There are several types of linear actuators: mechanical actuators, electro-mechanical actuators, linear motor actuators, piezoelectric actuators, hydraulic actuators, pneumatic actuators, wax motor actuators, segmented spindle actuators, moving coil actuators and moving iron controllable actuators. There are a variety of advantages and disadvantages regarding each type of actuator. Operators will select different actuators based on the application. Some actuators such as piezoelectric actuators are not as commonly utilized due to a number of factors. Piezoelectric actuators require large amounts of voltage which is expensive but the advantage with this unit is it provides very small amounts of expansion which is preferred for some applications. On the flip side, electro-mechanical actuators are widely used due to their high usability and functional properties. These systems are simple to use and the actuation process is easy to repeat.
Linear actuators will typically be designed for either standard or compact construction. Compact linear actuators are usually created with specialized motors to allow for high torque while occupying a small space. Pneumatic and hydraulic actuators are great for applications that require high levels of output. Hydraulic systems utilize fluids and unbalanced pressures to achieve the desired outputs of the mechanism. Pneumatic actuators has a similar process like hydraulic actuators but the difference is these actuators use condensed gas instead of liquids. Regardless of which style of actuator it will successfully provide linear motion for applications ranging from DVD drive openers to opening massive heavy duty doors. Manufacturers will work with users to determine which actuators are best for their application as well as options to ensure the longevity of these devices.
Linear actuators are devices that produce mechanical linear motion by converting various forms of energy into mechanical energy. Typically part of motion control systems in automated assembly processes, linear actuators are most often computer-controlled, although simple actuators may be powered mechanically by hand. The various forms of energy which power linear actuators include hydraulic, pneumatic, mechanical, electro-mechanical and piezoelectric. Linear actuators often act as servomechanisms to provide and transmit a precise amount of energy to work another mechanism or equipment part, or the actuator may do the actual work itself. Linear actuator manufacturers assist in robotic processes in a wide range of industries, including automotive, biotechnology, pharmaceuticals, food, packaging and electronics. Different types of processes use various actuator designs, including ball screw actuators, electric linear actuators (or electric cylinders), rotary actuators and miniature linear actuators. Piezoelectric and telescopic actuators are employed for specialty applications, with piezoelectric actuators supplying extremely small, precision movement, and telescopic, or spindle actuators providing vertical mechanical motion. Nearly all factory automation processes use linear actuators to push, lift, rotate or transport products or equipment during various manufacturing processes. Some linear actuators and units operate in vacuum, radiation, cryogenic, corrosive and underwater environments.
Actuators are not only powered by a variety of mechanical, electrical, pneumatic and hydraulic designs, but they also create motion based on several different principles. Many linear actuators use a ballscrew design consisting of a screw rod which rotates in and out of a housing, providing linear motion. Ball screw actuators, also called drive screws, are rotated using either a synchronous timing belt drive, worm gear drive or direct drive. The turning of the screw pushes a drive nut along the screw, which in turn pushes the rod out. Rotating the screw in the opposite direction retracts the rod. A cover tube protects the screw nut from environmental elements and contamination. Radial thrust bearings permit the screw to rotate freely under loaded conditions. Rotary actuators are not linear at all, although, like rotary tables, they serve purposes similar to those of linear actuators in assembly automation applications by providing radial motion. Most miniature linear actuators are electric, although some may use piezoelectric power for highly precise, short movement, while others are pneumatic actuators. Telescopic actuators utilize a fairly new "spindle" technology to provide linear motion; because they are telescopic, the length of the actuator can fit inside a fairly small housing, making telescopic actuators highly space-efficient.
