless they are properly aligned with each other. Battery alignment is the process of aligning all instruments and guns to a common system of reference points, lines, and planes. This process ensures that all associated gun bores, lines of sight, and radar beams are parallel when:
1. Dials are matched with correct follow-the-pointer indications,
2. No parallax corrections are made, and
3. No ballistic corrections are made.
It ensures further that they remain parallel throughout their operating motions, and that all instrument dials and automatic control equipment measure these motions correctly with respect to the proper reference.
The original alignment of a ship's battery involves (1) the mechanical alignment of the parts making up each element, and (2) the alignment of the various elements with one another. Alignment of parts within an element is largely a matter of design and installation; however, the gunnery officer should be able to check this alignment, and assure himself that it is accurate. Alignment of the various elements with one another is a primary concern of the gunnery officer, who should be familiar with methods employed in effecting the original alignment (in drydock) and should be able to make provision for routine checking and adjustment of the alignment when at sea.
The purpose of the train alignment is to adjust the battery so that the lines of sight of directors and guns and axes of gun bores are parallel at all angles of train, when no ballistics or horizontal parallax are considered and the dials are matched. The purpose of alignment in elevation is to adjust the battery so that at any angle of train and elevation the lines of sight of directors and guns and the bore axes will all be elevated at exactly the same angle above a common reference plane, provided that no vertical parallax is introduced, no ballistics are considered, and the dials are matched.
This chapter will discuss the alignment procedures employed in drydock and afloat. Under either condition, train alignment is conducted separately from and completed prior to elevation alignment.
21A2. Gun sight alignment
Gun sight alignment is that part of battery alignment which involves the mechanical alignment of the parts making up each element, namely, alignment within each mount, turret, or gun of the battery. Gun sight alignment is usually the initial step in battery alignment and consists of three procedures:
1. Boresighting. The pointer's and trainer's telescopes do not lie on the axis of the bore. For firing at short ranges in local control they must, therefore, be aligned so that their sighting axes intersect the extension of the bore axis at the target. Since, however, the gun bore is only a few feet from the sights, the error which may be caused by parallel alignment is very small. Thus, for battle use, gun sights may be aligned either to mean battle range for the battery involved or along parallel lines (converging at infinity).
2. Checking for lost motion. The values of sight angle and sight deflection as set on the sights must not contain inaccuracies due to lost motion in the sight mechanism.
3. Checking for parallelism of sights. Sight angle or range settings must elevate or depress the sighting axis of the telescopes in a plane perpendicular to the trunnion axes, and deflection settings must move the sighting axes of the telescopes in a plane parallel to the trunnion axes; in other words the sights must be aligned for parallelism.
21A3. Boresighting
The object of boresighting is to make the sight axes and the bore axis converge at either: (1) a specified range, or (2) infinity, with the range scale and the deflection scale reading their zero value in either case.
In order to adjust the sights in this manner, use must be made of a boresight telescope. This is a telescope which may be mounted in the bore of a gun and adjusted so that its line of sight coincides with the axis of the bore. Then by sighting through this telescope and through the sight telescopes simultaneously, the latter may be adjusted to obtain convergence or parallelism.
There are two main types of boresights: the breech-bar boresight and the boresight with self-contained optics.
A breech-bar boresight consists of (1) a boresight telescope, (2) an adapter for installing the telescope in the gun, and (3) a muzzle disc used for aligning the telescope on the axis of the bore. With bag-type guns, the adapter takes the form of a breech bar that can be installed across the screw box. A breech-bar boresight can also be used for boresighting a 5"/38 gun, but with a different type of adapter. The same boresight telescope can be used in boresighting all guns from 5-inch to 16-inch caliber (except the dual-purpose 6"/47 and the rapid-fire 8"/55), if provided with the proper adapters and muzzle discs.
Boresights with self-contained optics vary in construction and even in principle of operation, from those that consist merely of a fitting with a small hole for the breech end and another with a pair of cross-wires at the muzzle end, to those that convert the gun into a huge (but low-power) telescope. Self-contained-optics boresights are made to fit only the guns for which they are designed. No parts of them are interchangeable between guns.
The boresighting process with either major type of boresight is substantially similar, except that breech-bar boresight telescopes must first be aligned on the gun bore axis by sighting on the muzzle disc. This step is not necessary when using boresights with self-contained optics. This section describes boresighting with a breech-bar boresight, which requires this extra operation.
21A4. Boresight apparatus (breech-bar type)
The equipment required at the gun to boresight a bag gun includes, as noted in the preceding article, a breech bar, a boresight telescope, and a muzzle disc. The equipment and method of mounting on a gun are shown in figure 21A1.
The breech bar is a precision-machined bar which can be attached to the face of the breech by two screws. There is a hole through the midsection to receive the outer tube of the boresight telescope.
The boresight telescope tube is mounted within an outside adjusting tube which screws into the breech bar and is locked by means of the locking ring. Within the adjusting tube, the telescope is mounted in a spherical bearing which permits the telescope to be adjusted in both the horizontal and the vertical plane by means of four adjusting screws, so that the axis of the telescope can be made to coincide with the axis of the bore. The telescope has three rings near the eyepiece: (1) the reticle focusing ring for focusing the eyepiece to the individual eye, (2) the objective focusing ring for focusing the telescope on the target and eliminating parallax error (apparent target displacement when the eye is shifted about the optical axis of the eyepiece), and (3) the rotating ring, which permits rotation of the telescope about its axis within the outer adjusting tube.
The muzzle disc is a circular casting designed to fit snugly in the muzzle of the gun. Through the center of the disc is a small hole, and around it are four larger holes, arranged as shown in figure 21A1. Etched rings around the edge of the disc provide means for fitting the disc in the muzzle perpendicular to the axis of the bore. Notches are engraved on both disc and gun as index marks. With these index marks matched, one row of holes is aligned vertically and the other horizontally with respect to the gun. The purpose of the disc is merely to assist in the alignment of the boresight telescope axis with the bore axis.
Once aligned, the muzzle disc is removed and has no further part in the boresighting of the gun until the final stage, when it is remounted for a recheck of alignment to assure that no error has crept in during the process. Boresights with self-contained optics do not require the use of a muzzle disc in obtaining alignment with the bore axis.
21A5. Boresighting preparation
The general steps necessary in getting the gun ready for boresighting are as follows:
1. See that pointer's and trainer's scopes are clear, focused, and free from parallax.
2. Remove all evident lost motion from the sight mechanism. (See article 21A8.)
3. Lash back the breech plug so that motion of the ship will not swing the plug against the boresight apparatus. (For case guns, make sure that the breechblock is securely held down.)
4. Install breech bar, boresight telescope, and muzzle disc. (For boresights with self-contained optics, only the boresight components are installed. No muzzle disc or breech bar is required. Steps 5 and 6 below are also unnecessary, as is the final step in article 21A9.)
5. Focus the boresight telescope and center the crosshairs on the small center hole in the muzzle disc, using the four outer holes to align the crosshairs vertically and horizontally.
6. Remove the muzzle disc.
7. Set the range scale at zero (sight angle scale set at its zero value — usually 2,000 MIN) and deflection scale at the midpoint (usually 500 mils).
21A6. Choosing a target
A target with a clearly defined and visible point is suitable for boresighting in both deflection (train) and elevation. In the absence of such a target, two targets may be used: one with a clear vertical line for boresighting in deflection, and one with a clear horizontal line in elevation.
If the guns are being readied for a specific gunnery practice, the target selected should be at the range specified for that practice. For general use a target should be chosen at about the range at which the guns being boresighted are most effective, called the mean battle range. Or, if a parallel alignment is desired, a distant target like a star may be chosen. For elevation only, the horizon often makes an excellent target, especially for such guns as the 5-inch and 6-inch, for which it roughly corresponds to mean battle range.
21A7. Batten-board method
When a suitable target is not available, as often happens in drydock, or when it is necessary to boresight during rough or foggy weather, the batten-board method is used. It can be used either to set the sights exactly parallel to the axis of the bore, or to attain any desired angle of convergence.
A batten board is a screen set vertically on deck at any convenient distance normal to the bore axis of the gun. Marked on the batten are vertical and horizontal lines spaced at exactly the same distances that the telescopes are spaced from the gun bore(s) — these distances can be found in the Ordnance Pamphlet for the mount or turret. The markings on a batten for the 3"/50 pedestal mount are shown in figure 21A2.
If boresighting for a specified range, the separation of the lines on the batten board should be reduced. The required spacing can be readily calculated by the principle of similar triangles, as is shown in figure 21A3.
21A8. Looseness of parts and lost motion
Before beginning the actual boresighting, check for looseness of parts and for lost motion in the sight mechanism.
To check for looseness of parts, first sight on some convenient target. Then manually shake all adjustable parts, and recheck the crosshairs on the target. If the sights are off, tighten the linkage and try again. This check should be performed independently for the trainer's and pointer's telescopes.
