OT: Hydraulic Circuit Problem

Hi Everyone,

I have a problem on a new machine that I can not figure out, nor can anyone give me a plausible-sounding answer for. It is a *very* simple hydraulic circuit and I am frustrated.

There is a vertically oriented cylinder. 1 1/2" bore, 1" stroke, rod points "up". It is used to lift a fixture and press a part in the fixture into a forming tool. I have analog output to control the pressure to that cylinder and analog input to read the resultant pressure. I have an lvdt to monitor the displacement of the fixture which is the same as the displacement of the cylinder rod.

I had a quality problem that was the result of the part not being pressed up into the tool with enough force. During trouble shooting, I noticed that the cylinder was not fully extended when it should have been. We were applying hyd pressure, valve in the correct position, but cylinder only extended about 3/4". I used my laptop to drive the pressure control signal up and up and was eventually able to drive the cylinder to full extension with 1100 psi when it should only have required about 400. To skip a bit, I cracked open the opposite hydraulic line going to the other side of the piston, air was bled out, and then the cylinder went to full extension with no problem.

After reconnecting the line I opened, the cylinder works perfectly fine, extending to full travel with only 400 psi.

My question: If there was air trapped in that hydraulic line, why would it have not been pushed through the hyd. valve just as fluid would be? What is it about air that could possibly cause this?

When I ask hydraulic people this question, they tell me "oh yeah, it was air locked" but they can't explain how this is possible!

Does anyone have any insight into what could be going on here?

I've attached the hyd drawings. The cyl. in question is on page 3-15 called "compensation cylinder". Valves are plain vickers 3-pos valve on a manifold with a flow control (set wide open). Valve in the "up" position should shunt fluid back to the tank.

I am open to any ideas.

Thank you.

Bill

No attachment

Are both ends of the cylinder connected to A&B port of your directional control valve. Sounds like there was something blocking return flow from the rod end of the cylinder to tank, if that was the case, perhaps there was air trapped in the rod end, the air was compressed until the cylinder stalled out, then by raising the input pressure on the blind end you further compressed the air.

Lets see the schematic.

Argh, thanks.

I think I got it there now.

Okay, looks pretty simple, make sure you are not getting air in the circuit, noisy pumps, or frothy oil in the tank. Entrapped air can do strange things to hydraulics. Is this a reoccurring problem or has it only happened once?

Fluid Power Nerd said:

Are both ends of the cylinder connected to A&B port of your directional control valve. Sounds like there was something blocking return flow from the rod end of the cylinder to tank, if that was the case, perhaps there was air trapped in the rod end, the air was compressed until the cylinder stalled out, then by raising the input pressure on the blind end you further compressed the air.

Lets see the schematic.

Click to expand...

Yes, I do believe you are exactly correct. I think I had air in that end, but why would it become compressed if the valve was shifted to allow flow back to the tank? That what doesn't make sense to me.

Fluid Power Nerd said:

Is this a reoccurring problem or has it only happened once?

Click to expand...

It's only happened once. I had the manifold apart and almost certainly allowed air to get in.

I have not properly / fully bled that circuit to insure that I have oil all the way to the cylinder, so I need to do that. But it has been working fine since I burped it.

Sounds like entrapped air for sure, especially with a vertical cylinder. Its hard to get it all out, as to why it wouldnt go back to tank through the sandwich flow control you have there, I dont know unless something went through and it was blocked or closed off.

Fluid Power Nerd said:

Its hard to get it all out, as to why it wouldnt go back to tank through the sandwich flow control you have there, I dont know unless something went through and it was blocked or closed off.

Click to expand...

Ya, this is the part that worries me. I'm under pressure to explain exactly this point, and the fact that I can't has cast some doubt on my explanation for the root cause.

I'll get it bled out and write some logic to detect this before each cycle. That will hopefully cover me.

Thanks for taking a look at it. I really appreciate it.

Think of it like this.

Your cylinder holds 16 ounces of fluid, but your hydraulic line can hold 48 ounces of fluid.

Since air can be compressed by the hydraulics, is is reduced to the size of 6 ounces of space when compressed.

That same air will now expand to the 16 ounce size when the cylinder is shifted to the opposite direction of travel.

That's why you must bleed all cylinder lines first, then fill one side of the cylinder and bleed it. Then shift the cylinder in the opposite direction and bleed it.

You must repeat the process several times, let the system sit there for an hour or so and do it one more time.

Yes, its time consuming, but you get rid of the trapped air.

regards,
james

Fluid Power Nerd has a point about something being trapped . For your peace of mind , it would be worth checking the return filter for debris , particularly if this happened on the 1st run .

Paul

Air really does weird stuff to hydraulics, but to explain this. I will try, and I could be wrong but here goes. Fluid flows through the system by being pushed by itself, by that I mean it acts like a solid object if you apply enough force to one area of said fluid the another part of it will move. if you add an area of air in between parts of the fluid then the aforementioned movement is changed I would assume you have a flow control valve in the system and when all the fluid has passed through it the air then has to push the fluid through the flow control, air compresses and needs an inordinate amount of pressure to force a fluid through the flow control. Its the only explanation I can think of .

I don't see how air can impede the flow of oil. Air will only displace oil but as pressure increases the air bubble will become smaller.

Peter you are correct and when in the main cylinder there is a large surface area that the air can impact on. I was just thinking does the dynamics change as the area that the air has to work on decreases, as in a flow control or such of the like? I'm no hydraulic guru, just trying to apply logic allbeit fuzzy.

Reveiwing my previous post , a possibility is that a small bit of rubber from the 3/8 hose fabrication was cleared before assembly and lodged in actuating valve . When you increased the pressure the debris blew through the valve and normal operation resumed . If this IS the explanation ,then the rubber or whatever will be found in the return filter part reference 007 .

Paul