Description
This is a Navy mechanical impact tail fuze. It is used in 250 kg GP and anti-submarine bombs. The fuze is brass except for a steel sleeve, striker and pivot pin for lever arms. This fuze can be found with the A-3(a) nose fuze.
Principal parts are the body, the bushing which threads into the upper end of the body, the sleeve housed within the bushing, the striker which is threaded to take the vanes, and the arms which pivot on a steel pin extending through the body. When unarmed, the striker is prevented from moving down by the safety pin, the safety fork, the shear wire, and the arming vanes. The arms are prevented from moving by the sleeve which in turn is held in place by the arming vanes. The fuze body is internally threaded for the standard Navy gaine. Six spanner holes are drilled in the body.
The B-3(b) is similar to the B-3(a) except that all parts are larger and it lacks one minor safety pin hole at the top of the striker spindle. The diameter of the threads for the fuze pocket is larger than that of any other Japanese Navy fuze, but the fuze is internally threaded to take the standard Navy gaine. The fuze appears to be designed for use in large bombs - probably 500 kg. and over. Increased size of the fuze may provide greater certainty of arming and of firing despite the greater disruptive force of impact of the larger bomb.
Functioning
On loading in the plane, the safety pin and the safety fork are removed. The vanes are prevented from rotating by an arm on the bomb rack. On release, the vanes rotate up and off leaving the sleeve free to rise. The striker is held up by only a shear wire. On impact, inertia forces the arms down against the shoulder of the striker, the shear wire is sheared, and the firing pin pierces the primer. Because both arms pivot about the stationary pin and extend through the body, pressure upward against the arms would force the short ends of the arms down against the striker and thereby shear the shear wire. Two-way action is thus achieved.
After impact, a slight movement of the arms is liable to force the firing pin into the primer.
Hazardous Components
Standard Navy gaine.
Physical Data
Length, overall - 5.6 in, 142.2 mm
Width, overall - 2.4 in, 60.9 mm
See Also
Nose, A-3(a), Type 97 Mk 2 Model 2
Source(s)
OP 1667, Japanese Explosive Ordnance, Volume 1 (1946)
USNBD - Japanese Bombs & Fuzes (1944)
TM-E9-1984, Enemy Bombs and Fuzes, Section VII, Japanese Fuzes (1942)
