HST trans -- power relative to HP or hydraulic output?

[TSO]

So ... I have a question about HST transmission operations. Is the power put to the ground relative to the HP of the tractor? Or, does it have more to do with the system hydraulic output? I can presume that HP in a HST would still be directly related to PTO HP in the same way that a standard or shuttle gear drive system would.

I've had both shuttle & HST, and there's a clear & obvious difference in the power to the wheels. Not enough to make me switch back to shuttle of course, but it makes me wonder...

If two different tractors, with the same engine & HST transmission are equal in all ways except the hydraulic output, would there be a difference in the "power output" to the wheels? This would apply mostly to operations like plowing, pushing snow, etc.

 

[aczlan]

If two different tractors, with the same engine & HST transmission are equal in all ways except the hydraulic output, would there be a difference in the "power output" to the wheels? This would apply mostly to operations like plowing, pushing snow, etc.

IIRC, there is a separate pump for the HST (ie: an implement pump and a travel pump), so as long as you aren't using the hydraulics (and thus making more load on the engine) the power to the ground should be the same.

Aaron Z

 

[TSO]

IIRC, there is a separate pump for the HST (ie: an implement pump and a travel pump), so as long as you aren't using the hydraulics (and thus making more load on the engine) the power to the ground should be the same.

Aaron Z

I know they usually list total hydraulic output, then break it down between implement pump & steering pump. I've never heard of the travel pump.

 

[aczlan]

I know they usually list total hydraulic output, then break it down between implement pump & steering pump. I've never heard of the travel pump.

That's because the travel pump only provides flow inside the transmission and isn't relevant for those numbers.
Generally, the travel pump is a variable displacement pump which feeds a fixed displacement motor.
Here is a video showing how they are setup inside:

Aaron Z

 

[xtn]

Assuming 1:1 ratio of pump volume to drive-motor volume, the actual flow (volume of fluid pushed through) doesn't effect the ratio of flywheel HP to drive-wheel HP. Only the losses due to inefficiency do, same as mechanical drive line. Hydraulic setups are, at least generally speaking, less efficient than mechanical drive lines, so of course you get a bit less power at the wheels with one. But again, it's not directly related in any way to flow volume.

For example, you could design a hydraulic drive system with a very small volume pump and very small volume drive motor, and another system with a large pump and large drive motor. If they both held the same ratio, and had the same efficiency, they could both put the same power to the wheels. So just reading the gpm of the pump isn't telling you anything about how much power you'll get to the wheels.

And of course altering the ratio of volumes between the pump and the drive motor is like altering gear ratios. You can go slow and multiply power at the wheels, or you can trade power for speed, just like with gears.

xtn

 

[TSO]

So with my LS R4047 vs 4041 ... they use the same components, just one is tuned for higher HP output. But from what I understand, the tractors are identical otherwise. Are they putting the same "HP" to the wheels then? (HST only)

 

[aczlan]

So with my LS R4047 vs 4041 ... they use the same components, just one is tuned for higher HP output. But from what I understand, the tractors are identical otherwise. Are they putting the same "HP" to the wheels then? (HST only)

It depends. If they bumped up the relief pressure in the drive hydraulics, the R4047 would put more power to the wheels. If the relief pressure is the same, it might be able to go faster with the same load (due to the engine not bogging down) so it could be in that case too.

Aaron Z

 

[Gary Fowler]

My experience with HST (B26 Kubota and NH 2030) is that the engine will stall before the pump goes to relief in most cases or if in L range, the wheels will spin before the pump goes to relief. Having more HP at the engine (via highly tuned /turbocharged etc) even if everything else is the same will allow you to go a bit higher on the torque prior to the engine stalling.
Just my observations.

 

[xtn]

So with my LS R4047 vs 4041 ... they use the same components, just one is tuned for higher HP output. But from what I understand, the tractors are identical otherwise. Are they putting the same "HP" to the wheels then? (HST only)

Assuming you don't go into relief condition, the higher HP engine should result in higher HP at the wheels too. Efficiency losses can be considered as a percentage of input. For example 50HP might result in 40 at the wheels, and using the same components 60HP would then result in 48 at the wheels. Well, that assumes the same efficiency. The reality is that a particular hydraulic setup has an efficiency "curve" meaning that the number might vary a bit under different conditions, such as temperature, pressure, viscosity, etc. So, for example, the higher HP engine might warm up the fluid a bit more, turning it less viscous, resulting in slightly more internal bleeding, and ultimately having reduced efficiency under those conditions. So it's possible the 60HP machine would only get 47 at the wheels instead of 48. Or, it could be getting better efficiency because at the higher temp the machined tolerances are tightening up, resulting in less internal bleeding, giving that 60HP machine 49 at the wheels or something. Just depends on the specific component selection and conditions. But basically just consider it a flat percentage and you'll be close enough. The higher HP machine will have higher HP at the wheels, although still reduced.

xtn

 

[TSO]

Ok, I guess that all makes sense. I suppose a better way to have asked my question would have been to say, does HP mean as much to a hydro as it does to a gear when it comes to putting power to the ground.

It also makes me wonder if there's much difference between manufacturers when it comes to this same topic. for example, when I was looking up specifications, while shopping for different cab tractors in the 50 HP range, I noticed that the Massey tractors had much better hydraulic flow ratings than most of the other tractors in the group. Made me wonder, is the 47 HP Iseki able to put more power to the ground than the 47 HP Mitsubishi that's in my tractor, both HST, considering that the Massey hydraulic system runs at about 20 percent higher flow rate than my LS.