HST Power Consumption

[Redneck in training]

Every design is a compromise. At the end it boils down to bang for the buck issue. HSTs are popular for a reason. They can do almost everything as good as geared tractor but with lower efficiency resulting in marginally lower speed at given load or in higher fuel consumption. As I see it the limit how much force the tractor can produce is function of traction. So if you have either gear or HST system that can produce higher torque that what the wheels can transfer to the ground you will spin the wheels. In opposite case you will stall the engine or the relive valve will open. If you ballast the tractor to such a degree that the traction is always greater than what is the capability of the drive you stall the engine or something will break or relive valve will open. HST is protected by the relive valve but gear tractor is not. Then the protection is good judgement of the operator.
In example guy adds turbo to the engine to get more power, ballasts the tractor and tries to pull a tree out of ground in reverse and wonders why the shaft driving the front wheels broke. HST will relive and safe him the headache.
My conclusion is that HST is easier tractor to operate for most tasks at hobby farm or acreage. It excels at FEL work, mowing and any task where load on tractor varies (in ex. Taking a load of dirt where part of path is uphill). With HST you just ease on the pedal and keep going, with geared tractor you might need to stop and shift. HST loses the competititon at tasks requiring steady high pull. Like plowing.

 

[SPYDERLK]

I don't agree. Just as a gear and clutch system can be engineered to transmit torque without failure to some multiple of maximum engine output, so can an hst system be designed to transmit that same torque value without relieving. They need only be designed to the SAME output delivery spec.

The only inherent peak torque deficiency is one of efficiency. But you said we could disregard that for purposes of arguing potential drive system capability. Certainly with the same engine input maximum the hst will output less torque no matter how high a hydraulic pressure you design for, because they are less efficient. I didn't think that's what we were arguing. I though you suggested there was some inherent weak link in any hst system requiring it to give up before a geared system due to some imagined restriction on relief pressure... which just isn't so.

xtn

That is not design to a steady state sustained spec, but a design to match the peak force of the gear in a steady state manner. ... it has been designed as a higher force system to augment its inherent peak force deficiency. If higher sustained force is the aim then the gear would just be designed to have lower gears to shift to in all cases.
larry

 

[xtn]

That is not design to a steady state sustained spec, but a design to match the peak force of the gear in a steady state manner. ... it has been designed as a higher force system to augment its inherent peak force deficiency. If higher sustained force is the aim then the gear would just be designed to have lower gears to shift to in all cases.
larry

Okay, you're saying that the spread between design sustainable load and peak load is different for hst than for gears. Fair enough. But so what? You design a higher sustainable load into the hst and result in equal peak load capability. You're saying that's somehow a cheat? I can't think of any rule it breaks. The point is that an hst can be designed to perform as good as a gear. You have suggested that due to some inherent limitation it can't, but it can.

Besides. the hst could just be provided with a lower ratio also. So what? You don't think a designer could play around with the range selector gears, or even the ratio of displacements of the pump and motor, to achieve any possible gearing he felt was required just like the designer of a geared tractor can?