Abstract: The frictional loads involved in a friction welding operation, for joining metal parts, may be several tons and the mass of the welding apparatus is correspondingly large. Thus, in addition to the energy which is converted into heat at the weld interface a substantial amount of work has to be done simply to overcome the inertial loads of the apparatus. An inertial load cancelling system is proposed in which a double acting gas spring arrangement is coupled to the welding oscillator output so that the work done in overcoming the inertia of the apparatus is returned to the system when the oscillator motion is reversed at the ends of each stroke. Thus, after the initial system input the oscillator has only to supply work to overcome internal frictional and that converted to frictional heat at the weld joint interface.