Hydraulic Circuit help

[lukerobert]

I have a situation where i need to extend a large bore hydraulic rod and act as a clamp against a workpiece. But the workpiece will begin to move during the process and I need the hyd rod to follow with it (retract) and maintain a set, adjustable amount of pressure.

A simple pressure relieving circuit would seem to work but im not sure if it would allow retraction due to check valves in the directional valve control.

Any ideas?

Thanks!

 

[hydtools]

Is the relief valve not close to or at the inlet to the cylinder?
A schematic would be helpful.

Ted

 

[LittleInch]

I'm quite sure the hydraulics industry has constant pressure controllers, but an alternative is just a decent sized accumulator. How big a percent of the rod movement is backwards? How are you moving it forward? Motor driven pump or hand pump?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[lukerobert]

I am not sure how to post a picture here.

But to make this more simple, I will drive the cylinder forward (extend) until it clamps the workpiece. Then I want to filp a switch which activates a solenoid valve which brings a relief valve into play. The relief valve would be in parallel with the cylinder, I think. At this point I want the cylinder to act as a big spring with a constant rate. It needs to be able to retract when pushed upon with enough force, but maintain a constant clamping force while doing this.

IT will all be powered by a gas engine over hydraulic pump with accumulators.

 

[LittleInch]

To attach a picture save it somewhere you know, then click on the ...or upload your file to engineering.com at the bottom of the box you write in. This opens a dialogue box, click browse, find your file(s), click upload and then finally click the link in the box which attaches it to your post. Simples.

The problem with a relief valve is that is i=t only works one way. hence if t=you loose fluid, where is the replacement fluid coming from if your ram goes the other way. That's why you need to size your accullator so that the change in pressure from the movement of the ram is negligible. Job done??

what is wrong or doesn't work with your current set up?

Also simple spring reliefs have a variable opening pressure and can often "weep" or fail to fully close.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[lukerobert]

Littleinch,

Thank you for the ideas. The total gallons consumed during this process (2 cylinders) will be 13 gallons. How big of an accumulator do you think I would need to notice little pressure difference?

To answer your question, there is no current process in place. This is a one-off design I'm tasked with now.

I don't have a bunch of experience with accumulators, but I understand how they work as far as a hydraulic power units go. I suppose we could get an appropriate sized accumulator and plumb it in to the circuit and activate it with solenoid switches when we needed it.

 

[LittleInch]

Do you mean that 13 gallons is the amount which you think will be used between your initial movement to get the clamp distance / force and that occuring during the operation?

I can only guess that you need something like 10 to 15 times this volume in the liquid side of an accumualtor to maintain a pressure more or less the same. Of course the bigger the accumualtor or banks of accumulators the smaller the presusre change for the same volume being pushed back into it or taken from it. You'll really need to ask a vendor what they think with your pressures and volumes.

My idea is just that you either use it as presusre item or have it plumbed in all the time and pressure it up with a pump and then just isolate the pump.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[lukerobert]

Yes, 13 gallons is what I have calculated the cylinders will displace while they retract during the clamping operation. They are big 8" bore cylinders. I will take your advice and consult with a vendor for the appropriate sized accumulator in order to have little change in pressure.