unexplained movement


#1

I have an interesting observation that I would love someone to explain what is causing the issue.

Setup:

            Cylinder with rod pointing up lifting/lowering load, single poppet valve hard piped between full bore cylinder port and Bosch valve.

Components:

              Bosch Rexroth 4WRLE 25 V370m-3X/G24 ET K0/A1M
              Hydraforce SV20-28 2way poppet

When we shut down the pack, de-energise the poppet valve and have a 0volt signal to the Bosh valve the cylinder will slowly fall with the poppet valve unable to fully seat. When we shut down the pack, de-energise the poppet valve and remove all power from the Bosch valve including its 24v supply the cylinder stops almost immediately and holds position very well.

I am not 100% sure what is causing the cylinder to drift in the first scenario and not in the second. My best guess is that with a 24v supply and 0volt signal the valve still has just enough oil pressure/flow from the falling cylinder to keep the main spool central keeping back pressure trapped between the poppet valve and the Bosch valve. This pressure is almost equal to the pressure exerted by the load on the cylinders full bore port. This almost equal pressure stops the poppet valve from fully seating and allows leakage for the cylinder to fall slowly.

When power is completely removed from the Bosch valve it has no way of trying to hold a central position (apart from spring force) which the flow from the full bore end of the cylinder can easily push open slightly. This slightly larger flow rate creates enough of a pressure drop that the poppet valve seats almost instantly.

This is my best guess but I would appreciate anyone who could confirm this hypothesis or come up with another idea.

On power loss the Bosh pilot valves goes A/B-T with P capped. I’m not sure if this has much effect as the pump is also turned off. Maybe it allows the spring chambers on the main stage to drain and move quicker??

All thoughts much appreciated as I have no real idea why one way works and the other doesn’t.

Thanks
Pete


#2

Do you have a schematic of the circuit?

Is the pump turned off in both scenarios?


#3

Sorry for the late reply, yes the pump is turned off in both scenarios. Please note that the valve is using internal pilot and drain. System pressure is 130bar.

After much testing killing the 24v supply to the Bosch valve works every time and makes the poppet valve seat instantly but I am unsure why this doesn’t happen when removing the control signal.


#4

This seems to be a question for hydraforce. It is obvious the poppet valve is not doing its job unless there is no job to do. ( there is no pressure to block )
It would be interesting to see the pressures at point A and B when the poppet valve is failing.


#5

The Hydraforce is internally piloted. When the valve is de-energized, the oil in the pilot chamber needs to leave and go somewhere.

They don’t show you the piloting in the online information I found but if it is piloted from the 1 port, maybe the pressure from the load is preventing the pilot oil from leaving?

It would be interesting to see if you reverse the Hydraforce valve (usually much easier said than done) if it does the same thing.