Mk 18 Mods 0-4 (Obsolste)

Projectiles used in

Over-all length, inches

Diameter at base ogive, inch

Threaded length, inch

Threads

Weight, pounds

Material

Setting times, seconds

maximum, 45

maximum, 45

Description: In its assembled form, the fuze has a contour which corresponds to that of the Navy medium- and major-caliber projectiles. It consists of four main units, as fol-lows:

1. Movement assembly - The movement assembly is attached to the inside of the body by three holding screws.

2. Body - The brass body contains the magazine charge, 30 grains of black powder, and the bottom closing screw assembly. It is threaded to fit an adapter, which, in turn, is threaded into the nose of the projectile.

3. Lower cap assembly - The brass lower cap, or graduated rotative cap, is attached to the body by a joint, consisting of a steel wire leading through grooves in the cap and body. This allows rotative motion between the lower cap and the body, but prevents axial motion between the lower cap and the body. It has a tensioning feature wherein the torsional resistance which restrains rotative motion is adjusted by four screws during assembly of the fuze.

4. Upper cap - The brass upper cap screws into the lower cap and completes the nose contour of the assembled fuze.

For purposes of explantation, the movement assembly may be diveded into three main parts, as follows:

1. The timing-disc mechanism consists of the timing disc, a setting pin, a spring ham-mer assembly, and the central drive shaft. The timing disc has a firing notch on one side and, on the other side, a forked setting lug which engages the setting pin located in the top inside shoulder of the lower, rotative, cap. The timing disc is secured to the central drive shaft by a friction clutch, so that it may be turned independently of the central dirve shaft. Around the top of the timing disc is a retaining ring which prevents the tim-ing disc from riding forward when the projectile initially seats itself in the gun, and also prevents the hammer from driving the setting lug down too far. Beneath the timing disc is a safety disc, the projection of which bears against the elbow piece of the firing arm. This part is rigidly secured to the central shaft, so that it will rotate out of the way when the clock operates. Its purpose is to provide a safe and a minimum setting.

In setting the clock, the lower cap is rotated to turn the setting pin, which, in turn, will rotate the timing disc to the desired position, since the setting pin is engaged by the setting lug of the timing disc. Disengagement of these two parts is effected by the spring hammer assembly, which is fastened on one end to the top inside shoulder of the lower cap and (on the opposite end) has two small weights.

2. The clock mechanism consists of two centrifugal gear arcs, a series of reduction gears, and the escapment mechanism. The centrifugal gear arcs are geared to the cen-tral drive shaft and are weighted on one side so that they will turn in a counterclockwise direction, thus turning the central shaft and the timing disc in a clockwise direction. The gear arcs have starter springs on them which serve to prevent the gear from freezing. The reduction gears are similary geared to the central spindle, and their rotation is go-verned by the escapement mechanism which is connected to the lowest gear. The es-capement mechanism consists of an escapement gear, escapement lever, escapement-lever spring, safety-lever plate, and safety lever plate spring. In the assembled position, the escapement lever is prevented from moving by the safety-lever plate, which has a pin protruding from the bottom of it and engages the escapment lever. The safety-lever plate is pivoted with a weight on one end, and is held in position by the safety-lever plate spring. The escapement lever acts as a balance wheel and is caused to move back and forth by the escapement-lever spring, which is a hair spring secured at both ends and attached to the escapement lever.

3. The firing mechanism consists of the firing arm, firing-arm shaft, set-back pin, firi-ng-pin safety plate, and firing pin. The firing arm is pivoted and has a weight on one end and, on the opposite end, an elbow piece which bears against the outer periphery of the timing disc. Rigidly secured to the firing arm is the firing-arm shaft, which is prevented from turning in the assembled condition by the set-back pin. The set-back pin is held in position by the set-back pin spring and rests in front of a projection of the firing-arm shaft, thus preventing the latter piece from rotating. In the bottom of the firing-arm shaft is a notch. Bearing aganist the shaft in such a position that it will pass through the notch when the shaft is rotated, is the firing-pin safety plate. This safety plate is also pivoted and is fitting under a shoulder of the cocked firing pin, thus holding it away from the primer beneath it.

Operation: The fuze is armed by set-back, driven by centrifugal force governed by an escapement mechanism, and fired by a spring-driven firing-pin. When the projectile is fired from the gun, the force of set-back accomplishes two things:

1. The set-back pin overcomes its spring and drops into the bottom of the fuze. This action frees the firing-arm shaft for later rotation.

2. The hammer spring assembly pivots down the weights on one end to strike the set-ting lug, thus depressing it and freeing it from the setting pin. When the force of creep sets in, the spring returns the weights to their original position, in fron of the timing disc.

As the projectile rotates, centrifugal force accomplishes four things:

1. The safety-lever plate of the escapement mechanism is pivoted out of the way, thus releasing the escapement lever and unlocking the escapement mechanism. This ini-tial movement causes the escapement lever to oscillate, thus acting as the balance wheel and governing the speed of operation.

2. As soon as the escapement mechanism has been unlocked, the weights on the centrifugal gear arcs tend to move outward, thus causing the arcs to pivot and rotate the central shaft and, consequently, the timing disc. This rotation is slowed down by the series of reduction gears, and its speed is determined by the escapement mechanism. This rotary motion of the timing disc turns the firing notch around to the elbow piece of the firing arm.

3. When the firing notch has been presented to the elbow piece of the firing arm, the weight on the opposite end of the firing arm is moved outward, turning the elbow into the slot and turning the firing-arm shaft. This is now possible, since the set-back pin was depressed when the projectile was initially fired.

4. As the firing-arm shaft rotates, the notch in the bottom of it is presented to the firing pin safety plate, which will pivot through this notch, thus moving out from under the shoulder of the firing pin. The firing pin will then be thrown downward onto the primer by its compressed spring. The primer will then ignite the black powder charge in the base of the fuze.

Remarks: Mods 2, 3, and 4 are identical, but made by different manufacturers.

Figure 89. Centrifugal Timing Mechanism

Figure 90. Nose Time Fuze Mk 18