hyraulics question

[jerkyboy]

As a final note the one year warrantee really had me thinking last night to a bit of engineering advice I once heard (I am an engineer by training). It was you can predict how well something will work by how close the engineer is willing to stand next to it. If a lot of the car manufacturers can do a 4, 5, 10 ... year warrantee on a drivetrain, why can't a tractor manufacturer??

Matt, well said. Hope everything goes smoothly with the new HST. If and when you buy, please let us know where you end up purchasing as I'll be doing the same. Great thread:thumbsup::thumbsup::thumbsup:

 

[dfkrug]

Matt:

So it seems the proximate cause was some crud in the fluid that
affected the flow divider piston, which resulted in low charge pressure.
That, in turn, caused cavitation in HST axial piston pump.

I would also like to see more pix of the cleaned-up cylinder block and
pistons. The one of the assembled pump you posted earlier is amazing.

I have seen a light-colored sediment in a flow divider before, which caused
it to fail completely. I was able to fix it, but I could not see how the
crud got past the filters. And the tractor (a Bolens/Iseki G274) was
under frequent use. This flow divider was not for an HST, however, so
it did not cause any damage. So the thought that you may have a filtration
problem would only be correct if your crud was NOT initially dissolved in
your hyd oil.

That leads me to wonder why there's a separate charge pump on some
HST tractors (e.g. JD), and others use a divider.

Anyway, great post.

 

[EE_Bota]

I'm a chemical engineer in the pharmaceutical industry. I did my undergrad studies in Agricultural and biological engineering (grad in chem engineering), and worked on farms and at Agway from middle school through college so had a good amount of experience fooling with equipment. The engineer in me justs loves understanding how things work, just not paying for them when they don't work! In pharmaceuticals equipment failure has a very large negative effect so I tend to sweat the details.

That's a very impressive background. I know you fellows do indeed sweat the details. Your tractor is in excellent hands, I feel quite sure.

 

[MattJ]

I ordered the HST from big orange parts and hopefully it will be here soon along with a new fan as I'm missing a blade or two out of eight.

I also have a project I'm going to do as I reassemble and I'll post how it turns out. I'm going to go ahead and take the oil gauge setup I bought before for charge pressure and make it permenent on the tractor. The only thing I'll upgrade on the kit is the lines to braided (they're currently plastic) for durability. I want that mounted right on the dash so I can see if the HST is starved for oil. I figure that $30 in protection would have likely been real valuable back about 10-50hr ago.

 

[MHarryE]

Throughout my 44 year engineering career all of the equipment on which I worked had hydrostatic transmissions. It wasn't until reading your thread that I even realized there are hydro equipped vehicles that don't have a low charge pressure warning - all of our products had a low charge pressure warning light. I had to pull out the book on my new L5740 to check - it has a transmission warning indicator that I assume is a charge pressure warning but nowhere in the book does it say exactly what it monitors. It just says that if the light comes on, stop as quickly as you safely can and call your dealer. We wrote our manuals to say that it was due to low charge pressure, first change your hydraulic filter and if that didn't solve the problem call your dealer. I like your idea of adding the gauge so you can monitor the system, but I am still surprised at your piston barrel wear being caused by low charge pressure. I have had the unfortunate experience to examine parts from hundreds of failed hydros and I have never seen wear like that, but then again the hydros with which I have worked have been medium duty (about 5000 psi) and heavy duty (about 6100 psi) so I don't know the type of material they use on a light duty hydro.

 

[MattJ]

As an fyi I brought this to closure with a new HST I installed plus I added a gauge to watch the HST charge pressure for future peace of mind.

new hst install - TractorByNet.com

 

[rScotty]

As an fyi I brought this to closure with a new HST I installed plus I added a gauge to watch the HST charge pressure for future peace of mind.

"So it seems the proximate cause was some crud in the fluid that
affected the flow divider piston, which resulted in low charge pressure.
That, in turn, caused cavitation in HST axial piston pump."

