Hydraulic cylinder/valve question

Hi, Tom. Sounds like there may be a piece of trash in the control mechanism. Does your system have a very fine filter? Have you flushed the system with fresh oil?

Best regards, Paul

If I may tender a WAG: Hydraulic control valves have to "balanced" so that there is no feedback pressure to the controls. Think balanced valves as in a governor, etc. I would be suspicious of anythng fouling the ports or passages in valve innerds.

"Tom Gardner" wrote in news:X1bji.37664$ snipped-for-privacy@newssvr12.news.prodigy.net:

WAG here, but I would suggest checking for packing blowby. May have a worn spot in the cylinder allowing some leakage.

The first thing to suspect is air in the hydraulic system. Is the pump sucking air in on the intake line? I have seen this happen many times.

Once you bleed the air out that should be it. If you find more air in the system it is usually caused by a leaky suction hose to the pump or low oil level with the intake partially exposed to air.

If the check valve is working properly and there is no air in the system, then I would suspect the piston nut that holds the piston to the rod is loose, but since it worked for a while after you had the valves out that most likely is not the problem.

It has to be either air not getting bled out of the lines, more air gettin into the lines and cylinder, or a bad check valve.

John

In a full hydraulic system, being able to move a supposedly locked-up cylinder, generally means you've lost pressure on BOTH sides of the cylinder. I've been told it's bad design to attempt to use a cylinder's positioning for accuracy, that is not a part of their fundemental design. Also, in my somewhat limited experience, whenever this type of subject was brought up the consensus was valves also lacked the intergrity to perform this function reliably (prob' why we still use lead screws?).

You'd be better off using a positive stop of some sort, thereby bypassing your present difficulty and extend the working life of future cylinders while helping to eliminate waste.

Of course this will not work if you need every bit of the 10" but a work-around would be to install a slightly longer cylinder (if there's room).

I hope this helps.

dennis in nca

On fixed displacement pumps, don't forget the shaft seal.

Wes

I agree but it's not my design, the machine is 25 years old but not all the parts, if you know what I mean. That cylinder is one of three on the machine. This machine makes 6" knot-type wire wheel brushes. The other two cylinders/valves have no problems. This cylinder is attached to a pivoting arm that has another arm that is attached to a spoke of a big gear. There's a spur gear riding on the big gear that turns a shaft that has the wire twisting head that has two holes that the wire goes through. It sounds more complicated than it is. This actually works! But, with the cylinder acting up, the holes don't always line up with the brush disk perfectly. (With all these mechanical linkages even I'm amazed with the accuracy.)

The next generation machine will be servos.

Ok. If we can assume complete hydraulic integrity on either end of the cylinder is sufficent to keep the piston locked in place then perhaps consider re- plumbing/changing your control valve in order to keep pressure applied to the rod end of the cyl- inder to keep it retracted while this position is required? Or is the retracted/center blocked position re quired in another portion of the cycle other than what you've described?

dennis in nca

Nope, only full retract counts. The check valve is supposed to lock it all up. I have to keep in mind that this has worked for decades. The valves check out on the bench, that's why I'm stumped. It has to be in the plumbing, cylinder or such it just feels like air in the system...there's something I'm missing. Oh well, the hydraulic guru will be here Monday, maybe it'll stump him.

Sounds like a duff piston seal on the ram. If they start bypassing/leaking slightly, when you retract them, they can appear to 'bounce' back out, due to them going from acting like a two way cylinder, to a displacement cylinder. If bad enough, if you keep hydraulic pressure applied to the retract side, the ram will creep out, but sometimes the piston seal will seal after moving outwards slightly.

We had a similar problem on an elevating platform at work, which the main boom would creep out when you operated the stabiliser legs. After much studying of wiring/hydraulic diagrams, we discovered that when you operated the stabiliser leg control, it also pressurised the main boom to make sure it was fully retracted, but because the piston seal was leaking slightly, it was creeping out instead of being held in.

Makes sense. I should have taken the cylinder to the shop too. I'll let everybody know what the outcome is.

Any outcome?

dennis in nca