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United States. Army. Ordnance Dept.

Handbook of artillery : including mobile, anti-aircraft and trench matériel online

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model of 1917 MI, except in the method of securing the latch plate
to the gun. On the models of 1917 and 1917 Ml, the latch plate
is secured to gun by screws, while on the 1917 Ml the latch plate is
secured to gun by means of a lug.

The breech recess is rectangular in shape. Two extractor trunnion
seats, one in each side, are cut to the proper radius for the extractor
to rotate and slide. Two holes are drilled from the rear face of the
breech, one on each side, to accommodate a spring and plunger which
press against the hub of the extractor, keeping it in place and also
aids the extractor in ejecting the cartridge case.

The breech mechanism consists of the following parts: Breech
block, operating lever, operating handle, operating cam, operating
cam cover, trigger shaft, extractors, firing mechanism, latch, and
closing spring case.

The breechblock is of the drop-block type and is rectangular in
shape. Two grooves run lengthwise on the block, giving a wedging
effect against the end of the cartridge case when the block is closed,
and when opened insuring a clearance between the cartridge case and
the block thus eliminating any chance of the cartridge case jamming.
A venthole permits the escape of gas from a ruptured primer.

Two extractor grooves, one on each side of the block, are cut
parallel to the guide grooves and curve to a certain cam develop-
ment which permits the proper action of the extractors. At the top
of the block a radius is cut to permit of clearance when inserting the
projectile. The toe of each extractor is cut to a radius which will
just slide along the body of the cartridge when in place and engage
the run of the cartridge case.

The firing mechanism belongs to that type known as the con-
tinuous-pull mechanism; that is, no cocking of the firing pin is re-
quired other than a pull on the lanyard or trigger shaft. This
arrangement permits of repetition of the blow from the firing pin in
case of a misfire, as often as desired without the opening of the

mechanism.

Weights, mount, etc.

Weight of mount only pounds. . 12, 175

Weight of gun and mount do. ... 15, 280

Weight of gun and cradle do. ... 7, 105

Weight of cradle and recoil systems do. ... 4, 000

Weight of yoke with elevating and traversing mechanisms do. ... 5, 100

Weight of traversing rack, friction band, roller cage, and base plate do. ... 3, 000

Weight of sight and supports do. ... 75

The mount is emplaced in a concrete emplacement, in which 16
anchor bolts are set, and depressions provided for 8 leveling screw
thrust plates. A niche for an outlet box, through which electrical
connections are made to the main base, or for a storage battery when



348

generated current is not available, is constructed in the concrete
to meet the requirements of the mount.

This emplacement is constructed by the Engineer Corps, which
also furnishes and installs the necessary outlet or storage battery
and furnishes the plug box portable lamp, cable, and plug.

The principal parts of the mount are the base plate; racer; pivot
yoke; cradle (containing recoil cylinder and counterrecoil mechan-
ism) ; traversing mechanism, including traversing rollers and distance
ring; elevating mechanism; firing mechanism; illuminating circuit
and sight.

The base plate is a circular steel casting which rests on the concrete
emplacement with its upper surface machined to form the lower
roller path. Sixteen holes, equally spaced around the circumference
of the flange, are provided to receive the foundation bolts which
retain the base plate in its proper position in the emplacement. A
cylindrical projection in the center forms on its interior the housing
for the 360 electrical contact, and on its exterior receives the trav-
ersing rack.

The racer is a circular steel plate, upon which the pivot yoke is
bolted. The" under surface is machined to form the upper roller
path, and the upper surface to fit the yoke. The racer rests on the
rollers and rotates freely about the hub on the traversing rack. Two
clips, front and rear, are bolted to the under side of the racer, and
engage with a lug on the base plate to prevent the racer from leaving
the rollers, and overturning the mount, when the gun is fired.

The pivot yoke is a steel casting, consisting of two vertical side
frames joined in front by a transom. At the top of each frame is a
trunnion bearing and trunnion cap lined with bronze bushings.
Tapped holes are provided in the left frame for the depression and
elevation stop. The distance ring is a circular bronze ring provided
with spaces and bearings for the traversing rollers. The traversing
rollers, 30 in number, are interposed between the roller paths of the
base plate and racer, bearing the weight of the mount. The rollers,
roller paths, traversing rack, and pinion are protected from the en-
trance of dust, sand or grit, by dust guards. The oil grooves on the
circumference of the distance ring serve to distribute oil from the
holes in the flange forming the base of the yoke to the axles of the
rollers. A friction band, resting on the base plate, is made to grip
the traversing rack. To adjust this band, which allows slipping
of the traversing rack to protect the teeth of the traversing pinion
from too heavy a stress, a cover in the base of the yoke is removed,
giving access to the parts beneath.