To check the sight mechanism for lost motion, set the sights so that the crosshairs are on a target whose position with respect to the gun remains fixed. Use a point on the ship itself or, if the ship is in drydock, a target outside the ship. (If the target is close to the mount, some means of removing telescope parallax, such as a focusing cap, may be required.) While this check is going on, keep the gun stationary.
Set the sights at maximum range on the range scale and then return them to their original setting. If there is no lost motion, the horizontal crosshair will return to its exact position on the target. Make a similar check for the vertical crosshair by setting maximum sight deflection, returning to original deflection setting, and observing change due to lost motion, if any.
If there is lost motion in the sight mechanism, it can be removed by taking up excessive play or clearance between moving parts.
21A9. Boresighting the gun
To boresight in train, bring the gun to bear on the target so that the vertical crosshair of the boresight telescope is aligned with a vertical mark on the target. When the boresight is on, the man at the boresight calls “Mark.” If the pointer's and trainer's vertical crosshairs are not on target, adjust them until all vertical crosshairs are on at “Mark.” Similarly, to boresight in elevation, align the horizontal crosshair of the boresight telescope with a horizontal mark on the target.
The sight checker's telescope should also be adjusted in both deflection and elevation.
After boresighting, recheck for looseness of parts. Before securing, place the muzzle disc in the gun to make sure that the boresight line of sight is still coincident with the axis of the bore. If it is not, boresighting must be repeated.
21A10. Checking parallelism of sights
The check for parallelism is usually made while in a naval shipyard. Shipyards usually have available permanently constructed battens, similar to those used for boresighting but with the lines somewhat longer than necessary for boresighting only.
Secure the gun in train; install and adjust the boresight; and set the sight scales at zero range (no sight angle) and no deflection. Mark the points near the bottom of the batten where the lines from the boresight and the gun-sight telescopes appear to intersect the surface of the batten board (three separate points). Next, elevate the guns and mark a point near the top of the batten where the line of sight from the boresight telescope intersects the batten.
The upper and lower points for the gun determine the elevation line of the gun; run a fine piano wire between the pair of points to represent this line. From the lower points established by the pointer's and trainer's telescopes, mark off sight lines by stringing piano wires parallel to the elevation line of the gun.
With the gun elevated near the top of the batten, the vertical crosshairs of the pointer's and trainer's telescopes should, when range is set on the sights, move down along their respective sight lines on the battens. If they do not, the sights and gun do not elevate in parallel planes.
To make a similar test for parallelism of the motion of the sight in azimuth with the boresight, spot in points on a batten as the gun is trained to establish the gun-train line. Draw a second line parallel to the gun-train line at the level of the sights. This sight-train line must be located on the same level as the sights to be checked, to keep the sight plane horizontal; otherwise, when deflection is set, the LOS will fail to follow the horizontal line, even though the sights are in correct adjustment.
To complete the check, set deflection into the sight mechanism. As deflection settings are varied, the lines of sight of the telescopes should follow the sight-train line established on the batten.
For gun mounts with carriage-type gun sights an easier method of checking for parallelism utilizes a small batten board attached to the gun muzzle(s) (see fig. 21A4). With this board attached, start with the gun near zero elevation. Depress the sights by setting in some sight angle, and mark the points on the board where the lines of sight of each telescope meet it. Now elevate the gun. This will cause the lines of sight to move off the marks. If the sight trunnions and the gun trunnions are truly parallel, it should be possible to bring the lines of sight back on the marks by use of the sight-angle crank only. If there is an error, it can be measured by using the sight-deflection adjustment to get back on the marks.
B. Train Alignment in Drydock
21B1. Train alignment in drydock
The purpose of the train alignment is to adjust the battery so that the lines of sight of directors and guns and axes of gun bores are parallel at all angles of train when no ballistics or horizontal parallax are considered and the dials are matched. This is accomplished initially by aligning each element to the centerline of the ship, which is the line of 0° train.
Space limitations will not permit full discussion of all alignment procedures applicable to specific installations, for these installations are many and varied. The following discussion is limited to general procedures and principles used for batteries of 5-inch guns or larger, without reference to particular systems.
21B2. Preliminary work
1. Transmission check. Before proceeding, check the synchro transmission system to assure (1) that the various transmitters are sending out a correct electrical signal for a given mechanical input, and (2) that receivers are converting this electrical signal into a mechanical motion equal to that of the transmitters. It is first necessary to set the synchros to electrical zero. Any of several methods may be used, and each synchro dial should read zero when that synchro is on electrical zero. The next step is to check transmission, which is accomplished as follows:
a. Man the telephones at the stations to be tested.
b. Set the switchboard in turn to transmit from:
(1) The directors to plot.
(2) Plot to the guns.
(3) The directors to the guns directly (where possible, by-passing the rangekeeper or computer).
(4) The stable element to the directors (to transmit level and crosslevel).
c. Turn the transmitter to various readings, usually in 10° increments throughout its operating range, and compare the receiver readings with the transmitted values. They should check exactly. Any errors of position, direction, or firmness of position should be investigated by standard synchro methods. Particular attention should be paid to the action of the receiver dials when coming to rest. They should stop quickly and evenly in agreement, and there should be neither sluggishness nor long oscillation.
2. Establishing an offset centerline. The centerline itself cannot be used for alignment because of ship's structures in the way. Therefore, the first step is to establish an offset centerline. This is normally a line on shore which is parallel to the centerline of the ship.
The offset centerline is usually determined and definitely marked by the yard force. However, it is well for the gunnery officer or a representative of his department to check with the yard throughout the whole procedure.
The transit, which is an instrument employed in surveying, is used in establishing the offset centerline and for other steps in battery alignment. The instrument makes possible accurate measurement of horizontal and vertical angles. Though transits vary in details of construction, the basic principles are always the same. A plate with a graduated circle, or azimuth scale, is carried on a tripod support. This plate can be positioned in a true horizontal plane by means of levels and adjusting screws fitted to it. A separate plate, which can be rotated in the horizontal plane, concentric with the azimuth plate, supports a telescope. The telescope is mounted in bearings which allow movement in a vertical plane also, and therefore permit measurement of vertical angles.
To measure horizontal angles, the telescope plate is moved until its zero mark is lined up with the zero mark of the azimuth plate. It is then clamped to the azimuth plate in this position. The two plates are then rotated together until the telescope crosshairs line up with a distant point on the reference line being established. A plumb bob on the instrument locates the transit on the reference line at the vertex of the angle being measured. If the azimuth plate is now clamped to the support and the telescope plate is unclamped, angles can be measured from the established reference line at the chosen vertex point. Both the scale for reading these angles and the one for reading the vertical angles are usually equipped with vernier adjustments which permit readings to fractions of a minute of arc.
If there can be found on the actual centerline of the ship two points which are visible from each other, the procedure which follows is quite simple (refer to fig. 21B1).
1. Set up a transit over B, one of the points on the centerline. Lock the telescope plate at the zero point on the azimuth scale and sight on the other point, A, on the centerline. Clamp the azimuth plate to the transit support.
2. Select some other point on shore through which the offset centerline is to be established, such as C. (Either side may be used.) Unclamp the telescope plate from the azimuth plate and sight on point C. Read and record the corresponding angle, a.
3. Then set up the transit at C. With the telescope set on zero azimuth and locked, sight back on B. Lock the azimuth plate at this position. Then release the telescope plate, set it at the angle a, and lock. The transit telescope is now pointed parallel to the centerline of the ship.
4. Establish other points in the offset centerline by setting up stakes in the line of sight of the transit.
If no two points can be found on the ship's centerline which are visible from each other, the following method must be employed:
1. Select two points, one on either side of the ship (C and D), which can be seen from each other and also from two points on the centerline (A and B). (See fig. 21B2.)
2. Set up the transit over A, B, C, and D in turn, and measure angles a, b, c, d, e, and f.
3. The value of angle x may then be computed, using the data obtained in step 2. The following formula may be used in making this comparison:
Work sheets are available which simplify the computation of angle x by logarithms.
4. Set up the transit over D and sight on C. Rotate the telescope plate through the angle x, and the telescope is then parallel to the centerline of the ship. Other points on the offset centerline may now be established as in the preceding method.
3. Establishing the center of rotation. It is also necessary to establish the center of rotation of each battery element, so that a transit may be set up over a given center of rotation to measure the true angle of train of the mount or director in question. A center of rotation may be located by using either: (1) the transit, or (2) the plumb-bob method.
In the transit method, a transit is set up (on ship or ashore) at a point from which the top of the shield or mount can be seen. If the mount is of the open type, it is necessary to build a platform on which lines to determine the center of rotation can be drawn. The platform will also be needed later to support a transit at this point. With a helper stationed on the mount, sight on its estimated center of rotation and lock all motions of the transit. On top of the mount locate two points in the transit line of sight and draw a line connecting these two points. Then train the mount through two different angles, and in each case draw a line on the mount top through the transit's line of sight (see fig. 21B3).
