Friday, March 14, 2014

Advanced Design Considerations for Best-in-Class Scotch Yoke Actuators

Evaluating the design features of a Best-in-Class scotch-yoke pneumatic actuator, for example the Rotex Controls DRS.

Actuator Modularity

Actuator spring modules and pneumatic modules should be able to removed, swapped, rebuilt, or reinstalled on the frame module without disassembling the frame module. No need even to remove top cover of frame module, because all bolting for removal of the modules is external.

Actuators should be easily changed in the field from fail close to fail open by just swapping the positions of the spring and pneumatic modules. No need to dismantle the frame cover or cylinder assembly.

Actuators should be able to be modified from double acting to spring return easily in the field by adding a spring module.


Springs should be lifetime lubricated and epoxy coated.

Springs should be pre-compressed inside their welded spring modules for safety and ease of use.
Pneumatic cylinders should be lined with hard chrome plating. Smooth to eliminate wear of piston seals, the surface is very hard, for long life.

Inside the pneumatic module, dual oversized tie rods should be protected from external corrosion. Where the tie rods pass through the piston head, design should include a slide bearing. This bearing is important to help maintain alignment and assure smooth operation.

Frame Module

Inside the frame module, the yoke should be supported by life time lubricated bearings. Needle roller bearings ensure the most efficient torque transmission.

The actuator should include extra heavy duty travel adjustment bolts allow for +/- 10% adjustment of the 90 degree stroke. The area of contact is large, to withstand the impact forces.

The yoke thrust pin should be made of case hardened steel to resist wear.

The piston shaft should be supported on each side of the frame module by solid bearings that ensures proper alignment and smooth operation.


Each piston shaft should be equipped with two Flexijoints. These specially designed joints compensate for movement and changes in alignment to reduce internal stresses and increase output torque.

The thrust transmitted through the Flexijoints ensures no side thrust on the cylinder walls and bearings.

Unique Sealing Features

Piston seal should be an X design. Flexibility of the sealing material reduces friction, prevents leakage, and extends actuator life. The X shaped seal is wear compensating, and the unique shape of the seal retains the grease for longer periods. The piston should be guided by a PTFE thrust bearing, which helps keep the piston centered, and seal wear to a minimum.

The tie rod seal should also be X design. Old fashioned O-rings should be replaced by these flexible, low friction, resilient seals to reduce the failure rate of tie rod seals. Also reduces the wear and tear to the tie rods themselves.

Standard O-ring of the shaft seal should be replaces. In this position, a proven V-seal design should be provided. This self-energized, wear compensating seal guarantees low hysteresis and minimum friction. Like the X seal of the piston and tie rod seals, the inherent shape of the V-seal helps keep the grease where it needs to be. 

Optional Hydraulic Override

Advanced design scotch-yoke actuators should have optional hydraulic override, or the override may be retrofitted to an existing actuator.

The override is fitted into the spring cartridge, to keep the overall dimensions of the actuator and override to a minimum. It should be able to withstand up to 2,000 psi hydraulic pressure. 

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