Motion of the mount in azimuth is obtained by a traversing pinion
and shaft, the pinion meshing with the teeth of the traversing rack.
Power is transmitted from the traversing handwheel to the traversing



349




350




351

worm, thence to the mount through a set of gears and a clutch
mounted in the traversing gear case. Two speeds of traverse are
possible upon throwing the clutch in by means of a handle, so that
high or low speed gears are connected to the upper traversing shaft.

The elevating mechanism consists of an elevating rack keyed to the
underside of the cradle, having on its face teeth which mesh with the
elevating worm. The rack is of sufficient length to provide for ele-
vations from to 90.

The cradle is bored and bushed to receive the gun. Front and rear
liners are provided through which the gun slides in recoil. In addition
the cradle forms the housing for the recoil and counterrecoil systems.
The interior of the cradle has a cored recess to suit the firing mech-
anism.

Recoil mechanism is of the hydro-spring type. The recoil cylinder
is screwed from the front into a seat provided in the top of the cradle.
The piston rod is attached to the gun lug at one end and provided
at the other end with a piston, slightly smaller than the bore of the
cylinder. Three longitudinal throttling grooves are cut in the
interior surface of the recoil cylinder, each groove subtending an arc
of 30. With the cylinder in assembled position one groove is
located at the bottom. The recoil cylinder has a capacity of 6J
pints of hydroline oil.

Two cylindrical holes bored in the cradle form the housing for the
counterrecoil springs. Spring rods are attached at one end to the
gun lug and at the other end to the spring-rod piston. When the
spring compressor is first assembled it is secured against rotation by a
retaining screw. The counterrecoil plunger, designed to check the
recoiling parts as they return to battery, passes through the front
end of the recoil cylinder and enters the recess in the forward end of
the piston rod.

When the gun is fired it recoils to the rear about 12 inches in the
cradle, carrying with it the recoil piston and spring rods, thereby
compressing the counterrecoil springs. A portion of the energy of
recoil is taken up by the resistance the liquid offers to being forced
through the variable slots formed by the throttling grooves and the
constant clearances between the piston head and the interior surface
of the cylindrical bore, the remainder of the energy being absorbed
by the springs. The width of the grooves is uniform, but their depth
is proportioned so that the areas of the orifices, varying with the
position of the piston during recoil, will be such as to give, with the
aid of the counterrecoil springs, a constant resistance throughout the
length of recoil. The pressure in the cylinder is therefore a uniformly
decreasing one.

The counterrecoil buffer is tapered so that the escape of oil during
counterrecoil, through the varying diametrical clearances between
18322820 23



352

the plunger and the hole in the piston, will offer such resistance as
will control the motion of the gun during its return to battery posi-
tion after firing.

The firing mechanism consists of a firing handle whose shaft passes
through the center of the right trunnion and carries on its inner
end a lever which operates the firing shaft. Previous to the firing
the gunner pulls the firing handle which compresses the cocking
spring solid and moves the lever on the breech to the tripping posi-
tion; the gunner will know when the mechanism has reached this
position by feeling the increased pressure exerted by the firing spring.
To fire the gun, the gunner pulls the firing handle, compressing the
firing spring, thus tripping the firing pin. The two-stage movement
of the firing handle is intended to permit a shorter movement at the
moment of firing.

The illuminating circuit has a 360 contact mounted in the base
plate. Direct-current mains of either 110 or 220 volts are connected
with an outlet box located in the concrete emplacement.

Two circuits are led from the 360 contact, one leading to the
plug box for use as a portable lamp of line voltage, and the other
circuit leads to the switch box; from here it is led to the rheostat
from which two branch feeders are taken to the two receptacle boxes
and bolted to the yoke. A circuit is taken from the right receptacle
box with a spliced branch feeder to a candelabra receptacle for the
reticule lamp and deflection pointer lamp. Another circuit is run
from the left receptacle box with two spliced branch feeders to a
candelabra receptacle for the elevation pointer, range disk pointer,
and elevation correction lamps. These lamps are supported by lamp
brackets fastened to the trunnion and sight mechanism.

