Planetary rollers have the power of the force

The Planetary Roller Spindle combines the concepts of planetary gearboxes, roller bearings and lead screw nuts to deliver large forces from low torques with high levels of efficiency. It was originally developed by DLR, the German Aerospace Centre, for a robot gripper (ROTEX) for the German D2 Spacelab mission in 1993. The aim was to realise a compact and lightweight actuator. Mass limitations could not be met using conventional devices. The planetary rolls rotate about the spindle rod and are axially held in the nut by coarse grooves. The coarse grooves have no contact with the spindle rod. Contact with the helical grooves of the spindle rod is by fine grooves in the planetary rollers. The fine grooves in the rollers have different offsets in order to accommodate the thread of the spindle rod. The rollers are of greater diameter within the fine grooves than within the coarse grooves. There is no contact between the fine grooves and the nut. The use of rollers as opposed to a static nut thread greatly reduces friction. The device also looks relatively low cost to manufacture compared with a ball screw plus planetary gearbox. A spindle with a 1mm pitch can deliver 6kN force from a motor torque of 1.25 Nm with slightly more than 70% efficiency. For a roller screw with a 2mm pitch, a torque of 2.4 Nm is required. Efficiencies of up 75% are possible. Due to pre-stressing, efficiency is lower at smaller forces. The design has been licensed to Wilhelm Narr & Co who now manufacture it in two versions with two further models planned. Steady peak loads are from 1 kN to 10kN and Maximum loads are from 1.5 to 15 kN. Maximum nut rotation speed is 20,000 rpm. The largest size device is 34mm in diameter and 50mm long while the smallest is 25mm in diameter and 18mm long. Applications are seen in electromechanical drive by wire systems, particularly brakes, steering and suspensions. A project is under way with Knorr Bremse to use the devices in a mechatronic braking system for trains. Other possible applications include injection moulding machines and presses. Patents are held by the DLR. William Narr & Co Pointers * Very large output force versus input torque ratios can be achieved in a small space * Mechanical efficiencies are up to 75%. Highest efficiencies are achieved at highest forces. * The devices are simple and relatively inexpensive to manufacture