Chapter VI: French 75

GUN.

=Weights and Dimensions.=

Weight 1015 lbs.
Total Length (about) 107 in.
Rifling 24 grooves
Twist—right-hand slope, 7 degrees, 1 turn in 25.6 calibers.

CARRIAGE.

=Weights and Dimensions.=

Weight complete 1642 lbs.
Weight of gun and carriage complete 2657 lbs.
Weight at end of trail, carriage limbered 114 lbs.
Diameter of wheels 52.5 in.
Length of recoil (about) 45 in.
Maximum angle of elevation (338 mils.) 19 degrees
Maximum angle of depression (178 mils.) 10 degrees
Amount of traverse of gun on carriage (106 mils.) 6 degrees

=The gun= is of the built-up type and consists of a forged steel tube which extends from muzzle to breech. A breech hoop is shrunk over the rear of the tube and extends beyond it to provide a breech recess. This recess is threaded with seven threads to take the breechblock. A bronze jacket encircles the central portion of the tube. Inner and outer locking hoops screw to and firmly fasten the tube, hoop and jacket together and prevent them from separating under the stresses of recoil. A muzzle hoop is screwed on the tube at the end of the muzzle.

_MODEL OF 1897_ (_FRENCH_)]

MODEL OF 1897

(FRENCH)]

A recoil lug on the under side of the breech hoop forms a point of attachment between gun and recoil mechanism through the media of a piston rod and coupling key.

The rear sight is attached to the rear portion of the breech hoop. On top of the hoop at its rear end are two quadrant seat plugs. The front sight is fixed on top of the rear end of the jacket. A sweeper plate which sweeps and lubricates the roller is secured to the front end of the jacket.

=The breechblock= is the Nordenfeld rotating type, cylindrical with seven threads which serve to screw it into the rear of the breech. The breech is opened and closed by rotating the block 120 degrees around its axis. The block advances during the rotation due to the pitch of its threads and forces the cartridge case into the bore. The gun cannot be fired until the block has been completely closed, a condition which must exist before the striker of the firing mechanism is in line with the primer cartridge of the projectile.

=The extractor= consists of three parts:

1. Two arms connected by a hollow shaft.

2. A spindle which passes through the shaft and fastens it to the breech.

3. An extractor tang.

=The action of the extractor= is as follows: When the breech is closed the arms of the extractor are pressed against the face of the tube by the rim of the cartridge case which bears against them. The extractor tang projects into a groove in the inner face of the breechblock known as the loading groove. This groove is circular and its depth is equal to the projection of the extractor tang. It terminates in a helical guide surface called the “ejecting ramp.”

MODEL OF 1897

(FRENCH)]

The first part of the movement of opening the breech serves to move the grooves in front of the extractor tang. This pressure forces the extractor tang back into its slot in the breech hoop and as the extractor tang is firmly seated in the extractor, the arms of the latter are rotated around the extractor spindle. The arms being brought to the rear, press against the rim of the cartridge case, which is thereby started and ejected.

Inversely (the breech being open) when a cartridge case is smartly inserted in the chamber the rim carries the arms of the extractor forward. The extractor tang is thereby forced against the beveled surface of the ejecting ramp and automatically starts the closing movement of the breechblock.

A safety catch is provided to keep the breech locked between the time that the breech is closed and the shot is fired.

=The firing mechanism= consists of a striker or firing pin seated in the breechblock, a firing hammer, firing rack, spring and lanyard. By pulling the lanyard the hammer is drawn back and the rack moves forward against the compression of the spring due to its being geared to the hammer. When the lanyard is released, the spring forces the rack back which in turn causes the hammer to fly forward and strike the primer. A safety device is provided for locking the hammer while the piece is in the traveling position.

THE CARRIAGE.

The gun is mounted upon a cradle which encloses the recoil and counter-recoil mechanisms. The device for elevating the gun through the angle between the horizontal and the line gun-target (angle of site) is interposed between the trail and the rocker while the device for giving the gun elevation for range is placed between the rocker and the cradle. This arrangement is known as the independent angle of site or independent line of sight. It has the advantage over the three-inch type in that it allows the range elevation to be altered without disturbing the elevation for site.

_LONGITUDINAL SECTION_]

MODEL OF 1897 MI FRENCH

REAR VIEW]

MODEL OF 1897 MI(FRENCH)

LEFT SIDE]

MODEL OF 1897 MI(FRENCH)

RIGHT SIDE]

MODEL OF 1897 MI(FRENCH)

PLAN VIEW]

=The principal parts= of the carriage are: trail, axle, wheels, brakes, shields, angle of site elevating mechanism, range elevating mechanism, traversing mechanism, rocker, cradle and sights.

