OT- Need some hydraulics experts

Having some problems with a skidsteer loader, and I need some suggestions. NH LX665 skidsteer loader, sometimes it will lift the boom and hold fine. Other times it will lift slowly, and as soon as you let off the spool the boom will drop. I plumbed a gauge into the base(pressure side when lifting) of the cylinder. When lifting it will show about 1200 psi, then when I let off boom will drift down, but pressure will INCREASE at the base of the cylinder. The increase in the pressure would mean its NOT bleeding back though the valve. That tells me that the cylinder must be leaking internally? There wouldnt be any expanation for the boom going up with 1000 psi, but following while gaining pressure up to 2500 psi. Any input?

when you lift the boom with1300 psi the pressure on the top of the cylinder should be close to 0
the boom leaking internally would be less than 1300 psi,therefore the boom valve must be leaking on the down side pushing the boom down

The fact that this problem is somewhat intermittent suggests to me that the valve is at fault and not the cylinders. When you tear a center seal in a cylinder... it's torn and it doesn't heal. A bad valve can do a lot of weird things tho... so I'd suggest looking there.
If you really want to narrow it down I guess you could lift the loader up half way or so and unhook the return line from the rod end of the cylinder... then wait for it to settle and see where the oil comes from. If you get oil coming from the cylinder then you could plan on repacking... but if it just lowers with no oil loss or it spews from the hose... look at the valve.
Taking pressure readings off the cylinder is not really going to help you any.

Rod

What do you think could be wrong with the valve? The o rings on the spool only seal the spool to the outside right, not between the two different passages? I did notice there is some play in the pedal linkage. Maybe its not centering sometimes?

So you put a pressure gage in the line that feeds the lift side of the cylinder and the pressure goes up when the valve returns to the center position.

The only thing that would cause the pressure to rise like this is to apply pressure to the line that feeds the down side of the lift cylinder.

A leaking cylinder would never do this you would actually maintain or loose pressure as the arms go down.

I don't know the system or if someone played with how the hoses are hooked up.

I'd start with the valve and is it letting high pressure oil enter the down circuit when in the center position.

How many hours on the skid steer? If it is the valve leaking high pressure into the down side. It would have to be a scored valve or a cracked housing. The spool is a polished fit into the valve body. The orings or seals are to keep dirt out not oil in. Have you taken the valve stack out and disassembled it to check for wear. Also check any detent balls or groves.

I've never had one of those valves apart so I can't help you much. I'd start by checking the linkage and go from there. Get the parts schematic off NH's website and that will give you an idea of what's in there. Probably some springs and intricate parts and mabey you have a broken spring or something. That's only a guess.

Rod

I think you are correct. I would do some time testing first. Work the boom several times and time the lift stroke to get a base line on its speed. if it gets in a streak of drifting down, stop it part way down with a high lift jack, cherry picker, or chain fall to a beam. then remove and cap the lines going to the cylinder/s. now manually lower the boom, and if it is good internally, it will just move a little and then stop and stay put. If it drifts down, even a little, the piston (in one or the other cylinders) is bypassing. Simple, rule out one of two possible issues using the least difficult method. Jim

check the pedal for crap underneath anr if it rusted intwo then it wont move right seen one like that once
oldart

A lot of those valve setups have check valves, balls and springs, and popoffs that get some gunk in them that cause intermittant issues. Those spools can stick and linkage gets bent to where the spool doesn't move like you think it is.

I had a broken spring on my IH popoff valve. The broken end of the spring would hang the ball up sometimes and finally got stuck there hanging out the opening. Put it in the vise to open the ball and the piece fell back inside before I could grab it.

Just had to ask if you know the definition of an expert? X is an unknown quanity--- spert is a drip under pressure. LOL Just a joke guys nothing more. MY boss refers to me with customers as "his expert" I use this line often to describe myself, to lighten up the conversation.

Your pressure testing results are a perfect example of a leaking cylinder. It does not eliminate other problems you may have.