The rheostat used cuts down the voltage and makes the use of
low-voltage lamps and batteries practicable in case the line voltage
fails through accident.

The cable leading from the 360 contact with the plug, switch, and
rheostat is of the twin conductor, leaded, and armored type. The
two branch feeders leading from the rheostat to the receptacle boxes
and thence to the lamp brackets are of the portable conductor type.
The various cables are fastened to the mount by means of cable
straps and twisted hooks.

Sight for antiaircraft mount, model of 1917. This instrument
includes all parts used to direct the elevating and traversing of
mount so that the gun may be pointed properly in elevation and
direction. The parts consist of a sight proper (telescope), the sight
mount, range disk, correction scale, and pointer. For any visible
target the data necessary to properly lay the gun consists of fuse-
setter range, travel in elevation and deflection, and the required
arbitrary correction.



353

The target is brought into the field of view by turning the azimuth
and the angle-of-site knobs, imparting to the sight a movement in
azimuth and elevation, respectively.

A scale is provided whereby the sight proper may be set in eleva-
tion at any desired angle, whence the gun by means of the elevating
mechanism is also elevated to the same angle (corrections being zero).

An azimuth scale is also provided between the fixed and rotating
parts of the carriage so that the gun by means of the traversing
mechanism may be set at any desired angle in azimuth.



75-MILLIMETER ANTI-AIRCRAFT TRUCK MOUNT.

(MODEL OF 1917.)



To appreciate the difficulty of anti-aircraft fire it suffices to con-
sider that one is firing practically at a bird whose velocity is about
50 meters per second, i. e., one-sixth the average velocity of the
projectile itself in the case of the 75 -millimeter field gun. The
principal result hoped for by the anti-aircraft artillery fire is to prevent
airplanes from accomplishing their mission by obliging them to fly
at increasing altitudes, to continually change their direction, and also
to prevent their crossing certain regions.

At the present time the anti-aircraft artillery aims to keep airplanes
beyond the limit of their range. Observation airplanes are obliged
to fly out of range, reconnoitering airplanes continually increase the




TRUCK IN TRAVELING POSITION (RIGHT SIDE VIEW).

height at which they cross the lines, and battle planes must also fly
very high, except when they wish to attack trenches or batteries
with machine guns. Raids of this kind are almost exclusively
carried on at night. The result is that three kinds of fire have
become particularly important:

Fire against airplanes at a great range and a great height.

Fire against very speedy airplanes attacking positions.

Fire at night against bombarding airplanes.

The need of an anti-aircraft weapon to meet the above require-
ments led to the design of the 75-millimeter anti-aircraft truck mount.
This development involved the use of an American model 1916 field
gun, equipped with a hydro-spring recoil mechanism having throttling

(354)



55




356



1




357

valves cut in the cylinders. This gun and recoiling mechanism is
secured on an offset swivel gun mount, suitably mounted on a 2J-ton
White gasoline-driven truck, Model TBC, designed to receive the
base plate of the top carriage.

The gun (see p. 72) is carried on a cradle which rocks in elevation
about the trunnions of the top carriage, and, by means of the elevating
mechanism, a range of elevation from 31 to 82 is obtainable. The
piece has a recoil of 33 inches on its cradle and is provided with a
recoil cylinder, counter-recoil springs, and buffer.

The piece has a muzzle velocity of 1,830 feet per second, and uses
high-explosive and shrapnel shells, the former weighing 14.7 pounds
and the latter 14.3 pounds, employing a 20-second (maximum) time
fuze with each. With a maximum vertical elevation of 82 a
vertical bursting ordinate of 5,980 meters is obtained, and with the
minimum elevation of 31 a vertical bursting ordinate of 1,750




VIEW SHOWING TRUCK MOUNT IN ACTION.

meters is obtained. In both cases the bursting ordina^es were
limited by the time fuze.

The gun with the breech is located directly behind the driver's
seat, but as the length of recoil is fixed, the firing position is limited
to such horizontal position of the mount as would permit of the gun
recoil clearing the sides or rear of the truck chassis.

