Abstract: A drive mechanism (10, 110) for driving first and second displacement members (12, 14, 112, 114) disposed in tandem. The mechanism includes first and second driven sprockets (16, 18; 116, 118) for respective ones of the displacement members, a hydraulic motor (20; 120), and first and second driving sprockets (22, 24; 122, 124) disposed on the casing of the hydraulic motor so as to be rotated thereby. The mechanism also includes first and second chains (26, 28; 126, 128) disposed in a chain case (30; 130), each chain being engaged on a respective driven sprocket and on a respective driving sprocket. The hydraulic motor (20, 120) is a low-speed high-torque motor, having radial cylinders and a rotary casing. The motor is substantially contained in the chain case (30; 130).