Case 1: Sum of two train angles is less than 180°. In figure 21B4, the three original lines on the mount top are designated as 1, 2, and 3. Draw the bisector of the interior angle of the triangle formed by the first and third lines. In the figure this is ob, the bisector of angle cba. Next draw the bisector of the exterior angle of the triangle formed by the first and second lines. In the figure this bisector is ao, and the angle is cae. Finally, draw the bisector of the exterior angle formed by the second and third lines. In the figure this bisector is co and the angle is acf. The intersection of these bisectors is the center of rotation, o.
Case 2: Sum of the two train angles is greater than 180°. As shown in figure 21B5, the three original lines are labeled as in Case 1. Note also that the intersection of the bisectors of each of the interior angles of the triangle is the center of rotation, o. In the figure these bisectors are oa, ob, and oc.
The plumb-bob method can be used if a structure not attached to the mount is available over the mount. Suspend a plumb bob from this structure so that it hangs close to the center of rotation. As the mount is trained the plumb bob will describe a circle, the center of which is the center of rotation.
After a center of rotation has been determined, it should be checked by sighting from a transit to assure that this calculated center does not move out of the transit line of sight when the mount is trained. The center of rotation should then be permanently marked for future use.
21B3. Establishing zero train — director
The first step in effecting actual alignment in train is the establishment of zero train; that is, setting the train dials of each element so that when these dials read zero train the line of sight or bore axis of the element in question is parallel to the centerline of the ship and pointing forward. Zero train of a director is established as follows:
1. Select some point on shore (M in fig. 21B6) from which the director and some point (C) on the offset centerline can both be seen. This point (M) is the reference point, and will be used for zeroing all elements that can be seen from it.
2. Set up a transit over C, lock the telescope at zero on the azimuth scale, and sight forward along the offset centerline at some point (N). Lock the azimuth-plate movement, release the telescope plate, and sight on M. Read and record the angle (a).
3. Set up a transit over M, lock the telescope at zero on the azimuth scale, and sight back at C. Lock the azimuth plate, release the telescope plate, and sight on the center of rotation of the director (previously established and marked). Read and record this angle (b).
4. Train the director until the reference telescope (usually the pointer's), which in this discussion is considered as being located on the centerline of the director and properly aligned with all other parts of the director assembly, is sighted directly on M. Make sure that parallax is not present in the telescope, and also that no horizontal parallax is being introduced into the director dials by its parallax mechanism.
5. Set up a transit over the director's center of rotation, lock the telescope at zero on the azimuth scale, sight back at point M, and lock the azimuth plate. Now both the transit and the reference telescope are sighted on M.
6. Study of figure 21B6 shows that the director and the transit are trained from the true fore-and-aft direction by the angle e. This angle (e) is equal to angle a plus angle b.
7. Release the telescope plate of the transit, and turn it aft until it reads the angle e (or a + b). Lock the telescope plate.
8. Now train the director forward until the transit is again on point M. Obviously, the director has been trained forward through angle e, and now is exactly parallel with the centerline of the ship. Adjust the dials until they read zero train at this point, and adjust the transmitters so that they transmit zero train.
For purposes of checking director zero train at sea, a bench mark and a bench-mark reading are established. The bench mark usually is a small brass plate with crosslines etched on it. This plate is welded to a secure part of the ship within vision of the director sights. After zero director train has been established and the dials set, train the director and put the crosswires of the pointer's telescope on the bench mark, and read the train angle-reader dials. This is the bench-mark reading, which should be recorded. The same telescope must be used for obtaining all settings and readings. On multiple director installations the parallax corrector must be set for infinity — the point at which no parallax correction is entered into the train dials.
21B4. Establishing zero train — turrets
To establish zero train of a gun mount or turret, the guns (not the sights) are sighted on reference point M (see fig. 21B6). To do this, a boresight telescope is put in the barrel of a single mount, or in the middle barrel of a triple mount, and sighted on M. The boresight telescope must have its line of sight at the centerline of the bore. Then a transit is set up over the center of rotation, and sighted on M to establish zero. The transit is offset the sum of a and b away from the bow, and is secured. The mount is then trained until the transit is again sighted on M. This is zero train, and the dials may be set at 0 degrees (180° in the case of the after guns).
When a two-gun turret is at zero train, both gun bores should be parallel (within practical limits) to the centerline of the ship. Therefore, a somewhat different procedure is necessary to establish zero train, because both guns are offset from the center of the turret. The steps are as follows. Set up a transit over the turret's center of rotation and install a boresight telescope in each gun. Train the turret until gun A is sighted on reference M. Establish zero for the transit by training it on reference M. See figure 21B7 (A). Now train the turret until gun B is on M, sight the transit on M and read the angle d turned off by the transit. See figure 21B7 (B). Then d/2 is the angle at M formed by the lines of sight of the transit and gun B. Starting with the transit and gun B trained on M, turn the transit to the left through the angle equal to e − d/2 (see figure 21B8). The forward gun A could be used, but the angle in this case would be e + d/2. Then train the turret until the transit is again sighted on M, and the turret will be at zero train. The dials should then read zero. Be sure that there is no parallax input to the turret and dials. In the case of the after turret (fig. 21B8) the angle turned toward the bow by the transit is g − d/2; then the turret is trained aft until the transit is on M again. Dials should read 180 degrees.
21B5. Trams and tram marks
A reference point for each turret or mount must be established in order to check gun train at sea. Since very few guns can depress enough to point at a bench mark on the deck, a different type of reference is used. Such a reference is established when two marks or blocks, one on the fixed portion of the mount and one on the movable portion, are a definite distance apart when the mount is at a known angle. At any future time the mount can be trained until these two marks are separated by the specified distance, whereupon the mount will be at the previously established angle and the train dial reading may be checked. The distance between the two marks is established by a tram, and the marks or blocks are known as tram marks or tram blocks.
Trams are of two types: the older nontelescopic, and the newer telescopic. The nontelescopic tram consists of a single piece of steel with an offset machined point at each end. These points are aligned with the tram marks to establish the predetermined angle.
The telescopic tram (see fig. 21B9) consists of an outer tube within which slides a separate bar. Motion of the bar into the tube is resisted by a coil spring carried in the latter part. As shown in the figure a small window with an index mark is cut in the tube. Another index mark on the bar can be viewed through the window. When the marks on tube and bar line up, the tram is properly set for checking dial readings. This is done by inserting the tram between two tram blocks and training the mount until the index marks line up. The turret train dials should then read the value of actual turret train, which was determined at the last check of zero train in drydock. The tram can be checked by inserting it in the gage. If the scribe marks line up, it is correct.
A number of methods are used to effect initial establishment of the tram blocks, only one of which will be described here. The mount is set at a known angle of train (0° or 180° if possible), using transit methods. Then the tram blocks are welded in place — one on the moving portion of the mount and one on the fixed portion — the approximate length of the tram apart. The tram is then inserted, and the sliding or threaded tram-block rods, which pass through the tram blocks as shown, are adjusted until the scribe marks on the tram are aligned, whereupon such adjustable members are spot-welded or peened. The angle of the mount's train is then inscribed on the tram-block plate and entered in the battery logs for future checking.
21B6. Dial accuracy check
A dial accuracy check in train is made to ensure that the dials accurately indicate the angular movement of the mount. It is done by comparing the angular position of the mount at previously chosen intervals, as indicated by its dial, with these positions as measured by accurate survey methods. During this test the parallax input should be zero. The procedure is as follows:
1. Select a fairly distant point with a good vertical edge for a target.
2. Set up a transit over the center of rotation.
3. Train the director or mount to zero train.
4. Lock the telescope at zero on the azimuth scale; sight the transit on the target and lock.
5. Train the mount 10° by the angle-reader dial. Always come up to, but never pass, the desired mark on the dial.
6. Turn the transit back on the target; read and record the actual angle turned, which should be 10 degrees.
7. Train the mount to 20° by the angle-reader dial.
8. Turn the transit back on the target and read from the transit the angle trained, which also should be 10° (total transit reading 20°).
9. Continue the foregoing procedure for 360° (or to the limit of train) and then repeat in the opposite direction for 360° (or to the limit of train) to check for lost motion.
C. Elevation Alignment in Drydock
21C1. Purpose of alignment in elevation
A battery is aligned in elevation after it has been aligned in train. The purpose of alignment in elevation is to adjust the battery so that at any angle of train and elevation the lines of sight of directors and guns and the bore axes will all be elevated at exactly the same angle above a common reference plane, provided that no vertical parallax is introduced, no ballistics are considered, and the dials are matched. Before undertaking elevation alignment, each element involved should be properly adjusted, and a transmission check should be completed satisfactorily.
