Abstract: A positive displacement, constant speed, pressure sealing, wear compensating, hydraulic motor for driving a rock drill, comprising an exernally cylindrical rotor mounted for axial rotation in an internally cylindrical housing and connectible to a drill bit to transmit driving torque thereto, a plurality of driving members in the form of cylindrical rollers mounted in respective axially extending recesses in the cylindrical surface of said rotor, the rollers being floated by fluid flow and pressure only and without the aid of mechanical springs, into sealing engagement with said housing, and at least two circumferentially extending recesses in the cylindrical wall of said housing adapted to receive said driving members in succession during rotation of the rotor; each said recess having circumferentially spaced inlet and outlet means for drilling mud for impingement on said driving members to drive said rotor.