Straight talk on selecting linear actuators

Alan D.J. Augello
SKF Motion Technologies
Bethlehem, Pa.

Edited by Miles Budimir

Linear actuators come in a variety of shapes and sizes, with variable stroke lengths and a range of load capacities. Depending on type, electromechanical linear actuators can handle loads to 3,000 lb (13 kN) and deliver speeds to 6 ips (150 mm/sec) with strokes ranging from 2 to 60 in. (50 to 1,500 mm). Actuators can be self-contained in aluminum, zinc, or polymer housings for ready-to-mount, plug-in operation with an ac or dc power supply.

Modular actuators allow internal and external components to be interchanged according to specifications. Standard interchangeable components include drive screws, motors, front and rear attachments, controls, and limit switches. Power options include 12, 24, 90, or 180 Vdc or 110, 220, or 380 Vac.

Electromechanical linear actuators give precise, efficient, accurate, and repeatable motion. They are a relatively maintenance-free alternative to hydraulic or pneumatic equivalents.

Although every application is different, the selection process for electromechanical linear actuators is mostly the same. It typically begins by calculating several key parameters including electrical power-in, duty cycle, and actuator efficiency.

Obviously, getting mechanical power out of an electric linear actuator requires putting electrical power in. Mechanical power-out is probably the easier of the two to define, because it only involves load force and speed.

When parameters are in metric or SI units, multiply force (in Newtons) by the speed (in m/sec) to obtain watts. (To convert pounds to Newtons, multiply pounds by 4.448; to convert inches to millimeters, multiply by 25.4.)

Mechanical power-out, P_o, in watts is:

P_o = F v

where F = force, N, and v = velocity, m/sec. Information for electrical power-in is available from actuator supplier graphs and charts. Most suppliers include graphs for force versus speed and force versus current draw at some voltage. This is typically presented in two graphs or combined into one. In others, the current draw is in tabular form. Also, factors will be given based on a duty-cycle curve or in tabular form. The formula for electrical power-in, P_i, in watts is:

Pi = E X I
where E = voltage, V, and I = current, A.

Next, establish the duty-cycle factor, sometimes referred to as the derating factor. The duty cycle indicates how often an actuator operates in an application and the amount of time between operations. Because the power lost to inefficiency dissipates as heat, the actuator component with the lowest allowable temperature (typically the motor) establishes the duty-cycle limit for the entire unit. Of lesser concern are mechanical losses from friction in gearboxes and ball-screw and Acme-screw drives.

For example, assume an actuator runs for 10 sec cumulative, up and down, and then stops for another 40 sec. The duty cycle is 10/(40 + 10) 100% = 20%.

When duty cycle is increased, load or speed must be reduced. Conversely, a drop in load or speed means duty cycle can increase.

Another issue is a system's efficiency. It can predict how hot an actuator may get during operation, whether holding brakes should be specified in ball-screw actuator systems, and how long batteries may last in systems powered by them.

To calculate percent efficiency, simply divide mechanical power-out by electrical power-in and multiply by 100%.