A clamp (10) having a pneumatic cylinder (28) with an actuating rod (30) connected to one end of a drive arm (32) which has its other end attached to a crankshaft (19) which in turn has crankpins (20,22) eccentrically mounted thereon to drive inner and outer clamp arms (24,26) between retracted and advanced positions corresponding to open and closed positions of the clamp. The length of the drive arm (32), the extent to which the crankpins (20,22) are offset from the crankshaft (19), the specific portion of a quadrant through which the crankpins are rotated, and the distance between the application of the force to the clamp arms and the axes about which the clamp arms pivot all provide a significant mechanical advantage and an extremely large clamping force from relatively low pressure compressed air supplied to the pneumatic cylinder (28).