There is an appreciable change in the shape of a vessel between drydocked and afloat conditions.(1) While the taking of elevation alignment data when the ship is in drydock is described here, these data cannot be used to effect a final operating alignment afloat, because of the change in the shape of the ship and the consequent shifting of the relative positions of the various elements of the battery. Therefore the battery must be realigned when the ship is waterborne.
(1) Battery alignment is usually done with just enough water pumped out of the dry dock so that the hull rests firmly on the blocks; this condition is known as partially waterborne.
21C2. Gunner's quadrant
To obtain data for elevation alignment, use must be made of a gunner's quadrant. The gunner's quadrant is an instrument which measures the angle between its own base plate and the horizontal. The correct use and reading of a gunner's quadrant are rather difficult; accurate results can be obtained only after considerable practice. The following suggestions will prove helpful:
1. When using the quadrant, see that it rests firmly on a smooth surface, but do not clamp it to the surface so tightly that the quadrant becomes distorted.
2. Make sure the arc of the quadrant is properly positioned; e.g., if the elevation of a gun at a given bearing is to be measured, place the quadrant so that its arc is parallel to or coincides with the vertical plane through the axis of the bore.
3. Bring the bubble approximately to the center by sliding the arm along the arc; then clamp the arm.
4. Center the bubble carefully by means of the fine adjustment screw, allowing sufficient time for it to come to rest.
5. In any series of observations have one man use the quadrant for the entire set of readings.
Two types of gunner's quadrants are in use at the present time: the vernier type (Mark 3 Mod 1) and the drum type (Mark 7).
The gunner's quadrant Mark 3 Mod 1 (fig. 21C1) consists of a base, the bottom of which is ground to an accurate plane surface, an arc mounted perpendicularly on the base, an arm containing a spirit level, and a clamp arm.
The level arm and clamp arm are pivoted to the arc. The clamp arm serves to secure the level at any particular position on the arc by means of the clamp screw. The clamp arm is connected to the level arm by a tangent screw for making fine adjustments between the clamp and the level. The level in the level arm is a slightly curved graduated glass tube filled with colored alcohol and containing an air bubble. The upper surface of the glass tube is graduated so that the bubble can be accurately centered. The level arm also carries the vernier scale, which slides along the arc, and a magnifier glass.
The glass vial containing the spirit level is enclosed in a metal tube with only five inches of its upper surface exposed. The glass is graduated in units of ten seconds (10″) of arc.
The main scale is 100° long, and its smallest graduation (fig. 21C2) is 15 minutes (15′) of arc. (In some mods, the degrees are divided into six parts of 10 minutes of arc each.) Each main division of the vernier scale represents 5 minutes, and the next smaller graduations represent minutes. The smallest graduations represent 20 seconds of arc each.
To use the gunner's quadrant, place it carefully on a flat surface parallel to the axis of the gun bore or other object to be measured, and with the quadrant pivot toward the gun muzzle. Bring the level bubble approximately to the center of the bubble tube by adjusting the clamp arm and level arm positions on the arc. Tighten the clamp screw to hold the arms in position. Then turn the tangent screw slowly in the proper direction to bring the bubble exactly into the center of the tube. Then read the measured angle from the arc and vernier scales.
In reading the scale, count along the graduations on the arc scale to the zero on the vernier. This will give a ROUGH reading in degrees and minutes. To get the fine reading, count from the zero on the vernier scale up to the division on the vernier scale that lines up exactly with a division on the arc scale. This will give the fine reading. Add the coarse and fine readings together to get the total angle of inclination.
In figure 21C2, the indicated value is 9° 18′ 40″. This is read as follows:
1. The zero on the vernier falls between 9 and 10 on the main scale. The smallest graduations on the main scale are ¼° or 15′ apart. Since the zero graduation on the vernier falls between the first and second small graduations above the 9° graduation (which is not visible in the detail view of figure 21C2), the coarse reading must be 9° 15′.
2. Now locate the graduation on the vernier which lines up with a graduation on the main scale. (This is pointed out in the figure by the small black arrow.) Since, as the engraving on the vernier scale shows, the smallest graduations on the scale represent values 20″ apart, all that remains is to count on the vernier scale the graduations from zero to the matching one, and add this fine value to the coarse reading. This fine value is 3′ 40″. Adding this to 9° 15′ yields 9° 18′ 40″.
Often the difference between consecutive readings of a gunner's quadrant will be so small that it is not necessary to center the bubble for each reading. Instead, the second reading can be read directly in terms of the displacement of the bubble from its center position.
The Mark 7 gunner's quadrant (fig. 21C3) has been designed for easy reading. The vernier scale has been replaced by a micrometer drum mounted on the shaft of the tangent screw. For coarse adjustments, the level arm can be moved to any position by disengaging the tangent screw from the teeth cut on the arc.
The main scale is graduated as shown in figure 21C3. One revolution of the drum displaces the level arm by one degree. The finest graduation on the drum is 30 seconds, and each 5-minute graduation is numbered. Each graduation on the spirit level glass also represents 30 seconds of arc.
While the drum-type quadrant is much easier to read, it may be inaccurate unless certain precautions are taken.
1. Lost motion often exists between the worm and worm-wheel assembly that drives the level arm. Since the fine reading is read from the drum, any lost motion of the tangent screw introduces an error — often as much as 20 seconds. However, the lost motion can be eliminated if the tangent screw is always turned in the same direction when centering the bubble.
2. A second possibility for error results from inaccuracies in the teeth cut on the arc. For this reason all drum-type quadrants are calibrated and a correction sheet is issued with each quadrant. Whenever the quadrant is read, the corresponding correction factor should be applied.
3. Care must be taken in disengaging and reengaging the tangent screw. The spring holding the tangent screw against the arc is strong, and the teeth can be damaged if the tangent screw is allowed to snap into mesh.
Before the gunner's quadrant (either type) is used, it must be zeroed, as follows:
1. Adjust the bubble to correct length, if necessary.
2. Select a smooth plane surface that is approximately level. Place the quadrant on this surface, and draw thereon a line around the base of the quadrant. Center the bubble and read the quadrant.
3. Turn the quadrant around so that the inclination is measured in the opposite direction. Be sure that the base of the quadrant lies within the outline. Center the bubble and read the quadrant again.
4. The average of these two readings should be zero. If it is not, the quadrant must be adjusted (by the adjustment screw on the level tube) until additional tests show that the average of the two readings is zero.
21C3. Roller-path data with gunner's quadrant
A very important phase of elevation alignment is the procurement and interpretation of roller-path data. The purpose of taking roller-path data is to determine the actual relationship between the roller path and the horizontal plane. Then the necessary compensations can be made for aligning all elements of the battery to a common reference.
These data consist of a series of readings showing the relative inclinations of the planes of each roller path to some fixed plane. At sea this fixed plane is the reference plane of the battery, as explained in article 21C5, but in drydock it usually is a horizontal plane established by a spirit level. The data are used to determine the relationship in space of the various roller paths.
In all modern installations, it is assumed that the roller paths of all elements are true planes or so close to true planes that distortion is unimportant and not measurable. The relationship between a tilted plane and the true horizontal plane is shown in figure 21C4. It can be proved that the tilt (t) of the roller path at any point is equal to the maximum value of the tilt (T) between the planes (i.e., the tilt at the high point of the roller path) times the cosine of the train angle (B) between the high point and the point under consideration.
Therefore the readings obtained at points throughout the training limits of the element should vary in a sinusoidal manner, with some readings above and some below the horizontal. If they do not vary in this way, it is evident that the roller path has become distorted.
Procedure for obtaining drydock roller-path data with a gunner's quadrant is as follows:
1. Secure the quadrant to some smooth surface on the gun or director in such manner that the line of the quadrant is parallel to the axis of the gun bore (or the director's line of sight) and the pivot of the arm is forward (toward the muzzle).
2. Set the roller-path tilt corrector (if there is one) to zero tilt.
3. Elevate the gun to some convenient angle; this will make all readings positive. The difference between the readings is all that is important, so the amount of elevation is immaterial. In the case of a director, the forward end of the quadrant must be shimmed up to obtain the same results.
4. Train the gun or the director to 0°, level the bubble, and read the quadrant. Repeat this process for every 10° or 15° of train. Record the inclination and the angle of train in each case.
21C4. Interpretation of roller-path data
The relationship between any roller path and the horizontal is defined by the amount of tilt at the highest point of the path and the bearing of this point, and compensating devices are built to correct in these terms. Therefore in order to interpret the results, these two quantities must be determined for each roller path.
There are two commonly used graphic methods available for this determination: the sine-curve method and the 60° radial method. In order to explain these methods more clearly we shall assume a set of typical roller-path data shown above, and actually find the high point and its bearing by these methods.
Although the table shows inclination at 30° intervals of train, in actual practice readings would be taken every 10° or 15°.
Note that all readings are positive. This is due to the fact that the quadrant was slightly elevated, as mentioned before, to avoid the complication of negative numbers.
