An instrument drive unit for use in a robotic surgical system, the instrument drive unit comprising: a carriage configured to be coupled to a robotic arm; a plurality of drive shafts (66a, 66b, 66c, 66d) rotationally supported in the carriage, the plurality of drive shafts configured for interfacing with a corresponding driven member of an electromechanical surgical instrument; a plurality of drive gears (64a, 64b, 64c, 64d), each drive gear of the plurality of drive gears fixed to a corresponding drive shaft of the plurality of drive shafts; a plurality of motors (22a, 22b, 22c, 22d); and a plurality of motor gears, each motor gear of the plurality of motor gears operably coupled to a corresponding motor of the plurality of motors, wherein each motor gear of the plurality of motor gears is configured to rotate a corresponding drive gear of the plurality of drive gears in response to an activation of a respective motor of the plurality of motors to actuate a function of the electromechanical surgical instrument, further comprising a plurality of ring gears (62a, 62b, 62c, 62d), each ring gear of the plurality of ring gears operably coupling a corresponding motor gear of the plurality of motor gears with a corresponding drive gear of the plurality of drive gears, further comprising a plurality of drive belts, each drive belt of the plurality of drive belts disposed about a corresponding motor gear of the plurality of motor gears and a corresponding ring gear of the plurality of ring gears to operably couple the motor gears with the ring gears, characterised wherein at least a first ring gear of the plurality of ring gears has gear teeth on an inner periphery thereof and on an outer periphery thereof, wherein the gear teeth on the inner periphery of the first ring gear interface with a corresponding drive gear of the plurality of drive gears, and the gear teeth on the outer periphery of the first ring gear interface with a corresponding drive belt of the plurality of drive belts. a carriage configured to be coupled to a robotic arm; a plurality of drive shafts (66a, 66b, 66c, 66d) rotationally supported in the carriage, the plurality of drive shafts configured for interfacing with a corresponding driven member of an electromechanical surgical instrument; a plurality of drive gears (64a, 64b, 64c, 64d), each drive gear of the plurality of drive gears fixed to a corresponding drive shaft of the plurality of drive shafts; a plurality of motors (22a, 22b, 22c, 22d); and a plurality of motor gears, each motor gear of the plurality of motor gears operably coupled to a corresponding motor of the plurality of motors, wherein each motor gear of the plurality of motor gears is configured to rotate a corresponding drive gear of the plurality of drive gears in response to an activation of a respective motor of the plurality of motors to actuate a function of the electromechanical surgical instrument, further comprising a plurality of ring gears (62a, 62b, 62c, 62d), each ring gear of the plurality of ring gears operably coupling a corresponding motor gear of the plurality of motor gears with a corresponding drive gear of the plurality of drive gears, further comprising a plurality of drive belts, each drive belt of the plurality of drive belts disposed about a corresponding motor gear of the plurality of motor gears and a corresponding ring gear of the plurality of ring gears to operably couple the motor gears with the ring gears, characterised wherein at least a first ring gear of the plurality of ring gears has gear teeth on an inner periphery thereof and on an outer periphery thereof, wherein the gear teeth on the inner periphery of the first ring gear interface with a corresponding drive gear of the plurality of drive gears, and the gear teeth on the outer periphery of the first ring gear interface with a corresponding drive belt of the plurality of drive belts.