A linear actuator is any device that produces work by converting nonlinear energy in 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, output, potential, and power requirements. These include door openers, heavy duty machine actuators, and tiny, precision process controllers.
There are an astounding number of processes, machines, and appliances which use devices that supply linear actuation. These range from the humble DVD tray opener to giant hydraulic rams capable of producing tens of thousands of pounds of pressure. The basic principle behind these indispensable devices is the conversion of one, typically small, nonlinear energy source into linear motion of increased magnitude. There are several commonly used primary energy sources in linear actuators; each has its own particular conversion mechanism. Most often these are rotary motion or pressurized fluid inputs although several less frequently used types such as hot wax actuators exist.
The rotary source linear actuator usually utilizes an electric motor to supply its input energy. This actuator makes use of cams or lead screw arrangements to convert the motor's rotational energy to straight line motion. The lead screw example is a popular choice as it supplies a wide range of extension length potential. The motor turns a coarse threaded lead screw which passes through a nut attached to the actuator arm. The nut and actuator are prevented from turning and, as a result, advance up and down the lead screw as it turns providing linear motion.
A cam type linear actuator utilizes an eccentric cam connected to the actuator arm via a series of linkages or a slot in the cam face. As the cam turns, it pushes the actuator arm forward or draws it back. This variant does not have as large a range of motion as the lead screw actuator but is capable of very accurate inputs. Cam actuators often use electronic stepper motors which allow for increased advancement control and the added accuracy necessary for precision process control.
Hydraulic and pneumatic actuators use a pressurized gas or liquid to achieve linear motion and posses the greatest 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 forward or backward.
There are many other, less frequently used types of linear actuator such as thermal actuators which utilize the expansion of materials such as wax to supply linear motion. Other types include piezoelectric, magnetic, and rack and pinion actuators. Perhaps the simplest of all is the manual 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.