HST Power Consumption

[CJONE]

The hst pump/motor combination, separate from any geared range mechanism, is infinitely variable within it's own range. So if you push the pedal down 1/3, for example, you are getting an "effective" gear ratio that is 1/3 of the highest available. (The available range can be played with by specifying different displacements for the pump/motor.) So effectively by moving your foot you are choosing an input/output ratio.

The "safe" limit in a hydraulic system is mostly determined by how much you want to spend on the robustness of parts. If you want to go all out and use super strong hoses, fittings, coolers, valves, pumps, etc., then you can use a relief that is much higher.

Combine those two facts and the result is that an hst system can be designed to put down as much torque as you want and effectively geared as low as you want. You want enough torque on the ground to slowly spin the treads of the biggest track hoes... it can be done. Maybe your BX can't do deliver as much torque as its geared twin. But please stop suggesting that there is some quality inherent in hst systems that automatically prevent them from ever transmitting as much torque as a geared system can. After all, a gear box must be engineered to handle some design goal amount of torque too, and as it can be, so too can an hst system.

xtn

+1 CJ1

 

[milkman636]

One other thing, alot of people are using the PTO HP rating for HP loss on a hydrostat saying like 1.5 HP that is simply not true. That measurement is taken without the hydrostat being used. It is just parasitic loss from the hydrostatic charge pump. The real figures are like what has been stated 20-30% so take that in to consideration. CJ

I'd love to see some proof of this.

CJONE, you are obviously well versed in hydraulic systems and I am not contesting your point in any way. When I was buying my last tractor I searched hi and low trying to find published drawbar hp ratings for any modern CUT. Drawbar testing would tell the true tale of hp loss under load.

This debate will continue forever until U of Nebraska or some other group revives drawbar testing for small tractors.

 

[brokenot]

This debate will continue forever until U of Nebraska or some other group revives drawbar testing for small tractors.

I don't think that's going to help either. A geared and an HST-equipped machine might very well end up with similar results at the drawbar. HST proponents will say that that "proves" the efficiency numbers hydrostatic components manufacturers themselves provide are false.

I'm gonna go ahead and bring up the example I brought up earlier about the guy(s) that thought they'd stumbled onto "vehicle efficiency nirvana" when it occurred to them to install small, efficient diesels into commuter-esque cars. They knew all about the efficiency advantages diesels provide, and they also knew that an infinitely variable transmission was the way to exploit and maximize what the diesels had to offer.

The big, (and strangely unforeseen), stumbling block was that while a hydrostat is an infinitely variable transmission, the major components, (i.e. the pump and motor), have inefficiencies "built-in". And those "built-in" inefficiencies more than made up for any efficiency the infinitely variable speed feature ever gave them.

Will an HST machine provide gobs of driving torque to the wheels? Yes it will. Do hydrostatic component manufacturers falsify their own hard numbers in order to make their bits and pieces look more inefficient on paper than they really are? Nope.

Is every machine that could be hydrostatically-driven not hydrostatically-driven simply due to cost? Nope. There are other trade-offs as well.

 

[crazyal]

I don't know if anyone else has noticed this but with my HST when I start the engine and it's running at a fast idle 1200~1400 rpm) if I move the loader or raise the 3pt and put a load on the hydraulic pump the motor will load down. It doesn't matter how slow I try to raise the loader it still loads the motor. But when you use the HST it doesn't unless you push the HST pedal to the floor (which would be like trying to drive in top gear at idle in a gear tractor. Just an observation.

 

[jenkinsph]

I don't think that's going to help either. A geared and an HST-equipped machine might very well end up with similar results at the drawbar. HST proponents will say that that "proves" the efficiency numbers hydrostatic components manufacturers themselves provide are false.

I'm gonna go ahead and bring up the example I brought up earlier about the guy(s) that thought they'd stumbled onto "vehicle efficiency nirvana" when it occurred to them to install small, efficient diesels into commuter-esque cars. They knew all about the efficiency advantages diesels provide, and they also knew that an infinitely variable transmission was the way to exploit and maximize what the diesels had to offer.

The big, (and strangely unforeseen), stumbling block was that while a hydrostat is an infinitely variable transmission, the major components, (i.e. the pump and motor), have inefficiencies "built-in". And those "built-in" inefficiencies more than made up for any efficiency the infinitely variable speed feature ever gave them.

Will an HST machine provide gobs of driving torque to the wheels? Yes it will. Do hydrostatic component manufacturers falsify their own hard numbers in order to make their bits and pieces look more inefficient on paper than they really are? Nope.

Is every machine that could be hydrostatically-driven not hydrostatically-driven simply due to cost? Nope. There are other trade-offs as well.

I am a proponent of HST for most compact and subcompact tractor uses. That said I am quite willing to accept the fact that there are ineffcientcies in the power train compared to a gear tractor. For me it is a trade off of slightly more fuel costs for easier operation while grading and smoothing with precision. I can easily see and would expect the HST to loose 25 to 30 % through its input/outlput shafts.

At the end of a day working in the trees doing close quarters work the additional fuel used is easily offset by the increased work output I can get done. Now when working in a larger open field the gear tractor is a good choice and would certainly be more efficient to use.

 

[dennis52]

One other thing, alot of people are using the PTO HP rating for HP loss on a hydrostat saying like 1.5 HP that is simply not true. That measurement is taken without the hydrostat being used. It is just parasitic loss from the hydrostatic charge pump. The real figures are like what has been stated 20-30% so take that in to consideration. CJ

OK so I finally get enough money together to buy the 45HP tractor that is recommended for the range of jobs on my size acreage.

Then I find out that 9 to 13.5 HP will be consumed by a HST transmission.

That's OK ; just spend even more cash to size up to a 55 to 60 hp to accommodate that loss of power.

But wait there, that size of machine ( HST or Gear) is, for me, just too large to work in those tight places: the narrow creek crossings and tree plantings that separate large open fields.

Firstly, I want the HP I need but I also want a size machine that can be operated efficiently and conveniently in a fair range of conditions.

 

[Redneck in training]

HST will do fine. The limit is the traction. You will run out of traction before you run out of power. If you do plowing gear tractor will run marginally faster for the same load. Gear tractors have losses too so the difference is not as big.

 

[xtn]

HST will do fine. The limit is the traction. You will run out of traction before you run out of power. If you do plowing gear tractor will run marginally faster for the same load. Gear tractors have losses too so the difference is not as big.

Just as I have told SPYDERLK not to make generic blanket statements that hst won't do the same work as a geared unit, so shall I tell you not to make generic blanket statements that they will. The points he makes CAN be true. It depends on the particular system design.

xtn

 

[Redneck in training]

HST is mature technology. Search TBN and you will find very few complaints about HST performance. People who have them, with few exceptions, like them. The difference between gear and HST is marginal at the worst.

 

[CJONE]

I'd love to see some proof of this.

CJONE, you are obviously well versed in hydraulic systems and I am not contesting your point in any way. When I was buying my last tractor I searched hi and low trying to find published drawbar hp ratings for any modern CUT. Drawbar testing would tell the true tale of hp loss under load.

This debate will continue forever until U of Nebraska or some other group revives drawbar testing for small tractors.

I specifically said PTO not DRAW BAR. So yes once you put the HP through a tractors powertrain either trans gears or HST they will be close but you WILL loose more through a HST at full HP. That is the key how hard you are running the HST. They are very efficient at low flow levels. The higher the flow/pressure the less efficient they are depending on their design. CJ