Sine-curve method. As stated before, the inclinations from the horizontal will vary in a sinusoidal manner. Therefore, if the inclinations are plotted against the bearings, a sine curve will result. The curve for the sample data readings is shown in figure 21C5.
The bearings are read along the horizontal axis, and the inclinations are plotted vertically at these bearings. Then the sine curve is drawn by fairing-in a line through these points. The actual fairing-in of this curve is rather difficult, and for this reason the sine method is rarely used for drydock data.
After the curve is drawn, its zero axis is drawn in, parallel to the horizontal axis of the graph. It should pass through the curve halfway from the low to the high point, and its distance from the axis should equal the arbitrary elevation which was introduced to obtain positive readings.
To determine the inclination and bearing of the high point of the roller path it is only necessary to read the bearing and inclination of the high point of the curve. This is shown clearly on the figure. In this case the path has an inclination of 20 minutes at a bearing of 90 degrees. Note particularly that the inclination is the inclination from the axis of the curve itself, not from the axis of the graph.
Sixty-degree radial method. The second and most widely used method of plotting the roller-path data is the 60° radial method. There are several advantages to this method. Fairing a sine curve in rectangular coordinates from roller-path data obtained as described in article 21C5 is a difficult process. Also, in many cases the arc through which the element can train is limited, so that only part of the sine curve will be obtained. In all cases of limited train the radial method, which is based on the fact that a sine curve plots as a circle in polar coordinates, is preferable; and in extreme cases it is the only possible method to use.
An examination of the sine curve in figure 21C5 will show the principle on which the 60° radial method is based. First notice the point D at which the curve crosses the zero axis. If the inclination is read at a point 30° to the left of point D (point A on the curve) the inclination can be seen to equal one-half of the inclination of the high point C. (This can be proved by trigonometry, since the sine of 30° is .5.) The value of inclination at point B, 30° to the right of point D, is also equal to one-half of the inclination of the high point C; in other words the vertical distance between A and B is equal to the total inclination at the high point C, which is the maximum inclination of the curve.
If any other two points, 60° apart, are taken, the difference between their inclinations will always be less than the maximum. Thus, if a series of readings 60° apart are subtracted from each other and the differences plotted, using the median bearing, a new sine curve having the same amplitude will result, with the high point 90° from that of the original curve. To simplify the mathematics involved, instead of using the median bearing, we plot the values against the lower bearing of each pair. Now our new curve will be out of phase by only 60°, and the bearing of the high point will be obtained by subtracting 60° from that of our new curve.
Figure 21C6 shows a series of readings 60° apart arranged for plotting, with the differences indicated. The left-hand or lower bearing is used for plotting. The plot as shown is made on polar coordinate paper, and our new sine curve will appear as a circle passing through the origin. The diameter gives the maximum inclination, and 60° counterclockwise from the diameter is the bearing of the high point. For the data shown, the bearing of the high point is 90° and the inclination is 20 minutes, as was obtained above in the sine-curve method. These are the settings — bearing of high point and inclination of high point — that are combined vectorially with the inclination of the reference plane and then put on the roller-path tilt compensator.
21C5. Choosing the reference plane
The reference plane generally chosen is an actual plane on the ship; that is, it is the roller-path plane of one battery element. The plane is so chosen that the inclination between it and the roller paths of the other elements is as small as possible.
On larger ships having more than one director, the directors (with certain exceptions) are equipped with roller-path tilt compensators. In such case the roller path of any director or gun may be chosen as a reference. At installation an attempt is made to make all paths parallel. After installation, when roller-path data have been taken, the mean plane of all elements is determined, and the roller path closest to the mean plane may be chosen as the reference plane.
On destroyers, there is only one Mark 37 director, and it is not equipped with a roller-path tilt compensator, while each of the 5-inch mounts has a compensator. The plane of the director roller path must therefore be the reference plane of the battery. If all the guns and the director were installed simultaneously, with no special consideration given to roller-path inclination (other than the usual attempt to install the elements as nearly horizontal as possible), the plane of the director roller path might vary so much with respect to one or more of the guns that it would be unsuitable as a reference plane. Therefore, destroyer guns are installed first. Then the roller-path data for each gun are taken, and the mean of all the inclinations is computed. The director foundation is machined so that its bearing surface will be as nearly parallel to this mean plane as possible.
The radial diagram or polar coordinate graph may be used to compute the mean plane of all roller paths, as shown in figure 21C7. In this example there are five battery elements to be considered. The bearings and inclinations of their high points are indicated in the table below the graph. Bearings and inclinations are plotted on the radial diagram, being represented by the small circles numbered 1 to 5. The sum, as indicated, is obtained by adding the various inclinations vectorially. The resultant, represented by the line drawn from the center to the end of the last line (e), gives the solid line (f) on a bearing approximately 326° with an inclination of about 16 minutes. To obtain the mean, divide the total inclination (16′) by the number of elements (5). In this case the result is 3.2 minutes. The mean of the five inclinations, then, is an inclination of 3.2′ at a bearing of 326 degrees.
On modern ships, the roller paths of the individual elements are machined very close to parallel, and almost any of them could be used as the reference plane. Present specifications require that the roller paths of the major elements be within 6′ of one another. In addition, the roller paths of the 40-mm and 3"/50 caliber gun mounts and their directors must be within 10′ of the reference plane of the dual-purpose battery so that, when the guns of one system are controlled by the director of another, excessive errors due to misalignment will not be introduced.
The final selection of the reference roller path is influenced by the obvious advantage of choosing a roller path whose inclination will be relatively unaffected by waterborne changes in the ship's structure. This consideration often dictates the choice of an element near the center of the ship, such as one of the stable elements in the plotting room, as the reference element.
After the reference plane has been chosen and its tilt is known, it is necessary to determine how much and at what bearing each of the elements is tilted with respect to this reference. The radial or polar coordinate diagram is again used for this purpose. The process involves shifting the origin of the graph so that it lies at the reference point, or what amounts to the same thing — moving all the points without disturbing their relationship until the reference point lies on the origin of the graph. Figure 21C8 shows the points of figure 21C7 transferred so that element No. 3 is the reference. The inclination of any element to this reference can be read directly from the graph. It should be noted that the inclinations represented in this diagram have been assumed for purposes of illustration; lack of parallelism to such a degree is rarely encountered in modern ship construction.
21C6. Setting director elevation dials and establishing elevation bench marks
The bench mark is established to be used as a reference in checking director sights and dials at sea. The shore-transit method is as follows:
1. Train the director to either the high or the low point on the roller path. This is done so that the true tilt of the director may be obtained easily from the roller-path data.
2. Vertical parallax correctors must not be operating, or parallax range must be set for infinity. The roller-path compensator should be set at zero inclination.
3. Level the transit (ashore) so that the level bubble under its scope is centered when the stadia (elevation) circle reads zero. Illuminate the pointer's telescope from inside the director with a flashlight, and sight into this telescope with the transit.
4. Read angle a, as shown in figure 21C9. Read angle b from the roller-path data.
5. Depress the director telescope to sight on the transit. With the director telescope and transit aligned, read the director elevation dials. To this reading add angle c. Elevate the director line of sight until the elevation dials read the sum of the two. Angle c is equal to a plus b, which are respectively the angle below the horizontal through which the director telescope was depressed to sight on the transit, and the angle made by the director roller path with the horizontal plane. The director line of sight, therefore, will have been made parallel to the roller path, which is the condition desired. Set the elevation dials to zero with the director line of sight in this position.
6. Train the director and elevate or depress the pointer's telescope as necessary to put the crosshairs on the bench mark. Record the bench mark depression. Remember that the bench-mark readings were taken with roller-path compensator set to zero, and for future accurate checks it must be so set. To provide for quick checks with the compensator functioning, it is well to establish and record an additional bench-mark reading with the roller-path compensator set as for firing.
21C7. Setting gun elevation dials and establishing elevation tram marks
To ensure that gun elevation dials will read the true elevation above their own roller paths, it is necessary to set the dials to proper position while in drydock. This may be done in the following four ways:
1. Install a boresight telescope in the gun, and follow through the same process that was used for setting the director's dials, employing a transit set up ashore.
2. Make sure that the gun is properly boresighted, and set sight angle and deflection at zero on the gun sight. The procedure is then the same as in No. 1 except that the transit is sighted on the gun sight rather than the boresight telescope.
3. Set up a gunner's quadrant on the gun as when taking roller-path data. Train the gun to the exact angle at which roller-path inclination was read (roller-path data). Elevate the gun to any convenient angle and read the quadrant. Referring to figure 21C10, it is evident that the quadrant has read the angle A between the gun bore and the true horizontal. However, the roller path is inclined to the horizontal by the angle B. Hence, the true angle of gun elevation is A − B, if the roller-path inclination is positive (as in fig. 21C10) and A + B if the roller-path inclination is negative. Set the dials so that they read this value of A plus or minus B, as appropriate.
