Capstan directional proportional valve

When starting the capstan hydraulic pump, stbd capstan’s control valve is being pushed by oil pressure on one side thus turning the capstan in counter clockwise direction. We took out the valve and checked for the spool, lever linkage, etc. Everything seems to be normal. What seems to be the problem? Troubleshooting is still ongoing.

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Is there a centering spring for the spool? It would be on the end opposite the control lever or linkage. If this spring is broken, the spool would be free to move as gravity or some other force affected it. If this is a pilot operated hydraulic system, there would be springs on either end of the pilot operated spool to keep it in the neutral position until hydraulic oil shifted it. The only other explanation I can think of is damage to the lands on the spool or the valve body from debris passing through.

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This is highly technical so bear with me.

Did you jiggle the handle?


Yes that’s what we checked the first time. The springs on the opposite end of the lever though is fine. We are suspecting a blockage in the line.

Also it was working perfectly before we took out the spool to change the lever. We had the problems only after assembly.
We checked everything on the lever side including the proper positioning of the control guide. All is normal.

Yes. Before starting the pump, the resistance of the lever when jiggled to opposite ends is normal.
When the pump starts, the spool is being pushed upward by oil, and capstan is turning counter clockwise. When we hold the lever in the neutral position, the capstan stops turning. But when we try to force the lever on the opposite end, the capstan turns again in counter clockwise direction instead of turning clockwise.

This is stupider than jiggling the handle…

Did you put the spool in backwards?

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By backwards you mean rotated the spool 180°? Yes we did. We tried installing it with the control groove facing the end plate side and the opposite. Same result.

You didn’t answer question in post 2. Is this main directional valve pilot operated or only by a hand lever on the valve itself. Is there a remote station that can also pilot this valve? If there is no other force on the main spool but the manual operator take a closer look at your assembly methods. Are there any issues on the drain line / leak off ports in the valve body.

No it’s not pilot operated. Controlled by lever only. The lever and the spool were swapped with the ones we have on the winch and it worked perfectly. The capstan however still reacts the same. Seems like the valve block itself is the problem.

Is the valve open center or closed center. I guess you checked this but the positions and dimensions of the lands on the spool are identical to the one removed?

This should be on our often neglected engineering page but you didn’t answer KP Chiefs other question on if this had a remote station also???
My question in addittion to those above is: does it turn at rated speed? or rather, does it turn at the same speed when the valve is full on any direction?

Moved it thanks for pointing that out.