Valve Magazine - Winter 2011

BALANCE Thrust forces are equal and opposite resulting in zero bushing load.

TECHNOLOGY

Traditional Scotch Yoke Actuators

For larger ¼ turn valves, the scotch yoke mechanism provides an ideal torque characteristic due to the longer effective lever arm and angular contact at the ends of travel.

But a very high bending force acts on the rod, necessitating a supported through rod.

And while available from many sources, nearly all scotch yoke actuators for larger valves are based on a single lever, a through rod with springs, and pistons located at opposite ends of the assembly. Designs differ primarily in how the rod is supported—from simple rod bushings to more effective separate components.

LEVER
ROD
TRADITIONAL SCOTCH YOKE ACTUA TOR DEFICIENCIES:

• Force imbalance – 100% of the operating forces also act on the shaft bushings, creating wear and torque robbing friction.

• Proprietary, over-priced, rod seals (and in some designs, similar seals used internal to the pistons) that require removing the actuator from the valve and costly skilled labor to replace.

• Changing action from fail clockwise to CCW requires a complicated, labor intensive interchange of the piston and spring ends.

The Ingenious QTRCO Flat Yoke™

At QTRCO, we began with a scotch yoke mechanism but rather than modify existing designs, we threw them out and started with a blank sheet. We rationalized:

• If we used a two arm lever, shaft bushing loads, wear, friction and torque loss would be eliminated.

• Since we could not have a two arm lever and a through rod, we created an innovative slotted body design that absorbs all side loading and allows termination of the rod at the lever slot.

• RESULT We can locate identical force modules on each end of the body module
thereby achieving:
• Balanced weight
• Balanced forces
• Less wear
• Lower friction
• Smaller, lighter springs
• Smaller pistons
• Zero side loading of the piston rod

• We also rationalized that if a seal does not exist, it cannot fail. We placed the springs inward of the pistons and eliminated all dynamic seals except the piston o-rings.

• And, since the piston o-rings can be accessed while the actuator remains on the valve, the need to remove the actuator from the valve is eliminated. This prevents possible valve stem damage and eliminates the high cost of heavy lifting equipment, hours of labor, extensive downtime and lost process.

• ADDED BONUS If contaminants have scratched the cylinder bore, the cylinder can be turned end to end and re-installed with the unused inward end now serving as a free spare.

• Since both the top and bottom sides are identical, action reversal is achieved by simply turning the actuator top side down.

• And finally, our Flat Yoke™ actuators are manufactured in the USA from USA parts for optimum quality, with an unparalleled three-year performance guarantee (see our website for details).