4. Set up a gunner's quadrant as in No. 3. Train the gun 90° away from the high point of the roller path, thus eliminating roller-path inclination. Elevate the gun, read the quadrant, and set the dials in accordance with the quadrant reading.
After the dials have been set, the tram blocks are installed. The method used for installing train tram blocks described in article 21B5 is used for elevation tram blocks, except in the latter case one block is on the carriage and one is on the slide, as shown in figure 21C11. With the tram in position and its index marks lined up, the elevation-dial reading is recorded for use as a future check for proper elevation reading.
21C8. Dial accuracy check
A dial accuracy check in elevation is made for the same purpose and using the same basic principles as the train dial accuracy check. It will be more convenient to use a gunner's quadrant, however, to measure actual movement of the mount for comparison with the dial indication thereof. In the case of a director, the quadrant must be mounted on some surface, such as may be found on the rangefinder, that moves exactly with the line of sight in elevation.
21C9. Alignment of the stable element
After the reference plane of the battery has been established, it is necessary to align the stable element or stable vertical so that the plane of rotation of its sensitive element is parallel to the reference plane. Of course, if the plane of one stable element is used as a reference plane, it will still be necessary to align any additional stable elements.
The sensitive element is trained in accordance with director train and not in accordance with relative bearing in the horizontal plane. Its roller path, therefore, must be parallel to the reference plane, so that director train will produce the correct train of the stable element gimbals. Since these instruments are not equipped with roller-path tilt compensators, they must be aligned by shimming.
Two methods are available for aligning the stable element with the reference plane. Since the first of these two is invariably used with all late marks of stable elements and stable verticals, the second will not be discussed; it can be found in Bureau of Ordnance publications if required.
With modern instruments the first step is to take roller-path data with a gunner's quadrant mounted on the crosslevel gimbal-mounting bracket. (See fig. 21C12.) These data are plotted in the same manner as for other battery elements, and the bearing and inclination of the high point of the stable-element roller path with respect to the reference plane are obtained.
The number, thickness, and location of the shims needed to effect alignment is determined as follows:
1. On a plain sheet of paper draw a vertical line representing the ship's centerline (see fig. 21C13). Draw a triangle, ABC, representing the three mounting legs of the stable element drawn to scale and in their correct relationship to the centerline of the ship. In the example, the scale is ⅛ inch equals 1 inch, and the legs are drawn so that the single leg is to starboard.
2. From any point O in the line representing the ship's centerline, draw a line OZ at the bearing of the high point (with respect to the reference plane) of the stable element's roller path. In the example, it is assumed that the high point is on bearing 135° and that its inclination is 15 minutes.
3. Drop a perpendicular BX, from the vertex of the triangle in the direction of the stable element's high point, to the line OZ.
4. Drop perpendiculars, AD and CE, from the other two vertices of the triangle to line BX.
5. Measure the lengths of AD and CE and convert, according to scale, to the lengths represented. In the example, the length represented by AD is 25½ inches, and that represented by CE is 12½ inches.
6. Enter these lengths in the formula
S = 0.0003 LT, in which
S is the amount of shimming needed under the leg,
L is the length represented by the line dropped from that leg in the diagram, and
T is the tilt of the roller path at its high point in minutes.
In the example, leg A will require shims of 0.115 inch; and leg C, of 0.056 inch. Note that, in any case, only two legs will require shims.
After shims of the proper thickness have been placed, the taking of roller-path data should be repeated. If the new readings show a variation from the reference plane of as much as 2 minutes at the high point, the calculation and shimming should be repeated. (Extreme care should be exercised after the shims are inserted. The holding-down bolts are tightened evenly with a torque wrench to prevent canting the stable element.)
D. Battery Alignment Afloat
21D1. General
Since a ship is not a rigid structure, upon loading and putting to sea the space relationships between elements of a battery change, and correction for these changes must be made. The process involved is known as battery alignment afloat, and must be carried out while the ship is waterborne, by different procedures than those used for the original alignment in drydock.
Before initiating the actual alignment procedures, ensure that all elements are functioning correctly and that all transmission systems are properly adjusted. Have a routine transmission check carried out just prior to the alignment check.
The purpose of afloat battery alignment in train is to ensure that when the director is trained to any point and the gun dial pointers matched, with zero settings of sight deflection and parallax, the director and gun lines of sight and the gun bore axes are parallel (in the horizontal plane).
Since it is impracticable to use multiple targets, train alignment is checked on a single target; and when the gun dial pointers are matched, proper parallax set in, and zero settings of sight angle and sight deflection set in at the guns, the director and gun lines of sight and the gun bore axes converge on any given target at any range and on any bearing.
To accomplish this check, it is necessary to introduce parallax both into director train (in multiple director installations) and into gun train. It is therefore necessary to check the parallax system before beginning the actual alignment. Proper correction of parallax errors is important where there are a number of directors and large horizontal distances between units. Hence, all parallax correctors on guns and directors should be checked for:
1. Correct amount of parallax at various bearings and ranges.
2. Correct direction of applied parallax correction.
The purpose of afloat battery alignment in elevation is identical with the purpose of elevation alignment in drydock (article 21C1). This objective is attained by selecting some plane as the reference plane of the battery, so that the elevation of all units, when measured from that plane or a parallel plane, is equal. Again, a single target, the horizon, is used for the check.
21D2. Checking the directors on their bench marks
The first step in the actual battery alignment afloat is to check the directors on their bench marks. On some ships the space relationship between director and bench mark may show variations due to working of the ship in a seaway. The amplitude of this motion is usually about two or three minutes. The director should be checked on its bench mark about once a week, noting this movement. No adjustment is necessary unless checks show that the error is increasing in one direction, in which case something is wrong with the director.
The basic procedure for checking a director on its bench mark is as follows:
1. If the director uses parallax corrections, set these at zero.
2. If the director uses inputs of level and crosslevel, set these at zero.
3. Obtain the bench mark reading from the ship's records.
4. Train the director until the crosswires of the pointer's telescope are on the bench mark.
5. The train and elevation dials should now read the previously recorded bench mark values.
21D3. System alignment in train (train check afloat)
After the director is on the bench mark, it is possible to proceed with actual alignment of the various battery elements. Preferably, this should be done with the ship at anchor in smooth water. If the battery has never been aligned, a complete train check must be made, but otherwise a preliminary test may be made to determine if a complete check is necessary. The preliminary test is conducted as follows:
1. Establish telephone communication between director and guns.
2. Set switchboard for normal operation; i.e., director to plotting room, which in turn transmits to guns.
3. At the computer or rangekeeper, have time motor off, power switch on.
4. Set Vs and Ds at their zero values on their respective computer counters.
5. Set and lock level and crosslevel at zero.
6. At the guns, set zero values of Vs and Ds; put the guns in local, hand, or manual control.
7. Select a distant target off one beam. Train the director until the vertical wire is just off the target, so that motion of the ship will carry the wire across the target.
8. Obtain the range to the target by the most accurate means available, and set the parallax correctors to give the proper correction for this range.
9. Match pointers at the guns.
10. As the director line of sight swings on target, the director trainer calls (phone) “Mark” to the gun trainer. This is continued, the gun trainer meanwhile moving the gun, from one direction, until both gun and director telescopes are on the target at the same instant. The amount of displacement between the follow-the-pointer dials at the gun is the amount of error and should be recorded. This process is repeated, with the gun trainer bringing his vertical wire on target from the opposite direction, and the error recorded. The algebraic difference between the two errors is the lost motion of the gun. The mean of the two errors is the gun error. For example, if the errors are +2 minutes and −4 minutes, the lost motion is 6 minutes and the gun error is −1 minute.
11. Repeat the process, using a target on the other beam if practicable, and in any case a target at a widely different train angle from the first, and record the gun error and lost motion.
The gun errors should be equal and small. If they are equal and large (2 or 3 minutes larger than the lost motion), it is an indication that a constant error exists, and that this error may be corrected by adjusting the train response. In so doing, the dial which shows the actual train of the element (not the dial on the synchro receiver) must be moved. If the errors are not equal, a complete train check is necessary.
The complete train check is exactly like the test described above, except that a series of targets is used, at 10° or 15° intervals if possible.
The complete train check will furnish gun errors which, when plotted with their bearings as abscissas, should show a slightly ragged scattering of points. A line parallel to the abscissa which passes through the mean of these points (i.e., with approximately equal deviations above and below the line) can be considered as the zero error line. Its distance above the abscissa will be the constant error of the system, which can be removed by adjusting the response. If a sine curve results, it indicates errors such as improper parallax settings. If the points are erratic with large deviations from the zero line, it indicates damage to the dial drive shaft, such as a sheared coupling or slipping gears.
