M158 and M159 (Nose Mechanical Impact)

Bombs

M158

M159

Functioning

Armed condition

Fuzes used with

Arming time

Vane span, inches

Body diameter, inches

Over-all lenght, inches

M158

M159

Material

Aluminum body with steel safety blocks and striker

General: Thiese fuzes are essentially the same as the Nose Mechanical Impact Fuzes AN-M110A1 and AN-M126A1, except that they incorpoarte a spring-actuated detonator shutter which rotates into position after withdrawal of the firing pin, and is locked in pla-ce by a spring-loaded detent. Also, gear-reduction ratio is 40:1 instead of 34:1 as in the AN-M110A1 and AN-M126A1. The arming sleeve, striker, and firing pin move upward in-stead of downward. The pinion gear has a cut-out portion to allow the vanes to rotate freely after arming. The vanes require approximately 440 revolutions to arm (about 1,000 feet of air travel), whereas the AN-M110A1 and AN-M126A1 require approximately 260 revolutions to arm (about 725 feet of air travel). The new series does not have a safety block, and the striker and firing pin assembly are of heavier construction than in the AN-M110A1 or AN-M126A1. The striker assembly has a retaining ring, whereas AN-M110A1 and AN-M126A1 have a retaining pin. The M158 has the Detonator M20 located in the detonator shutter, a booster lead-in mounted in the lead-cup disc, and a booster below the lead-cup disc. The AN-M110A1 has its detonator (the M13) extending into the boos-ter charge.

The Nose Mechanical Impact Fuze M159 is the same as the M158, except that the Fuze M159 eliminates the lead disc (which carries lead cup) and the booster, but incor-porates a detonator holder (top of holder fits flush against shoulder of booster holder) which is essentially the same as the AN-M126A1 detonator holder assembly except that it has an all-tetryl detonator which acts as relay detonator. The Nose Mechanical Impact Fuze M159 has two arming-wire guides to facilitate positioning of the arming wire, where the M158 has one. The over-all length of the M159 is approximately the same as the AN-M126A1, but shorter than the over-all length of the M158.

Operation: When the bomb is dropped and the arming wire pulled, the vanes rotate. The arming wire pulled, the vanes rotate. The arming vane, nut, outer ball race, arming hub, and stationary gear rotate as a unit, but cannot move in a vertical plane because of a collar on the arming hub. The movable and stationary gears mesh with the pinion gear (idler gear), and (since the movable gear has one more tooth than the stationary gear) for each rotation the movable gear lags one tooth, thus screwing upward (right-hand therads) and toward the stationary gear. As the movable gear moves up, it lifts the striker and withdraws the firing pin from the hole in the detonator shutter. (Retaining ring in striker groove and under arming sleeve bevel enables simultaneous movement upward.) After approximately 440 revolutions of the vanes, the firing pin is completely withdrawn from  the detonator shutter and the detonator shutter is swung into position, lining up the detonator with the firing pin, thus arming the fuze. The detonator shutter is locked in position by a spring-loaded detent housed in the shutter which slips into a recess in the booster holder. The vanes continue to rotate, and, when the movable gear reaches the cut-out portion in the pinion gear, the vanes rotate freely. On impactm the striker, with its firing pin, is forced down, compressing the firing-pin spring and piercing the detonator. The detonator sets off the booster lead-in and booster successively.

Remarks: In the armed condition, the striker will protrude about 0.25 inch, beyond the vane nut, but any fuze in which the gap between the underside of the striker and the face of the vane nut exceeds 0.125-inch should be treated as armed.

Figure 335. Nose Fuze M158