Rotatable shaft lock apparatus

A lock apparatus selectively locks and unlocks a rotatable shaft from rotation. A brake disc is formed with opposed lock surfaces. First and second lock rings rotatably mounted on first and second bearing members are alternatingly moved by rotation along spaced projections on the adjacent first and second bearing members into locking engagement with the lock surfaces on the brake disc and away from the brake disc to an unlocked position. An outer member of each of the bearing members is held in a fixed position by engagement with first and second end caps mounted on the brake disc. An inner bearing member is axially movable with respect to the outer bearing member to bring bearings disposed in outer and inner races on the outer and inner bearing members into contact with thrust faces on the outer and inner members as axial force is imposed on the rotatable shaft to enable rotation of the shaft.