HST differences between brands

[CTyler]

There is only so much flow and yes the geometry will be different as you step on the peddle. I did not make my statement very well, way to broad.

I don't claim to be an expert by any means but have a fair understanding of how it works.

 

[getut]

Ok.. that leaves just the single question. Is a hydro transmissions output torque a function of simply the input pressure and nothing else or is it a function of all factors (rate, volume, and pressure)?

 

[WhiteRock]

I sounds like you would like to view this mathematically. That is a good way to look at it. I recommend you review the hydraulic formulas.

The drive train is driven by a hydraulic motor. Torque this motor can produce is a function of pressure X displacement. So if you know the displacement and know the pressure, you can compute the torque. However...

The engine drives a hydraulic pump that drives the motor controlled by the hydro pedal. As far as I know there is no relief valve in this circuit. Therefore if you lock the wheels, the pressure will try to approach infinity until all hp has been exhausted from the engine which will promptly stall the engine. This is what I call a "hydraulic lock" (just my term, have no idea if it is a valid term).

Flow only comes into the equation when you start evaluating hp.

 

[keeney]

It is easiest to think of it as the pedal controling the displacement of the variable displacement pump. For each revolution of the input shaft, it can push a variable amount of fluid through the wheel-drive half of the system. At the neutral pedal position, the pump has zero displacement - no fluid is actually pumped in either direction.

As the pedal is depressed, the pump increases its displacement. Assuming constant input RPM's (mechanical power source not lugging-down yet), the amount of fluid per rev increases. Conversely, the torque required on the input shaft for a given output hydraulic pressure also increases. To get maximum output torque on the wheels, the minimum displacement position of the pump will produce the most output pressure for the least input torque from the engine. This is the lowest "gear".

In a perfect HST pump, this holds true all the way down to zero displacement. At you approach that point, you would have max output pressure with zero flow.

In the real world, the pump is not perfect. Some of the fluid slips by the pistons (or wahtever mechanism it uses) and avoids being pumped. At low output flow rates (wheels not turning) and high pressures (trying to push an immovable object), the leakage becomes more than the output flow of the pump. This limits the lowest ratio the pump can operate at for a given RPM. This leads to a maximum in the torque curve similar to the graph posted.

Additionally, there could very well be a pressure relief in the system, limiting the maximum output pressure. Also, there could be pilot operated valves where a minimum flow is required before full pressure is developed or some other mechanism as was described to help produce a dead zone so as to prevent the tractor from creeping when the pedal is released.

- Rick

 

[Rip]

Very well put Rick. The "leakage" or bypass flow (that portion of flow not producing work, just heat) also occurs in the motor portion of the circuit as well. It will also change as componants and oil heat up or cool down and with age (wear).

 

[whitetiger]

There is no valve to open when depressing your hydro pedal.
Max tourqe is with hydrostate pump fully stroked.

 

[getut]

Thank you very much Keeney. That was pretty much exactly as it was described to me from Kioti. It does in fact create the torque curve as I posted. The numbers are made up, but the graph is very similar to what you would see in real life from what I understand.

I just see so many postings that seem to indicate that HST increases torque the lighter that you depress the pedal. To a point this is correct, but not entirely. Assuming a steady engine RPM for the all of the following.... both speed and wheel torque increase together for the first 10-20% of pedal input. Wheel torque reaches maximum somewhere in that 10-20% range of pedal input then starts declining again. Speed continues to increase through the entire range of pedal input.

For my tractor, which is HST, the "sweet spot" speed at which I can get full torque to the wheels is what I would guess to be around 1MPH. If I go faster I lose torque, and if I go slower I also lose torque. I have hit that magical borderline before where pulling an implement that is requiring just about everything my tractor could muster, I would like to go SLOWER than 1MPH, but couldn't because I would stall the HST.