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Nellie
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2002-08-05          41003

I recently purchased an LA402 for my B2910. Worked fine for the 1st week or so, but then I noticed the left side would compress when pushing into a pile. I noticed that the piston moves in when this happens. The dealer said to open the hydraulic line (1/4 to 1/2 a turn) to bleed any air out of the system. Anyone have any experience with this. Of course the tractor should be off, but how much pressure do I let out of the system prior to bleeding the system. Any other thoughts on this issue?

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Art White
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2002-08-06          41006

Nellie, your loader should have bleed down or out the first few times you used your loader. They don't have to be manually done. The action of cycling back and forth or raising and lowering will do it for you. Do raise and lower from top to bottom and as long as you are doing it cycle your dump to.. You will find some compression of a cylinder if the load is not even against the loader frame as both cylinders are plumbed together. Be sure when pushing that you are as close to the center of the bucket as possible and that might eliminate your problem. ....

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TomG
Join Date: Feb 2002
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2002-08-06          41030

I'm with Art. Far as I know loaders and 3ph's for that matter purge themselves of air, and bleeding isn't required. Air in cylinders, lines etc. is just pushed through the system to the sump where it bubbles out and escapes before the oil is re-circulated. It might require cycling though (moving a cylinder to the limits of its travels and holding the operating valve open until the relief valve opens).

One thought is that if heavy work is being done, there could be some oil cavitation if loads are dumped or lowered quickly. If the operating valve was closed when cavitation was present, air may still be present in the oil for the next lift or roll back operation. That's purely speculative, but I do think much the same happens to me if I don't let the hydraulics catch up and complete a dump when I'm working with a lot of weight.

Another thought is that the dealer might have said that care should be taken when loosening hydraulic fittings (Even it it doesn't seen necessary in this case). The load carried determines pressure in the lines and cylinder. Even if the tractor is off, there may be 2,000-lbs. pressure in a line if the bucket is off the ground and carrying a heavy load. That's enough pressure to cause serious injury. Fortunately, it's pretty difficult to loosen a fitting when a line is carrying that much pressure.
....

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MRETICS
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2002-08-07          41054

Nellie,
art and tom are right on the money. The bucket cylinders are plumbed together, and oil will flow the path of least resistance.

a bucket that has been sprung or twisted will sometimes cause the effect that you describe.

A twisted bucket will not allow one of the cylinders to complete it's full stroke. Because of this, the cylinder that does complete it's stroke will usually be the one that trips the releif valve in the system, leaving a little air in the other cylinder(s) in the circuit. In theory, the short stroke will be on oposite sides depending on the direction of travel.

I fought some engineers at one of our factories on this subject, he maintained that the above situation would not trap air in the circuit in question. It led to an experement at our shop.

I won ( I must admit here that I agreed with him at first, but we had exhausted all other possibilities)

After installing a new bucket, after a few strokes, everything went back to normal and worked fine. Re-install the twisted bucket, and within a few minutes of operation, the problem returned.

The engineer maintained this could not happen. Even though he wittnessed it with his own eyes.

After some more research in the matter, we found this only to be true once the oil was warmed to normal operating temps. We also found that if you waited a few moments between cylcles the problem disapeared, thus we came to the conclusion that tiny air bibbles were being trapped in the oil within the cylinder, and if you gave them some time, they would disapate.

Make any sense to you guys?? ....

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Nellie
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2002-08-07          41059

How can you tell if you have a sprung or twisted bucket? ....

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DRankin
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2002-08-07          41060

What makes sense to me is that some (certainly not all) engineers will only beleive what they are taught, even when their powers of observation tell them something different. I have seen it before. That "box" can be very confining and yet comforting at the same time. ....

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Art White
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2002-08-08          41063

Nellie you need a flat place to work from to begin with that is concrete or blacktop but it has to be flat. Make sure that the air presure in the rear tires is the same and of coarse it is critical that the rear tires are worn evenly. If they are not that you need to level the rear with a jack. At this point you could look at the cutting edge to see if it is touching the flooor across the full face or if one corner or side might be up. This would indicate that the bucket or the loader frame is bent if not than your loader is fine. If not take a tape measure and measure your pin locations off the ground at the bucket where the frame attaches and that will tell you if it is the bucket or the loader frame that might be sprung. Sometimes things are close and you might have to remove the bucket to see where it might be sprung. ....

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TomG
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2002-08-08          41072

Nellie: If you're operating the loader while at slow idle, you might increase the RPM a bit and see if they reduces the problem as well as checking for level as Art suggested. Maybe I haven't got my mind around the subject, but I'd think that equal tire pressures and wear on the front would be important and the front as well as the rear. It sort of reminds me of curing a headlight aiming problem on my 1/2-ton by having the rear leaf springs re-arched.

A side note about this sort of hydraulic issue is that I’ve heard of specialty cylinders called ‘indexing cylinders ‘ mentioned incidentally in a discussion. I think they’re used in platform applications where independent cylinders do not have a strong physical connection. The problem is that cylinders have different rates of leak down and the shaft lengths become different lengths after use. The idea as I understood it is that indexing cylinders equalize the shaft lengths when fully extended. I don’t know how they work, and I’d never expect to see them on a tractor loader.
....

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Art White
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2002-08-08          41074

Tom the front axle oscillates which unless the tires were that miss matched that one side is on the stops it would not make any difference. The rear axle is solid and tire heights are critical. Engine rpm after the cicuit is closed, unless the valve is leaking by that much that the cylinders don't have time to equalize will make little difference for the problem. ....

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TomG
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2002-08-08          41082

Thanks Art. Got it, I keep forgetting about the pivoting front axle. Regarding RPM, I was thinking that more might help reduce cavitation (if that's something that's going on). A control valve would be open at the time.

I could be wrong, but I think my loader is slightly spongy when I'm dumping heavy loads at low idle and close the valve before the pump catches up with the bucket. My loader doesn't have fast dump. My impression is that higher rpm reduces the sponginess. Whether or not that's right, more rpm never-the-less sure speeds up the loader work.
....

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