Abstract: A cylindrical body houses a disc-like pressure plate between a bottom of the body and a disc-like top plate. Ball bearing races are disposed between the body and pressure plate and between the pressure plate and top plate. A reversable ratchet mechanism interconnects the top plate and pressure plate for either conjoint rotation in one rotative direction or conjoint rotation in the opposite rotative direction; while permitting rotation of the top plate without the pressure plate in the opposite direction. A setting lever is provided to change the direction of conjoint rotation of the top plate and pressure plate. The ratchet mechanism includes a drive spur having teeth which coact with a ring gear formed in an opening in the upper surface of the pressure plate. An inflattable bladder is disposed between the body and the periphery of the pressure plate to selectively modify the force required to rotate the pressure plate conjointly with the top plate.