Specialist Load Limiting

Specialist Load Limiting

With more than 30 years experience in the structural testing industry FOX-VPS engineers excel in providing innovative solutions to everyday testing problems.

Calibrated Relief Valves:

When setting up test rigs it is important for the test engineer to be able to quickly and accurately adjust the settings of the safety relief valves without removing them the system or applying pressure to their associated loading actuator. To address this requirement FOX-VPS produce a detented, laser engraved, micrometer style adjustment conversion for standard Sun relief valves which has a graduated sleeve fixed to the relief valve body and an handwheel pinned to the valve adjustment screw. Each valve has approximately 100 detented positions covering the valve range with settings repeatable to +/- 1 division. These controls can be fitted to any of the relief valves incorporated into FOX-VPS safety manifolds, details of which appear in the Hydraulic Manifold section of this site.


Once assembled it is not possible to separate the control components. During build each valve is allocated a unique serial number and then undergoes a 12-point calibration on a purpose built test rig. Readings of pressure are logged at each calibration point and data is provided in a graphical format as shown below. A least squares fit is calculated for the linear portion of the valve operation with values for the characteristic equation noted on its test certificate.

Asymmetric relief valves:

Accurate load limiting of servo controlled asymmetric actuators has been problematic since the servo valve was invented over 50 years ago. Conventional relief valves, whether limiting the individual actuator chamber pressures or the difference between them (cross port), cannot be set to limit the actuator load because the output force is not proportional to the differential pressure across the piston.

To overcome this problem FOX-VPS have designed and developed a relief valve containing asymmetric operating areas, the ratios of which are made to match that of its associated actuator. When paired with a conventional balanced spool device, these relief valves provide stable, fast response, accurate load limiting of asymmetric actuators. 20 different area ratios are provided covering the range 1.33 to 5.26:1.

Asymmetric Relief Valves can be incorporated into a FOX-VPS standard intermediate slices to suit ISO 10372 servo valves sizes 04 and 05 or incorporated into more sophisticated safety systems.