A spring return(single acting) actuator is desirable in many instances where a power or compressed air failure can compromise the system. (Fail to close, Fail to open)
The actuator provides a single powered stroke, and since power is utilized for only one half of the working cycle, the actuator has lower energy use, easier installation and avoid pop-opening/closing.
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In a Scotch Yoke mechanism, the shaft of the piston is engaged with a rotary shaft via a pin and a slot, as shown in Figure 5. The linear motion of the shaft causes the pin to rotate. Scotch yoke actuators can be spring return or double acting. These actuators can also be equipped with two opposing pistons to double the amount of torque on the output shaft as shown in Figure 5 on the right. Scotch yoke actuators are heavy duty actuators producing torques ranging from multiple thousands to hundreds of thousands of Nm. A vane-type actuator shown in Figure 6 has a chamber with a shaft that passes through it and a vane is connected to it. When compressed air is supplied to one side of the vane, the differential pressure causes the vane and the shaft to rotate.