In recent years, power transmissions in the 0 to 3-hp range are mostly dc drives. However, one gearless transmission bucks this trend. It consists of aircraft-quality cable wrapped in opposite directions around driver and driven shafts. Cables are anchored to the largest shaft and preloaded to eliminate backlash, an advantage over geared transmissions. Cable drives work equally well as speed increasers or decreasers, but rotation is limited by the length of cable the shafts can handle. The drives are only suited where continuous rotation is not required.

Torsional stiffness is 105 lb-in./rad and higher for single cable drives. Stiffness varies with the square of the larger shaft radius; thus, doubling shaft size improves stiffness by a factor of four. Single-wrap cable drives can transmit up to 72 lb-in. and provide a useful life exceeding many millions of cycles. Torque capacity for multiple cable drives can range to 76,000 lb-ft, but speed is usually less than 1 rpm. The devices have found homes on turntables for silicon wafer saws, in positioning units for directional solar panels, and in robot-armjoints.