Linear actuators are used to move an object along a straight line, either between two end points or to a defined position. Actuators may be air or hydraulic driven using pressure or they may be driven by electricity. Air or hydraulic driven actuators are cheap and simple in design. They are also easy to control, but they require an air or hydraulic supply which is relatively expensive, especially for small systems.
Electrically driven linear actuators normally incorporate a rotating motor and some kind of transmission means to convert the relatively high-speed rotating motor to a low speed linear motion. This transmission means may incorporate a gear box and/or a screw shaft. One common type of linear actuator incorporates a screw shaft with a nut running thereon. The screw shaft extends over the full length of the actuator and sets the operating length of the actuator. Since the nut is held in a non-rotatable state, the nut will be displaced when the screw shaft is rotated by a motor. The nut may incorporate rolling elements, such as balls or rollers, between the screw shaft and the nut. This will allow for a high efficiency actuator with high load transfer and long life. The nut may also engage directly with the screw shaft, i.e. a sliding screw design. In this case, the nut is preferably made of a plastic material.
One disadvantage of using a plastic nut is the relatively short life of the nut, due to wear caused by friction between the nut and the screw shaft. The nut should be as short as possible in order to reduce friction and to allow for a long operating range. On the other hand, the nut must be long enough to take up the applied forces. Even though this type of actuator is relatively cheap and well adapted for low and medium load cases, there is still room for improvements.