OT: Hydraulic Circuit Problem

[scarince]

Okay, here's what we found once we could get into the machine today:

Reminder:

"A" side extends the cylinder.
"B" side retracts the cylinder.

Since the problem and focus has been on air or contamination on the B side, my first step was going to be to disconnect the hose from the cylinder and I was going to shift the valve to blow out whatever might be in that line. So, cylinder disconnected, line from valve put in a bucket, pumps on, shift valve.........and nothing comes out.

After another 30 min. of poking at things, some swearing, and pulling the drawings out again, I finally figure out that the drawing is wrong! There is an extra valve in the stack and I'm kicking myself now because it all comes back to me at once.

Remember I said that there is a heavy tool on top of this cylinder. Late in the machine build (actually after runoff) they added a "blocking valve". The valve is supposed to block the "A" side closed so that when we enter the robot cell (this is all in a cell) and when hydraulic power is killed, it holds the cylinder extended. When the machine is in cycle, the soln. on the valve is energized and it opens the blocking valve so that the cylinder can move as-required. I totally forgot about this thing. Apparently so did the guy who did the hyd. drawings.

So this valve body has two "holes" on each side for a valve to go into.....one for each circuit I imagine. One side has the blocking valve and the solenoid, the other side has what looks like a plug.

I think this "plug" looking this is totally blocking the "B" side in both directions. It would explain everything. I can't try removing it today b/c the line had to start back up.

I need to pull part numbers and look some things up and talk to the machine builder to put all the facts together, but I actually feel much better having found something so significantly wrong.

Once I confirm what I have vs. what I'm supposed to have, etc etc, I'll post an update.

Thanks again for all the support, it was very helpful.

B.

 

[pal]

Isn't our life fun ?

 

[Badger]

There is no way an air bubble is going to keep oil from flowing as long as the air bubble has some place to go. Oil will push air out of a upside down u pipe easily. It may not push all of it out but there is no way the air will keep the oil from flowing by.

Peter, oil will seal a gap as good as a gasket and prevent air from passing through. Rotary vane screw and even piston compressors would not work without this fact. It takes an inordinate amount of pressure for air to push oil through an orifice.

 

[Peter Nachtwey]

Peter, oil will seal a gap as good as a gasket

Not in a hydraulic cylinder. There had to be something else blocking the flow of oil. A blocking valve sounds like a likely culprit.

and prevent air from passing through. Rotary vane screw and even piston compressors would not work without this fact.

They leak. Hydraulic pumps and motors have tight tolerance but they still have case drains.

It takes an inordinate amount of pressure for air to push oil through an orifice.

Only if the pressure on the other side is high so the differential pressure is low.

You are making up stuff that shouldn't exist in a normal hydraulic cylinder.

 

[OdinIII]

Peter, the air in the pipe is compressible and the pressure applied to it will be converted to a reduction in volume until the air bubble pressure and the fluid pressure equalize at a lower pressure than would be present with the pipe filled with an incompressible fluid. Since there is a flow control valve that will only open when enough pressure is applied a large enough air bubble could cause the pressure to drop enough to prevent flow. The cylinder is only stroking 1" so it would not take even a very large bubble to reduce the pressure between the cylinder and the flow control valve enough to prevent the flow control from opening.

The piston could not stay stationary if the pressure dropped on the "B" side.

 

[Mark Snodgrass]

He said the piston is not stationary, only moves part of the stroke, but will not go full stroke. It only goes full stroke when enough pressure is applied to collapse the bubble and there is enough pressure to overcome the flow control valve spring tension.

 

[scarince]

I did some detective work over the weekend and this is what I think has happened:

As I mentioned earlier, the blocking valve was added to the "A" side of the cylinder to prevent it from falling when the hydraulic power is killed by entering the robot cell.

The original plan was to use a Vickers valve body A314A which has ports that go straight through for P, T, and B, and has a cavity for a valve for port A. The blocking valve is solenoid controlled, energized to open, and it allows the cylinder to move during the process but blocks it when I'm not in cycle.

There was a problem with the lead time for this valve body, so someone figured we could use a different valve body, a A314W, and put a plug in the "B" side. The A314W has a cavity for both port "A" and "B", but I think the plug that they send me (called C-10-2) is designed to actually block port "B".

So I think that the "B" port has been blocked the whole time, but I never noticed since the tool is so heavy that gravity pushes it down when I take pressure off of "A". I think we got air in the "B" side when the valve was installed and since that side was blocked it created the back pressure that caused all the symptoms I've seen.

I've sent all this info to my builder to see what they think, and in the mean time I'm running with the "B" circuit open (disconnected) to prevent any chance of back pressure.

I attached some tech info on the parts I referenced above.

 

[pal]

scarince This sounds as if you may be modifying a safety circuit . I would strongly suggest you check if there is a risk assessment document supplied with the machine and check out what it says regarding this press . Changing a valve to another type because of time constraints in a safety function could well come back to bite you and your company .

Paul

 

[scarince]

scarince This sounds as if you may be modifying a safety circuit . I would strongly suggest you check if there is a risk assessment document supplied with the machine and check out what it says regarding this press . Changing a valve to another type because of time constraints in a safety function could well come back to bite you and your company.

Hi Paul, I sincerely appreciate you pointing this out to me. In this case what I'm doing is actually going back to what the machine builder had originally specified. It was changed at the last minute as a contingency plan to recover from a long lead-time on the original body.

I spoke to them this morning and they recommended just going back to the correct body instead of trying to re-engineer this circuit.

I really do appreciate your cautionary statements. We're all in such a hurry it's easy to miss something important.

Thanks.

Bill