Traditional Scotch Yoke Design Compromises

Traditional Scotch Yoke

The Scotch Yoke Mechanism 

The basic scotch yoke actuator consists of a slotted lever through which a rod passes while applying a force to the lever that is generated by pressure acting on a piston.  The lever is longer at the ends of stroke so that a given force results in more torque at the ends of travel than at the mid-travel positions. Additionally, the angular relationship between the input force and the lever also acts to increase the torque output.

Scotch Yoke Torque Curve

A Price to Pay:

The increased end of stroke torque does not come without penalty which is a very high side loading force acting on the rod.

angular rod torque

Force on the piston rod

Traditional Scotch Yoke Actuators

As demonstrated, the scotch yoke mechanism provides an ideal torque characteristic for many types of valves and as the valve torque requirements increase, the scotch yoke becomes a relatively lower cost design to manufacture.  Because of the side loading forces applied to the rod, designers must contend with how best to provide rod support. Lacking an alternative means, designers (and those that have copied them) extend the rod through the actuator body to obtain support from the bushings on both ends.

Traditional Scotch Yoke

scotch yoke types

Traditional scotch yoke actuators are offered with two different yoke types commonly referred to as standard/symmetric and canted yokes.  The standard
yoke design is commonly used when the torque requirement at an intermediate travel position needs to be greatest.  This valve intermediate torque is commonly referred to as the run torque.  A standard yoke design is advantageous for plug valves and some ball valve applications. A canted yoke is not symmetrical and features an inclined yoke which shifts the actuator torque curve to have a greater torque at the beginning and end of travel.  The intermediate torque is reduced with a canted yoke design.  The canted yoke construction is desirable for butterfly and most ball valve applications

Design Compromises

Because the rod extends through the torque module the typical scotch yoke actuator is limited to a single lever arm and the force module(s) must be located on the axis of the rod. Usually the heavier and longer spring module is located at one end of the rod and the lighter, shorter piston module at the other end. This results in a weight imbalance which impacts lifting as well as stresses applied to the valve mounting. The single lever arm pivots on a shaft bushing which supports the full force of the spring and piston modules with friction and wear impacting and limiting the actuator cycle life.

 

A Disruptive Technology

Flat Yoke the next generation Scotch Yoke

The Flat Yoke actuator employs a basic scotch yoke mechanism but with a patented means to eliminate the side loading of the rods that permits terminating the rod at the yoke lever and not passing it fully through the body nor requiring rod support bushings. The spring and piston are combined into a one side mounted force module in a manner that enables mounting an identical spring/piston force module onto the torque module diagonally opposite the first. This results in a two lever design which balances the forces, eliminating the side loading of the shaft bushings and greatly increasing the cycle life of the actuator while also providing a weight balanced actuator for ease of lifting and reduced stress on the valve mounting.

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