Fluid Power Journal

Directional Valve Seals Failing

I was asked to help inspect a 4″ bore, 2″ rod cylinder that had a 40″ stroke and an 8″ stop tube. It had a D05 size mounting pad machined into the blind-end cap of the cylinder with a steel tube connection from the rod cap to the blind-end cap. The cylinder shop had repaired this cylinder about 2 months earlier, and, at that time, the cylinder did not have a valve mounted on the D05 pad, and the customer could not tell them where it was leaking. So, they did a light hone, installed new seals, tested and returned it to the customer only to have it come back again for warranty repair. This time, however, it had a 3-position proportional directional valve mounted on it. The cylinder shop was told, “It leaks,” but again, the customer could not say where.

The cylinder shop needed help to understand how to electronically stroke the proportional valve. They connected a pressure and tank line to the cap-end plate, and we connected our portable power supply and command generator, so they could stroke the cylinder electronically with the proportional valve mounted on it. Oil started leaking up through one of the mounting bolt holes when they reached about 200 PSI. They removed the valve and found that one of the O-rings on the valve had extruded and failed. All the mounting bolts were tight, and it looked like an easy fix. However, the cylinder came back again with the same problem 6 weeks later.

What do you think is the problem?

See the solution

Extruded O-rings usually are a result of pressure causing a small gap between two mounting surfaces allowing part of the O-ring to squeeze in between the surfaces. Then the O-ring gets pinched when pressure is removed, squeezing and nibbling away at the O-ring until it fails. The mounting bolts were still torqued to their proper values but were discovered to be too long. They had bottomed out in the manifold before securing the directional valve tight enough against the base plate. Replacing the mounting bolts with shorter bolts and applying the proper torque solved the problem.


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One thought on “Directional Valve Seals Failing”

  1. In my experience it would most likely be one of three things:
    1) The directional valve bolt is tight but because it is too long, it bottoms out on the female threads prior to applying enough force to keep the oring in place under pressure. This gap allows the directional valve oring to eventually extrude.
    2) The directional valve bolt is too short. Even though it is tight, it does not have enough thread engagement to resist the force of the pressure, allowing thread/bolt stretch leaving a gap for the oring to extrude. 3) the sealing surface of the valve mounting pad on the cylinder or bottom of the valve itself no longer has the proper finish or flatness, leaving a gap for the oring to extrude. These surfaces getting dinged or scratch is very common

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