Linear actuators are mechanical devices used to move items through a system. The device uses energy to develop force and motion in a linear manner, as opposed to a rotational motion in an electric motor. Linear actuators offer advantages including a simple design with minimal moving parts. They are self-contained and can achieve high speeds, and the actuator has identical behavior extending and retracting. The disadvantage to using a linear actuator is that it can only achieve a low force of actuation.

Methods

• Mechanical actuation is used to covert rotary motion of a screw thread or gear into linear displacement and force. Examples include a car jack, laser positioner, and other positioning instrumentation.
• Electrical actuation uses electrically driven motors to drive the actuator.
• Hydraulic actuation is performed by hydraulic actuators or cylinders. They use fluid pressure actuated by a linear motion piston within an enclosed cylinders. There are two isolated sides of the hydraulic piston. One side is pressurized or de-pressurized to achieve the linear motion of the piston, causing movement. A common example of a hydraulic actuator is an automotive repair lift.
• Pneumatic actuation, also called air actuation, uses air pressure to actuate a piston within a cylinder. Their function is similar to that of a hydraulic actuator except the actuating fluid is air.
• Electrohydraulic actuation is done with a self-contained electrically- driven hydraulic pump used to drive cylinders or actuators.

Types

Linear actuators can be divided into three types: screw, belt, and rod type.

Screw Type

Screw type linear actuators generate motion via a turning screw. Types of screws include a motor-driven ball screw, or lead screw. The load is attached to the end of the screw and is unsupported.

Lead screw actuators have a threaded nut which moves with respect to the screw. This generates motion in whichever element is not fixed. This technology is simple, economical, and widely used. All the screws in screw type actuators are made of lead, but term 'lead scrw actuator' is commonly used for this design. The disadvantages of this design include the amount of wear that occurs between the surfaces of the nut and the threads of the screw, which reduces lifetime, efficiency, speed, and performance. Lead screw actuators are best used when performance trade-offs are acceptable and when the application requires a lighter load and duty cycle.

Lead screw linear actuator. The lead screw is gray. Image Credit: Wikipedia

Ball screws are lead screw and ball nut combinations that enable the balls in the nut to circulate when the actuator is in motion. The motion of the nut around the screw is assisted by the ball bearings. This reduces friction, distributes the load, and increases the lifetime predictability over a lead screw design. The advantages of ball screws include the ability to take heavy loads, deliver precision positioning, and higher efficiency and thrust capabilities than a lead screw actuator. The disadvantages associated with ball screws is that they are more expensive, generate more noise, and the bearings can become contaminated reducing performance or causing failure.

Planetary roller screw uses a planetary arrangement of threaded rollers surrounding the main threaded shaft. This increases the surface area that takes the load and offers the highest possible thrust and lifetime of the screw type actuators. Planetary roller screws are the most expensive type of screw actuator but they have the best performance for demanding applications that require high thrust force.

Belt Type

Electric linear actuators with belt drives, geared drives, and direct drives are also available.

Belt drives connect the motor to the actuator with a belt. This type of actuator is best used when the application is horizontal and requires speed and force. They are generally not used in vertical applications since breakage of the belt could put the load into free-fall. The disadvantage of using a belt drive is that it has a lower repeatability than the alternatives and the belt requires regular tensioning. Belt drives have low noise and complexity.

Belt drives. Image Credit: MachineDesign

Geared drives connect the motor to the actuator with a set of gears. Gears used in motors are external type gears such as spur, helical, and herringbone designs.

Direct drives connect the motor directly to the electric linear actuator. This method takes the power coming from a motor without any reductions. Direct drives offer increased efficiency, longer lifetime, and less noise; however, direct drives do require a special motor that is best used for application with a relatively high torque at very low speeds and requires a more precise control mechanism.

Rod Type

Rod style actuators are distinguished by the thrust element (rod) moving out of the end of the actuator as the motion takes place. This type of actuator produces more force and excels at applications which require thrust since there is room inside the casing for a larger bearing mechanism. The housing is also sealed from dust and debris that could cause the actuator to wear or fail. Rod style actuators do not provide the support that some loads need since they are only supported by a nut at one end. In this design the mass from the rod can cause sag, compromising rigidity and increasing wear against bearing elements. Rod style is best used when the load does not need to be placed or held in a precise location.

Servo motor, rod type linear actuator. Image Credit: Machine Design

Rodless style actuators have the screw completely encased by the housing and this moves the load on a platform that rides along the top of the actuator. This style supports the load and provides a guidance structure since the device is supported by a nut at both ends. This smaller footprint is also less subject to screw whip, also known as critical speed. Due to the narrow slits that run the length of the actuator, rodless actuators cannot be sealed for wet environments. The narrow slits permit the load bearing platform to couple to and move with the nut.

Specifications

The applications of linear actuators are far reaching. They can be found in agriculture machinery, high-voltage switch gears, train and bus doors, and medical machines, just to name a few. There are many specifications to consider when selecting a linear actuator. Identifying the requirements from the application is the first step. This includes determining how much force the actuator needs in newtons or pounds-force. Also important to consider is how far and in what direction the actuator needs to move. The directions could include pushing, pulling, vertical, or horizontal, while the distance factors in both the stroke and retracted length.

Stroke is the maximum distance that the shaft travels from a fully extended position to a fully retracted position. Standard catalog options denote two inch increments. The longer the stroke, the longer the actuator will be when fully retracted.

Rated force or load is the force required to move the object. The actuator must produce enough force to overcome friction before motion.

System backlash, also known as backlash, is the position error due to directional change.