21D4. System alignment in elevation (horizon check)
To adjust the battery to its reference plane, it is necessary to compile data on the relative positions of all guns with respect to the reference plane as represented by the line of sight of the reference director. This is done by means of a horizon check, which compares the elevation angles on the dials of director and guns when all are pointed at the horizon, at a series of points completely around the horizon. To take the simpler case, where there is no uncorrected inclination of the gun's roller path with respect to the reference plane, if we compare director elevation and gun elevation at a common point (horizon), after accounting for any known angular divergences between the two units, such as that caused by the vertical distance between guns and director and the angle of the gun sights with respect to the bore axis (sight angle), their elevations should be equal.
In figure 21D1, if we subtract Vs and Dip Difference from Gun Elevation, we arrive at line CD of the diagram (see numerical example as well). If CD is parallel to AB, the gun and director are elevated at equal angles above (below) the reference plane, are aligned in elevation, and there is no system error. Or:
E′g − (Vs + Dip Diff.) = Eb
Now, by rearranging these quantities slightly, we obtain an array of values that lend themselves to checking with the least amount of time and effort. We simply subtract director elevation from gun elevation (values read at each different angle of train) and the result should equal Vs + Dip Diff. (values that are set and remain constant throughout the check). Any inequality is called the system error.
Steps in performing a horizon check are as follows:
1. Choose a day when the ship has little roll and the horizon is clearly defined.
2. Man stations and phones.
3. Make sure that the synchro transmission system has been checked recently, and that the director has been checked on its bench mark.
4. If possible, use the reference director with no roller-path inclination compensator.
5. Record the roller-path inclination compensator setting on the gun concerned. It should agree with the value determined during the last system alignment check in elevation.
6. Look up the height of gun and director, and compute the dip to the horizon from each. From this information can be computed the dip correction for each gun, by subtracting the dip angle for the gun from that for the director.
7. Set the dials of the computer or rangekeeper so that no corrections in elevation are introduced by its mechanism.
8. If the test is to be performed with the boresight telescope, ship the scope. If the gun sights are to be used (the normal procedure), they must have been boresighted recently. Set a positive value of sight angle at the gun and record this setting. The purpose of setting in this sight angle is to ensure that the elevation reading of the gun will be higher than that of the director at all bearings.
9. Train the director to a given bearing; elevate or depress the director line of sight so that it will move across the horizon as the ship rolls. Record for later reference the value of director elevation used on each bearing.
10. Train the gun to the same bearing as the director.
11. The gun pointer depresses his gun until it is approximately on the horizon. When the director sight crosses the horizon, the director pointer calls “Mark”, and the gun pointer turns his handwheels until his line of sight crosses the horizon simultaneously. When he is on, he checks back to the director exactly on the mark, so that when either one calls “Mark” the other will be exactly on the horizon. To eliminate lost motion, always move the director and gun lines of sight onto the horizon from the same direction.
12. When the gun is on, read and record both the mechanical and the follow-the-pointer dials. The follow-the-pointer dials will read the total uncorrected gun error, and this should equal the difference between the director elevation and the gun elevation as read from the mechanical dials.
13. Repeat the foregoing process at 10° or 15° intervals throughout the training arc of the gun.
14. Obtain the uncorrected gun error by subtracting the director elevation from the gun reading (never the reverse), and record the result for each bearing.
A sample of data obtained in the foregoing manner is as follows:
The foregoing results are most conveniently plotted by the sine-curve method as shown in figure 21D2. Note that the difference between gun elevation and director elevation varies with different angles of train. If the roller-path compensator setting had been proper (no uncorrected inclination) these differences would have been constant and the data would plot as a straight line. As it is, however, the differences vary, indicating uncorrected inclination, and the data will therefore plot as a sine curve. After the data have been plotted, find the zero axis of the resulting sine curve. Note that the example shown here is for the full 360° arc of train, which is a condition almost never realized in practice. Hence, while both a high point and a low point are shown on our sample curve, only one of these points may be present on the curves obtained in an actual installation. The method of obtaining the zero axis to be described is applicable if either the high point or the low point of the curve can be located. Simply take a point on the sine curve at a bearing 90° away from the high point or the low point and through it draw a line parallel to the abscissa. This line is the zero axis, and its distance above the abscissa represents the error due to all causes other than roller-path inclination, with respect to the horizontal. Figure 21D2 shows how this error is broken up into component parts. Sight angle and dip correction are known values; the remaining error represents the system error. This constant system error can be removed by adjustment of the elevation response at the gun.
The low point of the curve represents the bearing and inclination of the high point of the gun roller path, with respect to the reference plane (in this case, the director roller path). If no low point is shown on the plotted curve, it may easily be calculated, since it would occur at a bearing 180° from the high point of the curve. Further, it would occur at the same distance from the zero axis of the curve as did the high point. The bearing of the low point of the curve represents the bearing of the high point of the gun roller path. The distance of the low point below the zero axis of the curve represents the inclination of the gun roller path. In the example shown in figure 21D2, the high point of the gun roller path (represented by the low point of the sine curve) is at 30° train, and the inclination at that point is 17 minutes. Thus, for this example, the following desired data are available:
Bearing of high point — 30 degrees
Inclination of high point — 17 minutes
Constant error of system — 6 minutes
It may be difficult to understand why the low point of the sine curve is the high point of the gun's roller path. We know that the uncompensated roller-path inclination plots as a sine curve. When the gun is trained to the highest point on its roller path, the actual gun elevation to the horizon will be at its lowest value with respect to the reference plane. This occurs because the gun's roller path has not been completely corrected to the reference plane. In other words, the high point of the gun's roller path has raised the gun above the reference plane, and to elevate to a given target now requires less elevation angle between the gun bore axis and the gun roller path. With gun elevation at its lowest value, the difference between gun and director elevation will be a minimum; a minimum difference is the low point of the sine curve.
21D5. Calculating correct compensator setting
The horizon check is usually made with some setting already on the roller-path tilt compensator. The tilt found by the check, therefore, is not the total inclination but only the uncorrected inclination. It is an additional inclination to that for which the compensator has been set. This newly discovered inclination must be added vectorially to the inclination previously known to exist, in order to determine the total inclination for which the compensator must be set. This may be done graphically, as shown in figure 21D3. In this figure the results obtained previously were used to illustrate the method, which is as follows:
1. The line OA is drawn to represent zero train.
2. The original setting of the compensator (8.5′ at 150°) is plotted as line AC. This is done by measuring off the angle clockwise from OA, and measuring the inclination on that line to a convenient scale.
3. The inclination found (17′) is plotted as AB on bearing 30 degrees.
4. CD is drawn parallel to AB, and BD is drawn parallel to AC. These lines intersect at D.
5. A line AD is drawn from the origin to D. This line represents the total inclination. Its bearing (59°) and length (15′) may be read according to the previously established scale. These are the data that must be set into the compensator.
It should be noted that compensators are constructed to read the error rather than the correction. Thus, if the error is 15′ at 59°, the bearing scale is turned to 59°. Then the inclination of 15′ is set on the inclination scale, and the adjustment is completed.
21D6. Simple elevation check
When at sea, it is desirable to perform a simple elevation check at frequent intervals. The method is the same as that in the horizon check, except that each gun is checked at only one point on the horizon. The difference between gun and director reading after correction for sight angle should equal the dip correction. If it does not, an error of some sort is present and must be investigated. Before undertaking a complete horizon check as a result of such disagreement, however, check to see that the transmission system is functioning properly, and that the roller-path tilt compensator is at its proper setting, both for bearing and for inclination.
21D7. Other checks
After a battery has been aligned in elevation, a test of the automatic follow-up system should be made. This involves training on a target, setting up the problem in the computer and positioning the gun in automatic (using computed gun orders), setting the sights according to generated sight angle and sight deflection, and checking to see whether the gun telescopes are on target. If they are not on, the amount that Vs and Ds must be changed from the computed values to bring the sights on the target represents the error of the system. To eliminate trunnion-tilt errors when this test is made, it should be done when there is little or no roll.
The necessity for alignment of the fire control radar beam with the director optics should be mentioned. It is obvious that the radar line of sight must be parallel to the optical line of sight; otherwise false values of target position would be measured when tracking by radar. This alignment, the mechanical details of which vary with different types of radars and directors, is comparatively simple. Basically, it consists of placing the director exactly on the target optically and adjusting the position of the antenna until the pointer's and trainer's radar scopes give the optimum “on target” indication. When this condition has been satisfied, the antenna is locked in place.
The preceding discussion of battery alignment has dealt only with gun batteries. Proper alignment is equally important in any other director-controlled battery such as torpedo, rocket launcher, etc.; but the methods used will vary with the characteristics of the battery to be aligned.
E. Firing Stop Mechanisms
21E1. General
Theoretically the battery is aligned when, with the dials matched and parallax on zero, all the lines of sight and the axes of all gun bores are parallel, regardless of the ordered angles of gun train and gun elevation. In practice, however, the battery check is not complete until: (1) the firing cutout cams in each gun have been plotted, cut, and installed; and (2) the firing stop mechanisms have been checked, with the cams installed, to ensure that both the mechanical and the electrical firing circuits are interrupted properly whenever the guns move from a zone of safe fire to a danger zone.
