Linear actuators convert non-linear energy into linear motion using various primary energy sources such as electric motors, fluid and air pressure, and thermal expansion. They are used in a range of applications from door openers to heavy machine actuators. Different types of linear actuators include rotary source, linear cam, hydraulic and pneumatic, and less frequently used types such as thermal, piezoelectric, magnetic, and rack and pinion actuators.
A linear actuator is any device that produces work by converting non-linear energy into linear motion. There are many different types of primary energy sources used in linear actuators ranging from electric motors to fluid and air pressure and thermal expansion. Each type or class of actuator is suitable for different applications based on size, horsepower, potential and power requirements. These include door openers, heavy machine actuators and small precision process controllers.
There are a surprising number of processes, machines and equipment that use devices that provide linear actuation. These range from the humble DVD tray opener to gigantic hydraulic pistons capable of producing tens of thousands of pounds of pressure. The principle behind these indispensable devices is the conversion of a typically small and non-linear energy source into a linear motion of greater magnitude. There are several primary energy sources commonly used in linear actuators; each has its own particular conversion mechanism. In most cases these are rotary movements or pressurized fluid inlets, although there are several less frequently used types such as hot wax actuators.
Rotary source linear actuator usually uses an electric motor to provide the input energy. This actuator uses cams or lead screw arrangements to convert the motor’s rotational energy into straight-line motion. The lead screw example is a popular choice as it provides a wide range of extension length potentials. The motor turns a coarse thread screw which passes through a nut attached to the actuator arm. The nut and actuator cannot rotate and as a result they ride up and down the nut as it rotates providing linear motion.
A linear cam actuator uses an eccentric cam connected to the actuator arm via a series of links or a slot in the face of the cam. As the cam turns, it pushes the actuator arm forward or backward. This variant does not have as large a range of motion as the screw drive, but is capable of providing very precise inputs. Cam actuators often use electronic stepper motors which allow for greater feed control and the higher precision needed for fine process control.
Hydraulic and pneumatic actuators use a pressurized gas or liquid to achieve linear motion and have the highest output potential. These systems consist of a piston contained in a sealed tube with valves at either end. The piston is connected to an actuator rod which passes through a seal at one end of the tube. An external source of pressurized gas or liquid, typically air or hydraulic oil, is introduced into the cylinder via one of the valves. Depending on whether the fluid is introduced above or below the piston, the actuator will go forward or reverse.
There are several other less frequently used types of linear actuators such as thermal actuators which use the expansion of materials such as wax to provide linear motion. Other types include piezoelectric, magnetic, and rack and pinion actuators. Perhaps the simplest of all is the hand screw type similar to that used to adjust a vernier. This type of linear actuator uses the same principle as the lead screw varieties, but typically has a finer thread for more precise adjustment.
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