Basic Hydraulics question

[CowDoc1]

I have a hydraulic lift that raises normally, but rapidly drifts down. While isolating the problem, I ran into behavior I can't explain. I'm hoping someone can provide some of that good learnin'. The hydraulic diagram is attached.

I removed, cleaned and inspected the components in the power unit. They are all in good shape. The flow control valve and lift cylinder are harder to reach. Before removing those, I installed two valves to isolate the problem. The results are confusing.

Theoretically... if the lift is raised and allowed to begin drifting down, then valve B is closed, any leaks within the power unit are isolated and can no longer affect the drift. Indeed, after closing valve B, the cylinder still drifts at the same speed. Since the drift speed doesn't change, the power unit isn't the culprit. The cylinder seals must be leaking. BUT, when I reopen valve B (during the same run), the lift begins drifting at twice the previous speed. It comes down at an unsafe speed, much faster than the flow control orifice should allow. A stuck flow control check valve is possible, but why did the speed increase so dramatically?

Theoretically... if the lift is raised and allowed to drift, then valve A is closed, the lowering piston would create a vacuum on the rod side. That vacuum would prevent the lift from lowering... unless there is a leak past the cylinder seals. In which case there will be no vacuum and the lift will continue to fall. Indeed, closing valve A stops the drift dead. So... no leaking cylinder seals. Huh? Reopening valve A allows the lift to drift again at the previous speed.

I guess I'll have to pull the flow control valve and cylinder. The results have me scratching my head.

Can anyone explain what's going on here?

 

[Dagny]

more than one thing leaking?

 

[CowDoc1]

It's always possible there are two leaks, but it's unlikely. I cleaned and inspected the power unit components (check valves, etc.). They all appear to be OK. Also, there was no decrease in drift speed when I isolated the entire power unit using valve B. Oddly, the speed did change dramatically when I reopened that valve.

 

[American Locomotive]

There is something you missed. One side of the piston has more area than the other and the piston rod requires space. As the piston descends, the rod physically takes up some of the available volume in the cylinder, displacing fluid that needs to go somewhere. Normally with good piston seals this is not a problem, but if the seals are leaking badly enough, it becomes a problem.

If both valves are closed, the piston will eventually reach an point where it cannot travel further down just because the piston rod would be trying to compress the fluid - even if the piston had no seals left at all.

I suspect the piston seals are worn out, and fluid is actually pumping out of the the "A" connection as it leaks down - super counter-intuitive I know. As a test, pump the system up, close the "B" valve and disconnect the "A" connection from your other valve. I bet you will see oil coming out of it as the piston descends.

 

[CowDoc1]

Obviously, American Locomotive is correct. The drift stops because the volume around the rod is much smaller than on the under side of the piston. With valve A closed, the fluid below the piston simply has no where to go. That situation has the same affect on drift (it stops), but with an entirely different cause than I envisioned. Thank you for the insight. It appears the cylinder seals are surely leaking.

And the remaining question is, "Why does the drift speed increase dramatically when valve B is closed then re-opened?" I should note valve A was open during the test.

 

[Old Man Roger]

If you switch the lines on the ram/piston, and it still drifts down, then the seals in the ram would seem to be the problem.

 

[Old Man Roger]

To be positive, is there any way to remove the lines and cap the ram? If you can secure in the raised position, then cap the inlets, the ram should stay put once you release it. Unless the seals are bad.

 

[CowDoc1]

I believe capping the inlets is pretty much what I did with the valves. Open to operate. Close to cap that inlet/outlet.

Valve B isolates the rest of the system. It still leaks down when valve B is closed. Fluid has to be getting past the piston. There's no where else for it to go.

Closing valve A stops the out flow and the drift. Both test results point to leaky cylinder seals.

I'll pull the cylinder once I figure out how to turn the lift on it's side (per the service manual).

 

[Old Man Roger]

I believe capping the inlets is pretty much what I did with the valves. Open to operate. Close to cap that inlet/outlet.

Valve B isolates the rest of the system. It still leaks down when valve B is closed. Fluid has to be getting past the piston. There's no where else for it to go.

Closing valve A stops the out flow and the drift. Both test results point to leaky cylinder seals.

I'll pull the cylinder once I figure out how to turn the lift on it's side (per the service manual).

I was thinking it would eliminate the valves as a problem. If the drift stopped it would seem to point the finger somewhere other than the seals in the ram. Just throwing ideas out there.

 

[CowDoc1]

Thank you for your thoughts and ideas Old Man Roger. They do help.

 

[Strouty]

With the speed change, could you still have a flow control valve issue? When you shut off valve b, then the fluid in the system is still draining due to the other issue? When you open it back up, the fluid from valve b has lots of unencumbered space to flow to, so it goes faster?

 

[bigfunwmu]

Your flow control is probably fine as long as you haven't messed with it too much trying to trouble shoot.

Your cylinder piston seals are shot and allowing the table to drift/fall.

You may have a chunk of cylinder seal stuck in another component, I wouldn't worry about that too much unless you continue to have trouble after you repair the cylinder.