The importance of this phase of the procedure cannot be overemphasized. The numerous casualties that have occurred because a ship has fired one of her guns into her own superstructure testify to the seriousness of any misalignment of the firing stop mechanisms. It is equally important to note that in all cases these casualties could have been prevented. They resulted from negligence on the part of the ship's personnel: the cams were cut improperly and in some cases misaligned, or the firing stop mechanisms were inoperative through lack of preventive maintenance.
Firing stop mechanisms are designed to interrupt the mechanical and electrical firing circuits whenever the guns are trained or elevated to a position where firing the guns would endanger ship's personnel or damage own ship. They should not be confused with the frameworks of steel tubing or depression-stop cams that are used occasionally to limit the movement of light machine guns to safe zones of fire. Firing stop mechanisms do not interfere with the free movement of the gun; this is done by the train and elevation limit stops.
The Bureau of Ordnance has issued definite instructions for the guidance of the personnel responsible for plotting, cutting, installing, and checking firing cutout cams and mechanisms. In all cases these regulations must be adhered to strictly. In addition, special instructions govern particular gun installations; for example, the firing limits of 40-mm gun mounts.
21E2. BuOrd regulations for firing cutout cams
In accordance with Bureau of Ordnance instructions, a firing stop cam installed on any gun shall be so designed that it will prevent firing into fixed structure and into certain other areas under the following conditions:
1. When removable parts of the ship's structure such as stanchions, handrails, life lines, and davits, and equipment such as boats, chests, lockers, and hatches, have been removed or stowed so as not to obstruct the line of fire.
2. When, for the firing bearing (azimuth and elevation) under consideration, movable parts such as guns, turrets, cranes, and booms cannot be so disposed of as to clear the line of fire.
3. When, for the firing bearing (azimuth and elevation) under consideration, the personnel cannot be so disposed as to clear the line of fire.
Gun firing cutout cams designed in accordance with the above instructions will not restrict the firing of the guns so as to prevent damage to the following:
1. Personnel within the danger blast area of the gun.
2. Material that can be, but has not been, moved clear of the line of fire.
In order that firing stop cams may be designed for the maximum possible zone, the Bureau of Ordnance does not approve of cutting firing stop cams to provide protection of forestays, halyards, antennae, and such top hamper. Peacetime target practice must be arranged so as to keep the fire clear of these obstructions. In wartime these hazards must be accepted.
For guns of 5-inch and larger caliber a minimum clearance of one caliber shall be maintained between the extension of the axis of the gun bore and the fixed structure. For protection of other installations, this minimum clearance is computed with the other guns and directors at zero-degree elevation and the angle of train at which they are normally secured. For 3"/50 caliber guns a minimum clearance of 5 inches is required, while for 40-mm guns a minimum clearance of 8" must be maintained. This seemingly large clearance for 40-mm guns is due to the lag between the time the cutout mechanism functions and the time that the guns actually cease firing.
For 20-mm guns the safe zone of fire must be plotted with the ready-service lockers open. This is necessary because in time of battle these lockers are normally open, or may be open, so that the ammunition can be reached easily.
21E3. Turret firing cutout mechanisms
The majority of the main-battery guns of 6-inch bore and larger have separate firing cutout mechanisms for train and for elevation. In train the cam surface is fastened to the stand near the roller path, while the plunger and cutout switch are located on and move with the carriage. Cutout action is obtained by securing cam surfaces to the stand at those relative bearings coincident with the danger zones of fire. The elevation cutout is a flat cam-and-plunger mechanism mounted on the deck lug of the gun. Movement of the gun in elevation, through a mechanical connection, rotates the cam.
On these large-caliber guns only the electrical firing circuit is interrupted by the cutout cams. The percussion firing circuit is not cut out by the cams; however, warning lights are energized in the mount or turret to indicate that the gun is in a danger zone and should not be fired by percussion.
The proper settings for these firing cutout mechanisms are outlined in the applicable OP's. Usually the OP's will list the settings for the individual turrets in ships of a class. After the cams are installed, they must be checked by methods similar to those outlined for smaller guns.
21E4. Profile-cam mechanisms
Dual-purpose guns, and nonturret guns generally, incorporate mechanisms wherein one cam, referred to as a profile cam, controls the firing circuit when the gun is in or near a danger zone of fire in either train or elevation. This type is used on 40-mm, 3"/50, 5"/38, 5"/54, and some larger-caliber guns. The mechanical action of these mechanisms differs slightly from gun to gun, but in principle they are the same and can be considered collectively.
The cutout feature of a profile-cam firing stop mechanism is accomplished by the action of a plunger or cam pin on a lever and a circular profile cam. When the plunger rides up on a high point of the cam, which represents a danger or nonfiring zone, it pushes against the plunger lever, which in turn causes sufficient movement of another lever or levers to interpose a break in the firing circuit. In the 5"/38 dual-purpose gun, this movement interrupts both the electrical and the percussion firing circuits.
Figure 21E1 shows a cut-away view of a typical firing stop mechanism plunger and profile cam; this one is from a 5"/54 mount. The cam is turned by gun train order at one-to-one speed, while the plunger mechanism moves radially from near the center to the edge of the cam in accordance with gun elevation order. A point near the center of the cam represents maximum gun elevation, and the outer edge minimum gun elevation.
The rise from the cut-away to the raised portion of the cam is inclined to the face of the cam by an angle of 30 degrees. This permits the plunger to ride from the low machined-out surface of the completed cam to the high surface without excessive wear or scoring. Cutout occurs when the plunger is two-thirds of the way up the incline. This must be borne in mind when laying out the cam. It is necessary to scribe two additional lines, besides the line representing the danger zone of fire, on the cam. These lines represent the top and bottom of the incline.
21E5. Plotting the cams
The entire firing stop mechanism, with the exception of the profile cam, is assembled and installed on the mount during manufacture. As the profile of the cam will vary with the location of the gun, the cam must be plotted after the gun is installed aboard ship. The precise method of plotting the cam depends upon the type of gun. One of the methods which can be used for a 5"/38 gun will sufficiently illustrate the principles of the operation.
This method consists of mapping out the zone of permissible fire by means of a boresight in the bore of the gun, plotting the minimum safe angles of elevation at successive angles of train on a cam-plotting sheet, and transferring the plot to the cam blank. Cam-plotting sheets, such as illustrated in figure 21E2, are available at all naval shipyards. It is important to note that plotting sheets are made for each specific caliber, mark, and modification of gun, and that right and left guns of a twin mount use different sheets (one cam turns clockwise, the other counterclockwise). Before transferring the plot to the smooth copy of a plotting sheet, it is in each case necessary to consult the OP or OD for the particular type of mount. This will contain such information as the minimum radius of all curves in the plot, the radial distances of the top and bottom of the cam slope from the actual cutout line (see fig. 21E3), and other data essential to accurate plotting.
It should be noted that, in tracing the safe-fire zone of any mount with more than one gun, a separate cam must be plotted for each gun (each pair of barrels in the case of the 40-mm only). Thus one gun may stop firing upon training toward the edge of an obstruction while the other gun or guns will continue to fire until the mount has been trained a few degrees more toward the ship's structure.
21E6. Final steps
After a cam has been plotted by any one of the methods listed, it is turned over to the yard for machining. When the machine work is completed and the cam is returned to the ship, it must be installed and synchronized according to the instructions contained in the ordnance publication pertaining to that type of gun.
Finally the accuracy of the cam profiles and the adjustment of the firing stop mechanism must be checked. The following is a suggested procedure; however, the voltmeter recommended can be replaced by an electric lamp of the correct voltage rating, if desirable.
1. Make sure that the gun is not loaded.
2. Lay the gun so that it is outside of a danger zone.
3. Complete the firing circuit as for local firing.
4. Connect a suitable standard voltmeter between the firing pin and ground, or across the terminal of the firing-cutout switch.
5. Firing-circuit voltage should be indicated by the voltmeter, thus establishing continuity of the circuit.
6. Lay the gun so that it is within a danger zone. The voltmeter should now read zero.
7. Train and elevate the gun in and out of the firing zone, at various points, verifying that the operation of the firing stop switch, as indicated by the voltmeter, takes place at the proper points.
8. If percussion firing is also included in the cutout mechanism, check to see that percussion firing cuts out at the same points as those at which the switch in the firing circuit opens.
If a voltage is present in step No. 6, or during those parts of step No. 7 when the gun is in a danger zone, a defective firing stop switch, a defective or short-circuited plug, or faulty cable insulation is to be suspected, and immediate steps must be taken to locate and correct the defect.
The preceding section on firing cutout cams has briefly presented the theory of these mechanisms, but when it comes to plotting and cutting the cams, remember this: always consult the OP; there is no substitute for exact knowledge.