Abstract: A drum has a shell and tracks at opposite end of the shells. Inner and outer rings move within the tracks. The outer ring is constrained for vertical movement, and the inner ring is constrained for circumferential movement. At least one of the rings has a plurality of inclined planes, and the other ring has followers of the inclined planes. The followers have convex surfaces for fitting in concave surfaces of the inclined planes. A rack extending circumferentially on the inner ring has teeth shaped like ratchets. Turning gears turn the rack, and a latch successively engages the teeth. Lifting the latch permits the inner ring to move backward circumferentially allowing the follower rollers to slide down the inclined plane.The tracks and shells are covered by split hoops having inward hinges and latches.