An example of this class is the British Fuze No. 101 E. It will be seen in fig. 6 that the detonator is contained in the graze pellet. With this arrangement it is possible for the needle to fire the detonator when the cap is crushed in on impact (although the graze pellet may not have acted).
Detent Spring
Detent
Centrifugal ' Bolt
Cap
Shutter open
Shutter wi Detent
leedle inator 'Graze Pellet
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FIG. 6.
The shock of discharge causes the detent to set back, compressing its spring. Then the detent spring, reasserting itself, jams the point of the detent under the projecting shoulder, so that it cannot return to its original position blocking the centrifugal bolt. This bolt, actuated by the rotation of the shell, moves outwards, freeing the graze pellet. The latter is restrained from working forward during flight by a creep-spring. On graze or impact the pellet, overcoming the creep-spring, flies forward, and the detonator is fired by coming in contact with the needle. The flash passes into the " gaine " and this detonates the shell.
In the earliest models of the same class the detent alone was relied upon to give safety, but in the fuze illustrated and also in others, as an additional precaution, a shutter is introduced to mask the flash- hole until the shell is clear of the gun. This consists of a block held in the closed position, with its centre of gravity eccentric to the axis of the fuze, by a compressed spring and a detent with a weighted head. While the shell is going forward in the gun, the shutter is held in position by the spring, but on leaving the gun the rotation of the shell overcomes the spring, and the shutter moves outwards, uncover- ing the flash-hole; at the same time the tail of the detent is released from its recess in the shutter, moves to one side, and prevents the shutter from returning and masking the flash-hole.
Plug C.E.Pellets Detonator Powder' Pellets
FIG. 7. .
High explosives used as bursting charges, being comparatively inert and safe substances, require a violent detonation actually in contact with them to ensure that they shall detonate and not merely explode. This result is obtained by interposing a certain amount of less inert explosive between the bursting charge and the magazine of the fuze. In some cases this relay or part of it is placed in a steel or bronze container called a " gaine," which is screwed to and forms an integral part of the fuze.
Fuzes of the class described here are always used with a gaine (which is screwed into an adapter and so secured to the base of the fuze), the flash being inadequate in itself to secure detonation of the contents of the shell without being assisted by a relay. The internal arrangement of a gaine for use with these fuzes will be seen in fig. 7. The flash from the fuze ignites the perforated pellet. The flame from this passes through a flash-hole to the detonator, which, when fired, detonates in its turn two pellets of exploding composition (C.E.) or of picric acid, and these finally detonate an " exploder " bag placed choke downwards below the gaine in the cavity of the bursting charge. This train of three detonations detonates the H.E. in the shell. When a delay composition is included it is placed at the mouth of the gaine, above the powder pellets.
Base Fuzes. These are for use in shells having solid-pointed heads. That in general use for common-pointed and armour-piercing shells comes under the category of graze fuzes and is known as Fuze Percussion, Base large, No. 1 1.
The pellet is locked by a device which primarily releases it when acted on by the pressure of the propellant gases, its final release being accomplished by the rotation of the projectile. Safety shutters prevent the magazine from being fired should the detonator act prematurely.
The pellet is locked in its rearward position by a bolt projecting into a recess in its body, and is held in this position by the stem of the pressure-plate. This plate is fitted into a socket, and is made accident- proof by a steel protecting plate, perforated so that the gas pressures may act on the pressure plate.
Protecting
rl_ -P
Pressure Plate
Pressure Plate
Retaining Bolt
Sealing Powder
Bolt Pellet
When the pressure plate is driven in on firing, a recess in the stem is brought opposite to the fork of the retaining bolt, so that this is now free to move outwards. The mushroom-headed centrifugal bolt moves outwards on rotation being set up, and forces the end of the retaining bolt into the recess exposed by the movement of the pres- sure-plate stem. At the same time the tail of the centrifugal bolt which hitherto, by engaging in a recess in the body of the fuze, has prevented the rebound of the pellet on shock of discharge is with- drawn and the pellet is now free, its movement being only controlled by the creep-spring.
On impact the pcllet'moves forward and the needle penetrates the detonator. The flash from this passes through a passage in the pellet and centrifugal bolt, along a transverse channel, and ignites a vertical column of compressed powder leading to the magazine. This is a ring of compressed powder in a recess in the upper part of the fuze body, the powder being grooved on the under side so that the flash may pass all round and ignite the whole ring simultaneously. The flame from the magazine passes through holes in the cap to the bursting charge of the shell.
Premature action on this fuze is guarded against in three ways: First, the spindle of the centrifugal bolt masks the passage through the pellet until the shell has gained a suitable speed of rotation; secondly, the coned seating at the base prevents the escape of flash coming over or through the pellet; and thirdly, a ball in a seating at the end of the transverse channel is held in position by a spring- controlled plunger that moves outwards, the ball following it when sufficient rotary movement has been set up.
To prevent a " blind " being caused on impact by the rebound of the pellet there is a spring-controlled locking bolt in the side of the pellet so arranged as to enter a recess in the body of the fuze when the pellet has gone forward a certain distance; this permits of further forward but no backward movement or rebound due to the creep- spring.
One of the earliest and simplest of the base fuzes is the Base Hotchkiss Fuze (fig. 9) used mainly for small Q.F. guns.
Lead & Tin Alloy
Creep Spring Needle Holder/ Needle Spun in
/Detonator
crewed Cap
Percussion
Pellet FIG. 9.
On the shock of discharge, the pellet sets back over the needle holder, thus allowing the steel needle to project beyond it. The alloy -at the bottom of the pellet cushions against the bottom of the fuze, and a small portion of it dovetails into the undercut recess, round the base of the needle-holder. This forms a weak connexion between the pellet and fuze body, and assists the spring in checking rebound action. On graze or impact, the pellet and needle set for- ward, the needle pierces the detonator, and the flash passes through to the bursting charge of the shell.
Time and Percussion Fuzes, as their names imply, are intended to burst their shells either in the air in front of the target or on impact. Setting aside for the moment the newly developed clockwork fuzes,. theT. and P. Fuzes used in Great Britain depend for time of burning on rings of compressed composition which are ignited at the moment of discharge, and of which the flame is conducted by suitable passages to the magazine. The opening charge of the shrapnel is thus fired at a moment predetermined by the setting given to the time rings above
