Linear drive, also called linear actuator, is a power generating device. Different from the regular motor which creates energy by circular motions, the linear drive generates power by the motion in a straight line of moving between two points. Common applications of linear actuators are related to machine tools, such as industrial machinery, valves, and dampers, and computer peripherals like disk drives and printers. Sometimes, the linear actuators would operate by conversion of rotary motion into linear motion.
From different technologies involved, linear actuators can be divided into the groups below:
Mechanical linear actuators are operated by conversion of rotary motion into linear motion. The conversion can be made by few types of mechanism:
There is a principle of the simple machine known as the screw. Through the rotation of the actuator's nut, the screw shaft would move in a line and generate power. Under this working principle, leadscrew, screw jack, ball screw and roller screw actuators are applied.
● Wheel and Axle
The moving principle is that a rotating wheel moves a cable, rack, chain or belt to produce linear motion. Machines and equipment operating under this principle are hoist, winch, rack and pinion, chain drive, belt drive, rigid chain and rigid belt actuators.
The working principle of this kind is like the working principle of the wedge, but relatively limited. While the wheel-like cam rotates, thrust would be provided as the working power by its eccentric shape.
Some of the mechanical linear actuators, such as hoists, chain drive, belt drivers, etc., only pull while some mechanicals like cam actuators just push. Others can do both, for instance, pneumatic and hydraulic cylinders and lead screws, etc. To convert rotary motion of a control knob or handle into linear displacement, the mechanical linear would use screws or gears to which the knob or handle is attached. The jackscrew and car jack are examples of this kind.
Mechanical actuators are commonly used in lasers and optics for operating the position of linear stages, rotary stages, mirror mounts, goniometers and other positioning instruments. Some linear drives of this kind would have encoders and digital position readouts to help adjusting the position.
Hydraulic actuators or hydraulic cylinders are controlled by hydraulic pumps, usually containing a hollow cylinder with a piston inside. The unbalanced pressure applied to the piston generates a force to move an external object. Since the liquid cannot be compressed, linear actuators in this kind can help to provide controlled precise linear displacement of the piston that the movements are only along the axis of the piston. Hydraulic car jack is a familiar example of this kind.
The working principle of pneumatic linear actuators is similar to the principle of the hydraulic actuators. The only difference is that the pneumatic linear drive generates power by compressed air. They work similarly to a piston that the air would be pumped into a chamber from one side and pushed out from the other side. Pneumatic linear actuators would be, but not necessary, used on heavy duty machines.
The biggest advantage of pneumatic linear drives is that their power source is the compressed air which is not difficult to gain. This makes them can be used in most mechanical activities. However, most pneumatic linear actuators are designed large, bulky, and loud that they are hard to be transported from one place to the other. Moreover, a leaking problem is the issue that pneumatic linear actuators would face. This disadvantage troubles them a lot and makes them less good than the mechanical linear actuators.
Some of the certain materials contain the property of piezoelectric effect which would cause the expansion while applying the voltage to the material. However, high voltage only creates tiny expansions. It caused a result that although piezoelectric linear drives can achieve extremely fine positioning resolution but would also be limited in their motions. Piezoelectric materials contain hysteresis which makes it difficult to control the expansion in a repeatable manner.
Electro-mechanical actuators are similar to mechanical actuators. However, the control knob or handle of the previous one would be replaced with an electric motor which the rotary motion has been converted to linear movements.
When meeting the situation of powering a motor which converts electrical energy into mechanical torque, electro-mechanical actuators would also be a good choice. Manufacturers produce electro-mechanical actuators plenty and all of them tend to develop their own design. Therefore, types and modes of this kind would be various. They can be briefly divided as the groups below:
● Simplified Design
● Compact Construction
Linear actuators of this design are specialized linear actuators used when the space restrictions exist. A common form of telescoping linear actuators is made of concentric tubes of approximately equal length. These concentric tubes extend and retract like sleeves and the telescopic cylinder. Examples of telescoping linear actuators include:
● Helical Band Actuator
● Rigid Belt Actuator
● Rigid Chain Actuator
● Segmented Spindle
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