When traveling or resting, the tube rests on the =cradle= which supports it by means of the jacket. When firing, it recoils on the cradle by means of the rollers. The jacket has two pairs of rollers, and the muzzle hoop is provided with a single pair of rollers. On the upper part of the cradle are the lower slides, on which the jacket rollers, supporting the tube, roll during the recoil. When the jacket rollers are about to leave the lower slides, the muzzle rollers come under the upper slides; the tube is then supported until the end of the recoil by the muzzle rollers and the more forward of the two pairs of jacket rollers. This device gives the gun a long recoil upon short slides. Inclined planes are used in such a manner that when the gun returns into battery the rollers rise from the lower slides thereby relieving the slides from the weight of the tube when the tube is in the traveling position.

The carriage supports the cradle which in turn supports the tube. The cradle and the tube together are displaced, during the laying for elevation with respect to the carriage which remains stationary. The carriage is held steady on the ground by means of the trail spade which with abatage prevents the carriage from recoiling on the ground.

=Abatage= consists of elevating the wheels on the brake shoes which are provided with small spades which prevent lateral movement. The brake shoes are fastened to brake beams attached to a sliding rack beneath the trail in such a manner that the abatage frame may be placed under the carriage during travel. In preparing to fire, the frame may be adjusted to allow the brake shoes to slip from a position in rear of the carriage wheels to a point directly beneath the wheels.

Abatage is accomplished as follows: (1) The brake shoes are dropped to the ground in rear of the point of contact of the wheels with the ground; (2) The trail is lifted, turning around the axle, until the spade is about five feet in the air. Tie rods and a slide working on a rack beneath the trail move forward in this action; (3) The trail is then brought down. The rack prevents the slide from moving to the rear and the carriage turns on the abatage frame until the wheels rest upon the brake shoes. This gives the gun a three point support, two small spades under the wheels and a larger one at the end of the trail.

=Laying in direction= is accomplished by traversing the piece on the axle. The trail spade is fixed and the axle is straight and rigid so that in the movement of the gun to the right and left on the axle both wheels must turn—one to the front and one to the rear. The device for laying for direction is composed of a threaded axle, which is prevented from rotating by a spur and a sliding nut which is contained in a box fixed on the left flask of the gun. This nut bears one of the bevel gears, which is put in motion by the hand wheel. The traverse is three degrees either side of the center or a total of 6 degrees or about 105 mils.

=Laying for elevation.= To obtain greater accuracy and speed in firing the 75 has an independent angle of site. A rocker with two trunnions is interposed between the cradle and the carriage. The rocker trunnions are seated in the cradle trunnions and support them. This gives the same rotating axis to both rocker and cradle. This is necessary for the mechanical addition of the angle of site elevation and for the range elevation. When the angle of site handwheel is revolved it turns a pinion, which meshes in the rocker rack and thus causes the rocker to move in relation to the carriage. This gives the cradle through the rocker the elevation equal to the difference in elevation between the target and the gun. It is independent of the angle given to the gun for the elevation due to range to the target.

_RANGE ELEVATING MECHANISM_]

The angle given the gun for range is effected through a telescopic screw. This screw is fastened at one of its extremities in the rocker and at the other in the cradle. The nut which receives the elevating screw is seated in an oscillating support which allows it to always remain perpendicular to the axis of the bore at any elevation.

The angular displacements of the cradle with respect to the rocker (angle of elevation) are recorded by the elevating system composed of a graduated arc and a range drum. The lower part of the arc is connected with the right arm of the rocker. The arc is graduated in meters. When the range handle is turned the arc does not move, but a brass slide block connected with the gun and the cradle and bearing an index slides along the arc. It is thus possible to set the range in meters.

However, the arc graduations are not very legible and it has been supplanted by a graduated range drum having more legible readings.

The black part of the arc bears a rack which meshes with a pinion, which in turn rotates around an axle fixed on the cradle. When the cradle moves, the pinion rotates and carries with it the range drum.

The elevation on level ground varies from a minus 11 degrees to a plus 20 degrees. Greater elevation may be obtained by sinking the spade.

=The Recoil and Counter-recoil mechanisms= are of the Hydro-pneumatic type. Their accurate description is a secret. The following brief description will give only a general idea of the working of the mechanism. The whole apparatus is inside of the cradle through which are bored two cylinders: an upper cylinder 40 mm diameter, and a lower cylinder 66 mm diameter. These cylinders may communicate through a large hole. A piston moves in the upper cylinder, the piston rod, 24 mm diameter, being fast to the gun.

In the lower cylinder are: (1) The valve carrier pipe screwed in the rear part of the cylinder supporting =spring valves=; in the inner walls of the pipe are cut two grooves; the valve carrier pipe is ended by a circular ring. (2) =The diaphragm= with its hollow rod. (3) =The loose piston= with its small rod, which may come in contact with the upper rack of the gauge. The two cylinders are full of liquid, usually Russian oil. The front part of the upper cylinder in front of the piston may communicate freely with the air through the Front Plug. The front part of the lower cylinder is closed by a plug and contains compressed air at a pressure of 150 kg. per sq. cm.

