A linear actuator is a mechanical device that uses mechanical energy, often in the form of a rotating crank or an electric motor, and creates motion in a straight line. But, instead of using the circular motion of a motor via a pulley and some sort of a belt or other connecting device, a linear actuator converts circular motion, seamlessly within its mechanism into motion in a (linear) straight line.
There are three types of linear actuators, an electric or electro-mechanical linear actuator that uses a motor or a hand crank, a pneumatic linear actuator that uses air pressure to move the piston in or out in a straight line, or a hydraulic linear actuator that uses hydraulic pressure on liquid to move the piston linearly.
How Does a Linear Actuator Work?
As you can see above, using the thread of a lead screw in the center that acts as a continuous ramp that allows a small rotational force (such that of a crank or a motor) anything attached to the slide block can be moved in a linear fashion. Such linear motion can be used for anything from lifting, descending, pushing or pulling, blocking, clamping, or ejecting.
When you connect a circular force such as a hand crank or an electrical motor to the end of the lead screw, depending on the thread, the slide block will move up when the circular force is applied clockwise and move down when applied counterclockwise.
The external force applied need not be necessarily rotational in kind, it could be hydraulic or pneumatic. So, for example, a steam engine where the steam pushes a piston back and forth to create linear motion is also an example of a linear actuator.
In case of both the pneumatic and hydraulic linear actuator, the slide block as shown above as the linearly moving part is replaced with a piston. Instead of using a threaded screw, the pneumatic linear actuator will use air pressure and/or springs to move the piston back and forth and similarly a hydraulic actuator will use some sort of liquid to apply the pressure on the piston to move back and forth.
As you can see above that in a pneumatic linear actuator, high air pressure is applied through an inlet and excess air is allowed through an outlet to push the piston back and forth. So in the pneumatic actuator above, in order to move the rod outwards (piston to the left), air pressure will be applied via the ‘Extend Flow Port’ while leaving the ‘Retract Flow Port’ open and in order to move the rod inwards (piston to the right) air pressure will be applied to the ‘Retract Flow Port’ and the ‘Extend Flow Port’ will be left open.
Another very familiar example of a pneumatic linear actuator is a “height adjustable” office chair that we use on a daily basis. The chair rests on a pneumatic linear actuator, and when you pull the lever on the side to raise the chair, the chair is being lifted up by the spring in the actuator while simultaneously filling the actuator cylinder with air through a one-way valve.
When you sit back down on the chair the one-way valve prevents the chair from sinking back to its original height by keeping the air from escaping. When you pull the lever again to lower the chair to a different height the pull opens the one-way valve allowing a limited amount of air to escape until you let go of the lever shutting down the valve.
This way the spring only helps to expand the pneumatic linear actuator to fill it with air, but it is the air and the one-way valve that hold the chair at the desired height.
In conclusion, a linear actuator is a very useful conversion of rotational force or air or hydraulic pressure to create linear movement.
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