|Scotch Yoke |
The Scotch Yoke principle is characteristic for its high torque when required - at the beginning and end of each operation. This increases safety and reduce the energy consumption.
Increased torque where required.
Compact measures. Ca.30 % smaller than rack and pinion actuators.
Low air consumption.Ca.30 % less than rack and pinion actuators.
Safe to mount, the pistons cannot be wrongly meshed.
Good regulating features. No play in force transmission and low friction because the transmission passes over bearings.
Favourable force transmission, gives favourable action in the regulation area. This provides lower turning speed when approaching the end positions.
DA - Double acting = air in both directions
SR - Single acting = air in one direction, spring return
The diagram above compares the torque between Scotch Yoke actuators and rack and pinion actuators at the same stroke volume = the same air consumption. The Scotch Yoke actuator gives ca. 50 % higher starting and closing torque (0°) compared to the rack and pinion actuator.
The diagram for single acting actuators with spring return shows that the Scotch Yoke provides an increased torque in the spring return position = safety position where a safe function really is required.
The Scotch Yoke in the RC actuator has oblique grooves in order to provide the highest torque in closed position where most valves require the maximum power.
The normal sizing of actuators requires a 20-30 % safety margin to handle valves which age and wear out. The actuator sizing torque corresponds to the valve torque requirement in closed position (0°) for both starting and closing. These requirements apply to both Scotch Yoke and rack and pinion actuators. The diagram clearly shows that a rack and pinion actuator increases 1,5 times in size compared to a Scoth Yoke actuator in order to operate the same valve.