Product Management Specialist
Cost-effective actuators capable of precise linear motion are in growing demand by machine builders. Two of the more widely used actuator drives fitting this description are toothed belts and ball screws. Both convert rotary motion of a drive motor to linear motion, though they do so with markedly different methods.
When should engineers choose one actuator technology over the other? The short answer is, it depends on the application. A quick review of the basics of both designs is a good first step in the selection process.
Ball-screw-driven actuators use a ball screw and a train of recirculating ball bearings contained in a nut to convert rotary motion to linear motion. Bearing balls travel in opposed, hardened ball tracks located in an axially translating ball nut and on a stainless-steel rod. The rod track or groove is cut at a particular helix angle.
Screw pitch scales with the rod helix angle and is defined as the distance between adjacent screw threads. The lead defined as the pitch of the screw multiplied by the number of threads specifies the linear travel of the ball nut per revolution of the screw. Screws with greater pitches axially move a nut faster for a given screw rpm. Smaller-lead screws lower the amount of drive torque needed to move an object and therefore produce greater linear force (thrust).
Either an internal reversing system or a series of external return tubes circulate the balls. Balls are in compression when riding in the ball track, and unloaded in the return system. They are preloaded to eliminate axial play between the nut and screw. Actuator manufacturers typically use oversized ball bearings to adjust preload.
Ball-screw actuators tend to be rigid, highly accurate and precise, because bearing balls have little compressibility, and the actuators themselves are held to tighter manufacturing tolerances. A low rolling friction of the recirculating balls means drive mechanisms are typically over 90% efficient, which lowers energy consumption and wear.
Ball-screw actuators can have lifetimes exceeding 5,000 km when operated at low speeds and loads. Higher loads and speeds approaching 1 to 1.5 m/sec shorten lifetimes to about 3,000 km. Ball-screw actuators are length limited by the critical speed of the screw. Critical speed scales with screw diameter and is inversely proportional to the square of (unsupported) screw length. In other words, smaller-diameter screws for a given unsupported length have a lower critical speed than larger-diameter screws. In addition, manufacturers employ different methods of supporting the screw to increase critical speed.
Belt-driven actuators, in contrast, are optimized for high speeds. They are limited in length only by the actuator profile itself, or about 3
3 that of equivalent ball-screw actuators.
Belt-driven actuators use a synchronous, toothed timing belt to run two geared pulleys mounted at each end of an actuator profile. One pulley acts as the driving gear, and the other as the driven gear. A motor coupled to the drive pulley converts rotary torque to tangential force. Actuator thrust scales with pulley diameter. Linear speeds and accelerations are mostly limited by the guiding mechanism, not by the belt or pulleys.
Belts made of a synthetic rubber, such as polychloroprene, surrounding a core of either metallic or fiberglass strands are lightweight, flexible, and have an extremely high tensile strength. Belt teeth come in different shapes, the most common of which are a rounded tooth or a trapezoidal-shaped tooth. Trapezoidal teeth can transmit more power, while rounded teeth, or selftracking circular teeth, reduce noise and vibration. Regardless of design, teeth are covered with a nylon fabric for low friction and wear resistance. Belt drives require little maintenance except for the belt itself, which generally lasts about 2,500 to 5,000 km (up to 15 km in some cases).
Belt-driven actuators typically employ either a recirculating ball-bearing guide or a roller guide. Recirculating guides combine high dynamic load capability with high speeds (up to 9.8 ft/sec). Roller guides excel in light-duty, extremely high-speed applications.
An automated drilling station uses a linear actuator to move a drill head horizontally along 15-ft-long aluminum extrusions at programmed locations. The needs for high linear speeds and a long travel length point to a belt-driven actuator.
A parts-transfer station is another example where belt-driven actuators make sense. This particular machine moves a 10-lb part 5 ft in 500 msec, dwells for 500 msec, then returns to its original position in another 500 msec. Belts and pulleys have low inertia, making them well suited for such high speeds and accelerations. Also, the machine is located in a relatively quiet assembly area so noise is an issue. Optimized belt-tooth geometry and tight manufacturing tolerances of gear pulleys keep noise levels between 55 and 65 dBA, about 8 to 12% lower than equivalent ball-screw actuators.
Ball-screw actuators get the nod for a paint-spraying machine used in the automotive industry. The spray head moves vertically at a relatively slow speed to insure a proper finish. Use of a fine-pitch screw gives slow constant-velocity motion, and lets the engineer spec a smaller motor.
Another example is a machine that winds a cable around a spool with predictable and consistent motion. A ballscrew actuator acts as a reciprocating guide to move the cable back and forth as it coils.
Some applications use both beltdriven and ball-screw actuators. For example, a step-and-scan machine that visually inspects PC boards moves the X axis (step axis) with a belt-driven actuator because accuracy is not as critical as speed. This axis steps 6 in. in 300 msec to position the Y axis (scan axis). The scan axis moves a vision system slowly over the part to be inspected, then returns to home position for the next part. Here, the metrics of repeatability and constant velocity make a ballscrew actuator a good choice. In general, ball screws are repeatable to about 0.001 to 0.0001 in. For belt drives that number is 0.004 in.