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Counterbalance valve Diagnosis

joelmartin

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On a 944E-42 Lull, the boom seeps down, no load, about one inch per 3 minutes.
I took both hoses off and the small line leaks fluid out. If I cap it, the boom stops dropping after about a one inch drop, then when I open the cap and bleed pressure off, it will resume dropping.
JLG said to replace counterbalance valve, which I did. Same thing.
Anybody run into this ? Could it be the cylinder packing?
 

GaryHoff

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I would have agreed with the dealer on this one. Does it leak down with the engine running, or with it off, or both? Check your cylinder packings, as they can do odd things (place boom all the way down, relieve pressure, remove the barrol end fitting, cap the hose only, now stall the boom down. if significant amounts of oil come out, the the packings are worn out)
 

Per Eriksson

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If the cylinder have an internal leak it will inrease it's internal pressure since it is then only the area of the rod that holds the weight and perhaps that increased pressure is to much for the counterbalance valve to handle?

Capped the cylinder connections on 972H that had a leaking tilt piston and the pressure after it had been dropping for a while was waaaay over the relief settings.
 

John C.

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I am a bit confused by your use of the "counterbalance valve" in this application. I've never seen counterbalance valves on a cylinder before. They are generally used on motors that can be turned faster by the load they carry than the pump can supply oil to.

Most of the man lifts I have worked on had a load lock valve that carried the circuit relief for the piston side of the boom cylinder. The relief was a cartridge that unscrewed out of a manifold block and was easy to replace.
 

CAT793

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NO EXPERT Here but in my World ........Large CAT Mining Trucks we have Counterbalance Valves on the Hoist Cylinders! They are used to regulate the Rate (Read - SMOOTH) the Tub Lowers when the Tub Weights 50T and the Cylinders have different Volumes on each side of the Piston due to the Rod.
 

joelmartin

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In response to John C. yes they have it listed as a counterbalance valve in the PDF parts book and yes it threads into the cylinder. My thought is if your lift hose blows, it closes the circuit so boom does not slam down without warning.
I did the Test Gary Hoff suggested , took larger lift hose off, capped the hose, left cylinder tube open, the boom lowered just a touch which shot a handful of oil out the tube, after the boom could no longer travel down, a tiny stream of oil ran out as long as I held the valve open to lower it. I know what packings look like, they would allow a miniscule amount of oil by. I decided that JLG / Lull sent us a defective counterbalance valve, which has happened in the past. Will install and update in a few days.
 

willie59

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Counterbalance valves are used on load holding cylinders, such as boom lift cylinders on cranes and manlifts, and with machines like haul truck bed hoist cylinders as mentioned by Cat793 previously. There main purpose in these applications are to control the rate of descent, to prevent runaway of cylinder/load when control valve spool is shifted to lower the load.

When used in these applications, it requires pressure to build on the retract port of cylinder to open the counterbalance valve to allow oil to exit the extend port of cylinder and allow it the cylinder to retract and bring the load down. In this application, it would also control the rate of descent of load/cylinder in the case the extend line/hose were to blow as it still take pressure on the retract line to open CB valve and bring load/cylinder down.

Here's a diagram of the hyd circuit of a JLG 80HX boom lift cylinder showing the internal pilot operated CB valve that incorporates internal pilot lines from both the retract port of cylinder as well as internal pilot from the oil on the piston side of cylinder.




JLG 80HX boom lift circuit.JPG
 

John C.

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From your description it sounds more like the valve is a load lock which is required for safety purposes. Usually you have to have down pressure on the cartridge in order to open the spool to allow oil out of the cylinder. It may have some throttling function as well which might explain why Lull calls it a counterbalance valve.

Usually to test the cylinder you would pick up the boom and have pressure on the piston side of the cylinder. If the packing is leaking through the piston seals the boom will drop and you will get oil leaking when you loosen the hose that feeds the rod side of the cylinder.

This will also test your cartridge valve. Basically if you don't get any oil from the rod side open hose and the boom still drops, your load valve is leaking.

Good Luck
 

willie59

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From your description it sounds more like the valve is a load lock which is required for safety purposes. Usually you have to have down pressure on the cartridge in order to open the spool to allow oil out of the cylinder. It may have some throttling function as well which might explain why Lull calls it a counterbalance valve.

You got it John C. Since the "throttling" is directly related to pressure being required on the rod side of cylinder to overcome the spring keeping the valve closed and thus allow oil to exit piston side of cylinder. The amount the valve opens is directly proportional to the amount of pressure being applied on the rod side of the cylinder. In this arrangement, it would be considered a counterbalance valve. :)
 

John C.

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I'm thinking we are on the same track but have learned different definitions for this function. What I see in the diagram is a variable throttling valve. It is a little more sophisticated than a one way restricting check valve.

Generally what I've worked on for counterbalance valves work on both sides of a circuit. The valve will work the same way on an excavator traveling down hill no matter which way the motor is turning.

The method I describe for troubleshooting the cylinder and the valve though is as I described previously. I do have one question though on the extra line going into the base of the cylinder block and to the sensing line for the piston side of the cylinder. It is connected into piston side of the circuit and it has a restriction in it. Is it a dead man circuit for lowering the boom when the engine won't run?
 
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willie59

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I do have one question though on the extra line going into the base of the cylinder block and to the sensing line for the piston side of the cylinder. It is connected into piston side of the circuit and it has a restriction in it. Is it a dead man circuit for lowering the boom when the engine won't run?


