The first patent for a two-stage servo valve was filed in 1946, almost 75 years ago.1 Ten years later, a patent was granted for the double nozzle-flapper two-stage valve. In those early years, electrohydraulic servo valves (EHSV) didn’t perform very well, suffering from poor accuracy and slow response times. By the 1950s, inventors had largely solved these issues with the adoption of permanent magnet torque motors which replaced solenoids in actuating a first stage spool which in turn drove a rotating main stage.2

Today, closed-loop servo and proportional valve automation is used in a wide breadth of machinery from nuclear energy generators to aircraft engines to blow molding machines which make plastic containers.

That is why when a machine needs balancing and recalibration of its servo valve, the technical expertise of the repair facility is critically important in returning the valve to its original equipment manufacturer (OEM) specifications.

To understand why technical expertise is so critical in servo valve repair it is important to understand what a servo valve is and the job it performs. Moog, one of the first companies to introduce a servo valve for industrial use back in 1963, defines a hydraulic servo valve as “a device that meters the flow of oil in response to an electrical input signal or command to control position, velocity, pressure or force in some type of machine or device (usually through a piston or cylinder.)3 Servo valves come in single, two- and three-stages and are closed-loop control devices, meaning that they do not require manual input, but automatically regulate a process variable to a desired state or set point with no human interaction.

A qualified repair facility will totally field strip and ultrasonically clean every valve. All internal parts will be measured and examined for wear or damage and the fit-relationship of the bushing and spool will be verified. Testing will be done, and test results will be recorded and provided to the customer.

Using a non-OEM repair facility to repair a servo valve is often necessary as many OEMs no longer repair their servo valves or the lead time for a repair is too costly for a business to withstand. That is where outside repair facilities come into play. Lead times are significantly reduced, especially if the repair facility is within your local geographic area or is able to rapidly ship to your area.

While you are researching which repair facility to select for your servo valve repair, questions to ask include the following:

• How long has the facility been repairing servo valves?

• How long does it take to get an evaluation / quote?

• Who are the facility’s previous servo valve repair customers?

• How long have the servo valve technicians been working for the facility?

• What is the training and/or factory certification of the servo valve technicians?

• What type of repair and calibration equipment does the facility have?

• Does the facility have custom-built drive electronics to test all the functions of the servo valve or is installation of the valve on the customer’s machine the final test of the valve?

• What is the testing process and does the customer receive computer generated reports and graphs verifying the test results?

• Does the repair facility meet industry recognized quality control standards, such as ISO 9001:2015?

• Is the facility experienced in repairing servo valves from the major manufacturers such as Rexroth Bosch, Moog, Parker, Star Hydraulics, Schenk Pegasus, Vickers and others?

• Do repairs meet OEM test specifications?

• What kind of OEM parts inventory does the facility have on hand?

• Can the facility expedite delivery and how costly is this service?

• Is there any warranty on the servo valve repair?

Since machine downtime can play havoc with production and cause critical delays, taking the time front-end to select an experienced and qualified repair facility can be vital to ensuring that the servo valve repair will be done as economically and as quickly as possible.

Contact a Servo Kinetics team member who can provide answers to all of the above questions, or browse the Servo Kinetics website for additional information about our commitment to customer service and achieving the highest quality standards at the lowest possible cost in the shortest possible time.

As a full-service hydraulic repair, inspection and rebuild facility that has been in business for over 45 years, we have worked with industries that have very exacting standards and include some of the most tech savvy companies in the world.

Our expert hydraulic repair services include Vickers classic factory rebuilds, Beach Russ vacuum pump repair, reverse engineering services, new pumps and motors, and more! Call and speak with an experienced engineer today and ask about our Special Offers!

1 Electrohydraulic servovalves — Past, Present and Future, by Professor Andrew Plummer, 2016

2 Electrohydraulic servo valve,

3 Servo Valves and Proportional Valves for Industrial Machines,