A heavy base plate is secured to the rear end of the chassis, and
the top carriage swings in azimuth about a central pintle bolt on
rollers. By means of the traversing mechanism, the carriage,
carrying the gun and its corresponding mechanism, can be traversed
through 240, which is the field of fire. The chassis is equipped with
firing and stability jacks to relieve the rear springs and truck of all
firing strains. With the jacks properly placed and leveled, the
vehicle is supported on a rigid horizontal platform formed by the
firing jacks and the base plate. When in action the stability equip-
ment .functions to prevent the mount from overturning when the
gun is fired at low angles of elevation.



358

Weights, dimensions, and ballistics.

Caliber inches.. 2.953

Total length of gun do 90. 9

Length of the rifled portion of the bore do 72. 72

Length in calibers 28. 4

Weight of projectile:

Shell pounds. . 14. 7

Shrapnel do 14. 3

Weight of full powder charge do 1. 625

Service muzzle velocity feet per second . . 1, 830

Horizontal range at 45 elevation (14.3-pound shell) yards. . 10, 595

Maximum elevation degrees. . 82

Minimum elevation do 31

Total traverse do 240

Weight of gun and breech mechanism pounds. . 750

Weight of gun, breech mechanism, and carriage do 3, 300

Weight of chasis, including attachments and accessories do 4, 500

Weight of top carriage, and its corresponding mechanism, etc do 4, 250

Total traveling weight of unit (fully equipped) do 9, 500

Overall length of vehicle (traveling position) inches . . 209

Overall width of vehicle (traveling position) do 74$

Weight of chassis only (without attachments) pounds . . 4, 100

Wheel base inches. . 157$

Wheels, front do 36 x 4

Wheels, rear do 36 x 7

Road clearance do 9$

Rating of load tons. . 2$

Tread inches . . 62

The principal parts of the carriage are: The base plate, top carriage,
recoil mechanisms, cradle, elevating and traversing mechanism,
angle of sight mechanism, firing jacks, stability jacks, and rails.

The base plate is a rectangular steel casting secured to the truck
chassis, and, serves as a support for the top carriage. The traverse of
the top carriage is limited by lugs provided on the top of the clip
surface. The clip prevents the traversing rollers from leaving their
path due to the shock caused by action of recoil or counterrecoil. Four
lugs radiating from the pintle bearing serve as points of attachment
for the firing jacks and rail tie rods. At the forward end of the base
plate are two lugs, one on either side, which project out at right
angles to the truck chassis, serving as points of attachment for the
stability jacks. The firing strains produced during action are
transmitted through these lugs and jacks, thus eliminating all unneces-
sary strains from the rear springs and wheels of the truck.

The top carriage is a steel casting, comprising a base and two vertical
side frames designed to mount the cradle to permit sufficient clearance
for the recoiling parts at high angles of fire. A pintle bolt projecting
from the base plate through the case of the top carriage forms an axis
about which the carriage is rotated in azimuth upon four rollers resting
on the roller path of the base plate. Between the two side frames is
housed the angle of sight mechanism. The traverse of the carriage



359

is limited to 240 by means of a traversing stop plunger, which
engages the lugs on the clip surface. When preparing for travel the
stop plunger can be raised to clear the limit stop, thus permitting the
proper position of carriage when en route. Four seats (two on each
side) riveted to swinging arms, for the use of the cannoneers during
action, are secured to the top carriage.

Recoil mechanism. The recoil mechanism is of the hydro-spring
type, with recoil cylinder mounted above the gun. Three longitudi-
nal ribs, or throttling bars, of uniform width but varying height, are
formed on the inside of the cylinder and engage with corresponding
grooves in the piston head. The clearance between the bars and
grooves determines the amount of oil which may pass from the back
to the front of the piston head, thereby regulating the amount of recoil.

The piston rod is a hollow steel tube and is fitted with a bronze
head. The rear end of the piston rod is bored out to accommodate
the counterrecoil buffer, which fits into the bore with a small clearance.
This clearance depends on the taper of the buffer, as the hole in the
piston rod is of constant diameter.

In each spring cylinder, three coils of inner and outer counterrecoil
springs are assembled over the spring rod. The inner and outer
springs are wound in opposite directions to prevent nesting, and each
pair of inner springs is separated from the next pair by a bronze
separator.

