Design and operation of conventional hydraulic torque converters (HTC) are described. At the Central Research Institute of the French National Coal Board (CERCHAR) theoretical and experimental investigations of the application of HTC for drives of underground chain conveyors were carried out. In particular there has been investigated the effect of HTC: on start-up; on limitation of torque in case of conveyor jamming; and on distribution of the load among several motors. Results of the investigation and conclusions are presented.