Tail Fuze No. 871 Mk I (Soon in service)

Description: This fuze is a combination long-delay, anti-disturbance tail fuze, and consists of a No. 881 fuze mechanism placed in a No. 28 pistol body. There is a deep V-slot cut through the base of the upper body, which is covered with adhesive tape. The detonator in this fuze is not shuttered.

Operation: When the bomb is released from the plane, the arming fork is rotated by the vanes and reach ord located in the standard British tail unit. Rotation of the arming fork screws the arming sleeve downward, until the rubber washer in the shoulder of the sleeve is firmly set against the top of the fuze body. This action brings the delay ass-embly adjacent to the extended portion of the screw in the top of the bayonet sleeve, and also brings the chamfered edge of the arming ring opposite the large locking ball, al-lowing the ball to move inward and partially disengage the bayonet sleeve from the lock-ing seeve. Variation in the delay times of the fuze is accomplished by varying the length of the buffer spring. This varies the distance that the bayonet sleeve must force the de-lay assembly upward before the plunger begins to puncture the plastic delay disc.

On impact the bayonet sleeve moves downward by force of its inertia until stopped either by its main spring or contact with the top of the ball cage. This allows the small locking ball to move into a recess in the bayonet sleeve to disengage fully the locking sleeve from the bayonet sleeve.

Although the main spring is assembled under considerable torque tension, rotation has previously been prevented by the ball lock between the locking sleeve and the bayonet sleeve. The latter is firmly positioned by a pin extending down from the fuze body. The three upward-extending fingers of the ball cage engage in similar notches cut in the bot-tom edge of the locking sleeve. When the locking sleeve and the bayonet sleeves are disengaged, the main spring exerts its torque influence and commences to rotate the ball cage and locking sleeve in a clockwise direction. However, this rotational movement is slowed down by the brake plate and knife assemblies. The brake plate assembly consists of two perforated plastic disc held in a metal housing. Tabs in the outer edges of the brake-plate assembly are engaged by the three fingers of the ball cage, causing the as-sembly to roatate with the cage. The three knife egdes of the brake knife extend down-wards through the perforations in the brake-plate assembly and into three holes in the top of the bayonet sleeve. Therefore, the brake knife cannot rotate, but must cut through the plastic disc of the brake-plate assembly when the ball cage rotates. A small screw with a long projecting head holds the brake-plate and knife assembly loosely in place. This screw threads into the top of the bayonet sleeve.

Located inside the ball cage are the striker and six retaining balls. The lower end of the release spring is fitted into a screw-driver slot in the top of the striker. The upper end of this spring engages a hole in the base of the bayonet sleeve. The spring itself is loaded under torque tension, and attempts to rotate the striker, but the striker is pre-vented from rotating by the guide fork. A notch cut in the upper portion of the striker is engaged in a slot cut in the guide fork, and two upward-extending prongs on the guide fork engage in similar grooves in the base of the bayonet sleeve. The six retaining balls are arranged, one on top of the other, in two layers of three balls each. In the unarmed position, these balls are held in place around the striker by the inner walls of the ball cage. The ball cage is prevented from rotating more than 60° since the fingers of the cage come up against the ends of the slots in the base of the bayonet sleeve at the end of that distance. Rotation of the ball cage through 60°, however, presents cutaway por-tions of its inner walls to the balls. The balls are then prevented from scattering only by point contact between themselve, the steel ring placed around the striker above them, and the steel plate placed around the striker below the balls. When the ball cage has completed its 60° rotation, the fuze is fully armed.

A delay assembly in the top of the fuze consists of a housing containing a metal plun-ger and a plastic delay disc. When the bayonet sleeve is disengaged from the locking sleeve, the main spring forces the bayonet sleeve upwards, bringing the extended port-ion of the screw into contact with the plunger, which gradually forces its way through the delay disc. When the bayonet sleeve has risen sufficiently to disengage the prongs of the guide fork, the striker is allowed to rotate under the torque tension of the release spring. The flick motion imparted to the striker scatters the balls and allows the striker to be forced into the detonator by the main spring, working through the ball cage.

Disturbance of the fuze prior to its long-delay functioning will scatter the balls from under the striker, and the ball cage, under the compression of the main spring, will move downward, bringing the striker into contact with the detonator.

Remarks: The detonator in this fuze is not shuttered, but is at all times aligned with the striker. An internal weaking groove is cut around the inside of the head of the fuze, completely severing the fuze head except at four equi-spaced places, which are 0.2 in. wide. Alternatively, instead of the internal weakening groove, a deep V-groove may be cut around the outside of the fuze. The external open portions of the groove are cove-red with adhesive tape. The weakening groove ensures that the fuze head will shear at the weakened point rather than at another point where fuze functioning might be ob-structed. It also increases the difficulty of identification by means of external fuze cha-racteristics, and eleminates surfaces to which a wrench might be applied for extraction purposes.

Figure 168 - Tail Fuze No. 871 Mk I

Figure 169 – Exploded view of Tail Fuze No. 871 Mk I