hydrostatic transmission

[csr0831]

ok guys...I'm about to ask ANOTHER stupid question. I have a tractor with a hydro (GR2100). I am concerned about over taxing the tranny. Lets take a hypothetical situation. I would not do this, but for illustration purposes....if I were to take a chain and secure my tractor to a tree, take up the slack slowly and then when it was tight, push the pedal to the floor, what is happening internally? Assuming no wheel slippage, are there clutches inside that are slipping, is it a pure fluid drive??? IF anyone knows, I would appreciate them enlightening me : ) Thanks!

Chris

 

[aczlan]

ok guys...I'm about to ask ANOTHER stupid question.

the only stupid questions are those that dont get asked.

if I were to take a chain and secure my tractor to a tree, take up the slack slowly and then when it was tight, push the pedal to the floor, what is happening internally? Assuming no wheel slippage, are there clutches inside that are slipping, is it a pure fluid drive??? IF anyone knows, I would appreciate them enlightening me : )

inside the transmission is a hydro pump, that is connected to a hyd motor, inline with this is a "pressure relief valve", if you were to do as you say, that valve will go into bypass mode and prevent too much pressure (probbaly more than ~2500psi) from building up.


does that make any sense???

for more info try here: Hydrostatic transmissions or here: Hydrostatic Transmission,Hydrostatic Transmissions

Aaron Z

 

[Fordlords]

I have seen puller guys remove the bypass valves on old Sunstrands and just plug the holes to see what they can actually do in the "more power" fashion. In that event when over stressed, they most often start blowing out oil seals until the thing no longer can move the tractor. In normal working mode on garden tractor hydros with the pump-line-motor setup they just don't over stress even under very heavy use, and under regular use will last for decades. The same does not hold true for the fully integral hydros found on light lawn tractors- they will not withstand much abuse and are often just about toasted at the end of their typically projected 500 hour lifespans.

-Fordlords-

 

[Bird]

Aaron answered correctly, but I'll add one comment. Having the relief valve open in that situation won't hurt anything for a few seconds occasionally, but if you just sat there with it forcing the relief valve open, the hydraulic oil will get hotter than normal; i.e., don't do things that cause the valve to stay open for extended periods of time.

 

[csr0831]

Thanks guys. As I understand it then, no clutches in the conventional sense. The reason I asked is that I've got a Jonny Bucket on the thing and when shoving it into a pile of hard packed dirt, the machine will come to a stop and sometimes the front wheels spin, and sometiems they dont. The tranny will whine loudly and I let up on it and take another approach. Based on what I have heard this type of activity on and occational basis will not do signifiant damage to the machine, correct? Thanks again : )

Chris

 

[Bird]

Yep, nothing to worry about. That loud whine is when the relief valve opens, and as you said, it's time to take another approach.

 

[dfkrug]

If your HST goes into relief, and you hear the telltale squeal, your HST
pump is pushing oil at max pressure thru a small RV orifice. This heats the
oil up very fast, and when it exceeds a certain temp, you will get localized
boiling, which then scores your pump's swash plate. I don't know the GR1200,
but my Kioti, which has a 2500psi relief for the implement pump, has a 4000
psi relief for the HST. If you were to disable the HST RV, you will probably
stall the engine first, even at low swash plate angles.

 

[TimS]

Some hydro tractors (like mine) have an implement relief valve made specifically for use with a loader. My manual indicates that if I use a loader with the regular, non-loader relief valve, it may damage the transmission (and voids the warranty). Might be worth checking if you're in the same situation - awfully expensive to replace a transmission.

Tim

 

[dfkrug]

Some hydro tractors (like mine) have an implement relief valve made specifically for use with a loader. My manual indicates that if I use a loader with the regular, non-loader relief valve, it may damage the transmission (and voids the warranty). Might be worth checking if you're in the same situation - awfully expensive to replace a transmission.

Is this a small garden tractor? I wonder what a "regular non-loader relief
valve" is?

A compact tractor will have an implement pump to power the PS, as well
as any implements and/or the 3-point. Downstream from the implement
pump will be a RV on the tractor somewhere, then usually a RV comes with
each spool valve, whether the FEL valves, or any remotes. On these CUTs,
the HST hydraulics are entirely separate, except for the reservoir.

 

[TimS]

Is this a small garden tractor?

Relatively large garden tractor - JD455

I wonder what a "regular non-loader relief valve" is?

The one that came with the machine.



Don't know what type of tractor the original poster was referring to; just trying to make sure he didn't bust something. I know someone who bought an aftermarket loader for his 455, didn't think to swap the relief valve, and needed a new transmission the following week.

Tim