Broke Hydrostat twice? Why?

[dfkrug]

After splitting the case, I first removed the motor (1st, 2nd photos). On first inspection, it
looked OK. The slippers looked smoother than I am used to seeing. Other HSTs
have pre-cut scratches in the slippers to help retain an oil film. I will check the
thickness with a micrometer later, after getting the spec from the manual.

I meant to report on the slippers, but I do not have a good close-up of the
pump slippers. I did mike them.

The manual has no spec on the slipper thickness, as I would expect, so I
just compared the thickness of those from the motor to those from the
pump.

They were the same. The pump slippers do have the pre-cut micro-scratches
that I expected to see on the motor slippers. This indicates to me that
none of the slippers had experienced significant wear.

Why the maker did not use micro-scratches on the motor slippers, I do not know.

 

[dfkrug]

Sure was cavitation, I was going ask how it was getting air but Roy posted the
reason.

Whether the proximate cause of the erosion of the valve plate was boiling or cavitation, which are very
similar, heat was a major factor.

Cavitation or boiling occurs when the vapor pressure of a fluid exceeds the ambiant pressure. Get any
fluid hot enough and it boils, even when the fluid is in a closed system under high pressure. In a
dynamic system with fluid flowing, any interruption in the flow causes lower pressure regions in the
fluid, sometimes resulting in bubbles of vapor to form locally (cavitation). Usually those bubbles
immediately collapse, which can cause damage to the surrounding surfaces of the fluid passages.

Cavitation can also occur at the inlet of any pump, as it attempts to move fluid by applying
low pressure to it. That is one of the reasons there in a charge pump to pressurize the
inlet on the HST pump. A pressure test by Bob's mechanic showed it was working.

The system RV, passing full flow of the implement pump at 2000psi, to the tank at 0psi ambient is
going to cavitate the fluid, as OnS says. After minutes of this abuse, the fluid in the tank is
going to probably aerate with all that agitation. Will the whole 5-7 gals of fluid get a bit foamy?
Possibly.

Entrained bubbles can even explode (Diesel) when pressurized by the system, if they
contain the right combo of oxygen and oil vapor. It is possible that the fluid was
dieseling at the high-pressure region near the valve plate. I suspect that dieseling
would foul the fluid rapidly.

But Bob reported that the loader still worked fine while he was experiencing loss of HST power.
Was it under-performing due to entrained air? Maybe, maybe not.

To me, that points to more localized boiling at the point of highest pressure on the fluid, at the
pump valve plate. Just my opinion. I welcome others, too.

 

[dfkrug]

So what about the broken spline, what does that indicate? Any part of cause
and effect on this particular HST, or is the manufacturer rebuilding used/broken/returned under warranty, HSTs
and selling them as replacement units, possibly?

I am scratching my head over the broken spline. The adjacent splines are fine. I suspect it just missed
QA at the factory.

 

[dfkrug]

It may be me, but I don't see the wear you refer to on the port(s) etc. I do see subtle scratches on some metals you
showed and some darkening of the metal discs in others. Can you possibly circle the exact areas you're referring to,
if not too much trouble, for us feeble minded older guys?:confused3:

See yellow dotted line.

When I refer to a "fingernail" test, if I can feel any ridge with my nail, that is a failure. Also used
with engine cylinder bores, cylinder heads, other critical mating surfaces, etc.

 

[Billrog]

dfkrug thank you for your time and explanations I now have a lot more knowledge on what actually goes on inside all I knew before was the caused damage just not what the damage actually was. I've ran tractors with the remote hydraulics engaged before but only for a min. or so until I figured out the reason for pour hydraulic pressure to the loader or transmission.

 

[dfkrug]

I've ran tractors with the remote hydraulics engaged before but only for a min. or so until I
figured out the reason for pour hydraulic pressure to the loader or transmission.

I suspect a newer tractor with a good engine and an implement pump RV set at 2000psi or less is going to bog
down less on relief, vs. a tired older machine with a tired engine.

When I operated my old Kubota with the 3-pt cyl valve at the top (on relief), it was very noticable. My
tired old rental tractor with 2000h on it really bogged down, so I did not use it that way for more than
maybe a minute. A minute on relief should not hurt anything.

That was my first tractor, back in the 90s, and I was a CUT newbie.

 

[oldpilgrim]

I cannot hear/feel my tractor FEL or BH go into relief. It just stops working.

If I drive into a pile of dirt that I can't move, it will either spin the wheels or stall the engine. I sense no relief at all.

 

[dfkrug]

I cannot hear/feel my tractor FEL or BH go into relief. It just stops
working.

If I drive into a pile of dirt that I can't move, it will either spin the wheels or stall the engine.
I sense no relief at all.

That is the HST relief, not the implement pump RV. The latter is set at 2000-2500psi, and
will only affect the FEL, 3-pt, and AUX valves. The former HST RV is set at 4000-6000psi, and
you may not feel it go into relief. I don't always on my tractor.

This is an important distinction. Bob's tractor saw the implement pump circuit go to relief, not
the HST pump circuit.

 

[dfkrug]

If I drive into a pile of dirt that I can't move, it will either spin the wheels or stall the engine.
I sense no relief at all.

In the case you describe, your HST RV is not opening. For that to happen, try driving into
that immovable pile in M or H range. THEN, tell us what you hear. If you are bogging the
engine, then turn up the revs to max.

 

[C4Ranch]

Great thread! I've always wanted to see inside of HST. May not fully understand how it works but thanks for this!

As the easiest failsafe, wouldn't plugging in a loop hose be simple and easy? No need for any lockout plates, etc.