hydrostat vs gear hp loss

[kensfarm]

If you push the pedal down more you get more wasted flow and very little more push.

You get less torque & more speed the farther you push the pedal down.

 

[_RaT_]

You get less torque & more speed the farther you push the pedal down.

You really don't suddenly hit a wall where a hydrostatic suddenly stops because of a relief valve popping, in fact, I don't know if those talking about hydrostatics in such a way have ever used one or at least a Kubota. On both my L48 and L3830, both HST, I could and did kill the engine without ever "popping" the pressure limit much like you do with a gear. I am not even sure how the HST in the Kubota works other then to say much of what I see folks posting here seems inaccurate as to how mine operates. In medium and low I can kill the engine or spin the wheels by depressing the pedal. I would like to drive a L3830 with gears to see just how much more effort I get transferred to the wheels, something I do not doubt will be inevitable. On the otherhand, as was mentioned, it is easy to quickly recover by backing off or removing your foot from the hydro pedal.

 

[SPYDERLK]

You get less torque & more speed the farther you push the pedal down.

This does not fit with my experience. Deducing from the ones I have driven, I believe the pedal controls a variable displacement pump. From the operating characteristics it seems that this pump supplies flow to a fixed displacement hydraulic motor. The flow pressure rises according to the load placed on the hyd motor until the pressure exceeds the setting of the relief valve. At that point the wheels will push no harder and you have a happily running engine that cant get to the ground because the pressure to the hydraulic motor has hit a limit. What makes me think that the variable displacement pump interpretation is correct is that if you floor the pedal you do get a little more push as the engine bogs. This would be explained by the higher pump flow and the increased pressure needed to cause this added flow thru the relief valve. If the pedal controlled a variable displacement motor you would lose torque if you floored it like you said, but I havnt seen that effect. Its time for somebody to talk who really knows how these work.
Larry

 

[_RaT_]

If the pedal controlled a variable displacement motor you would lose torque if you floored it like you said, but I havnt seen that effect. Its time for somebody to talk who really knows how these work.

Hi Larry, I think that is how it works. The farther you push the pedal the greater the flow and the more load on the engine yet the less torque which I assumed was because the engine was losing RPM. I know the HST in the Kubota (and others I think) use a swasher plate to vary the flow. I think your right though, someone with a whole lot more understanding of how this stuff works is needed because I don't.

 

[Doc_Bob]

I have a TN70A with R1. I run out of power before my tires loose traction. Might be different if I had R4s. 70 HP with R4s might spin a little easier than R1s. The only time I spin is in very loose or wet soil.
Bob

 

[dieselsmoke1]

I don't have a lot of understanding, but I do have a lot of experience.

Lot's of folk's have raised the point that a hydro gives up drawbar HP to an essentially similar geared, and they are right.

A CUT, whether geared or hydro, should develop enough dbhp to spin it's wheels on demand. Personally, if I ever got one that would not turn the tires I'd get rid of it post haste.

One of the most significant features of an HST is that you don't "have" to spin the tires to get the maximum amout of force delivered to the ground that the unit, traction and conditions will allow. I have pushed mine to the ragged edge of breaking traction and held it there many times while other activities took place, such as a buddy sawing down a tree that was heavily leaning towards the house or working the bucket against a stump. No worries of spinning or of smoking a clutch, when the load and traction allows I will move forward.

 

[_RaT_]

I have a TN70A with R1. I run out of power before my tires loose traction. Might be different if I had R4s. 70 HP with R4s might spin a little easier than R1s. The only time I spin is in very loose or wet soil.
Bob

Boy thats for sure, R1's do indeed take more to spin and when they do, they make a hole fast.

 

[john_bud]

In a Kubota, the peddle controls a hydraulic servo piston connected to the variable swashplate. There are 3 main elements to the hydro, variable displacement piston pump, fixed displacement piston motor and a charge pump.

The charge pump runs all the time off the input shaft. The swashplate angle dictates the amount and direction of fluid pumped from the variable displacement pump (on the input shaft). The motor takes the fluid from the variable displacement pump and turns it into motion on the output shaft.


See the Kubota WSM page 3-NM10-12

 

[Soundguy]

Hmm... wasn't there a fellow with a kubota at the bottom of a hole that the kubota wouldn't crawl out of.. relief would pop.. tires wouldn't move, and engine didn't stall??? Was a couple years ago when this discussion came up. Anyone got that saved thread??

Soundguy

You really don't suddenly hit a wall where a hydrostatic suddenly stops because of a relief valve popping, in fact, I don't know if those talking about hydrostatics in such a way have ever used one or at least a Kubota. On both my L48 and L3830, both HST, I could and did kill the engine without ever "popping" the pressure limit much like you do with a gear. I am not even sure how the HST in the Kubota works other then to say much of what I see folks posting here seems inaccurate as to how mine operates. In medium and low I can kill the engine or spin the wheels by depressing the pedal. I would like to drive a L3830 with gears to see just how much more effort I get transferred to the wheels, something I do not doubt will be inevitable. On the otherhand, as was mentioned, it is easy to quickly recover by backing off or removing your foot from the hydro pedal.

 

[_RaT_]

Hmm... wasn't there a fellow with a kubota at the bottom of a hole that the kubota wouldn't crawl out of.. relief would pop.. tires wouldn't move, and engine didn't stall??? Was a couple years ago when this discussion came up. Anyone got that saved thread??

Soundguy

He apparently did NOT hit a pop off or relief. The only thing that was deduced was that the tractor was on a steep enough hill that the pickup tube pulled air (now that would be a very steep hill). The final conclusion was never 100% other then the tractor would not move but then, the engine would not load up either. Bad linkage as John Miller III sugests, maybe, we may never know. I know I have been in similar circumstances with my front end buried down hill to where it took all my bucket force and spinning tires to get me out. No popoff popping. Heres the thread for folks to browse.

http://www.tractorbynet.com/forums/showthread.php?t=20041&highlight=L4610