=Operation.= In recoil the piston of the upper cylinder compresses the liquid, which has to pass through the spring valves and between the circular ring and the hollow rod of the diaphragm. The passage of the liquid through these different openings constitutes the braking effect. In so moving the liquid opens the valves, which are widely opened at the beginning of the recoil and gradually close in proportion to the decrease of the speed of the recoil. At the same time, the air of the recuperator is compressed by the action of the liquid on the diaphragm.

When the recoil is finished, the compressed air pushes back the diaphragm. The liquid thus compressed acts on the small cylinder piston and obliges it to come back into its initial position, bringing with it the tube.

The liquid in flowing back completely closes the valves and must pass between the diaphragm rod and the inner wall of the pipe. At beginning of the return into battery, the space between the rod and the bottom of the groove is large. This space decreases in proportion to the progress of the return. The passing of the liquid through this constantly decreasing space causes the braking which at the end reduces the speed of the return to nil.

CARE AND PRESERVATION OF FRENCH 75.

Dismountings.

=Cannoneer Dismountings.=
A. Breechblock.
1. Safety piece.
2. Striker.
3. Lanyard.
4. Striker hammer.
5. Hammer spindle.
6. Spring assembling pin.
7. Rack springs.
8. Rack.
9. Latch pin. (Pawl Pin.)
10. Latch (Pawl).
11. Latch spring. (Pawl spring.)
B. Extractor.
1. Clow. (Tang.)
2. Spindle.
3. Arms.
C. Level.
D. Sight case.
E. Wheels.
F. Limber pole.
G. Fuze setter from caisson.
H. Luggage frame from limber.

=Mechanic Dismountings.=
A. Coupling keg.
B. Push gun back on slides.
C. Safety bolt.
D. Friction piece. (Sweeper plate.)
E. Wiper. (Guide piece.)
F. Front plug.
G. Filling hole plug.
H. Elevating screw pin.
I. Elevating screw.
J. Trunnion caps.
K. Rocker Trunnion caps. (Half Rings.)

=Daily Cleaning and Lubricating.=
(By cannoneers.)
1. Clean sight support and socket.
2. Lubricate oil holes 20, 21, 22, 23, 24.
3. Clean base of sight column.
4. Clean levels.
5. Lubricate range mechanism. (Holes Nos. 1 and 2.)
6. Clean and oil rocker trunnion caps.
7. Oil holes 7 and 8.
8. Clean and grease exposed parts of axle.
9. Dismount and clean breech and all its parts.
10. Clean and grease the bore if the gun has been fired.
11. Clean and grease the slides if the gun has been fired.
12. Grease the wheels if the gun has been on the road.
13. Clean fuze setter.
14. The life of a gun depends on “Daily Care.”

=Forbidden Practices.=
1. =Readjustment of French Sights=.
2. =Fitting with files=.
3. =Forging and Riveting=.
4. =Unauthorized Dismountings=.

Care of Recoil Mechanism.

(_Chief Mechanic Only_)

=Caution.=—Never remove piston rod nut, as piston is under pressure and would pull piston rod into cylinder.

The recoil apparatus proper cannot be dismounted.

If properly taken care of the recoil mechanism will not go wrong for years; but if neglected, its destruction is only a matter of very little firing.

The position of the gauge finger is the index as to whether or not the recoil is being properly absorbed.

The joints are not absolutely tight, the slight leakage which takes place during fire or even when gun is at rest is not important.

In normal firing conditions the recoil apparatus contains a slightly greater quantity of oil than absolutely necessary; this quantity is called the “reserve.” When the reserve is exhausted any loss is liable to reduce the quantity strictly necessary. The loss may prevent the gun from fully returning to battery when firing.

The amount of reserve is indicated by the position of the gauge.

No reserve: The gauge finger is down deep in its recess.

Full reserve: The end of the gauge finger is level with the gauge index.

Excess reserve: The gauge finger projects beyond the index.

_No firing should be done with an excess reserve._

_The gauge finger should be between the index and ⅛ inch below the index._

It is the duty of the Chief Mechanic to see that the gauge finger is in the proper position before firing.

It is the duty of the Executive and the Chief of Section to see that the gauge finger is in the proper position during fire; if it moves from this position the Chief Mechanic will be called to make the proper adjustments.

When the gauge finger has been brought to the proper position there are only two conditions which call for tampering with recoil apparatus.

1. The gun goes into battery too slowly, or has to be pushed in.

In this case the gauge finger will usually be found too deep in its seat, and the pump will have to be used. If the gauge is in the proper position look at the slides. Either they will be found dirty or bits of the wiper may be nicked off. In the latter case the gun can be fired without the wiper.

2. The gun jumps badly. In this case the gauge finger will generally be found beyond the index. When this is the case the oil extractor must be used until the gauge finger is in the proper position.

If after cleaning slides, putting gun in abatage and adjusting gauge finger, the gun still jumps badly—complete draining of the reserve and refilling will frequently remedy the trouble.