BOMBS USED IN

FUNCTIONING

ARMED CONDITION

FUZES USED WITH

ARMING TIME

VAN SPAN

OVERALL LENGTH

MAX. BODY DIAMETER

2.4"

DESCRIPTION:

The No. 28B is a pyrotechnic delay, serial burst fuze, with possible time settings from 0 - 22 seconds, though 7 seconds is generally the minimum employed. The fuze consists of a body, an igniting mechanism, a fixed ring and setting ring, each containing a pyrotechnic delay element, a valve mechanism to prevent premature functioning of the fuze, and a magazine containing a gunpowder charge for igniting the flash or flare.

The fuze body has a flange, on the upper surface of which the fixed ring and the setting ring rest. Below the flange the body is externally threaded for insertion into the flash of flare, and a securing ring screwed on to the threads locks the fuze in position.

A spring-loaded detonator is housed in a chamber in the fuze body, opposite a fixed striker, the detonator is held in position by the firing rod, movement of which is impeded by a spring loaded ball engaging a groove in the rod. A flash channel extends from the detonator chamber to the delay element in the fixed ring.

The movable setting ring rests of a flange on the fuze body, and is placed below the fixed ring, which is secured by a screw to the fuze body. The delay elements are plac-ed on the under surface of each ring in a circular groove, broken by a metal bridge. Communication between the grooves is effected by a hole in the top of the setting ring. An igniter pellet is placed in the upper end of this hole, and when the fuze is set on SAFE, this pellet is masked by the bridge on the fixed ring. When the fuze is set live, the setting ring is moved to a position where its delay powder is above a flash pellet, located in a hole in the body. This hole communicates with a horizontal flash channel, which opens into a vertical flash channel. Freedom of movement of the setting ring is restricted by a tensioning ring, which is locked in position by fixing screws.

A spring loaded valve is located in the vertical flash channel, and is supported in a valve guide carried by a threaded arming vane spindle. To the upper end of the spindle an arming vane collar, with vanes attached, is secured by a split pin. Rotation of the spindle is prevented by the firing rod, part of which is located in a groove in the arming vane collar. A retaining plate prevents the spindle and the arming vanes from being se-parated from the fuze. To prevent the valve from binding, should the vane be rotated downwards, a stop screw in the vane collar can engage with a similar screw in the re-taining plate.

Two safety pins connected to a spring clip prevent movement of the spindle and the firing rod.

OPERATION:

When the firing rod is withdrawn, upon release from the plane, the spring-loaded de-tonator is released and forced against the fixed striker. The flash from the detonator passes through the flash hole and ignites the delay powder in the fixed ring. The delay powder burns for an interval the igniter pellet in the setting ring is fired and ignites the delay powder in the setting ring. The delay train in the setting ring burns until it reach-es the flash pellet, which is then ignited.

While the fuze powder has been burning, the arming vane spindle has been rotating until the valve has been raised from its seating, so that, when the flash pellet is ignit-ed, the flash passes through the horizontal and vertical channels and ignites the maga-zine pellet. The pellet fires the magazine charge and ignites the flare or photoflash into which the fuze is inserted.

REMARKS:

1. This fuze is obsolescent, and is similar to early No. 28 fuzes which are now obso-lete.

2. Although the fuzes are calibrated from 0 - 22 seconds, generally the numerals from 0 to 4 are painted out, and in some cases the numbers start at 4. In actual pract-ice, no setting below 7 is authorized to be used, due to the possibility of "blinds" or misfires.