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.

Electric cylinders, from Industrial Devices Corp., are either direct drive units or use gears or belts to match motor power to load inertia. Also available is the newest type of linear actuator, the linear servo motor. With speeds over 300 ips, it's one of the fastest actuators around.

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.

Combining servo performance with pneumatics results in this system from Festo Corp. With a positioning accuracy of 0.008 in., it handles general applications that don't need extreme position accuracy. The system includes a single or multiple axis controller, servopneumatic valve, standard pneumatic actuator, and a position feedback transducer.

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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