How does an HST compare in pulling power with the same tractor and a gear trans?

[4570Man]

Absolutely. Weight is traction. Case closed? Well-l-l no when the development does not cover a common exception that is a norm for most of us. We have tractors equipped with loaders. We ballast and counterweight by necessity to improve the force application platform. This weight is more traction and more potential force ... if you can get requisite torque to the axles.

The feature of HST allowing infinite speed control from 0 to top speed of each range presents the possibility of "infinite"/huge wheel torque at creep speed in any range. Creeping speed requires so little of the engines power that the operator has no indication of the tremendous forces that may be being applied. Wisely, the HST designers include a relief valve between the output of the variable displacement pump and the hydraulic motor that drives the wheels through the range gears. So when the relief operates and the wheels wont turn you shift to a lower range. When the wheels wont turn in the lowest range that is the limit of motive force that the tractor will apply.

With a gear tractor the link between engine and wheels is fully mechanically defined. Engine speed and wheel speed are always directly proportional. There is no variable between engine and final drive that the operator can control. The engine feels wheel torque and the operator can gauge it by sound and gear down where needed. The hidden "infinite" torque possibility of a zero to xspeed is removed, so the automatic resetting "fuse" in the HST isnt needed. The wheel torque is limited by the gearbox amplification of clutch slip torque. Presumably the designers choose components with accommodating safety factors. Apparently so, at least in above creeper gearset reduction transmissions.

So, Which transmission on a comparably powered tractor will provide greater motive force (wheel torque/r) ? Absent a specially contrived test you can only find out by equipping the tractors with loaders and requisite counterweight and ballast for safety and prudence. Then use them on the high side of their loader capability. I think you will find as I have, that you will occasionally find the wheel torque limit of the HST tractor but never the one of the gear tractor. Indeed, the manual trans seems to spin wheels nonchalantly in 1st gear on loads it wont move.

If you chained the tractor wheels to the ground so they can’t just spin and chained to front end down so it can’t flip the gear tractor would absolutely make more torque. Probably so much so that it would self destruct. But if you did a more realistic test and chained both tractors to an unmovable object and tried to pull it both tractors will break traction and spin out making the drawbar pull very similar.

 

[SPYDERLK]

If you chained the tractor wheels to the ground so they can’t just spin and chained to front end down so it can’t flip the gear tractor would absolutely make more torque. Probably so much so that it would self destruct. But if you did a more realistic test and chained both tractors to an unmovable object and tried to pull it both tractors will break traction and spin out making the drawbar pull very similar.

Well, we could be a bit cautious and actually find out the true motive force limit of the tractor power assemblies. Weight the tractors heavily and pull a trailered load up a moderate hill ~ 20 degrees. Add more weight equally until the wheels stall. With the force limiting choices made in a 3 range HST I think the geared force limit could be as much as 2 to 1. ... The BIG challenge is how to make the tractor brakes strong enuf to control the limiting case.

 

[npalen]

Well, we could be a bit cautious and actually find out the true motive force limit of the tractor power assemblies. Weight the tractors heavily and pull a trailered load up a moderate hill ~ 20 degrees. Add more weight equally until the wheels stall. With the force limiting choices made in a 3 range HST I think the geared force limit could be as much as 2 to 1. ... The BIG challenge is how to make the tractor brakes strong enuf to control the limiting case.

Well heck, sounds like we need to set up a full blown TBN tractor pull!

 

[sd455dan]

Well heck, sounds like we need to set up a full blown TBN tractor pull!

make mine a 1961 Oliver 1900 MFWD for an Old school puller A big old Minneapolis Moline would also do. If it has to be a hydrostat a Ford 9030 Bi-Di would do nicely.

 

[4570Man]

Well, we could be a bit cautious and actually find out the true motive force limit of the tractor power assemblies. Weight the tractors heavily and pull a trailered load up a moderate hill ~ 20 degrees. Add more weight equally until the wheels stall. With the force limiting choices made in a 3 range HST I think the geared force limit could be as much as 2 to 1. ... The BIG challenge is how to make the tractor brakes strong enuf to control the limiting case.

A more realistic test would be to just put a regular weight on the wagon that’s within its safe working load and have both tractors pull it up the hill side by side. That’s the fairest way to demonstrate the power put to the wheels. Without the same gear ratio which they don’t have testing the absolute max torque doesn’t prove anything.

 

[5030]

4500+ hours never touched the HST or motor. Wore out most other parts......

How about 6200 meter hours on my open station M9 with a 12-12 gear transmission and wet pack hydraulic shuttle and only this last spring I checked the lockup pressure and it was still within WSM specs.

I fully expect it to outlast me actually.

 

[npalen]

I was curious if there would be a noticeable difference in engine RPM on my Kubota B9200 (22 HP) hydro with the clutch engaged and disengaged at various engine speeds. So I did a little test with the hydro pedal in "neutral" and there was a perceptible difference of maybe 50-75 RPM on the tach. I'm wondering if that difference would be very similar to that noticed on a typical geared tractor of that size. (I remember years ago always pushing in the clutch pedal when starting geared transmission vehicles to reduce the load on the starter in cold weather.)

My thinking on all of this discussion with regard to hydros consuming more power brings up questions like at what tractive load does the hydro begin to consume more power.

I was doing some tilling last week on a tee box at the local golf course and the second pass running deep was challenging the guts of the little B9200 even barely creeping along at nearly a full stop. I don't think the hydrostat is at any disadvantage in this type of condition.
Having said all this, I agree that gear drives and hydrostats all have their place and there is a lot of overlap regarding their capabilities.

 

[SPYDERLK]

A more realistic test would be to just put a regular weight on the wagon that’s within its safe working load and have both tractors pull it up the hill side by side. That’s the fairest way to demonstrate the power put to the wheels. Without the same gear ratio which they don’t have testing the absolute max torque doesn’t prove anything.

Speed is not the issue. Ultimate Motive Force from an equal but differently transmissioned platform is.

 

[LouNY]

Yep don't want that hydro having to move AND pull a load at a reasonable speed

 

[4570Man]

Speed is not the issue. Ultimate Motive Force from an equal but differently transmissioned platform is.

If that was the main goal just drop the gear ratio of the hydro trans. With enough gear ratio you can shear the axel shafts which is probably what would happen if you chained down the gear trans so it can’t loose traction. You could fix that problem with planetary hubs which most larger equipment has already.