I have a liftmoore 3200ree truck mounted crane that leaks down on its own when weight is on the hook. There is what I would assume is a holding valve in the cylinder, would this be suspect over the internal seals of the cylinder leaking? The serial number is not legible anymore and liftmoore says they have built this crane for years and can’t help without it. My understanding is that the holding valve needs a pilot signal from the rod end of the cylinder to allow oil to leave the barrel end as a safety. So if the cylinder internal seals were leaking the check in that holding valve would still hold the oil is this correct? I am thinking I need to identify and source this holding valve. Thanks for any suggestions
That is a counterbalance valve and it has failed. I strongly suggest you remove that crane from service until it is repaired. That valve is there to preclude that boom from coming down unexpectedly and suddenly. If it is leaking, there is no predicting when it will fail catastrophically. Sun hydraulics is a large supplier of them as are several other brand names. Get that cylinder to a qualified repair shop before you hurt, (or even worse) yourself or equipment.
Is your crane ran through lever and linkage or solenoids on stems? I would stowe the boom, and pull the stem, check for damage to the o-rings. I would do the same for the load holding valve with the boom stowed, you should be safe.
It’s controlled with electronic solenoids, I’ll take a look at the o-rings. To clarify I won’t be using the crane until the issue is fixed.
Check for damage to the o-rings and back-up rings. Exercise the plunger with a pic or small screwdriver. Make sure it's free and not hindered by bits of vinyl or rubber. Good luck
I no longer work with systems employing counterbalance, or overcenter, (same thing, different name) valving due to life/safety issues I don't carry insurance coverage for. This work however simple is sent out sublet to shift the liability exposure. Commendable move to not use till repaired properly.
Electric solenoids may control the operation of the cylinder, but that cylinder, since it's a boom hoist cylinder, will most definitely have at least one if not two counterbalance valve (holding valve). And yes, they're are internally ported to open them to allow movement of the cylinder. If there's just one holding valve on the cylinder, then yes, a leaking cylinder piston seal can cause cylinder drift. But if there's two holding valve cartridges on the cylinder then piston seal leakage won't cause drift. Those cartridge valves do have o-rings on them, and a failed o-ring can cause drift, I'd start with that. But I will add this caution, I always hated messing with removing cylinder holding valves as it's damn near impossible to fully relieve the pressure on those things for removal. Be very very careful removing those things.
I replaced the counterbalance valve and it didn’t fix it you can visually watch the boom drift down from just the weight of itself. At this point it must be the internal seals of the cylinder?
Probably. I have had more piston seals, than counterbalance valves, be the issue. But the valve is pretty easy, but piston seals won't be too bad. Pop the pins and get the cylinder torn down, or take it to a hyd shop and have them rebuild it.
If you're boom hoist cylinder only has one holding valve, which will block the piston side of the cylinder, then a failed piston seal will for sure cause drift as there's no holding valve on the rod side of the cylinder.
There’s only 1 holding valve yes. So the oil bypassing into the rod side is also leaking past the closed control valve? Is this normal for the valve to allow that much leakage that you can see the cylinder move?
You have to understand how holding valves work on a load bearing cylinder. Holding valves, also known as counterbalance valves, do two things, 1) they provide a "throttle effect" to prevent rapid drop when cylinder control is activated, controlled descent, and 2) they "lock" the oil inside the cylinder. Look at it this way, you have two hoses going to that cylinder, imagine putting a ball valve on both lines where they connect to the cylinder. When you're not moving (using) the cylinder you close both ball valves, oil is locked in the cylinder, it can't move. That's what the holding valves do, no cylinder drift because oil can't escape. But in your case, you only have one holding valve, which will obviously lock oil in the load bearing side of the cylinder, the piston side. With no holding valve on the rod side of the cylinder a failed piston seal will cause oil to escape because there's no holding valve on that hose to stop it.
I believe I’m understand the theory your explaining. The part I’m not getting is for oil to leave the piston side and enter the rod side oil needs to be displaced from the rod side. Where is that going? Is it leaking through the control valve?
Absolutely leaking back through the control valve spool. Control valve spools are not tight enough to stop oil flow. If they were that tight they wouldn't move freely to do their operation. Oil in load bearing cylinders have to be locked by holding valves, control valves don't do that.
I appreciate the explanation I’ll tear apart the cylinder next thanks
No worries, I'm more than happy to help if it fixes your problem.
The drifting issue is resolved. The seals must have been originals they crumbled into pieces when removed. Cylinder had an aluminum gland in a steel barrel with a wire lock setup holding it together. Real fun to get apart! Thanks again
thanks for reporting back. Close the loop!