Description
The Type 2 Mark 3, Model 2 fuze is a Japanese Navy clockwork delay, aerial burst tail fuze used in incendiary shrapnel bombs. It is brass except for a few internal parts made of steel. It is most likely found with the A-3(a) fuze.
The D-2(a), D-2(b), and D-2(c) are all similar in design, construction and operation. The clockworks in all three fuzes is almost identical with the clock in a Japanese artillery projectile and with the fuze in a German artillery projectile for the 88mm gun. All of these fuzes require a rotation of 1,000 revolutions per minute before they can function. The parts of the fuze can be broken down into five principal categories according to their function: the body, the arming mechanism, the timing mechanism, the firing mechanism, and the safety features.
The body consists of an upper part and a lower part held together by a retaining ring which fits over the upper body and threads into the lower body. The upper body has a lip on its lower edge which is held under the retaining ring, permitting rotation of the upper body for setting the time. An adapter fits into the lower body, secured by two screws.
The arming mechanism consists of six vanes press fitted into the vane nut and an arming sleeve externally threaded to take the vane nut. The arming sleeve houses the arming spindle which is secured at the base to the pressure cap. The top end of the arming spindle is externally threaded (left-handed) to take the locking nut. A cover is secured to the vane nut with three grub screws.
The timing mechanism consists of a spring driven clockwork which turns a timing rotor and slotted disc at a predetermined rate. The pressure cap is keyed to the upper body and has a setting stud which engages the notch in the timing rotor. The parts of the timing mechanism which start the clockwork are the spring-loaded starter plunger, the spring-loaded pawl, the claw, and the escape wheel.
The firing mechanism consists of the spring loaded striker which has a beveled shoulder that bears against a beveled edge on the trigger arm.
The five safety features are: safety pin, safety spring, safety block which overcomes the pressure of its spring (not shown in the drawing) and swings out from under the striker by centrifugal force, shipping safety pin which locks the starter plunger, prevents the safety block from swinging out during shipping, and locks the trigger arm by the starter plunger until it rises during flight and presents a notch through which the end of the trigger arm can pass.
The time of delay is set before leaving the ground, and the bomb must be dropped from a specific height above the target. The bomb usually explodes about 100 to 175 feet above ground.
Functioning
The fuze is set by rotating the upper body, which is calibrated up to 50 seconds, and matching the desired setting with a line on the retaining ring. The setting stud in the pressure cap - which is keyed to the upper body - is thus rotated, turning the timing rotor - in which it is engaged - and the slotted disk so that the slot of the disc is positioned with respect to the trigger arm. At the expiration of the set time, after the bomb has been dropped, the disk will have rotated so that the slot will be opposite the trigger arm.
Shipping safety pin is pulled when bomb is loaded in plane. On release, safety pin is pulled, vanes and vane nut rotate up. Initial impact of the vane nut against the locking nut, plus the continued rotation of the vanes and vane nut, lifts the pressure cap, overcoming the resistance of the safety spring and freeing the setting stud from the timing rotor. As the pressure cap is lifted, the spring-loaded starter plunger rises until the spring-loaded pawl slips into the groove in the plunger. This action of the pawl rotates the rod to which it and the claw are keyed, freeing the claw from the escape wheel and starting the clockwork. The clockwork turns the slotted disk so that the slot is rotated toward the trigger arm. The pawl locks the starter plunger up so that the notch in the plunger is opposite the end of the the trigger arm. The starter plunger no longer obstructs the end of the trigger arm, since it can now pass through the notch when the trigger is forced into the slot of the disk.
Angled fins on the bomb cause rotation which, when 1,000 revolutions per minute is attained, is sufficient to swing the safety block out from under the spring-loaded striker by centrifugal force. The spring-loaded striker is now held by the edge of the trigger arm only. After the set time has elapsed the slot in the disk comes opposite the trigger which is forced into the slot by the pressure of the beveled shoulder of the striker on the beveled edge of the trigger arm. The spring-loaded striker impinges on the primer, sending a flash through the flash channel which sets off the gaine.
Hazardous Components
A primer detonator Is fitted in the fuze. The flash from it sets off a standard Japanese gaine.
Physical Data
Length, overall - 5.75 in, 146.1 mm
Width, overall - 2.44 in, 62 mm
See Also
Nose, A-3(a), Type 97 Mk 2 Model 2
Tail, D-2(b), Aerial Burst
Tail, D-2(c), Aerial Burst
Source(s)
OP 1667, Japanese Explosive Ordnance, Volume 1 (1946)
OPNAV 30-3M, Handbook of Japanese Explosive Ordnance (1945)
USNBD - Japanese Bombs & Fuzes (1944)
TM-E9-1984, Enemy Bombs and Fuzes, Section VII, Japanese Fuzes (1942)
