The simple solenoid is a good example of a spring return actuator.
A spring return actuator is a control device that provides unidirectional motorized movement with the drive for its return stroke provided by a spring. For example, a spring return door actuator will only open the door by its own force, and the door will close again by a spring arrangement. The simple solenoid is a good example of a spring return actuator, with the solenoid plunger returned to its neutral position by spring tension. The fact that the actuator provides only a single power stroke simplifies the actuator control system with proportional reductions in unit cost and maintenance requirements. The spring return actuator typically has a longer life, further adding to the cost savings in such systems.
Conventional bi-directional actuators provide an actuation force for both return and work strokes. This is usually accomplished by reversing the direction of an electric motor or, in the case of hydraulic and pneumatic systems, by pumping compressed oil or gas into the opposite side of the actuator cylinder. In contrast, a spring return actuator uses only one motorized stroke in the middle of its duty cycle. The impetus for the return stroke that restores the mechanism to its neutral position is provided by a spring arrangement. One of the best examples of this concept is the linear solenoid which uses a spring to return its plunger when power to the coil is cut.
There are many types of spring return actuator mechanisms available for linear and rotary output applications. In some cases, the return spring is an integral part of the actuator mechanism, and in others, a separate unit. The speed at which the return spring moves the secondary mechanism is often governed to produce a specific reset speed. In many cases, this steering function is achieved by a separate hydraulic damper, typically equipped with an adjustable damper valve mechanism, which allows for precise speed adjustments.
The single stroke of a spring return actuator offers several benefits including low installation costs, long life and reduced maintenance. The benefits can be attributed to the relative simplicity of the systems and the reduced number of control elements and duty cycles required for their operation. This means slightly lower start-up costs and reduced running costs. It also ensures superior longevity of the actuator and its power supply, as only half the normal duty cycles are performed for each actuation.