DRUM UNIT ASSEMBLY

Live ammunition and expended cases are stowed in the drum unit assembly. This assembly has four major parts-drum unit, entrance cover, exit cover, and scoop disk, Refer to figure 6-12 as you read about these parts.

Drum Unit. The drum unit is a cylindrical structure that consists of an outer drum and an inner drum helix, The live ammunition rounds and expended cases are stored radially around the longitudinal axis of the outer drum with their bases in an outward direction. The rounds are suspended by their bases in partitions. These partitions are mounted lengthwise with respect to the inner surface of the outer drum. With the outer drum mounted stationary to the aircraft's structure, the rounds are moved along the length of the partitions by the rotation of the double-lead helix (inner drum), It's easy to understand this movement if you think of the inner drum helix as the threads on a screw. The projectile end of the casing protrudes into the threads. As the inner drum helix is rotated, it produces a screwing-type motion, causing the rounds to slide along the partitions from one end to the other end of the outer drum.

Entrance Cover. The entrance cover is stationary mounted to the entrance end of the outer drum. The entrance cover contains a retainer gear and 252 steel ball bearings that support the scoop disks and the inner drum helix. The retainer partitions are mounted to the retainer gear that controls the position of the rounds as they are passed from the entrance cover to the scoop disk. A spring-loaded timing pin on the entrance cover is used to index the drum for installation of the entrance unit.

Exit Cover. The exit cover is stationary mounted to the exit end of the outer drum. The exit cover is constructed like the entrance cover. It controls the position of the rounds as they pass from the scoop disk to the exit cover. A spring-loaded timing pin on the exit cover is used to index the drum for installation of the exit unit.

Scoop Disk. A scoop disk is mounted on each end of the inner drum helix. Each scoop disk has two sets of sprocket spur gears located 180 degrees apart. These spur gears mesh with the retainer gear in the entrance/exit covers that provide rotating support for the inner drum helix. The sprockets attached to the spur gear transfer rounds from the entrance cover retainer partitions to the drum partitions, and from the drum partitions to the retainer partitions in the exit cover.

EXIT UNIT

The exit unit is attached over an opening in the exit cover and geared to the exit cover retainer gear. The exit unit contains two gear-driven sprocket assemblies. The sprocket assemblies remove live rounds or expended cases from the retainer partitions in the exit cover and place them in the conveyor elements. Before you install

Figure 6-12.-Ammunition handling component (exploded view).

the exit unit to the exit cover, press and hold the exit unit timing pin and the exit cover timing pin to make sure there is proper gear alignment between the two components. Once the exit unit is properly attached to the exit cover, make sure that the spring-loaded timing pins release.

The exit unit also contains a last-round switch that is electrically connected to the gun control firing circuits. The switch is actuated by the projectiles of the 20-mm rounds. The switch prevents expended rounds (empty cases) from being fed into the gun and jamming it, Before the gun will fire, the ammunition must be cycled through the ammunition drum until the first round actuates the last-round switch. When there is no more ammunition present or when expended rounds are present, the last-round switch is released. This automatically initiates the gun clearing cycle and terminates gun firing. Before the gun can be fired again, live ammunition must be manually positioned by the ground loading crew to actuate the last-round switch.

ENTRANCE UNIT

The entrance unit is attached over the opening in the entrance cover and geared to the entrance cover retainer gear. The entrance unit contains three gear-driven sprocket assemblies. The sprocket assemblies remove live rounds or expended cases from the conveyor elements and place them in the entrance cover retainer partitions. Before installation, press the spring-loaded timing pin and align the scoop sprocket stud pin with the entrance unit timing mark. With the entrance unit properly timed and aligned, you must press the entrance cover timing pin and align the timing mark on the scoop disk with the timing mark on the entrance cover. You need to properly time and align both the entrance unit and the entrance cover to ensure proper gear alignment between the two components. Once the entrance unit is attached to the entrance cover, you should make sure that the spring-loaded timing pins release.

TRANSFER UNIT AND ADAPTER ASSEMBLY

The transfer unit and adapter assembly are actually two separate components bolted together to form one unit. They are never separated at the organizational maintenance level. The function of each component is discussed in the following paragraphs.

Transfer Unit

The transfer unit is gear-driven by the gun and attached to the M61A1 gun housing by quick-release pins to make maintenance easier. The gear-driven sprocket assemblies and guides maintain positive control of the rounds and conveyor elements passing through the transfer unit. The transfer unit removes the rounds from the conveyor and places them into the extractor lip of the gun breech bolts. The transfer unit also receives expended cases and unfired rounds from the gun breech bolts and places them in the conveyor elements. When you install the transfer unit and the adapter assembly as a single unit, press and hold the timing pin on the transfer unit and the gun housing.

Once the transfer unit/adapter assembly is properly attached, you must make sure that the spring-loaded timing pins release.

Adapter Assembly

The adapter assembly bolts directly to the transfer unit. During downloading and loading operations, the adapter assembly interfaces with the linkless ammunition loading system (LALS).