A rack actuator is a device designed to produce a linear activation movement for a secondary mechanism using a rack-and-pinion gear arrangement. This type of gear arrangement consists of a narrow, flat strip cut with gear teeth along one edge known as the rack. A circular pinion gear cut with corresponding teeth is then attached to the shaft of the power source and, mated to the rack gear, serves to move it back and forward. Rack-type actuators can only produce bi-directional linear, or straight-line, motion. They are ideal for applications requiring actuation motion in a long, straight line such as sliding gates and doors.
Rack-and-pinion gears are one of the most widely used gear types for directly translating the rotary input motion of a motor to linear output motion. A rack-and-pinion gear set consists of two parts — the round pinion gear mounted on the drive source and a long, flat rack gear attached to the driven element. Although of different shapes, both the rack and pinion gear feature teeth of a similar pitch and shape. The two gear elements are orientated to mesh so that, when the pinion gear turns, it pulls the rack gear along in a straight line. To change the direction of the output motion, the motor is simply reversed.
This basic principle is the driving force for the rack actuator, which employs either closed or open rack-and-pinion arrangements to produce straight line outputs. One of the most common applications for the rack actuator is the door and gate operators used on residential and security vehicle access points. These operators consist of a stationary pinion gear equipped motor with a long rack gear mounted on the gate. When the motor is activated, it pulls the gate on rollers or casters along a track to open and close it.
The rack actuator used on gates is typically an example of the open type of actuator. This means that the whole arrangement is open to the elements with the rack attached along its entire length to the actuated secondary device. Closed rack actuator types feature a sealed internal rack-and-pinion mechanism typically housed within a cylinder. The rack component is joined to an actuator arm, which, in turn, is connected to the device that the actuator is intended to activate. When the motor turns, the rack moves either backward or forwards, thereby supplying the required activation movement.
Although rack actuators are limited to producing linear outputs only, they do have the benefit of having few internal restrictions on the extent of those outputs. As long as the motor is rated to move the weight of the actuated device, the only limiting factor in how far it can be moved is available space. This allows a relatively small motor to open very long gates, for example.