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Going linear

Oct. 1, 2000
Looking for a linear actuator? Here's a quick review that will help you sort the many options.

Whether you're guiding, pushing, or positioning a load, there's a linear actuator that fits your application. In fact, given the range of electromechanical, hydraulic, and pneumatic actuators, there's probably more than one viable solution, as many types overlap each other in speed, accuracy, and other performance factors.

In addition, new designs have entered to fill a few application gaps. Yet, despite their increasing capabilities, there are crucial differences that make one actuator type more suitable than another for a particular operation.

Put it there

When you need to position small or supported loads, or loads that pivot, rod-type electric cylinders can be a cost-effective choice. The key in choosing this actuator is that other devices support the load.

Rod-type actuators use an electric motor to rotate a ball screw or acme screw, which translates the motion to extend the rod or tube. In non-direct drive designs, a timing belt or gear drive matches the motor's power to the needed linear speed and thrust. Reversing motor rotation usually retracts the rod.

Some cylinder models can also consecutively position a load. For example, if a load riding on a conveyor must undergo several assembly processes, a rod-type cylinder can move it to a position and stop, then move to the next position and stop while the load undergoes various operations. This go-and-stop process can continue through the cylinder's stroke range.

Positioning accuracy for either single or multiple positioning is usually ±0.0005 in. (±0.013 mm). For loads that can't handle jostling, there are electric cylinders that will gently decelerate before each stop.

Rod-cylinders are also well suited to guide loads in harsh surroundings. Because the rod portion extends out from the main body, it can go to work while the rest of the cylinder and motor remain outside that damaging environment.

For applications that require rapid, point-to-point positioning of light loads, consider pneumatic cylinders. Adding digital sensors and valves will assure precise positioning. But the newest sensors to use with pneumatics output signal that varies linearly with a cylinder's piston movement. Therefore, you escape having to reposition the sensor every time you move the signal source.

When weighty loads need support as well as positioning, consider rodless actuators. Operation is similar to electric cylinders. Instead of an extending rod, though, there's a moving carriage that's supported by either the linear bearings of a screw or by a belt. The carriage can support a load, the amount of which depends on the driving motor's torque capacity and, to a lesser extent, on the screw diameter. The need for roll or pitch motion will also affect load-carrying capacity. In general, these actuators support loads ranging from a few pounds to about 5,000 lb.

Rodless actuators carry the additional advantage of being smaller than extending rod cylinders.

A recent pneumatic introduction to rodless devices is the servopneumatic actuator. With positioning accuracies to 0.008 in., which is a little less than that of electric cylinders, this device can be used in about 80% of industrial positioning applications.

The key to its development is the proportional pneumatic valve. It receives position signals from an axis controller, which it converts to flow outputs that are directly proportional to those signals. Conversion takes less than 5 msec. The flow output signals then feed to the pneumatic actuators, which move to the required stop positions.

These devices move loads from 180 to more than 1,600 lb without producing shock in the mechanical system or workpiece. They also offer intermediate-stroke positioning where the stop point is adjustable along the actuator's entire stroke length, not just at the end.

For positioning heavy loads (over 5,000 lb) hydraulic cylinders are the common choice. Like pneumatic systems, adding position sensors assures reliable accuracy.

When micron positioning accuracy is needed, the newest solution is tubular linear motors. The permanent magnet secondary is the thrust rod, with the primary moving back and forth over it.

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In addition to its accuracy, this device will reach commanded positions fast with an acceleration rate of 5 g. However, fairly stiff machinery is needed to accommodate those rates.

Movement is smooth with tubular linear motors, even at low speeds. And an extra bonus is that the one rod supports two or more thrust blocks so they can position multiple loads simultaneously.

Fast and powerful

Once you determine where a load should go, you must decide how quickly it should get there. And depending on the weight of the load, you'll need to figure how much power, or thrust, to supply. The load, speed, and thrust may affect positioning, which can alter your actuator selection.

Electric cylinders generally move the rod at speeds ranging from 12 to more than 50 ips, with thrusts from 400 to over 5,000 lb. Speed limits are affected by whether you select a ball screw or acme screw design. Cylinders using a ball screw will offer faster speeds than models with acme screws. But if it's important to hold loads after power is removed, then you need acme-screw based electric cylinders. Ball screws have a tendency to back drive when power is removed.

For speeds even faster than those obtained with screws, such as more than 100 ips, consider belt-driven rodless actuators. Just remember, compared to screws, you'll trade off speed for repeatability and thrust.

An application's duty cycle will also affect whether you choose acme, ball screw, or belt-driven actuators. Ball screws have a higher duty cycle. Thus, when the cycle rate exceeds 60%, go with a ball-screw driven actuator. Belt models also handle high duty cycles, on a par with ball-screw models.

For power, rod-type cylinders offer higher thrusts than rodless (which range from 100 to 1,200 lb), because all the transmitting components are in line. In rodless systems, loads are offset from the lead scew. The mechanical limitations of components between the load and screw face can reduce available thrust to less than 50%.

Hydraulic actuators are generally not chosen for speed. Their largest advantage is thrust and stroke capability. Of all actuators, hydraulic devices offer the most thrust. But speed control is a difficultly, due to stickslip problems and thermal effects.

Pneumatic systems, which can move at speeds to 200 ips, are also susceptible to stick slip. And varying load weights will increase or decrease their speed.

The speeds of servopneumatic actuators, however, range to 70 ips. These devices offer very high accelerations - to 20 g -, which is higher than tubular linear motors.

Tubular motor actuators offer the highest speeds at more than 350 ips. Their thrust capability, though, is tied to their motor size, which usually limits them to about 300 lb. Beyond this, the cost of motor magnets will force engineers to consider other options.

Linear motors also offer the benefit of long operating life. Because they are almost frictionless, there is very little wear, which enables them to easily handle high duty-cycle applications.

Going the distance

High precision, speed, and load handling are fine, as long as an actuator has the necessary stroke length. For travel from a few inches to almost six feet, you can choose from electric cylinders, pneumatic and servopneumatic actuators, tubular linear motors, and even rodless devices.

Between six to seven feet, rodless actuators and linear motors are usually best. Screw driven actuators, though, will be forced to operate at lower speeds if you go beyond seven feet. Screws are limited by their critical speed and by the maximum recirculating speed of the ball nut circuits or sliding nuts. At travel lengths over seven or eight feet, the critical speed becomes a factor. This is where the screw starts to "vibrate," reducing positioning accuracy. If the application requires more distance, consider belt-drive rodless actuators, which easily go nine feet and beyond. For greater distances, hydraulic becomes the best choice.


Most linear actuators are designed for horizontal positioning. But if the application calls for vertical or inclined movement, consider the electric cylinder or a hydraulic or pneumatic system. Also, some rodless actuators are possible candidates. Other actuators can be used, but they will require brakes if power is removed.

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