Linear actuators: It's what's inside that counts

Two general rules for selecting a linear actuator are: Use a belt-driven actuator if the linear stroke must be over 8 ft, and choose a ballscrew-driven actuator if the application needs precise positioning. But there's more to the process of choosing between these actuators than specifying stroke length or positioning accuracy.

What if the stroke length is 4 ft, the speed is attainable with either actuator, and precise positioning is not a factor? Does the choice automatically rest on cost? It shouldn't. Other factors engineers need to consider are the load, the move profile, the amount of tolerable backlash, and needed resolution. And, in those situations where the cost differences between these two types of actuators are negligible, these factors assume greater importance.

Near and far

Even though the needed stroke length shouldn't be the main selection criterion, it can quickly reduce the choices to a more manageable level. When engineers must go long, belt actuators are available in lengths to 20 ft. Some manufacturers will make them to 60 ft, or offer them in segments that engineers can piece together. The limiting factor on length is the cost of extruding the belt.

Most ballscrew actuators, on the other hand, are limited to 6 ft or less. Occasionally, a manufacturer will make a ballscrew to 8 ft. Beyond this, maintaining straightness over the length of the screw makes the manufacturing cost prohibitive.

Fast or faster

Along with more length, beltdriven linear actuators generally offer higher speeds than ballscrew versions. Most ballscrews are limited to travel speeds of 50 ips for shortstroke lengths and to a few inches per second at strokes from 6 to 8 ft. Beltdriven actuators achieve speeds to 180 ips. Belt actuators, however, have limits on the acceleration rates they can withstand. The belt material is subject to deformation, fatigue, or fracture at high acceleration and deceleration speeds.

Ballscrew speed is affected by several interrelating factors; the length and straightness of the screw rod, its diameter, and the pitch of the threads. Any sag or deflection along the length reduces the maximum achievable speed. To compensate for some of the deflection, a few manufacturers offer self-aligning bearings in the nuts. Depending on the load and positioning accuracy needed, these bearings can offset a fair amount of sag and still attain high speeds. Radial load, however, will reduce the amount of compensation from these bearings.

The screw diameter must be large enough to accommodate the force to be transmitted through the screw as well as the load the actuator will carry. But a large screw diameter can limit the speed of the actuator, because it determines the speed of recirculating balls in the nut. The screw is also subject to a limit known as critical speed. At this speed, the ballscrew begins to vibrate about its axis. Larger lengths and smaller diameters lower the speed at which this vibration occurs.

Screw pitch also affects speed. A screw that has a low lead or a high pitch will give a slower linear speed for the same rotational speed than a screw with a high lead or low pitch.

In addition, pitch influences the resolution and positioning accuracy of the screw. Resolution is the smallest incremental move needed. Generally, the higher the pitch, the more accurate the positioning. Ballscrew actuators offer more precise positioning than belt actuators. Also, their repeatability is higher. They return to a specific position with greater accuracy than a belt actuator.

Carrying the load

Ballscrew actuators can handle the highest thrust loads, to thousands of pounds. Belt actuators, on the other hand, are limited to a few hundred pounds.

Moment loads, however, can be a factor in multiaxis applications. Here engineers need to define the position based on how the second or third axis will move as a result of the moment load that is applied through the primary axis.

Some actuators transmit the load through the cylinder body instead of the screw or belt. Such actuators offer good general purpose load capacity and tracking accuracy, plus they control friction well.

The prime mover

Belt and ballscrew-driven actuators can be powered by general purpose, servo, or step motors. However, keep the motor to load inertial mismatch to within a 10:1 range. The choice between a servo and a step motor depends on how precise the positioning must be and whether feedback is required. Servos are more precise.

With ballscrew linear actuators, the ballscrew tends to affect the motor much as a gear reducer does. Belt actuators react like direct drives.

In motion

Another factor to examine is the motion profile of the application. Unlike ballscrews, belt actuators do not have backlash, but they do have elasticity. (Backlash in some ballscrew systems can be set low.) Usually, belt actuators are not the best choice if the application requires a series of very small incremental moves. When starting a move, the motor must execute enough small moves to take up the elastic stretch and overcome the static friction before the belt moves. If a belt actuator must be used in such an application, one solution is to use a microstep or servo motor with a high encoder line pulse count.

Some of the newer belt actuators use belts made of stiffer materials. These versions are capable of good positional accuracy.

A third option

Sometimes, neither a ballscrew or beltdriven actuator results in the best fit. Another option may be pneumatic rodless linear actuators. These devices handle highly repetitive, back-andforth moves that don't require complex positioning. They also offer high velocity and high load capacity, in some cases, for about 50% less cost per axis than other actuator choices, especially if you already have access to a compressed air source.

Easier selection

Calculating all the parameters to find the right solution can be tedious. To make it easier, most manufacturers offer selection software. These programs usually require engineers to specify the load, travel distance and speed, the motion profile, and duty cycle needed in the application.

Several software packages provide a range of solutions, although they may be limited to just the manufacturer's products. Some programs are sophisticated enough to indicate parameters engineers can change to achieve a better solution, such as altering the duty cycle or lowering or increasing the acceleration or deceleration rate. A few programs will even let engineers perform a "whatif" analysis, varying multiple parameters and simulating the effects on the application.

Some programs will even help engineers select the actuator, motor, motor mount, drive, control, gearbox (if needed), and feedback device.

Thanks to Derrick Alcock, vp of Electric Motion Products at Tol-O-matic Inc., and Don Alfano, chief engineer at Warner Electric for their help with this article.

Axidyne electric linear actuators can handle loads to 2,000 lb and speeds to 100 ips. Tol-O-Matic also offers motors and drives for their actuators.