To show why it must be a cylinder consider the following:

Assume a cylinder with an area of 2.5 square inches and a ram cross section area of 1 inch. Since the ram always extends the entire length of the cylinder above the piston we have 2.5 sq in on the bottom and an effective 1.5 sq in on the top of the piston.

Raising the load requires 1000 PSI. 1000 PSI X w2.5 sq in results in the force required to lift 2500 pounds. There is no force being applied to the top of the cylinder so the pressure there is 0 PSI.

If everything works as designed the pressure on the bottom of the piston will remain at 1000 PSI and the top will remain at 0 PSI.

If we have a leak at the lowering port of the valve nothing happens because there is no pressure there.

If we have a leak on the raising port of the valve the pressure will remain at 1000 PSI but the cylinder will retract as the oil leaks out.

If we have a leak on the piston seals in the cylinder the pressures will equalize on top and bottom BUT because oil is leaking by the piston it has the effect of not having the piston there at all. Now the effective size of the piston is only the size of the ram, 1 sq in. We still have 2500 lbs pushing down on the ram but the piston is now only 1 sq in resulting in a pressure of 2500 PSI. As long as the valves are not leaking the cylinder will continue to hold the load but now at 2500 PSI rather than 1000 PSI.

As far as the loader drifting down, that indicates a leak through the valves. With a leak in a cylinder, a leak through either the raise port or the lower port will allow the loader to settle. You are also exposing the valves to 2.5 times the pressure the would have if the cylinder was not leaking.

I would repair the cylinder(s) first and then decide if there was enough problems to go into any valve problems.

I am with you on your line of thought. The loader actually only drifts down untill I reach about 2500 psi, then the boom holds. So maybe, the cylinders are leaking, and once they equalize pressure inside the boom doesnt drop anymore.

I was going to explain to you just what Owen did but could not have done it any better. when you said it only leaked down till it got to 2500 that was the clue needed. If you would place the gage in the upper end of the cylinder it would demonstrate that the pressure is equalizing across the cylinder and for all practical purposes you have a system that is being held up by a effective piston the size of the cylinder rod. If you warm the oil in the system then hold the system against relief anf feel the inlet fittings on the cylinders the one leaking through should get hot while the good one will stay cool, You could double check by lifting the loader up , allowing it to settle till it stops and then with the engine shut off pull the lever to the raise position and it should go down as you have opened a return on the rod end of the cylinder to sump. I believe you said it was the lift circuit so I assume two cylinders. Your senses are your best tool when diagnosing hydraulics, I have worked since 62 using methods such as this. mEl

(quoted from post at 22:39:49 12/01/10) The fact that this problem is somewhat intermittent suggests to me that the valve is at fault and not the cylinders. When you tear a center seal in a cylinder... it's torn and it doesn't heal. A bad valve can do a lot of weird things tho... so I'd suggest looking there.Taking pressure readings off the cylinder is not really going to help you any.Rod

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I agree with Rod that if a cyl packing is leaking the boom will drift down any time it's raised ie boom wouldn't stay up at all. It has to be in the valve or linkage

All the pressure testing in the world will tell you absolutely NOTHING in this situation because pressure is related to load which is variable. To make this simple... unhook the lines from the rod end of the cylinders and raise the boom to full extension and dead head the valve. If oil shoots from the cylinder ports then the cylinders are bypassing. If it doesn't, they're tight. It's that simple...

Rod

I did reseal some of the pressure reliefs with new o rings per the service manuals recommendation when there is this problem. There is still the possibility that the spool isnt centering all the time...

The definition I heard of "expert" was someone who is more than 25 miles from home and brings slides and a projector.

Jerry

TX Jim, I don't want to come across as a know it all but I have made my living doing this and am comfortable in my knowledge. Look at it again, you have a cylinder with a leaky packing, it will raise but leaks by, when the ports are closed on the valve the oil from the cylinder base or rod end has no place to go, so........... pressure builds on both sides of the piston as the cylinder leaks down as the upper end oil cannot return to sump as neither can the lower end of the cylinder oil return, so......... pressure builds on both sides of the cylinder due to the unequal volume of oil as the rod moves down and displaces oil from the lower end of the cylinder The cylinder acts like a simple displacement cylinder whose actual surface area is only the rod diameter as the piston size is negated by the leak effectively removing its diameter from the equation. That is why he said it only leaked down till the pressure went to 2500 lbs, when it only took 1000 lb to raise it. Think about it. Not to be smart but that is the way it is. mEl

(quoted from post at 09:41:19 12/02/10) All the pressure testing in the world will tell you absolutely NOTHING in this situation because pressure is related to load which is variable....