Rated speed is the maximum actuator speed; this is typically a low or no load amount. The actuator will need to move fast enough for the application. The combination of speed and the rated force required will help determine the mechanical power required by the motor.

Duty cycle- A critical specification is the duty cycle or lifetime of the actuator; it indicates how often an actuator will operate and how much time there is between operations. The life verses load relationship should be evaluated based upon the desired result of application demands. Applications which will have a higher load would be expected to reduce the life of the bearing system and vice versa. Except for high end servo units, most actuators will not be able to run continuously without overheating. To calculate duty cycle, assume an actuator runs for 10 seconds cumulative, up and down, and then doesn't run for another 40 seconds. The duty cycle would be 10/ (40 + 10), or 20%. If duty cycle is increased, the load or the speed must be reduced.

Linear Actuator Specifications

Linear actuators vary in terms of motor type, power, and features. A linear motor is very similar to a rotary electric motor. In a linear motor the rotor and stator (metal ring of insulated wire) components are laid out in a straight line. The magnetic field structures of linear motors are physically repeated across the length of the actuator.

DC Motor Types are powered by direct current such as from a battery or DC power supply. They are able to change rotational direction when the polarity of the input current changes.

• DC brush motors feature built-in commutation so that as the motor rotates, mechanical brushes automatically actuate coils on the rotor.
• Brushless DC motors use an external power drive that allows commutation of the coils on the stator.
• DC servomotors have an output shaft that is positioned when a coded signal is sent to the motor.

AC Motor Types are electric motors powered by alternating current. They convert electrical energy into mechanical energy in order to do work in a system.

• AC motors include a wide class of motors including single/ multiphase, universal, induction, synchronous, and gear motors.
• AC servomotors are permanent magnet synchronous motors that have low torque-to-inertia ratios for high acceleration ratings.
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• AC stepper motors use a magnetic field to move a rotor in small angular steps or fractions of steps.

Linear motors generate force only in the direction of travel, and do not utilize a rotary mechanism to transfer power.

Motor specifications

• Motor voltage denotes the voltage applied to the motor and includes AC, DC, and stepper type motors.
• Continuous power, also known as sustainable power, does not include short-term peak power ratings.

Motor features

• Motor encoder feedback provides continuous output position in an analog or digital signal.
• Linear position feedback provides continuous output of position in an analog or digital signal.
• Position switches have a switch that outputs the limit travel.
• Integral brakes hold the current position of the motor.

For more information on motor specifications, read How to Select DC Motors, and How to Select AC Motors.

Actuator Mounting Options

There are several mounting options for electric linear actuators.

Clevis mount. Trunnion mount. Lug mount

Image Credit: UHAUL | FBValve | Garvin

• Clevis or eye attachment connects the cylinder to the extended end of the piston via threads.
• Double end mounts have a nose and rear which contain threaded bosses for nut attachment.
• Flange mounts are brackets placed on the cylinder
• Floating mounts are for more convenient installation of the cylinder.
• Foot brackets are flanges that rest underneath the cylinder.
• Lugs are short blocks with holes that attach to the side of the cylinder and allow mounting to another surface.
• Face mount have threaded holes on the front face for attachment.
• Nose mounts are threaded for mounting through a hole with a nut on the other side.
• Rear mount have tapped holes or mounting flange on the rear.
• Threaded mounts are designed to mount to bolts via threaded holes in the cylinder head or cap.
• Trunnion mounts are specially designed mounting blocks which can be located at the cap, end, or an intermediate location along the cylinder.

Actuator Features

Linear actuators provide many optional features.

• Adjustable stroke- Adjustable stroke allows for the end points or the total stroke length to be adjusted.
• Bumpers and cushions-Bumpers and cushions are used to soften the impact at the end of the stroke.
• Closed loop control- Also known as servo control, the cylinders have external devices that send back a signal to the pump control giving it position information.
• Holding brakes- Holding brakes work in conjunction with the self-locking feature to increase holding force.
• Shock absorbers- Shock absorbers are used in pneumatic of hydraulic fluid absorption of shock.
• Double-ended rods- Double end rods extend from both ends of the cylinder with attachment features such as threads on both ends.
• Multi-position end-plate- A multi-position end plate can be actuated to different positions along its stroke, not just the endpoint.
• Integrated overload slip clutch or torque limiter- An integral flow control incorporates a flow control valve that limits the amount of air or fluid that enters the cylinder.
• Protective boot- Protective boots are a cover that protects moving parts against environment damage.
• Self-locking- Self-locking actuators lock in the current position when there is a loss of signal.
• Integral sensors- Integral sensors are equipped within the cylinder to monitor position and proximity.
• Integral flow- Integral flow control incorporates a flow control valve or device that limits the amount of air of fluid that enters the cylinder.
• Non-rotating- Non-rotating denotes multiple rods to prevent the plunger from rotating.
• Magnetic switches- Magnetic switches such as Hall Effect sensors indicate if the thruster is in the retracted or extended position.
• Thermal overload protection- Thermal overload protection is used to trip a switch when a preset temperature is exceeded.
• Intrinsically safe- Intrinsically safe electric linear actuators can be used in hazardous environments such as chemical processing facilities.
• Water resistant- Devices are sealed to prevent corrosion due to water or liquid entering the housing.

Typically, body materials consist of aluminum, steel, plastic, or stainless steel.

References

• Choosing the Right Linear Actuator
• Linear Actuator Review
• Selecting a Linear Actuator
• Making Sense of Manufacturer Specifications
• What is a Linear Actuator?
• How to Choose the Right Linear Actuator

Image Credit: Del-Tron | Thomson | AeroTech