HST Question/Concern....

[MHarryE]

Not sure about Kioti but some hydros have a pressure override valve that destrokes the swash plate when it approaches max pressure. The result is very low flow at high pressure. Since power is a function of pressure and flow very little power is used. It makes new owners concerned that something is wrong because they can push against a wall and the engine doesn't lug down. Beyond that some hydros have calving the destrokes the pump when it senses the engine is being pulled down below a selected speed and destrokes the pump so the engine will maintain peak operating power even if you mash the pedal to the floor. Bosch-Rexroth, probably the worlds leading supplier of hydros by monetary volume, calls this their DA valve. Now most suppliers are going to computer controls on the large hydros but purely mechanical controls like the POR and DA are still common in the less expensive machines.

 

[DT86]

I'm not sure that's the way it works.

 

[arlen4720]

Beyond that, some hydros have valving that destrokes the pump when it senses the engine is being pulled down below a selected speed, and destrokes the pump so the engine will maintain peak operating power even if you mash the pedal to the floor.

That's how John Deere's LoadMatch works in their eHydro transmissions.

 

[xtn]

Any pump with a tilting swash plate will push more volume at a higher tilt angle, but can make more pressure at a lower tilt angle. This is why you can get more torque at the wheels if you lift off the HST pedal some. Of course there are some other factors that weigh in to the equation of whether or not this will work for any one specific tractor in any one specific set of circumstances: whether the rest of your system is up to it, whether you're making such low volume that you're not exceeding the leak-back rate of the pump, whether your relief setting is high enough to make use of it, etc. But the basic premise is true.

xtn

 

[dfkrug]

Some HSTs have an electrically-controlled dual-angle swash plate on their hydraulic
MOTOR, e.g. Kubota's HST+, JD's eHydro, etc. I am not aware of such a system on
the tractor's HST PUMP.

 

[SPYDERLK]

Any pump with a tilting swash plate will push more volume at a higher tilt angle, but can make more pressure [ ___ ] at a lower tilt angle. This is why you can get more torque at the wheels if you lift off the HST pedal some. Of course there are some other factors that weigh in to the equation of whether or not this will work for any one specific tractor in any one specific set of circumstances: whether the rest of your system is up to it, whether you're making such low volume that you're not exceeding the leak-back rate of the pump, whether your relief setting is high enough to make use of it, etc. But the basic premise is true.

xtn

At a given drive HP.

 

[xtn]

Some HSTs have an electrically-controlled dual-angle swash plate on their hydraulic
MOTOR, e.g. Kubota's HST+, JD's eHydro, etc. I am not aware of such a system on
the tractor's HST PUMP.

Well, I don't know if they're electrically controlled or dual-angle or any of that stuff, but I think most HST pumps in use on tractors these days have a variable swash plate. Yes... in the pumps. But even if the pump were constant volume at a giving input rpm, and the variable swash plate were only in the motor... the resulting output at the wheels would still be subject to the same effect.

xtn

 

[arlen4720]

Xtn is right. All of the tractors that I know of, including the JD eHydro are a variable displacement pump coupled to a fixed displacement motor. Here is a hydraulic schematic of an eHydro.

 

[dfkrug]

All of the tractors that I know of, including the JD eHydro are a variable
displacement pump coupled to a fixed displacement motor.

The "LoadMatch" feature of your eHydro HST is the system that changes the swash plate angle of
the HST drive motor. The computer monitors various things like engine RPM, go-pedal position,
etc, and automatically changes the HST output. On some JDs, you can't even turn this feature off.
New Holland has this feature, too, tho I don't recall the trade name. Kubota's is "HST Plus".

If you have your service manual, look up the LoadMatch info. I don't know if ALL eHydros have
the LoadMatch feature, but it is integral to JD's HST.

 

[arlen4720]

The "LoadMatch" feature of your eHydro HST is the system that changes the swash plate angle of
the HST drive motor. The computer monitors various things like engine RPM, go-pedal position,
etc, and automatically changes the HST output. On some JDs, you can't even turn this feature off.
New Holland has this feature, too, tho I don't recall the trade name. Kubota's is "HST Plus".

If you have your service manual, look up the LoadMatch info. I don't know if ALL eHydros have
the LoadMatch feature, but it is integral to JD's HST.

What you are saying is right except the part about the hydraulic motor varying its displacement. It is the pump that changes its swash plate angle. You can see that in the schematic that I attached. LoadMatch compares thottle position with actual engine RPM, then decides if you are lugging the motor, and if so, reduces the current to the servo valves that control the swash plate angle of the pump, which of course, destrokes the pump.
The reason that the variable pump/fixed motor is popular is becuase it will produce a "constant torque", variable speed and power system. A fixed pump/variable motor would give you a constant power system, which, as you can imagine, wouldn't be real controllable.