FEATURE


LINEAR MOTION your application? W


hile the selection of linear motion technology depends on the application, roller screws can have


advantages over hydraulic or pneumatic cylinders, ball or lead screws. If we take efficiency as the primary criterion


for selection, the roller screw is over 90% efficient, and only the ball screw can compare. Furthermore, life expectancy is very long for a roller screw, typically 15 times longer than a ball screw, and only the hydraulic or pneumatic cylinder options give similar service life. These, however, both need maintenance to retain long life. The roller screw requires very little


maintenance as the friction created by the rolling screw design is minimal compared to that generated by sliding friction. However, the roller screw should still be lubricated to minimise wear and to dissipate heat. Providing sufficient protection against contaminants is also critical to long functional life, so wipers can be added to the front or back of the nut to scrape particulates from the threads throughout the screw stroke. Maintenance intervals will depend on two main factors, the operating conditions and the screw diameter. By comparison, both hydraulic and pneumatic cylinders need much higher levels of attention; and ball screws can suffer from pitting in the ball groove, and the ball bearings can be lost or need replacing. The load ratings of a roller screw


can only be matched by an hydraulic cylinder. In performance, roller screws have, in some specific applications, been able to move up to 20,000 times their own weight, so


50 DESIGN SOLUTIONS JULY/AUGUST 2021


are capable of carrying greater loads in relation to their size. They can also carry heavy loads for continuous duty and in the most arduous of conditions. The combination of speed and acceleration can only be equalled by that of a pneumatic cylinder. Both the stiffness and the effects of shock loading also make the roller screw an attractive option.


space saving


Automated industrial production lines are being designed to take up less space. One of the biggest advantages of the roller screw is that it requires only minimal space requirements as it is much more compact and is much easier to install than hydraulic or pneumatic cylinder alternatives. At the same time the roller screw achieves considerably better positional accuracy compared with a ball screw. Environmentally, the roller screw is less


noisy in operation, which compares well to pneumatic systems, but also has the obvious advantages of leak free functionality (air and


option for the right


Despite the first patent for a roller screw being granted in 1949, why is roller screw technology a less recognised option when compared to other mechanisms for conversion of rotary torque into linear motion? Mark Moore of Moore International examines their advantages


hydraulic fluid) and less power consumption. The return on investment by purchasing


roller screw technology is offset through total cost of ownership and life cycle costs. It is realised through the reduction in the cost of maintenance, early replacement and resulting downtime.


How do roller screws function?


Roller screws use rollers to transmit force from the nut to the shaft. The main elements are the screw, the nut and the satellite rollers. Moore International supplies the ROLLVIS Swiss range of roller screws which are used to transform rotary movements into linear movements and vice versa. The rolling elements are threaded rollers between the screw and the nut. The high number of points of contact enable satellite roller screws to support very heavy loads. The ROLLVIS range comprises satellite


are roller screws


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