Broke Hydrostat twice? Why?

[paulfun9]

I have thought about this a bit since you posted. My final thought is that if you cant find a clear cut answer to why I would sell it before blowing an other one and replace it with something else. If temps really were the problem I would suggest that you would have noticed it running/working differently as well as smelled the fluid overheating which gives me the indication it could well happen again without you realizing it.

 

[MHarryE]

Usually if the problem is heat the oil viscosity drops too low to prevent metal to metal contact and things start to wear. I tried taking a picture of 2 hydrostatic transmission pistons from an overheated transmission but my iPad camera sucks at getting the detail. Anyway the result is often slowly degrading performance due to internal leakage. Yours have been sudden catastrophic failures. Even more unusual is that with your latest failure the tractor went fine in reverse but wouldn't pull itself forward. I don't know your particular hydro and it is much lighter duty than the ones with which I am familiar so it's hard to really define a likely failure point but if it were one of the ones I am familiar with the first failure would likely be a blown piston due to a material defect and the second one either a failed high pressure relief in the forward side or a plugged control/ other control failure. Any idea why the dealer happened to have a used hydro available? Seems unusual unless he rebuilds failed hydros during the winter to be available for customers like you who need one immediately during the peak season.

 

[Gregster613]

Bob, I, also, have a TYM273 and have not had any transmission troubles as of yet...knock on wood! I have 500 hours of mostly trouble-free operations. I have experienced several (numerous) times of engine overheating -- noticed through spitting out of the radiator overflow tube -- NOT via the temperature gauge!! The gauge "always" stays within the normal operation range?!? I have come to realize that I need to blow off (via air) the radiator "each and every" time I put it away after operating it!! I also "flush" out the radiator fins with water and I "always" include the oil cooler each time!! With both air and water -- as it is directly in front of the radiator -- it is covered with debris first!! This has been my normal ops procedure for quite awhile now and has thus been successful. It is a PITA bothersome task, but one that I gladly partake of rather than having $$$ breakdowns. Has worked so far -- again -- knock on wood!! I await any results you may learn of as to your 2 transmission failures!
Good luck. Greg

 

[Bob Christensen]

Thanks for your reply, paulfun9.
Selling is not a first option for me because my best dealer option is in Idaho, alooong drive for me.
Plus I couldn't get much in trade for mine.
I almost have to stay with TYM because I have a backhoe that wouldn't fit any other brand.
And the size/ power is perfect for me.
And my wife won't let me.

 

[Bob Christensen]

MHarryE,
I' not clear just how quickly the problem developed. I don't remember smelling hot oil but I don't know if I would recognize it anyway.

It can' have been the forward relief valve because I took it out and switched it with the reverse one and there was no difference. Reverse but almost no forward.

The dealer had a near new hydro because someone else burnt their tractor and traded what was left for a new one. There was no singed paint on it and I payed less than half new. It wasn't entirely a coincidence because I checked around and this was one thing that influenced me to go to Jordan.

 

[Bob Christensen]

Greg.
I've always wondered about the temp guage. After a brief warm up period it goes to the half way point and stays there no matter what. I will now surely adopt your cleaning regimen!

 

[dfkrug]

A few weeks ago, almost all forward motion stopped. Reverse was fine. Implements were fine.
I reversed the relief valves and still no forward.
Checked pressures:
Rear (backhoe) 1800 lbs - this is correct.
2 check valves at Hydrostat - 500 lbs each - should be 2500 lbs.
Feed pump check point - 125 lbs - correct.

Interesting problem, Bob. It is no secret that I love working on HSTs.

I never had a TYM, but would love to get a broken one. My initial thoughts:

Sounds like you have done some good initial analysis. Do you have the service manual?

The charge pump pressure of 125psi sounds about right, but the HST relief pressure
should be more like 4000-5000psi. To get that measurement, you need to put the
HST in high range, and drive it up against an immovable object. In reverse, since
you say that still works fine. What did you do to get the 500psi measurement? And
the 2500psi spec...is that from the manual?

Since REV still seems to work OK, your HST valve plate and slippers are OK.
Does your HST use a direct-acting swash plate-to-pedal connection, or a
servo?

 

[paulfun9]

Thanks for your reply, paulfun9.
Selling is not a first option for me because my best dealer option is in Idaho, alooong drive for me.
Plus I couldn't get much in trade for mine.
I almost have to stay with TYM because I have a backhoe that wouldn't fit any other brand.
And the size/ power is perfect for me.
And my wife won't let me.

I can understand that, so my next train of thought would be to find out exactly what went wrong with the old one even if it means paying for the place you had the repair done to tear into it and find the exact cause. Myself I just couldn't feel right about not knowing and having the potential for an other breakdown to happen. We use our tractors all year long and simply put I couldn't rest not knowing especially if it can be avoided or if its maybe even a simple fix. Many folks would be surprised at how simple the fixes were for some known problems with machines/cars /trucks over the years.
Good luck and I hope you don't have anymore problems with it.

 

[Bob Christensen]

dfkrug -

I do have a service manual (not that I'm a good mechanic and can actually understand all of it) it says:
"The hydrostatic system is a closed loop fluid power system that consists of a charge pump and a THI piston pump/motor assembly, which is driven by a flexible coupler. Attached to the flywheel. Fluid flow is controlled by changing the angle of the cam plate. This angle is controlled by the operator through a mechanical linkage"
Does this even answer your question?

I tested the pressure at the rear remotes, but I soon after gave up and brought it to a local mechanic/engineer who I have used previously.
He tested the hydro at the test ports and got about 500 psi.
I believe he followed the manual which states:
"Place range transmission shift lever in neutral. Start engine and run at full throttle. Slowly depress forward directional pedal. Gauge should slowly rise to approx. 130-170Kgf/cm2, 1848-2417 psi and relief valve will open with an audible squealing noise if the pressure rises to over 350Kgf/cm2. Repeat with reverse"

They go on to say that if charge pressure is good (it was) and you can't reach relief pressure, the hydro static pump is worn or damaged (but remember I got 1800 psi at the rear remotes).

So, I believe my local mechanic got about 500 psi at 2700rpm and expected about 18-2500psi and never tried to reach relief pressure of about 5000 psi.

Jordan was expecting something simple like linkage, driveshaft or such and were forced by their examination and tests to agree that the Hydrostat was shot.

One thing I forgot to mention was that in forward the tractor would make a noise like a pig killing a cat.
Now it is as quiet as I can ever remember.

Thank you for your interest.
Bob

 

[MHarryE]

Your transcribing from the shop manual proves it is a regular piston pump and motor hydro. Forget the rest of the hydraulics - steering, 3 point, any auxiliaries. They are powered by completely different pumps. In neutral, the swash plate controlling piston displacement is straight up and down - no fluid pumped. As you increase forward or reverse the swash plate is tilted to a degree equivalent to how far you push your pedal. Since the same major components are driving forward and reverse, it doesn't follow that you have normal reverse and no forward. That's why I suggested a problem in the control section. Now that you described the noise in forward, that does sound bad.

My engineering basic reference on hydrostatic transmission design is to use 30% of available power as the amount of heat needed to be rejected by the oil cooling system. Some of this is transferred directly from the reservoir (transmission case) to the air and the rest by the oil cooler. When we have tried downsizing in order to reduce cooling package size we always regretted the decision. That's why I have this sack of failed pistons here to remind me penny wise, pound foolish.