Basic Hydrostatic Transmission Question

[JackJ]

I'm wondering what the practical difference is between high range and low range on an HST when running at the same ground speed and the same engine RPMs.

For example, if I've got my engine at 3100 RPM (which corresponds to 540 PTO speed on my BX) and I'm travelling at 3 MPH (mowing speed), does the load on the engine or transmission vary depending on what range I'm in?

I ask because last night I mowed about an acre while in high range, but not intending to be in high range. (I always mow in low.) In hindsight I should have noticed a difference in how I was using the pedal, but engine temps and transmission temps (measured with an IR thermometer) seemed if anything a bit lower than normal, and certainly not hotter, even though it was a warm evening and I was mowing relatively tall grass.

Thanks for any insights.

 

[85Hokie]

The simple answer to your question is :

TORQUE.......
LOW (turtle) = slow go, but higher pushing/pulling force - torque
HIGH (rabbit) = faster speed, lower torque

this video is all about tires and why one is better for pure traction than the other....

https://www.youtube.com/watch?v=c9EVpXjzHsY&t=341s

however - it shows the "torque pressure" on the tires - which answers your question in a round about way.

 

[NoJacketRequired]

From an operational point of view, most of the HP you are using is being burned up by the mower. Very little HP is being used to move the tractor. IF you were mowing up a steep hill you might find you would need more power to be available to the wheels, thus low range might be more appropriate, but it's not likely until you're really pulling or pushing something that you'll find you need the torque of low range.

I mowed several acres this past weekend on the B2410 with 54" mid-mount mower. All of it was done in high range with no adverse effects, other than the shaking my kidneys received when I went too fast over bumpy parts!

 

[twomany]

Your engine turned fewer Revolutions during your mowing.

And the hydrostatic and mechanical transmissions experienced greater "stress" at the same time.

A hydro drive built around the "high" speed mechanical drive does not require a low speed option. But.. the low gear drive gives the operator great "feel" and control over low speed operations.

Think of it, The hydro unit puts out it's MAX torque output at the minimum fluid displacement position of the "go pedal". If the tractor can handle that, that's the end of the "wear and tear issue".

Bolens added the two speed rear end to the HT series as a market concession. Consumers wanted a low speed for tiller operation. Of course they could have just pressed the speed pedal less. Which is MUCH MUCH easier on the hydro unit.

Hydros thrive on lots of flow at the lowest possible fluid pressure.

That;s why one should always run a hydro tractor at rated RPM. Engines are cheap compared to hydro units.

 

[hagrid]

Basic Hydrostatic Transmission Question

I have a basic answer: my pappy has a 1974 John Deere 140 H1 with a Sunstrand hydrostatic transmission. No auxiliary reduction unit like Kubota has.

We change the filter once a year and have never changed the fluid (runs on type F automatic trans fluid). That tractor has 1550 hours of grass cutting on it and the transmission functions the same as when he bought it.

 

[JackJ]

Thanks for the insights. I was feeling like there was no practical difference between high and low range, other than ground speed.

But the torque discussion reminded me of an application where I do notice the issue: I've got a short but steep hill connecting two different sections of lawn. If I'm in high range, I hear the HST whine more than it normally does when I hit that hill, and indication that it is working harder to deliver the engine's power to the wheels.

 

[troverman]

Low range is a reduction on how hard the HST has to work. You will use more horsepower to move the tractor in high range than in low range. As you noticed, trying to go up a steep hill in high causes a lot of hydro whine and the tractor struggles. If you were mowing tall grass, you'd probably notice the mowing wasn't going well either.

Just use the range that "feels" best. If you think the tractor is being worked to hard, use a lower range. I have only ever used high range for driving fast with no implements operating. But then again I've never had a two range hydro, only three range units.