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#4457315 - 07/12/17 02:17 PM Kubota Hydrostat - what range?
Alex_V Offline


Registered: 04/10/16
Posts: 1155
Loc: Campbellsville, KY
I regularly operate several pieces of Kubota equipment with hydrostatic transmissions. In a system such as this, which puts the fluid under more stress at the same ground speed: wide open pedal in low range, or, say, 1/3 pedal in high range?
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#4457332 - 07/12/17 02:41 PM Re: Kubota Hydrostat - what range? [Re: Alex_V]
Rand Offline


Registered: 08/20/03
Posts: 12641
Loc: NE,Ohio
I wouldnt be worrying about "stress" on the hydraulic fluid.
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#4457365 - 07/12/17 03:11 PM Re: Kubota Hydrostat - what range? [Re: Alex_V]
dlundblad Offline


Registered: 09/30/13
Posts: 10028
Loc: Indiana
The pumps speed depends on the throttle. Hydrostatic transmissions are ran by a disk with hydraulic cylinders running 360 degrees around the disk. (For a rough visual, think of a wind chime with long chimes parallel to the ground. The chimes being the cylinders and the top being the disk.)

As the angle changes, so does the speed. The more severe the angle, the faster the speed. If the disk is perpendicular with the ground (or driveshaft depending on the design), the vehicle isn't moving. That said, I don't think the fluid would ever know what stress is. Then again, the faster and longer it's moving, the hotter it gets.


Edited by dlundblad (07/12/17 03:13 PM)
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#4457418 - 07/12/17 04:17 PM Re: Kubota Hydrostat - what range? [Re: Alex_V]
Syntheticuser Offline


Registered: 06/11/17
Posts: 147
Loc: Waco, Tx.
Alex,
In my experience, with a little 3011 tractor, low range and rev'ed high produced the least slippage and ran cooler than high range light throttle, which produced the most slippage and heat. Change the fluid often, they can run super hot when under load like dragging a plow.
Similar to a auto car's torque converter I think. If it's not locked up, the converter produces the least slippage on the rev limiter...ergo the higher you rev it the less it slips and generates heat.i used torque pro and my Focus to verify this about an auto trans.

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#4457512 - 07/12/17 06:09 PM Re: Kubota Hydrostat - what range? [Re: Alex_V]
doitmyself Offline


Registered: 06/03/02
Posts: 6217
Loc: MI
Originally Posted By: Alex_V
I regularly operate several pieces of Kubota equipment with hydrostatic transmissions. In a system such as this, which puts the fluid under more stress at the same ground speed: wide open pedal in low range, or, say, 1/3 pedal in high range?


That's an interesting question that I don't know the answer for. I think that as long as you have proper engine RPM for the HST system to run and cool correctly, it won't matter too much regarding fluid stress in either scenario (just my opinion). You get more torque in low range and more speed in high range. I was taught to run the engine rpm at a minimum of 1700rpm to ensure proper HST cooling and operation. I generally don't run mine lower than 2000 rpm when doing work. You adjust the engine RPM above 2K, select low vs. high range, and foot pedal to match your work conditions (Low speed high torque plowing vs. high speed low torque road driving.). You don't want to lug the engine AND you do not want cavitation and excess HST heat with too low of engine rpm (with resulting low HST pump circulation).

Hope I'm not preaching to the choir here, and you are just asking your question, already knowing about proper engine/HST operation. Maybe it will help someone else here new to HST operation. It looks like the newer Kubotas have an ECM that adjusts engine speed and HST pump operation somewhat automatically.

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#4457561 - 07/12/17 06:58 PM Re: Kubota Hydrostat - what range? [Re: Alex_V]
Syntheticuser Offline


Registered: 06/11/17
Posts: 147
Loc: Waco, Tx.
I've had hydros get so hot working, when unhooked, they barely move the tractors under their own power. Keep em'cool and change the fluid regularly. Again, high rpm, low gear, light pedal position...if possible.
Experience with:B3011 Kabota compact tractor
46hp John Deere compact tractor
1970s International 866 Hydro 6 cylinder.- this tractor was crazy fast...
The bigger ones I use are not hydros.

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#4458104 - 07/13/17 10:35 AM Re: Kubota Hydrostat - what range? [Re: Alex_V]
Alex_V Offline


Registered: 04/10/16
Posts: 1155
Loc: Campbellsville, KY
Ditto on the higher flow concept. I'd have guessed that, too, but I suppose the operating principles of the hydraulic pump that generates the pressure in this case made me doubt what I know about fluid temps in auto applications - that maybe the higher flow>better cooling through the hyd. motor would be mitigated by higher flow and friction through the pump to satisfy flow needs. No worries about lugging here, either - when the engine gets loud and bogs it's time for more throttle.

The main machine in question is a 3-4 year old L4400 that's used for mowing duty 90% of the time with a 3 pt. Woods 7 ft., rear discharge finish mower. So it runs 2,300-2,600 RPM the whole time, but the best ground speed can either be obtained at full pedal in low range, or about half pedal in mid range. Sounds like full pedal in low range might be better. There's also an older diesel/hydro UTV (1,700 hours) that sees some use spraying weeds at a very slow, but consistent pace, and at times it's a draw whether to run about 1/2 throttle in low or barely off idle in M. The 3 cyl. engine right behind me makes me prefer idle in M, but oh well. :P

Sounds like I should do the trans. fluids in all of them at some point. They all get the filter changed every 50-100 hours, but AFAIK the fluid has just been topped off until now.
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#4464153 - 07/19/17 04:11 PM Re: Kubota Hydrostat - what range? [Re: Alex_V]
CT8 Offline


Registered: 10/09/14
Posts: 10967
Loc: Idaho
How does the 7 ft mower work? I may be buying a new mower and a 7 foot with 1 gear box vs an 8 foot with 2 gear boxes and blades are on my mind.
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#4465866 - 07/21/17 08:18 AM Re: Kubota Hydrostat - what range? [Re: Alex_V]
DriveHard Offline


Registered: 11/24/03
Posts: 1063
Loc: Middle of Iowa
Lots of mis-information on here...

If it is a true hydrostatic transmission with axial piston pumps and motors, they operate most efficiently at higher displacements. We are constantly proving over and over again that low engine speeds with the pumps at higher displacements operate more efficiently. If they are operating more efficiently they are changing less of the engine power into heat, and you are getting more power to the ground.

This does not mean that the tractor has the capability to cool the hydraulic fluid at that low speed though. If the fan speed is dictated by engine speed, then everything might be running hotter simply due to reduced air flow through the coolers. I also don't know if your loop flushing is engine speed related either...so you might be getting less cooling fluid into your loop at lower engine speeds.

If you have the service manual and can shoot me some pictures of the hydraulic schematic I can give you some more details on how your system works.
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