Hello all. I just stumbled on this thread while searching under pumps.
I'm really impressed by the rational approach to a mechanical problem....but am concerned that the case for the cause of failure is so weak. Just what is this "crud theory" based on?

I know I'm coming in after the fact, but I've got to say that's sure a lot of well-polished symmetical wear for a little bit of crud to cause. In my experience, wear from crud tends to be more assymmetrical and includes scoring. And cavitation leaves a telltale specific erosion pattern. What the pictures posted look like to me is just excessive wear which usually has a simpler cause: poor metallurgy, poor fits, or poor lubrication.

Metallurgy is easy to test so I'm going to go out on a limb and say that it probably isn't the hardness or alloy of the pump parts because those are obvious enough to leave clues. And of course the fit of the original parts is gone forever.

So if those causes of failure are out, that just leaves the oil itself. Maybe I missed it, but was there ever anything odd about the oil or filter? Any tests done on either? The reason I ask is when I switched to SUDT in my M59 last year several problems developed immediately, problems which just as promptly went away by switching back to a standard hydraulic oil.
Enjoyed the thread,
rScotty

 

[dfkrug]

I know I'm coming in after the fact, but I've got to say that's sure a lot of well-polished symmetical wear for a little bit of crud to cause. In my experience, wear from crud tends to be more assymmetrical and includes scoring. And cavitation leaves a telltale specific erosion pattern. What the pictures posted look like to me is just excessive wear which usually has a simpler cause: poor metallurgy, poor fits, or poor lubrication.
.
.
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The crud referred to would be in the flow divider, not in the HST pump.
I and others here would like to see more pix of the failed HST pump, incl
the slippers, and the valve plates. Cleaned up.

I don't have personal experience with cavitation damage on HST pumps...
what kind of pattern do you find with this type of failure?

 

[rScotty]

The crud referred to would be in the flow divider, not in the HST pump.
I and others here would like to see more pix of the failed HST pump, incl
the slippers, and the valve plates. Cleaned up.

I don't have personal experience with cavitation damage on HST pumps...
what kind of pattern do you find with this type of failure?

Cavitation damage looks a lot like little irregular corrosion pits. Under a microscope the pits have sharp edges where tiny bits of metal were fractured away from the surface and edges of the pit. This is thought to be caused by the explosive force as the cavitation induced gas pockets form and collapse. In extreme form it can cover the whole surface where fluid pressure was too low to prevent the cavitation bubbles from occurring. Cavitation is said to be able to drill a hole through steel - given enough time. What I've seen looked more like tiny pick points all over a surface and edge. Or sometimes like mice had been nibbling at the metal. The sharp edges are the clue.

Cavitation itself is caused when the pressure on a fluid is low enough to be beneath the vapor pressure of that fluid at that temperature. It is different than foaming and I don't know the relation between the two. If any. Anyone?
rScotty

 

[MattJ]

Here's my guess. I don't think it was crud. I think the flow priority valve's spring fatigued. It, along with the tiny port in it balance the flow and under lower flow cut off the loader/3pt and give priority to the power steering and HST. When I hooked up a pressure gauge for charge, and for the FEL and drove it up a hill under load the charge pressure went way down and the FEL had full bypass pressure (~1800 psi) and the 3pt raised quickly as well. If you look at that flow priority valve if the spring wore out it would puch back under high pressure and block the PS/HST.

I think the loss of charge pressure while I was working it hard starved the HST of oil, and it burned it up. Kubota's engineers said a few minutes under load without adequate charge and its all she wrote.

In the HST the fluid level observation and heat is also backed by the color of the pump and motor assemblies. The motor is a nice shiny steel color (and is in the bottom of the HST where oil would remain the longest. The top of the HST has the pump and it was a blued color, like a shotgun, and I think it heated up and lost whatever temper or hardening it had.

When I put in the new flow priority valve it behaved correct. Head up a hill the charge stays fine the FEL and 3pt slow down. So far its been acting great and my main concern is to stop watching the charge pressure so much on the new gauge! Everytime I put it under load I'm watching it, but it always stays in the same spot just north of 50 psi, just like it should.