Rod

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But in this case the load remains the same but the pressure still changes. Since we know that PSI times the area gives us the total force, then if the force remains the same but the pressure changes, the area to which the pressure is applied must have changed. The way that happens in a cylinder is when the oil leaks by the piston the pressure is still trying to push the ram out of the cylinder but now effectively has only the area of the ram itself to push against.

Loader cylinders normally have rather large rams compared to cylinder borer so a pressure difference of 2 to 3 time is normal when they leak. With larger bore cylinders where the difference may be 10 to 1 it is possible to blow hoses or even destroy the cylinder itself if it were to leak without any relief valve to protect it.

mEl
This is a quote from Formerly PaMike original post[/quote]

How can the boom cyl have a leaky packing and stay up and hold fine sometimes and then drop sometimes???????????

I'm by far not a qualified hyd repair man but I've been around/diagnosed hyd problems for over 40yrs. Maybe I can see the "forest for the trees" but I still think it's a VALVE problem not a cyl problem

mEl
I just read this new thread by Formerly PaMike[/quote]
So for those of you that saw my post last night, here is an update. The boom started drifting so I ran the boom up. I pulled the ROD side lines to see what would happen. Boom doesnt sink, and normal pressure (about 1200 psi) is maintained on the Base end. This shows the cylinders are OK. NOW THE WIERD PART. When I go to raise the boom up with the rod lines off, fluid sprays out of the rod lines. Nice big streams from the lines on both sides. Streams start and stop with the pedal movement.Somehow the pressure is getting out the wrong port. Instead of going to the cylinder port certain times it is going out the rod port. Ideas? Maybe a cracked valve block? I gotta go study the book somemore.

(reply to post at 04:51:33 12/03/10)

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I am watching with great interest now as the dynamics seemed to have changed from what was posted originally, it was said that the boom leaked down and the 1200 lb base pressure needed to lift the boom raised to 2500 lbs which is consistent with what I explained. That scenerio would be consistent with oil leaking by the packing and the diameter of the rod displacing oil into the top of the cylinder which had no return to sump. Now we've gone to a no leakdown situation and a steady 1200 lbs in the base of the cylinder so the info that I and Owen posted on no longer fits. If the original post was correct that the pressure rose as the boom lowered with the engine shut off then a mechanically induced pressure rise is the only possibility. Now that the boom no longer leaks down I will wait and evaluate further as more information is gathered. In the modern Case backhoes it would be considered normal for oil under some pressure to come from the hoses even with the spool not activated, probably from a regen circuit that another poster pointed out. Interesting discussion, keep the info coming. mEl

mEl
This example is why it's very easy to misread a post and/or misdiagnose a problem without being there in person.

Granted I can make a mistake and have but as long as I keep trying it helps keep my pea brain working.

TX, I just reread your original post and it is very clear as to your description and there is no misunderstanding on my part. You have obviously not solved the problem so for your own satisfaction why do you not tee the gage into the top port and see if you can replicate the problem and watch the resulting pressures on the gage? I would be interested in the result. Owens excellent explanation of why it would take a certain pressure to raise and an increase in pressure as the boom settles is right on the money. BTW Older Case backhoe steering cylinders use a ORING that can wear flat over years and cause severe steering wheel creep as it is a fully hydrostatic system, Oftimes you can jerk the steering wheel and it will push the oring enough to get a good seal and the creep will stop so it is possible in that system to have an intermittent internal piston leak but sometimes work normally. mEl

(reply to post at 14:58:11 12/03/10)

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Sorry TX , just read your post and responded without geting all my eggs in a basket, interesting thread tho . sorry, mEl