Linear actuator provides rigidity and accuracy of a linear stage

Although there are no industry standards that define linear actuators and linear stages, generally accepted terminology indicates that a linear actuator is typically constructed with an aluminum extrusion or base, while a linear stage is typically built on a flat, machined steel or granite base. This distinction implies that linear actuators can provide longer strokes and use a variety of drive mechanisms (belt, screw, rack and pinion), while stages generally have higher rigidity and use high-precision linear guides and drive mechanisms (typically a ball screw or linear motor) for excellent travel and positioning accuracies.

But one actuator design — the U-shaped linear actuator — defies these specifications, using an extruded steel base to provide rigidity and travel accuracy specifications that rival some linear stages.

This actuator design has various names, depending on the manufacturer. But for this discussion, we’ll refer to it as a “U-shaped linear actuator,” in reference to its defining feature — a U-shaped, extruded steel profile.

Image credit: THK America Inc.

The use of a steel (rather than aluminum) profile makes the U-shaped design extremely rigid and allows manufacturers to offer a linear actuator with the high travel and positioning accuracies typically found in more precise — and more expensive — linear stages. The steel base can also be machined to provide a reference edge, for precise alignment with other machine components, or with other actuators in a multi-axis system. And with very high rigidity, the U-shaped linear actuator is much better suited than other designs for applications where the actuator is supported only on one end, such as 2 and 3-axis Cartesian systems.

U-shaped linear actuators are constructed with an extruded steel base. The linear guide raceways are ground into the inside of the base, and the slider, or carriage block, is similar to a bearing block turned inside-out, allowing it to house the ball nut in the center. This design makes the entire package extremely compact and rigid.
Image credit: THK America Inc.

In the U-shaped actuator design, the linear guide system is integrated — there is no guide rail. Instead, the raceways that would normally be found on the guide rail are ground into the inside of the base. The carriage, or table, is analogous to a linear bearing block turned inside-out, with the balls riding on the outside. This leaves the center portion of the carriage available to accommodate the ball screw nut. This construction principle makes the entire actuator extremely compact, with a width-to-height ratio of approximately 2:1. For example, a U-shaped actuator with a width of 60 mm is only 33 mm high. The most common cross-sections (width x height) are 40 x 20 mm, 50 x 26 mm, 60 x 33 mm, and 86 x 46 mm, although other sizes are offered as well.

Despite their compact dimensions, U-shaped linear actuators have very good load and moment capacities. This is because the raceways are spaced relatively far apart, so the geometry of the carriage is similar to that of a bearing block much larger than the actuator could accommodate in its standard form.

Some manufacturers offer U-shaped linear actuators made from extruded aluminum profiles, with steel inserts for the linear guide raceways. Aluminum versions lack the rigidity of steel designs, but they offer a very compact profile and are often dimensionally interchangeable with steel versions, for a lower-cost option. While steel versions of U-shaped linear actuators use ball screws drives almost exclusively, aluminum designs are more likely to be offered with both ball screw and lead screw drive options.

Many of the original applications for U-shaped linear actuators were in very clean environments, and sealing wasn’t a concern. But as their usage has expanded to different applications and industries, protecting the actuator from debris or liquids has become more important, so manufacturers now offer versions with a protective cover plate (left) or a fully-enclosed strip seal (middle).
Image credit: Bosch Rexroth Corp.

Originally developed for high-precision applications such as semiconductor wafer handling and medical diagnostic dispensing — for which space constraints don’t allow a typical linear stage — U-shaped linear actuators are now used in a wide variety of industries and applications. These include plasma welding, automated assembly, and optical inspection.

One of the driving factors behind the widespread adoption of U-shaped actuators is that they are the only linear actuator design with dimensional interchangeability between manufacturers. It’s important to note, however, that due to differing guideway and ball screw designs, technical specifications (such as load capacity, speed, or rigidity) can vary between manufacturers and product lines, even for products with the same cross-sectional size and mounting dimensions.

Feature image credit: THK America Inc.