Mk 237 Mod 0 and Mk 238 Mod 0 (Tail, Lead-Shear-Wire, Long Delay, Time)

Bombs

Mk 237 Mod 0

Mk 238 Mod 0

Functioning

Armed condition

Fuzes used with

Arming time

Vane span, inches

Body diameter, inches

Over-all length, inches

Material

Zinc or cadmium-plated steel

General: This fuze differ only in the length of their arming stem. Delay arming is ob-tained by means of the 30-to-1 gear-reduction system, which is like that of the Tail Fuze M115 Series. Upon completion of arming, the gear-reduction assembly and the arm-ing stem do not separate from the fuze, but cease rotation. Functioning time of these fuzes is determined by a lead shear wire (50% lead, 50% tin). The various delays are ob-tained by varying the diameter of the wire, i.e., increasing the diameter for longer de-lays. The slider carries a Detonator Mk 23, which is made up of three explosive compo-nents; namely, azide priming mixture, lead azide, and tetryl. The fuze incorporates a te-tryl booster lead-in, but does not have an integral booster.

Operation: The complete arming of the fuze is effected in two stages: air tarvel and impact. Upon withdrawal of the arming wire, the vane assembly rotates, causing the arming stem to rotate via the gear-reduction system. The lower end of the arming stem is encased by the firing-pin housing nut, which, in turn, is pinned to the firing-pin hous-ing. To the bottom of the firing-pin housing is attached the slider stop. In addition, the firing pin is locked to the firing-pin housing by two balls, and the firing-pin housing, in turn, is locked to the ball-retainer sleeve by two balls. As the arming stem screws up-wardm the entire firing assembly moves upward under the action of the ball-retainer spring. After approximately 150 revolutions of the vanes, the firing assembly has risen sufficiently to withdraw the slider top and firing pin from the slider slot, allowing the slider to align its detonator below the firing pin. (A detent locks the slider in position.) Continu-ed rotation of the vanes causes the O-ring gasket on the firing-pin housing nut to seat and stop rotation of the vane assembly.

On impact, the ball-retainer sleeve is forced down by the inertia freeing the retaining balls. At the instant deceleration ceases, the ball-retainer sleeve is forced upward by its spring, allowing the firing balls to jump out of their recess, and the spring-loaded striker is restrained only by the lead shear wire. The fuze is now fully armed. The firing pin, un-der action of its spring, exerts pressure on the lead shear wire and causes the wire to shear when the proper length of time has elapsed, depending on temperature conditions. The firing pin strikes the detonator, which, in turn, sets off the booster lead-in.

Remarks: These fuzes incorporates an anti-withdrawal feature, but no booby trap device like the M123 series. Under normal handling and installation, the anti-withdrawal feature does not function, since the locking ball and its carrier are held in place in the deep part of the eccentric groove, by a carrier pin and spring. Upon impact, however, the force of inertia is sufficient to withdraw the carrier pin from the fuze body, and allow the carrier spring to pull the ball carrier into the shallow part of the eccentric, locking the fuze in place. The fuze body is case-hardened to prevent removal with a wrench after impact. Since these fuzes require impact to lock in place, bombs with these fuzes can be safely returned to the base or carrier, and the fuzes removed for stowage.

Since these fuzes have a two-inch thread diameter, it is necessary to remove the in-ner sleeve of the Adapter Booster M115A1 before installation. Fuzes should be screwed securely into the adapter booster; otherwise a dud may result. A special short-length locking pin is shipped with these fuzes for securing the Adapter Booster M115A1 to the base plate.

Bombs fuzed with these fuzes should not be released at an altitude lower than that specified by the Chief of Naval Operations as the minimum safe altitude of release for instantaneous action fuzes; for, if the bomb is subjected to multiple impact, the first im-pact would completely arm the fuze, and the second might shear the lead shear wire, allowing the fuze to fire without delay.

These fuzes are detonator safe. In the unarmed position, the detonator is lined up with the safety expansion chamber. If the detonator should function prematurely, the force of detonation is dissipated in this cavity, and will not set off the booster lead-in charge, or adapter booster charge.

Figure 373. Long-Delay Tail Fuzes Mk 237 Mod 0 and Mk 238 Mod 0