Buying Guide for Linear Actuators

Linear Actuators are electromechanical or mechanical devices that produce straight-line motion. This motion is used for many tasks, from aircraft-control actuating to precision-assembly positioning. Although ordinary hydraulic and pneumatic cylinders are linear actuators, for the most part, they are simply called “cylinders.” This article will focus on electromechanical forms that produce either direct linear motion or convert rotary motion to linear motion through some combination of motor and power transmission element. It will also discuss rodless pneumatic linear actuators, as they are one step beyond ordinary air cylinders.

The following types are discussed here:

Electric Rod-Style Linear Actuators
Pneumatic Rodless Linear Actuators
Electric Rodless Linear Actuators
Precision Electric Linear Actuators
Compared with cylinders, electric linear actuators are costlier to purchase. Specifying them requires closer engineering evaluation than specifying cylinders because oversizing electric actuators can greatly affect their cost. Among the necessary considerations for selecting electric linear actuators:

Static and dynamic loading matched to peak and continuous load capabilities
Duty cycle
Force and velocity
Pneumatic actuators will share some of the same attributes without the same penalty for upsizing.

Rod Style Electric Linear Actuators
These actuators, sometimes called electric cylinders, are intended as replacements for hydraulic and pneumatic cylinders in motion control systems. Most actuators use lead- or ball-screws driven either directly (in-line) by stepper motors or servomotors or through toothed-belt transmissions (reverse-parallel). The envelopes of these actuators do not exactly match the outlines of cylinders but the attachment styles for the actuator bodies and rod ends are similar. The reverse-parallel design can often be fitted with a clevis at the cap-end and closely approximate the length of an air cylinder, with some space needed for the motor which lies parallel to the actuator body. In-line designs are necessarily longer than air cylinders of equal stroke since the motors sit behind the cap-ends. This is not necessarily a problem for fixed mounts. Apart from their higher initial costs, rod-style actuators count positioning capability and motion profiling as advantages over air cylinders (where needed). Maintenance is less, too. Another advantage over cylinders is the ease with which their strokes can be shortened, with changeover for various production scenarios handled by the machine controls.

Rodless Pneumatic Linear Actuators

These actuators, sometimes called rodless cylinders, use traditional pistons and air cylinders but forgo cylinder rods. Instead, a sliding carrier rides the exterior housing of the cylinder and is coupled to the piston by either magnets or cables, or directly to it through a flexible band/seal. These arrangements produce more compact linear-motion systems than can be achieved with ordinary cylinders and slides. Manufacturers offer them in a variety of carrier-support configurations, such as sintered bushings or linear bearings. They can be specified for horizontal and vertical loads.

Rodless Electric Linear Actuators
These actuators are the electromechanical equivalents of their pneumatic cousins. Generally driven by timing belts, ball screws, or lead screws, the design eliminates sealing concerns associated with the pneumatic versions. Depending on the applications, motors may be either stepper or brushless DC servo. Belt drives are preferred for long-stroke applications with high speed/acceleration requirements. Ball screws are preferred where accuracy is important and stroke lengths are shorter. They also provide higher thrust than belt drives. Lead screws are less expensive but also inefficient and prone to heat build-up at high speeds/loads. An advantage of lead screws is they are self-locking. As with rod-style actuators, these electric versions offer positioning, motion profiling, and pain-free changeover for motion systems that warrant them.

Precision Linear Actuators
This group of actuators employs a range of motor styles to produce linear motion over generally short spans but with very high positional accuracy, often in the micro- and nano- ranges. In addition to stepper and servomotors, which often drive through gearheads to produce fine movement, precision actuators employ linear motors, piezoelectric motors, voice coils, etc. whose positioning accuracies are suited to the demands of semiconductor manufacture, optics control, etc.