A counterbalancing and sheet clamp retention system for high speed sheet handling drums (24) of the type in which leading and trailing edge clamping bars (46, 48) extend along the length of the drum periphery and are capable of movement between retracted and sheet clamping positions for automatic loading and unloading successive sheets to and from the drum (24). The clamping bars (46, 48), supported at opposite ends for movement between their respective retracted and clamping positions, are engaged centrally along their length by a retaining device (72) which moves to a clamping bar engaging position under centrifugal force as the rotational velocity of the drum (24) reaches a predetermined value. The retaining device (72) is a self-contained unit insertable through a radial opening in the drum (24) and contains a pair of bell crank retainers (88, 90) each having a clamping bar engaging claw (98) on one arm and a weight (100) on the other arm so that centrifugal force operates to pivot the bell crank and bring the claws (98) thereon into engagement with the closed sheet clamping bars (46, 48). The drum (24) is further counterbalanced by a fixed weight (104) opposite from the movable bell crank weights. Further, the ends of the clamping bars (46, 48) are moved against the drum periphery to their sheet clamping positions by centrifugal force acting on counterbalancing weights carried by radially movable brackets (38, 40) from which the ends of the respective clamping bars (46, 48) are supported.