Description
The B-7(a) is a Japanese Army mechanical impact tail fuze. It may be used in the 250 kg GPHE bomb, and the 50 kg, 100 kg, and 250 kg anti-submarine bombs. The fuze is brass except for the striker, springs, anti-binding studs, trigger and 3 spacers, which are steel.
The fuze consists of the vane assembly, striker body and housing, lower fuze body, explosive train and sealing ring.
Vane assembly: Three vanes are mounted on a hub retained in a housing by two retaining screws, which engage an annular groove in the hub and permit the hub to rotate. Inside the vane hub, a key fitting into a longitudinal keyway in the arming spindle allows the spindle to thread itself upward. The hub housing threads onto the. striker body housing and constitutes the top part of the fuze.
Striker Body Housing: Threaded internally at the top to take the arming spindle, and internally at its base to thread onto the lower fuze body. The lower, external portion is enlarged to form a shoulder for a rubber gasket. The striker is mounted on the tapered striker body which is keyed to prevent rotation. The body rides on a creep spring and when unarmed, is retained by the arming spindle which threads into its top.
Lower Fuze Body: Contains the primer, delay ring, relay and is threaded to receive the gaine. The delay ring is a friction fit against a flange on the lower fuze body. A Belleville spring washer, fitting between the delay ring and the striker housing, maintains tension on the ring. A diagonal flash hole leads through the flange of the lower fuze body to a relay of black powder above the gaine. The base of the lower fuze body has external threads and internal threads of the same size as those of the B-1(a).
Functioning
On release from the plane, the arming vanes rotate the hub, which threads the arming spindle out of the striker body. On impact the striker body is driven forward by inertia, compressing the creep spring and piercing the primer. The weight of the striker body and the lightness of the creep spring makes the fuze very sensitive.
The flash from the primer goes through the lead -in hole to the delay ring, burns aground to the lead-out hole, and sets off the relay and gaine. A red setting’ line on the lower body flange gives a delay of about 3.5 seconds; an uncolored line sets it so that the whole delay train must burn, giving a delay of about 25 seconds. Any setting has to be made before the fuze is sealed.
The sealing ring serves to waterproof all the explosive elements of the fuze which are outside the fuze pocket.
Hazardous Components
Primer, delay ring, relay, gaine, booster.
Physical Data
Length, overall - 5.88 in, 149.4 mm
Width, overall - 2.13 in, 53.9 mm
See Also
Nose, A-8(a), Type 4 Two Second Delay
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)