When fired, the gun moves back on its slides carrying with it the
recoil cylinder and counterrecoil springs. The piston rod is secured
to a nonrecoiling part of the carriage, thus when the recoil cylinder
moves to the rear, the oil in it must pass from one side of the piston to
the other. The energy of recoil of the gun is absorbed by the resist-
ance which the oil offers to being forced through small openings past
the piston, also by the compression of the counterrecoil springs. The
energy stored up by the springs returns the gun to battery position.
The return movement is eased and regulated by the counterrecoil
buffer, which prevents any undue shock to the recoiling parts by
offering resistance due to the fluid in the hole of the piston rod being
forced out as the buffer gradually enters the hole.

The cradle comprises the counterrecoil spring cylinders with their
component attached parts. The spring cylinders are below the gun
and in the form of two cylinders joined at the center. Above the
cylinders are the bronze lined gun ways or slides. The trunnions
are secured to the cylinders, and the elevating arc is bolted to lugs on
the bottom. Riveted to the right side of the cradle is the elevation
stop and the depression stop.

The rocker, a U-shaped forging, is journaled upon the trunnions.
Bearings are provided in the bottom of the rocker for the elevating
mechanism and a rack is cut on the exterior of its yoke for the angle
of sight worm.



360

The traveling lock is located under the rear end of the spring cylinder.
This device functions to lock the cradle at an angle of approximately
32 in elevation when preparing for travel, thus the elevating angle of




GUN MOUNT SHOWING GUN AT MAXIMUM ELEVATION.

site and traversing mechanisms are relieved of all unnecessary vibra-
tions when the vehicle is en route. At the front end of the base plate
is hinged a lock bar and brace, which engage a lug provided on the rear



361

end of the cradle. In order to bring this lug in proper position to
receive the lock bar, the cradle must be traversed to its traveling posi-
tion, that is, to bring the axis of the gun into a vertical plane with the
center line of the truck chassis.

The elevating mechanism consists of an elevating worm, an elevating
arc or rack, and a train of miter gears mounted on the right side of the
rocker. The shaft on which the elevating handwheel is mounted
extends through the side frame of the top carriage.

The worm is operated through gears and a shaft by means of a
handwheel, and rotates the cradle about the trunnions. Any move-
ment imparted to the worm by the handwheel will cause the cradle
to move with relation to the top carriage. One turn of the hand-
wheel will cause the gun to be elevated or depressed approximately




TRUCK IN TRAVELING POSITION (LEFT SIDE VIEW).

1.7. The elevating arc permits a change of elevation from 31 mini-
mum to 82 maximum.

The traversing mechanism located on the left side of the top carriage*
consists mainly of a traversing handwheel and shaft, worm, worm
wheel, clutch, bevel pinion, traversing pinion, and rack. The worm
engages the worm wheel and can be either engaged with or disen-
gaged from the shaft by means of a clutch operated by a foot lever.
If the clutch be disengaged, the worm and worm wheel are released
from the gear train, and the top carriage may be traversed about the
pintle bolt by hand. The traversing pinion engages the traversing
rack which is secured to the base plate. One turn of the handwheel
will cause the gun to be traversed approximately 2.

The angle of site mechanism consists of a handwheel, handwheel
shaft, and angle of site worm. The angle of site worm is secured
to the top carriage and engages in a rack cut in the face of the yoke of
the rocker. To one end of the shaft which extends out to the right



362

side of the top carriage is secured the handwheel and to the other end
is fixed a miter gear.

The angle of site worm is a one-piece forging comprising a worm,
a shaft, and a miter gear of the same size as that on the bracket at
an angle of 90 to the handwheel shaft and with the miter gears at,
the ends in mesh. One turn of the handwheel will cause the rocker
and the cradle to be elevated or depressed approximately 25 mils.
The rocker allows a correction of 124 mils depression and 200 mils
elevation.

Firing and stability jacks. The principal parts of the firing jacks
are the jack body, jack screw, foot, and spade. There are two firing
jacks, the bodies being hinged at the rear corners of the base plate
as shown on page 363. At the end of the jack screw the foot, or float,
is secured by a ball joint which enables the jack to be seated on
inclined surfaces. The foot is provided with a sharp-pointed spade,
which is driven into the ground. A tie rod is used between the two
jacks to keep the jack bodies from spreading out when the load is
put on the screws. The firing jacks act as rigid supports for the base
plate at the rear corners, and relieve the truck chassis of firing strains.

The stability jacks are essentially the same in construction as the


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Online LibraryUnited States. Army. Ordnance DeptHandbook of artillery : including mobile, anti-aircraft and trench matériel → online text (page 17 of 19)