Loader Valve Leaking Down

[oldnslo]

I don't see how the problem can be in the cylinder. Think about the loader control valve for a moment. Until you move the levers, the control valve is simply an on/off switch in the OFF position. In that OFF position with the the levers sitting in the neutral center position the control valve blocks all flow to and from the cylinders.

Now picture the cylinders. In order for the loader arms to drift down , the lift cylinders have to become shorter, which they can only do by forcing fluid that should be trapped inside the cylinders to go somewhere else. Without a leakage path through the control valve and back to the sump, the lift cylinders cannot shorten and the loader arms cannot drift down.

It won't hurt to look at the cylinders and do some tests, but I think you should also look at the loader control valve. If it is the type of control valve that has a built-in relief valve then that is where I would look first.

rScotty

Scotty
Unless the relief valve is a work port relief it is not in the loader circuit and would have no effect on loader drift.

 

[dieselscout80]

No enough information. There is an acceptable leak down RATE. Are you talking about the loader dropping 4 feet in 1 minute or 4 inches in 1 hour? There is a difference. Quantify please. All hydraulics leak down.

Like 4” down in a 1-3 seconds.

 

[rScotty]

Scotty
Unless the relief valve is a work port relief it is not in the loader circuit and would have no effect on loader drift.

I agree with that.

 

[rScotty]

Like 4” down in a 1-3 seconds.

Now that is interesting.
Does it then hold for awhile and then repeat the sudden drop?
Will it eventually stop dropping?
And how does the amount of load in the bucket affect it?

I figure that when it drops like that your instinct is to move the lever and raise it back up. How does that work?

Oh, and is it just the lift arms or does the bucket try to uncurl or whatever at the same time?
rScotty

 

[Dman1981]

I don't see how the problem can be in the cylinder. Think about the loader control valve for a moment. Until you move the levers, the control valve is simply an on/off switch in the OFF position. In that OFF position with the the levers sitting in the neutral center position the control valve blocks all flow to and from the cylinders.

Now picture the cylinders. In order for the loader arms to drift down , the lift cylinders have to become shorter, which they can only do by forcing fluid that should be trapped inside the cylinders to go somewhere else. Without a leakage path through the control valve and back to the sump, the lift cylinders cannot shorten and the loader arms cannot drift down.

It won't hurt to look at the cylinders and do some tests, but I think you should also look at the loader control valve. If it is the type of control valve that has a built-in relief valve then that is where I would look first.

rScotty

So if a seal
Is blown it will displace
Oil from one side of cylinder to other also unless you hydraulic locking mechanism plus in forty five years don’t see to many valves that are totally leak proof
Reason for hydraulic locks on cylinders
Be pretty scary if you were a hundred feet in the air and cylinder or hose blew
Worked aerial ladder trucks a fire department also

 

[dieselscout80]

Now that is interesting.
Does it then hold for awhile and then repeat the sudden drop?
Will it eventually stop dropping?
And how does the amount of load in the bucket affect it?

I figure that when it drops like that your instinct is to move the lever and raise it back up. How does that work?

Oh, and is it just the lift arms or does the bucket try to uncurl or whatever at the same time?
rScotty

That’s with a load on it.

Not sure if it would stop as I was moving about 700 pounds on my pallet forks when it happened.

It will move up when I raise it, but it starts down before going up.

 

[rScotty]

So if a seal
Is blown it will displace
Oil from one side of cylinder to other also unless you hydraulic locking mechanism plus in forty five years don’t see to many valves that are totally leak proof
Reason for hydraulic locks on cylinders
Be pretty scary if you were a hundred feet in the air and cylinder or hose blew
Worked aerial ladder trucks a fire department also

Suppose that the cylinder is half way through its stroke and the piston is is somewhere near the center of the cylinder. The piston has a bum seal, so fluid can get past it, but fluid cannot get into or out of the cylinder because it is blocked by the loader valve being in the OFF position.

The thing to notice is that whatever oil is in the cylinder has nowhere to escape, and that the piston divides that fluid into two different volumes. The side with the ram in it has less fluid.

OK? Now what happens as the seal leaks or maybe we remove it altogether.

The cylinder can extend a little ways as fluid moves from the ram side past the damaged piston and into the new larger volume on the other side created as the cylinder extends. But the cylinder cannot ever compress, because to do that would mean oil on the large volume side would somehow be forced to occupy the smaller volume on the ram side. It is smaller on that side because the ram is already taking up part of the space, and we know that fluids don't compress. So seal or not, trapped oil can't move that way,

rScotty

 

[rScotty]

That’s with a load on it.

Not sure if it would stop as I was moving about 700 pounds on my pallet forks when it happened.

It will move up when I raise it, but it starts down before going up.

Doing it just once sounds like there is a possibility that air in the hydraulic fluid created a pocket of air in the lift cylinders - air WILL compress and can move where fluid cannot. Next time you get the hydraulic fluid working and up to temperature, take a look at the hydraulic fluid dipstick and see if the fluid is clear or foamy.

 

[oldnslo]

Like 4” down in a 1-3 seconds.

That sounds like a load check not seated. The load check is supposed to prevent oil from flowing out of a work port from load induced pressure into the pressure port. With open center system you need to move the spool far enough to block flow in the pressure port and direct it to the work ports partially shifted the pressure port with minimal pressure build up and can be connected to work port allowing the reverse flow if the check is not seated.

 

[rScotty]

That sounds like a load check not seated. The load check is supposed to prevent oil from flowing out of a work port from load induced pressure into the pressure port. With open center system you need to move the spool far enough to block flow in the pressure port and direct it to the work ports partially shifted the pressure port with minimal pressure build up and can be connected to work port allowing the reverse flow if the check is not seated.

I'm beginning to wish we had a hydraulic schematic. Does it have check valves? We need to know more about the configuration of the loader control valve(s). We've been assuming that it is the most basic type control valve - but it may not be.