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Linear Actuator Terms

Accuracy – The difference from the precise value of the intended velocity or position.
 
ACME Screw – A threaded screw utilizing sliding friction surfaces between the nut and the screw. It is self-locking and is about 30-40% efficient.
 
Back Drive – Torque produced by the applied load on a drive resulting in the reversal of rotation of the nut.
 
Backlash – The space between the interactive elements in a drive train or leadscrew assembly that creates a mechanical “deadband” when shifting directions.
 
Ball Bearing Screw – A screw that operates on ball bearings. Ball bearing screws have a low starting torque, are approximately 90% efficient and can be back driven. (http://www.ball-screws.net)
 
Bi-directional Repeatability – The divergence in the ending position attained by moving away and then returning to a regular point from both plus and minus directions. The error or non-repeatability factor is determined from the sum of the hysteresis, the backlash and one unit of the system resolution.
 
Cantilevered Load – Loads or forces that are not symmetrically placed on the center of the positioner table.
 
Compression Load – A load that leads toward compressing the positioner.
 
Continuous Motor Torque – The torque created by the motor at rated constant current.
 
Cycle – A complete positioner extension and retraction returned to the beginning point.
 
Duty Cycle – The amount of time a positioner can run and how much time it needs to cool. It is on time to cooling time, meaning a duty cycle of 25% is a cycle in which a positioner operates continually for ten seconds must rest for thirty seconds.
 
Dynamic Load Rating – A design constant used in calculating the estimated travel life of the roller screw; the dynamic men load is the load at which the device will perform one million revolutions.
 
Efficiency – The ratio of input power to output power.
 
Error – The difference between the actual and the intended condition. Error typically refers to the position but could refer to velocity.
 
Extension Rate – The speed at which the positioner extends or retracts. Extension rate differs with the load on DC positioners but differs very little on AC positioners or step-motor positioners.
 
Force Rating – The linear force created by the actuator at constant motor torque.
 
Hardwired Signals – Electrical signals traveling between two control devices that are connected with dedicated conductors.
 
Holding Brake – A brake that works against backdriving to hold the positioner in place under compression loads or tension.
 
Hysteresis – The opposing force accumulated in an elastic material or mechanism after the outside forces acting on it have been changed (e.g. the mechanical wind-up in the lead-screw assembly).
 
Jog – Moving or positioning a load in incremental steps.
 
Lead – The distance the leadscrew nut travels for every rotation of the leadscrew.
 
Limit Switch – A switch that limits the travel or motion in a specific direction.
 
Linear Movement – Movement in a straight line.
 
Linear Position Accuracy – The error between the intended shift and real position attained by a linear positioning component or stage system. The linear accuracy of components and stage systems, which includes motor accuracy, leadscrew accuracy, stage accuracy (pitch and yaw) and thermal expansion, varies with complexity and number of components.
 
Linear Rate – Rate of movement of a linear component.
 
Load – The amount of force axially put on the positioner.

Max Velocity – The linear velocity the actuator will attain at a given motor rpm.
 
Maximum Static Load – The mechanical load limit of the actuator if recirculated oil or other cooling method is used to allow higher than rated torque from the motor.
 
Microstepping – The technique of electronically subdividing every complete step of a stepping motor.
 
Multiplex System – A system that utilizes two lead-screws in order to actuate several three-piece pump modules, the combination of which drives the pistons in a linear motion to create displacement. Each system uses a pneumatic rotary actuator to drive its main function.
 
Optical Encoder – A linear or rotary element that has alternating opaque and clear spaces. Detectors calculate the light and dark changes, and the position is determined by counting the amount of changes.
 
Pneumatic – Operated or actuated by compressed air or other gases.
 
Resolution – The lowest exact positioning movement attainable from a system.
 
Stroke Length – The complete movement of the positioning table from complete retraction to full extension.
 
Thrust – The complete force necessary to move a load, taking into account friction, acceleration and gravity.
 
Unidirectional Repeatability – The capability of a system to return to an intended position, nearing that position from a plus and minus direction.