Directional valve problem 2

[lukin1977]

Please help me solve a problem in a double acting cylinder system

The system is very simple. An oil pump, a directional valve (manually operated ) and the cylinder

Normal operation: valve position es moved by hand (lever) to forward or reverse and the cylinder moves accordingly. One can release the lever at any of those position and the valve stays in it

Problem: Valve is put in forward, cylinder moves forward
valve is put in reverse and the cylinder moves reveres but the operator has to strongly hold the lever because if he release the lever the valve position goes back to forward and so the cylinder goes forward too

Hope my explanation is clear

According to your experience, what is the problem? Internal wear of the directional valve? What should we do?

 

[HPost]

The pilot valve is a CETOP 3 or NG 6 valve. It has only A B P and T ports. The A and B ports serve as the pilot ports that send oil to the main spool.

On the main spool, the X may be connected to the main pressure gallery or the pilot pressure may be supplied from elsewhere. The X port will connect to the P port on the pilot valve. The Y port will connect to the T port on the pilot valve.

There are cast identifiers on the side of the block (pilot valve). They will tell which port is which.

It is likely that your main spool is getting a pilot pressure from the pilot valve body. I'd suggest either removing the pilot valve and replacing it with a blanking plate. Or you can put a plug in the X and Y ports.

It will be worth checking to see where the pilot pressure is coming from. Either the X port or via an internal connection into the pressure port on the main valve.

It'a looking very much like you have an unwanted interaction with your pilot pressure. The fact that it has worked for so long may just be down to luck. Slight wearing of the spool can change the pressure drop across the spool and change the pressure acting on the spool lands and cause the spool to move unexpectedly.

You don't need any pilot pressure, so you should just blank it off.

 

[lukin1977]

Thanks again HPost

X port is blocked
Y is connected to return

We just finish testing and now the directional valve works fine. We are making a blanking plate to replace the pilot valve and probably test it tomorrow

Thanks

 

[HPost]

Sounds good...

In summary of the above then...

It could be that difference in the pressure in the tank line, caused by the flow amplification in the cylinder (head to rod volume) is fed back via the Y port and then into the A and / or B port and on to the main spool. Capping the pilot valve ports will tell you whether that is happening.

Good luck...and get that pipework checked.

 

[lukin1977]

update:

After a few days the valve problem slowly started again. So we installed the blanking plate to replace the non operating pilot valve. The result was that the directional valve got "blocked" and it was very hard to change its position on any position, so we re-installed the pilot valve and went back to the original problem

So after an intensive check of the whole system we found out that one valve in the return line was 90% closed (due to vibration). This was creating an over pressure in the return line and it was piloting the directional valve through the Y port!

Now the found closed valve is fully open and the directional valve is working correctly