Mk 61, Mk 62, Mk 63

Projectiles used in

5"/38 W.P.

14"/45/50 H.C.

16"/45/50 H.C.

5"/25/38/51 A.A. Com.

5"/25/38/51 Ill.

5"/38 W.P.

5"/38 Window

5"/51 H.C.

6"/47/53 H.C.

6"/47/53 Ill.

8"/55 H.C.

Over-all length, inches

Diameter at base of ogive, inches

Threaded length, inch

Threads

Weight, pounds

Material

Setting times, seconds

Description: The Nose Time Fuze Mk 61 is a modification of the mechanical time fu-zes of the Mk 18 and Mk 50 type, designed specifically for the 5"/38 projectile when the special reduced charge with initial velocity of 1,200 feet per second is employed. The modifications, allowing the fuze to function at lower rotational velocities and decreased set-back, are as follows:

A weaker hammer spring is used.

The set-back pin has been eliminated.

A centrifugally operated firing-pin safety block has been added under the firing pin.

A centrifugally operated detent or plate, retained by a flat spring, holds the safety-lever plate, which locks the escapement mechanism.

Heavier centrifugal weights are placed on the driving gears to drive the clockwork mo-vement.

Stronger centrifugal "kick-off" springs are incorporated on the centrifugal gears. These springs are actually driving springs, and exert a strong force throughout the entire cycle.

A spring is added to the firing-arm shaft.

Weight is added to the safety-lever plate.

The fuze body is slightly altered to provide clearance for the heavier driving weights.

The safety setting is changed to 0.9-1.0 second.

The Nose Time Fuze Mk 62 is almost identical to the Mk 61, but is slightly further mo-dified to allow its use in the major-caliber H.C. projectiles, where extremely low set-back force on firing is encountered. The additional modifications incorporated in the Mk 62 are as follows:

A still weaker hammer spring is used.

A safety post is provided beneath the hammer spring, preventing arming of the fuze as the result of accidental dropping.

The Nose Time Fuze Mk 63 is designed to replace the Nose Time Fuze Mk 18 and Mk 50 in all projectiles where the Mk 50 is now in use. Its internal construction is identical to that of the Mk 61, except that the escapement movement has been regulated for a hig-her spin rate - 12,600 revolutions per minute - the average spin rate of the 5"/38 projec-tile at service velocities.

Remarks: These fuzes, like the Mk 50 type, have complete moisture-resistant featu-res incorporated. These measures include gaskets between the time-setting ring and the upper cap and body assemblies, a silica-gel capsule in the base of the fuze to absorb ex-cess moisture, and joints and openings coated with approved luting or bakelite varnish.

The Nose Time Fuze Mk 62 is designed to replace the spring-driven Nose Time Fuzes Mk 42 in all assemblies.

The Nose Time Fuze Mk 62 differs in operation from the Mk 61 only in the functioning of the safety post. This safety post consists merely of a stud fastened by a screw to the uppermost ring of the clockwork mechanism. With the fuze set on "Safe", this stud is located beneath a projecting end of the hammer spring, preventing the hammer from dis-engaging the setting lug from the setting pin as the result of accidental dropping. When the fuze is set off the "Safe" position preparatory to firing, the hammer assembly is mo-ved away from above the safety post, allowing the hammer to move down on set-back.

Figure 94. Nose Time Fuze Mk 61

Figure 94. Nose Time Fuze Mk 62