Valve Magazine - Spring 2010

DESIGN CONSIDERATIONS

With so many manufacturers supplying rack and pinion actuators, it is important to consider several aspects of an actuator’s design before making a selection. Some considerations include:

Housing: Hard anodizing or other surface treatments can increase abrasion resistance, and using materials such as stainless steel increases corrosion resistance.

Fasteners: Corrosion-resistant options, such as stainless steel, can ensure longer life.

Springs: Two main spring configurations are available: concentrically nested springs and parallel springs. No significant differences exist in performance, but there are considerations on parts inventories. For example, it may be attractive to have the same housing with or without springs to simplify inventory if both spring-return and double-acting actuators are used throughout an installation. Also, multiple parallel springs might have merit in both stocking and configuration because it is possible to load in enough springs to achieve the required closing torque. Long fasteners allow the spring to be relaxed fully before removal.

Travel stops: As discussed earlier, adjustable travel stops are beneficial if the actuator is being used with a position-seated valve and fully adjustable-travel stops are beneficial if the valve must never close or must never open past a certain point.

NAMUR and ISO mounting dimensions: These standard mounting options readily accept a wide range of accessories. They also ensure adaptability for use in any application.

Replaceable components: Replaceable pinion journals and guide pads simplify maintenance.

Rack and pinion design: The actuator design should ensure gear engagement is full-length between the racks and pinion to maximize the contact area and minimize wear and specific loading of the teeth surfaces. Surface treatments such as nitriding will harden the gear’s surface and decrease wear.

High-temperature options: Heat can cause premature actuator failure so special elastomers and lubricants can be used in severe service applications not possible with standard construction.

Pistons: Quality piston materials and manufacturing ensures reliability in standard applications while stainless steel offers maximum corrosion resistance for severe applications.

BEYOND ON-OFF APPLICATIONS Rack and pinion actuators are a proven technology well suited for modulating control applications as well as traditional on-off applications. Though many success stories of these actuators for modulating control have been told, in general, they are underused in these applications. The compact and efficient design of rack and pinion actuators has advantages for many installations, and these actuators can meet facilities’ control needs with reduced cost but increased reliability. VM

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