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A screw actuator is a mechanical device that converts the rotational motion of an electric, pneumatic, or hydraulic motor into a linear output motion. This is achieved by transferring the motor's rotation to a round bar threaded with a coarse, lead screw type thread. The bar passes through a similarly threaded moving collar that moves up and down the bar when it rotates in either direction. The collar is attached to an actuator arm that transfers the collar movement to a secondary mechanism, such as a door or vent. The screw actuator is one of most efficient, simple, and cost effective mechanical actuators, with its only real disadvantage being limited torque output when compared to hydraulic ram types.
Lead screws are a commonly used method of converting a rotary input motion to a linear, or straight line, output motion. A good example of this concept at work is the bench vice found on the workbenches of most do-it-yourself enthusiasts. When the vice handle is turned in either direction, the vice jaws open or close. The basic principle that underpins this functionality can also be seen when a nut is turned onto a bolt. As the nut is turned, it advances along the threads of the bolt shaft until it pulls up tight.
The lead screws used in the vice and a screw actuator work in exactly the same way, but feature a different thread type to that found on the bolt. These threads are generally of a coarser, square profile, allowing the nut or actuated item to move faster while supplying a more powerful motive force. The square threads are also more robust and less prone to strip under intense loads. Other than that, the basic principle of motion transfer remains the same.
The lead screws used in a screw actuator are generally machined to closer tolerances than those used on a vice, which produces a more predictable, smoother output action. The moving collar that travels up and down the screw shaft is attached to an actuator arm, which is, in turn, attached to the mechanism that the actuator is intended to operate. The lead screw is turned by an electric, pneumatic, or hydraulic motor, and, as it does so, the collar moves the actuator arm and activates the secondary mechanism. To reverse the direction of the actuated stroke, the motor is simply reversed. The screw actuator is extremely efficient, accurate, and robust, although generally not as powerful as hydraulic piston type examples.