September 2021

28 n ROLLERSCREWS September 2021 www.drivesncontrols.com Rollerscrews: an under-appreciated technology? W hen designers consider the options for controlled linear motion, they probably think initially about hydraulic or pneumatic cylinders, or ball or leadscrews. But there is another technology, the rollerscrew, that offers potential performance benefits over all four. If we take efficiency as the primary criterion for selection, rollerscrews are more than 90% efficient, and only the ballscrew can compare. Life expectancy is typically 15 times longer than for a ballscrew, and only hydraulic or pneumatic cylinders give similar service lives – however, they both need maintenance to achieve these long lives. Rollerscrews need little maintenance because the friction created by the rolling screw design is much less than that generated by sliding friction. However, rollerscrews still need to be lubricated to reduce wear and dissipate heat. Protecting against contaminants is also critical to a long life, so wipers can be added to the front or back of the nut to scrape particles from the threads throughout the screw stroke. Maintenance intervals depend on two main factors – operating conditions and the screw diameter. By comparison, both hydraulic and pneumatic cylinders need much more attention, and ballscrews can suffer from pitting in the ball groove, while the bearings can be lost or need replacing. The load ratings of a rollerscrew are matched only by hydraulic cylinders. In some applications, rollerscrews can move up to 20,000 times their own weight, so can carry greater loads in relation to their size. They can also handle heavy loads for continuous duty and in arduous conditions. The combination of speed and acceleration can only be equalled by pneumatic cylinders. Stiffness and the effects of shock loading also make rollerscrews an attractive option. One of the biggest advantages of the rollerscrew is that it is much smaller and easier to install than hydraulic or pneumatic cylinders, both of which can be complex and costly to fit. At the same time, rollerscrews achieve much better positional accuracy than ballscrews. Environmentally, rollerscrews operate quietly – like pneumatic systems – and are also free of leaks of air and hydraulic fluid. They also consume less power. The return-on-investment when buying rollerscrew technology is offset through total cost of ownership and lifecycle costs. This is realised through lower maintenance costs and reduced downtime. How do they work? Rollerscrews use rollers to transmit force from a nut to the shaft. The main elements are the screw, the nut and the satellite rollers. The rolling elements are threaded rollers between the screw and the nut. A high number of points of contact enables satellite rollerscrews to support heavy loads. There are various types of rollerscrew available, including satellite rollerscrews with no recirculation, inverted rollerscrews, differential rollerscrews and screws with recirculating rollers. Rollerscrews where the rollers do not recirculate offer robust, precision assemblies with stable driving torque. Special gears on the rollers and nut maintain a good kinematics even in severe conditions. The internal thread of the nut is identical to that of the screw. To ensure no axial movement occurs between the nut and the rollers, a single-start thread can provide precise angle accuracy between the nut and the rollers. In rollerscrews with recirculating rollers, the rollers are guided within a cage and their motion is controlled by a set of cams. This design combines high positioning accuracy, resolution and stiffness, while capacity ratings are also high thanks to a robust thread geometry. These are ideal for applications that need high accuracy at low or moderate speeds. Inverted rollerscrews are based on the same principle as satellite rollerscrews with no recirculation, and deliver similar performance. But the rollers do not move along the shaft and stroke is travelled within a longer nut. This supports higher capacity ratings with smaller leads, reducing the driving torque, optimising compactness and making direct guiding of the shaft possible. Differential rollerscrews are suitable for high-accuracy applications, especially those with short strokes. Their components are designed and adjusted to allow thin leads down to 0.02mm. Moore International supplies Swiss-made Rollvis rollerscrews, including satellite rollerscrews with no recirculation (RV and HRV types), inverted rollerscrews (RVI), differential rollerscrews (RVD) and screws with recirculating rollers (RVR) . n Even though the first patent for a rollerscrew was granted in 1949, the technology is less well-known than other mechanisms for converting rotary torque into linear motion. Mark Moore, managing director of Moore International, examines why. A satellite rollerscrew without recirculation can deliver stable driving torque Rollerscrews with recirculating rollers offer high positioning accuracy, resolution and stiffness