Linear Actuators

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 actuators such as 12 volt linear actuators and other electric linear actuators 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, DC actuators, servo actuators, valve actuators, electric 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. Piezoelectric actuators, in particular, achieve linear motion as a result of electricity that is produced from the mechanical pressure that is exerted on certain crystals, typically either quartz or Rochelle salt.

Images Provided by Del-Tron Precision

Image Provided by Haydon Kerk Motion Solutions, Inc.

Linear Actuator Types

• 12 Volt linear actuators provide mechanical linear motion by converting 12 volts of direct current (DC) electrical energy.
• Ball screw actuators are devices that produce mechanical linear motion by converting rotary motion into mechanical energy through the use of ball screw and ball nut combinations.
• DC actuators are devices that produce linear motion through conversion of electrical direct current (DC) energy to mechanical energy.
• Diaphragm pneumatic linear actuators achieve valve actuation by using a diaphragm in a closed piston to enable control from a low pressure pneumatic (air) supply, similar to the way air cylinders work. Coiled springs of diaphragm pneumatic linear actuators provide quick and dependable valve shutdown that is independent of flowline pressure, ensuring a fail-safe position.
  
• Electric actuators convert electricity into mechanical energy.
• Electric linear actuators or electric cylinders convert electricity into linear mechanical energy.
  
• Electrohydraulic thrusters are linear actuators that consist of motors and closed-loop hydraulic systems for the driving and operating of brakes, levers, dampers and safety mechanisms for a variety of industrial equipment.
  
• Electromechanical actuators, or electrical linear actuators, are basic linear actuators. These terms denote that electrical power is used to achieve mechanical force through linear actuators.
  
• Hydraulic linear actuators utilize some sort of hydraulic fluid to achieve the positioning movement. Typically, electricity is used to start the positioning process.
  
• Linear cylinders are simple cylinders that have a pin-ended rod connecting to a crank arm, which then rotates the shaft. A fail-safe linear cylinder is spring-loaded to ensure the return of the shaft to a safe position.
  
• Linear thrusters provide cyclical linear motion via double-acting air or hydraulic cylinders attached to plates and shafts.
• Microactuators are microscopic devices, with physical dimensions ranging from submicrometers to millimeters, used to provide mechanical linear motion to another mechanism or part through the conversion of various types of energy. 
• Miniature linear actuators typically used in small spaces that require large payloads.
  
• Piezoelectric linear actuators produce a small displacement with a capacity for high force when voltage is applied.
  
• Planetary linear actuators consist of a piston assembly and shaft assembly and have planetary rollers on a piston located between the helical shaft and housing grooves. Piston movement causes the roller to follow the helical grooves into the housing, forcing piston rotation, while the rollers follow helical grooves into the shaft, causing shaft rotation.
• Pneumatic actuators are devices that position control valves using linear motion by converting various forms of energy, typically air pressure, into mechanical energy.
• Rack-and-pinion linear actuators make use of fluid pressure to move a piston connected to a gear rack, which rotates a pinion. The output torque of rack-and-pinion linear actuators can be doubled with two parallel piston-rack units.
  
• Rod linear actuators are electric linear actuators that have an output rod which provides linear motion through a motor-driven ball or ACME screw assembly. The load of rod linear actuators is typically unsupported but could be attached to the end of a screw or rod.
  
• Rodless linear actuators have a barrel of extruded anodized aluminum and are formed with a longitudinal slot, permitting a connection between the piston and mounting carriage. A hardened stainless steel band pneumatically seals the cylinder, while a second stainless steel band on the exterior closes the slot and avoids contamination to the interior of the cylinder; a system of slide rails divides the two bands in the pressure-free zone between the two piston seals, allowing the mounting carriage to move.
  
• Rotary actuators are compact, simple and efficient linear actuators that rotate an output shaft through a fixed arc to produce oscillating power. They require limited space and simple mountings and can produce high instant torque in either direction.
• Servo actuators are used to provide position control, utilizing linear motion in order to maintain proper functioning of another mechanism or equipment part. 
• Valve actuators are mechanisms that provide linear movement to valves through the use of various screw assemblies.

Linear Actuator Terms

Accuracy - The difference from the precise value of the intended velocity or position of electric linear actuators.
 
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.
 
Ball Bearing Screw - 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.
 
Efficiency - 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).
 
Jog - Moving or positioning a load in incremental steps.
 
Lead - 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 actuators.
 
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.