Flow divider with check valve problem

[RobertHasty]

Hello,

Can anybody maybe point me to a problem solution which I have on an attached circuit.

There is a problem occuring during lowering. We experience "load vibration" or check valves tilting. I assume that p.o. check valves aren't opening at the same time although we have a counterbalance valve at the bottom which is set up in a way that his pilot pressure is larger than a pilot pressure of p.o. check valves.

Does anybody know what could be a problem?

Best regards,
Robert

 

[braddles90]

Possibly a problem with the pressure differences between the counterbalance valve setting and the check valve setting? As the counterbalance valve opens and closes it could build up enough pressure to quickly close the check valve, then the pressure across the check builds up again and the check valve bounces?
Is there a reason the check valves can't be replaced with counterbalance valves, and run separate return lines around those motors? There's a lot of restrictions in that line back to tank. Are those motors/pressure reliefs/counterbalance valves restricting the return flow too much and helping cause the shudder? Could you explain how the circuit actually works/what it's meant to do?

One final thought I had at the end of writing this-is this a problem from having one counterbalance valve servicing two cylinders? Normally you would have one counterbalance per cylinder, there could be something up with them not coming down in sync, with the counterbalance valve bouncing back and forth as the cylinder load varies during the retracting?

Hope these (slightly scattered) thoughts help!

 

[RobertHasty]

Hello Brad,

I appreciate Your answer.

Possibly a problem with the pressure differences between the counterbalance valve setting and the check valve setting? As the counterbalance valve opens and closes it could build up enough pressure to quickly close the check valve, then the pressure across the check builds up again and the check valve bounces?

There is a difference. Counterbalance setting is bigger, i.e. we need a higher pilot pressure to open a counterbalance valve.
We do also experience hydraulic hammers.

Is there a reason the check valves can't be replaced with counterbalance valves, and run separate return lines around those motors? There's a lot of restrictions in that line back to tank. Are those motors/pressure reliefs/counterbalance valves restricting the return flow too much and helping cause the shudder?

We shouldn't replace p.o. check valves with cb valves because gear flow dividers generate quite a lot backpressure which in result has a big pilot openning pressure. What do you mean by seperate lines? To have a additional line under cap side?

Could you explain how the circuit actually works/what it's meant to do?

The circuit is used for lifting a huge pipe. The cylinder movement should be synchronized while extracting. That works fine, but when we go down, we experience problems.

 

[3DDave]

Looking at a cylinder pair:

When pressure is applied to the rod end of the cylinders, it is also applied to the piston end check valves. If the valves were ideal and the piston end pressures identical the check valves would open at the same time.

If the check valves don't open simultaneously, then one side of the flow divider is exposed to the piston end pressure, but the other is not so the flow divider stops the flow in both sides. The other side tries to reduce the pressure to the cylinder piston end with the closed check valve.

I'm guessing, but the pressure required to pilot open the check valve must change at least a little with the delta-p across it.

It is a fight between the pilot pressure opening the check valve and delta-p holding it closed.

I presume at some system inlet pressure the check valve opens, allowing flow to the previously unloaded side of the flow divider, causing an instantaneous drop in pressure and allowing the check valve to shut again. It also causes the differential inside the flow divider to relax at a high rate.

I don't know an answer yet, but is this a reasonable explanation of the behavior?

 

[RobertHasty]

Hello, I've found a solution from one of the manufacturers on the market. Can anybody tell me what is the function of the orifice shown on an attached diagram?

 

[hydtools]

It is a meter-out arrangement. It controls the return pressure of the flow exiting from the cylinders.

Ted

 

[RobertHasty]

It is a meter-out arrangement. It controls the return pressure of the flow exiting from the cylinders.

Are you sure? When cylinders retract they have a non-restricted flow to filter? Or I'm wrong.

 

[hydtools]

On second look, it appears to be a bypass to allow more or less flow to be delivered to the cylinder rod ends for perhaps speed control.

Ted