Servo valves are similar in function and purpose to conventional proportional valves or control valves. However, they are more complex and even more precise in their control of pressure and flow rate. They also control flow direction in closed loop applications, such as in hydraulics.

Servo, in general, refers to any device that automatically adjusts to error via an electrical feedback. Servovalves have these types of features. Specifically, a gain-compensated servovalve is the most self-correcting valve and comes closest to providing a perfectly even flow. All sorts of specializations exist for servo valves, such as the jet pipe servovalve, and there are many actuation methods for servovalves. However, here are the three basic servo valve designs:

1. Single-stage servo valves have direct drives.

2. Two-stage servovalves have an initial pilot stage, followed by the main stage.

3. Three-stage servo valves have a two stage servo valve as their pilot, followed by the main stage. These are typically only used for very high volume applications.

Action Steps
The best contacts and resources to help you get it done

Find a quality servovalves supplier that fits your field

Look for a servovalves manufacturer that either addresses your specialty, or one that has a very large selection to fit a comprehensive list of applications. This will help provide you with some options.

I recommend: For many sectors, the answer is Moog. The company manufactures precision control equipment for medical, aerospace, aircraft, military, and, yes, industrial applications. The Oilgear Company is another source for quality servo valves.

Look for servovalves with the actuation method and ratings that best fit your systems

You should be able to find a servo valve that fits with your overall system plans. Choose an actuation method that works well with what you already have going. You can choose from a pneumatic servovalve, hydraulic servovalve, electrohydraulic servovalve, or some specialty types. Of course, you'll also have to find an industrial servo valve that has the right flow (usually GPM) and pressure (usually PSI) ratings.

I recommend: Attica Hydraulic Exchange sells a nice range of equipment, including both proportional valves and servo valves effective for many applications, including hydraulic circuits. They also carry a number of manufacturers, so there are plenty of choices. There are, however, many other places to look if you want to do some price or product comparing. For instance, Ellis/Kuhnke Controls carries pneumatically actuated spool servo valves.

Repair your servo valves

Servo valves can take more knowledge and technique to repair or refurbish than other types of valves, but there are companies capable of doing a good job for you.

I recommend: Furmanite is a serious company that specializes in taking care of pipe systems and the valves that control them. Also, A-1 Servomotor Repair specializes in servo devices in general, which may be useful if the repair issue is not with the valve mechanism itself.

Tips & Tactics

Helpful advice for making the most of this Guide

• •  Use an external transducer in an external loop for the highest accuracy in servovalve speed and position regulation.