MK 346 MOD 0 MECHANICAL LONG-DELAY TAIL FUZE

The mechanical long-delay fuze Mk 346 Mod 0 is an all-mechanical clockwork fuze that is used in the tail well of the Mk 80 (series) LDGP bombs. The functioning delays are preselected for the total number of hours, in increments of 15 minutes. The range of the functioning delay time is from 30 minutes to 33 hours.

Description

The Mk 346 fuze (fig. 1-5) is approximately 6 3/8 inches long and threaded for a 2-inch adapter booster well. For purposes of description, the terms outer end and base end are used. The outer end couples to the arming assembly; the base end fits into the adapter booster well.

The input shaft and the delay functioning setting shaft are located on the outer end of the fuze. The input shaft is coupled directly to a centrifugal clutch that engages the speed reduction gear assembly when the input shaft reaches 1,200 rpm. The input shaft is fitted with a key that engages a locking slot to prevent accidental rotation. The key is automatically freed when the arming assembly is fitted to the input shaft.

The delay function setting shaft is located in the outer end and may be rotated in either direction by a screwdriver. The setting shaft is coupled through a gear train to the dial, which is seen in the time delay observation window.

The safe-armed band, visible through the safe-arm window, is coupled to the speed reduction gear. The red band covers the window when the input shaft and clutch have rotated 45 revolutions, thus completing the first stage arming. The red band starts to appear after the input shaft and the clutch have rotated approximately 22 of the required 45 revolutions. A slip clutch lets the

Figure 1-5.-Mechanical long-delay fuze Mk 346 Mod 0.

setting shaft rotate past the minimum or maximum delay setting without further rotation of the indicator dial or damaging the mechanism.

The base end of the fuze houses the timer, firing pin, detonator-rotor, lead-in, arming shaft, and the functioning delay indicator dial. On the face of the base end plate is the toothed cam, which operates as an antiremoval device. The arming shaft moves forward at impact to initiate the timer mechanism; this allows the detonator-rotor to align with the firing pin and release the antiremoval cam.

Operation

After the bomb is released from the aircraft, the arming assembly is freed from the arming wire and allowed to rotate. After approximately 45 revolutions of the input shaft at 1,200 rpm or more, first stage arming is complete. Usually this takes about 1.5 to 2 seconds.

Upon impact, the second stage arming is initiated, which aligns the fuze firing train, starts the delay-to-burst timer, and releases the antiremoval cam. After the preselected time on the delay-to-burst timer has expired, the bomb detonates.

NOTE: First stage arming must be completed before impact, and impact must be sufficient to initiate second stage arming.

Arming Assemblies Mk 3, 4, and 5 Mods

Mk 3, 4, and 5 arming assemblies are used with the tail assemblies of LDGPs (except Mk 84) and Snakeye weapons. They provide arming action for the Mk 346 fuze. The arming assemblies consist of a Mk 5 Mod 1 or Mod 2 fuze drive, an adapter, an appropriate drive shaft, which couples to the input shaft of the fuze, and a funnel guide. The Mod of the drive unit denotes the Mod of the assembly.

Figure 1-6.-Mk 346 fuze with Mk 68 thermal shield installed.

The Mk 68 thermal shield (fig. 1-6) protects the fuze and adapter booster when used in the Mk 82 Mod 2 thermally protected bomb. The fuze is hand-pressed into the booster. The base of the shield covers the exposed portion of the adapter booster after fuze/shield installation.

Identification of a Fully or Partially Armed Fuze

Normally, the Mk 346 fuze is removed from an unused bomb, returned to its shipping container, reset, and reinstalled for a subsequent mission. However, if an arming wire is lost or the arming wire installation is faulty, first stage arming can occur while the bomb is aboard the aircraft. You can identify the first stage arming condition by the red band covering all or part of the safe-arm observation window on the body of the fuze. You should examine all unused bombs returned from a mission as soon as possible. If evidence of a malfunction is present, immediately remove the fuze from the bomb while on the flight deck. A partially armed fuze is safe for handling and disposal; however, if the fuze is jarred or dropped, complete arming may result.

Second stage arming of the fuze does not occur unless first stage arming is complete and the fuze is accidentally jolted or jarred. Then, the fuze is locked in the bomb and isn't easily removed. This condition is unlikely, but it could happen with a hung bomb, or with a bomb that is inadvertently released because of a bomb rack structural failure. This is a dangerous situation. If a safe condition is in doubt, immediately notify explosive ordnance disposal (EOD) personnel.