When choosing from linear actuator manufacturers, several factors are important for the success of the actuator within its application, including the speed, stroke length and load rating of the linear actuators. The duty cycle accuracy and programmability requirements must also be measured, as well as desired lifetime of the linear actuator system, particular safety requirements, environmental concerns and space constraints. If the linear actuator system is not battery-run, the size and kind of motor (AC, DC or special) are important considerations. Different available electric motors, which include stepper, brushed DC or brushless servomotors, give different levels of torque and accuracy. Rotary actuators and linear actuators may be powered electrically, hydraulically or pneumatically. Electric linear actuators are typically powered by DC or stepping motors. Hydraulic actuators have brute strength, essentially no compressibility and excellent power-to-weight ratio. However, they tend to leak, have lower reliability, are higher maintenance, expensive and loud, use flammable fluids and generate heat. Even though pneumatic actuators are inexpensive, have rapid response and are simple and easy to control, they are also loud, and their position is difficult to control.
Electromechanical actuators are quickly replacing pneumatic actuators because they save money by reducing unnecessary energy consumption within plants, have vastly improved control and flexibility, are especially beneficial for multi-positional tasks and provide no health and environmental issues due to high noise levels. However, the tendency of these electrical linear actuators to spark limits their use in hazardous environments, and they have lower power and torque-to-weight ratios. Research has been moving forward on piezoelectric linear actuators and other forms of technology, which use short high voltage bursts to create small-scale movement, but this has been primarily focused on micro-actuators and micro-manipulation.
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Linear Actuator Types
Linear Actuator Terms
Accuracy - The difference from the precise value of
the intended velocity or position of electric
ACME Screw - A threaded screw utilizing sliding friction surfaces between the nut and the screw. These screws are used in linear actuators and are 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 in many linear actuators.
Backlash - The space between the interactive elements in a drive train or leadscrew assembly that creates a mechanical "deadband" when shifting directions.
- A screw that operates on ball bearings. Ball bearing screws (or ball screws) have a low starting torque, are approximately 90% efficient and can be back driven.
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 of linear actuators. The error or non-repeatability factor is determined from the sum of the hysteresis, the backlash of linear actuators system resolution.
Cantilevered Load - Loads or forces that are not symmetrically placed on the center of the positioner table in rotary actuators.
Compression Load - A load that leads toward compressing the positioner in electric linear actuators.
Continuous Motor Torque - The torque created by the linear actuators motor at rated constant current.
Cycle - A complete positioner extension and retraction returned to the beginning point in rotary actuators.
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 of electric linear actuators operates continually for ten seconds and then must rest for thirty seconds.
Dynamic Load Rating - Linear actuators design constant used in calculating the estimated travel life of the roller screw; the dynamic men load is the load at which this linear actuators device will perform one million revolutions.
- The ratio of input power to output power.
Error - The difference between the actual and the intended condition of linear actuators. Error typically refers to the position but could refer to velocity of many linear actuators.
Extension Rate - The speed at which the positioner extends or retracts in rotary linear actuators. Extension rate differs with the load on DC positioners but differs very little on AC positioners or linear actuators step-motor positioners.
Force Rating - The linear force created by linear actuators at constant motor torque.
Hardwired Signals - Electrical signals traveling between two control devices of linear actuators 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 of rotary actuators.
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 of linear actuators).
- Moving or positioning a load in incremental steps.
- The distance the lead screw nut travels for every rotation of the lead screw.
Limit Switch - Switches found in linear actuators that limit the travel or motion of rotary actuators in a specific direction.
Linear Movement - Movement in a straight line as seen by the movement of linear actuators.
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 in linear actuators.
Linear Rate - Rate of movement of linear actuators components.
Load - The amount of force axially put on the positioner in rotary actuators.
Max Velocity - The linear velocity that linear
actuators will attain at a given motor rpm in electric
Maximum Static Load - The mechanical load limit of linear actuators 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 - An electric actuator 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 electric actuator system uses a pneumatic rotary actuator to drive its main function.
Optical Encoder - Linear actuators or rotary actuators 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 - Pneumatic actuators are 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 rotary actuators positioning table from complete retraction to full extension.
Thrust - The complete force necessary to move loads of linear actuators, taking into account friction, acceleration and gravity.
Unidirectional Repeatability - The capability of electric linear actuators systems to return to an intended position, nearing that position from a plus and minus direction.