Roller screws have the edge on ball screws in demanding, continuous-duty tasks.
Principal Design Engineer
Planetary roller screws are efficient and compact devices that convert rotary motion into linear motion. They perform the same types of tasks as do Acme or ball screws. A comparably sized roller screw can, however, carry higher loads than a ball screw, has higher efficiency than an Acme screw, and can turn significantly faster than either. This makes them an excellent choice for demanding, high-duty-cycle applications.
Roller screws' advantages stem from how the mating parts interface. The radiused flanks of the rollers deliver point contact like that of balls on a raceway. In a roller screw, however, only the contact region of the radius is part of the profile. Therefore, a much larger radius and a high number of contact points can be packed into the available space. The result is much lower stresses within the components. Comparatively, roller screws have a load capacity as much as 15 times that of similar-size ball screws. Furthermore, low-friction rolling contact between components yields high efficiencies. Because the rolling members are fixed relative to each other and never come into contact with adjacent rollers, roller screws can turn at speeds to 5,000 rpm.
Roller screws generally have ground screws that offer high precision. The units are typically grouped in tolerance classes according to DIN 69051, Part 3. The standard expresses manufacturing tolerances as lead errors per distance of linear travel. Typical tolerance classes are G1 (0.006 mm/ 300 mm), G3 (0.012 mm/300 mm), G5 (0.023 mm/300 mm), and G9 (0.200 mm/1,000 mm).
Three different nut types add to design flexibility. Standard single-nut roller screws maximize life and have backlash of about 0.01 to 0.03 mm. Split-nut or double nut configurations eliminate backlash entirely. With the split-nut design, the nut is split transversely and a precision-ground spacer inserted between the front and back halves. Double nuts, as the name suggests, use two nuts preloaded against each other on one screw. They provide the high load rating and long life of single nuts with the zero-backlash attributes of split nuts. In both zero-backlash designs, preloading is generally 5% of the dynamic load rating.
As with other leadscrews, regular grease or oil lubrication helps maximize roller screw performance. There is no standard for lubrication intervals screw diameter, lead, and operating conditions dictate the interval.
Higher duty cycles may require oil lubrication. In such instances, use high-performance gear oil with extreme-pressure (EP) additives and ensure that the nut components are thoroughly lubricated. Lubrication rates depend on screw diameter, number of rollers, and amount of heat to be dissipated. Oil is usually sprayed on the screw shaft during operation. Horizontal mounting lends itself to immersion lubrication, but oil levels must be deep enough to fully submerge the bottom-most roller.
Contaminants such as nonlubricating fluids, metal chips, and abrasives adversely affect all leadscrews. The first level of protection is adding wipers to the front or back of the nut. Wipers capture grease within the nut and scrape particulates from the threads as the screw cycles back and forth. This configuration requires adequate lubrication to load-bearing parts of the nut assembly. The optimum sealing option is to package the roller screw within a force tube. A fully enclosed force tube keeps lubricants in and contaminants out. Force tubes can meet IP67 ratings and mount in a wide variety of arrangements.
Designers often think of ball screws first when it comes to linear motion, but they may not be the wisest choice. Here's a look at how roller screws compare in some key performance areas.
Loads and stiffness. Due to design, ball size limits the number of contact points in a ball screw. As a result, planetary roller screws have more contact points than possible on comparably sized ball screws. More contact points equate to higher load-carrying capacities (up to 779,000 lbf in Exlar units) plus better stiffness. In practical terms, this means typical roller-screw actuators need less space to meet specified load ratings.
Travel life. With higher load capacities, roller screws deliver major advantages in working life, which is usually rated in total "inches of travel" the unit delivers in service. For instance, a 1.2-in.-diameter, 0.2-in. lead roller screw under a 2,000-lb average load will give predicted service life approximately 15 times greater than for a same-size ball screw.
Speed. Ball-screw speeds are typically limited to 2,000 rpm or less due to the balls colliding with each other as the race rotates. In contrast, the rollers in a roller screw are fixed in planetary journals at the ends of the nut and, therefore, do not have this limitation. Hence, roller screws run at speeds exceeding 5,000 rpm and produce comparably higher linear travel rates.
Fluid-power comparisons. When applications involve high loads or fast cycling, roller screws are an attractive alternative to hydraulic and pneumatic actuators. They do not require complex support systems of pumps, valves, tubing, filters, and sensors. Thus, they take up less space and last a long time with virtually no maintenance. Hydraulic leaks are not a concern and the units significantly reduce noise levels. Electromechanical units using roller screws typically have much simpler controls. They also offer the flexibility of computer-programmed positioning, a definite plus for many applications.
The Comparing linear-motion technologies chart gives an overview of the basic motion technologies in terms of performance characteristics, efficiencies, and economics.
Rollers screws can be found in many critical, demanding, and precise linear-motion machines, ranging from artificial hearts and automated medical syringes to machine-tool presses and broaching devices. Food and beverage filling; cartoning, sealing, and palletizing equipment; as well as glass and plastic-molding operations are among other significant users.
In riveting applications, for example, roller screws can increase machine service life more than tenfold over machines using ball-screw technology. The ability to withstand high shock loads lets roller-screw actuators act directly as the upset force when riveting. Ball screws, on the other hand, have low shock resistance. Roller screws can also move rapidly several cycles per second on a continuous basis. Attempting similar movements with ball screws usually results in quick failures. Combining riveting with electro-mechanical servo-based systems ensures consistent fastener heights and minimal scrap.
Roller screws on servocontrolled weld guns accurately exert weld-tip force and tip position, characteristics not possible with pneumatic actuators. Compared to air cylinders, roller-screw electric actuators' position and speed can be adjusted without resetting switches or changing offsets. They are also more compact and virtually noise-free. Because roller screws provide high stiffness, weld tips do not "bounce," which extends service life. And all-electric actuators are more energy efficient than pneumatics, operate for millions of weld cycles without relubrication, and require less maintenance.
Sawmill and forestry applications provide excellent opportunities for hydraulic cylinder replacement. Eliminating the need for oil keeps the environment clean. With hydraulic systems, flow and speed can vary considerably with temperature, but this is not an issue with electromechanical systems that run consistently regardless of temperature.
The efficiency of converting electric power to linear motion exceeds 90% with electric actuators, compared with about 40% for hydraulic drives. Canter, edging, and merchandising saws, as well as high-speed fences that rely on roller screws can survive rugged sawmill environments while generating fast, accurate, and high forces. The flexibility of servocontrolled, electric systems also makes cutting a variety of patterns simple and cost effective.