Whoa, easy John C, you're very perceptive noticing that line. But let's not confuse the readers, rather, let's say this is not an illustration of the Lull being discussed, but that of a JLG boom lift. That line you noticed is a "manual descent" line that allows you to open a manual operated valve and bring the boom down in case of engine/machine failure, something the Lull likely does not have. :)
 

John C.

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Willie,
If one manufacture has a circuit like that, I would think it certainly possible others would do something similar. I'm thinking you just might have identified another possible way for that boom to drift down without there being something wrong with the cylinder or the valve :)

Nice work!
 

maytag

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Counterbalance valves are used on load holding cylinders, such as boom lift cylinders on cranes and manlifts, and with machines like haul truck bed hoist cylinders as mentioned by Cat793 previously. There main purpose in these applications are to control the rate of descent, to prevent runaway of cylinder/load when control valve spool is shifted to lower the load.

When used in these applications, it requires pressure to build on the retract port of cylinder to open the counterbalance valve to allow oil to exit the extend port of cylinder and allow it the cylinder to retract and bring the load down. In this application, it would also control the rate of descent of load/cylinder in the case the extend line/hose were to blow as it still take pressure on the retract line to open CB valve and bring load/cylinder down.

Here's a diagram of the hyd circuit of a JLG 80HX boom lift cylinder showing the internal pilot operated CB valve that incorporates internal pilot lines from both the retract port of cylinder as well as internal pilot from the oil on the piston side of cylinder.




View attachment 81784

The symbol shown is a counterbalance valve that is externally piloted from the other cylinder line. In heavy industry a counterbalance valve will be referred to as a
"load control" valve but not a load lock or load holding valve as they are not
zero leak. Sun and Eaton Vickers both offer a wide variety.

Tom
 

willie59

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The symbol shown is a counterbalance valve that is externally piloted from the other cylinder line.


Now maytag, you're an experienced hydraulic tech, that had to be an oversight on your part, but for the sake of others reading the thread we must make an effort to be accurate and avoid any confusion. You yourself know that box that surrounds the circuits/valve components that consists of long lines separated by dashes indicates a manifold on a hyd schematic. That indicates all the pilot lines are within the manifold making it internally piloted. ;)



JLG 80HX boom lift circuit edited.JPG
 

icestationzebra

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Another reason for counterbalance valves on telehandlers is to control the extend cylinder. Some have then on both sides of the extend to prevent a run-away condition when the boom is pointed down hill. ISZ
 

maytag

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internal/external

You are correct that that is a manifold block but the pilot pressure source is still being directed from the opposing cylinder line/port-would bave been easier to designate A and B. If it was a pilot operated valve and deriving its pilot fluid from the same line/port it was controling it would be an internally pilot operated valve.
At least that is the terminology used in industry.
Some of the C/B valves that Sun and Vickers offer can be used either way and are termed direct operated/pilot assisted. With a 3 to 1 pilot ratio, let's say that when maxxed out, the valve, when no pilot assist is used opens at 3000psi, when configured with the pilot assist it would open at 1000psi.

Tom
 

joelmartin

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Well guys it has been a good learning experience for me. Gotta say I wish JLG Lull would hire tech support personnel with real world experience. Spoke with several and none of them could clearly explain how to isolate packings versus counterbalance valve malfunction.
We borrowed a known good valve, still drifted down; pulled cylinder and it will need some work internally. thanks for all who posted, the day I stop learning is the day I look for a new job with new challenges................Thanks again.
 

maytag

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more C/B

cbed.jpg

Willie 59,

Not sure how successful posting this print will be BUT if it does show I've circled the C/B valves that are configured similar to your earlier posting-only difference is this one has an EXTERNAL physical connection for the pilot pressure where as your pilot pressure was routed thru the manifold. Does not change the functionallity of the valve, both are piloted from the opposing cylinder lines/ports.

Tom
 

willie59

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Yep, that's pretty much the same valve configuration Tom, only difference is the external pilot lines. I fully admit I may have it all wrong, it's just the way I learned it, maybe in error, that "external" meant just that, and external source, outside of the manifold, as shown in your drawing. But hey, I'm never too old to learn anything.

Think I find interesting is the cylinder is controlled by a 4-way, two position valve, causing the cylinder to be fully extended or fully retracted. Does that system operate by a hydrostat pump that maintains a high standby pressure? Nice find. ;)
 

maytag

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C/b

Its not a question of being right or wrong and like other people have mentioned that learning is a life long process. The key to hydraulics from a maintenance prospective is to know enough to be able to read between the lines so to speak as you can't depend on the designer to get things right, even down to the symbol as he may click and drag the wrong one. This equipment is powered by fixed displacement vane pumps-I inherited this area in a steel mill years ago. Not uncommon to lose 2 cylinders during a week of running, usually blown rod gland, sometimes we would blow a hose on the rod end. Me being lazy decided I'd do some investigating, we were seeing intense pressure intensification on the rod end, over 4000 psi where system pressure was only about 1800. Looking the circuit over I found that the external pilot lines had never been installed-these were the
Vickers valves that are direct operated/pilot assist. Ran the pilot lines and cut the cylinder failure rate significantly-"Even a blind hog can find an acorn"
BTW, I did change the 2 position valve to 3 position float center
Tom
 
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