hen designers consider the options for controlled linear motion, do they fully examine the performance
benefits that the roller screw offers in relation to hydraulic or pneumatic cylinders, as well as ball or lead screws? Each designer may have differing selection
criteria, which will be determined by the application. So, in examining the major selection concerns, here is how the roller screw performs: The roller screw is over 90% efficient, and only the ball screw can compare. Life expectancy is also very long, typically 15 times longer than a ball screw. While only the hydraulic or pneumatic cylinder options give similar service life, 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; 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 a hydraulic cylinder. In performance, a roller screw has, in some applications, been able to move up to 20,000 times its own weight. They can also carry heavy loads for continuous duty and in 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. One of the biggest advantages of the roller
screw is that it is much more compact and easier to install than hydraulic or pneumatic cylinder alternatives. At the same time the roller screw achieves considerably better positional
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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 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.
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 enables satellite roller screws to support very heavy loads. The ROLLVIS range comprises satellite
roller screws with no recirculation (RV and HRV types), inverted roller screws (RVI type), Differential roller screws (RVD type) and screws with recirculating rollers (RVR type). Bearing units are also proposed to equip the different roller screw designs. The RV screw is a high precision, robust
assembly where the rollers do not recirculate, thus enabling a very stable driving torque. Special gears are designed on the rollers and nut to maintain good kinematics even in the most 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
The RVI roller screw is an inverted roller
screw. Whilst it is based on the same principle as the RV design and delivers the exactly the same level of operational performance, the rollers do not move axially along the shaft and stroke is travelled within a longer nut. This concept follows specific design rules which enable higher capacity ratings with smaller leads which consequently reduces the driving torque, optimizes compactness and makes a direct guiding of the shaft possible.
rollers, there is a single-start thread with precise angle accuracy between the nut and the rollers. The RVR roller screw is a design which
incorporates recirculating rollers. The rollers are guided within a cage and their motion is controlled by a set of cams. This design combines extremely high positioning accuracy, resolution and stiffness while capacity ratings are at the highest thanks to a robust thread geometry. This concept is perfect for all applications that need a very high accuracy under small or moderate speeds.
The RVD roller screw is ideally suited
for high precision applications when high accuracy is required. Its components are specifically designed and adjusted to allow extremely thin leads down to 0.02mm. Shorter stroke applications are best suited for this type of roller screw. The HRV roller screw is designed for very high
load applications and for extended life. This design provides more contacts in the assembly than the RV type and has a different thread profile. The nuts are also longer than in the standard RV version. The HRV series is suitable for larger diameters and consequently the range is not available in split nut format or with internal preload. Only standard or reduced backlash options are available.
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