There's always an ongoing debate over the pro/cons of hydro vs. gear tractors. If hydro's go into relief, then there's a black mark for them.
I understand why a hydro would have a relief valve, but good god, shouldn't it be rated/set higher than the maximums torques that the engine could produce? (essentially protecting the tranny if someone made post-factory modifications like boosting the fuel pump or adding a bigger turbo etc..) That is, the tranny should be able to handle any torque the engine can dish out and more.
If I buy a 40 hp tractor, I'd expect that all the torque would be able to get to the tires and the tires should either spin or the engine should stall. I WOULDN"T expect the tranny to go into relief at some lesser value! Otherwise I haven't bought a 40 hp tractor.
This is like having a gear tractor and expecting the clutch to slip if the wheels get too much traction and the load is too heavy.
TOTALLY UNACCEPTABLE!
In such a configuration when the drive train can take full power. Now when the hydraulic loaded is operated the engine will bog. Now some operators are going to complain about an under powered tractor . Can't win.
Sure it is possible to design an HST for the loads that a gear can benignly support in an overload engine stalling condition. However; due to the nature of an infinitely variable transmission, it must be either made hugely robust, or include built in protection against itself. This protection thwarts extreme hi torque performance.
...Regardless of tires/traction there will always be a situation where they cannot be spun on any tractor. Universally, to my knowledge, this will happen first with a fluid drive. This is because you dont have a mechanical connection between engine and wheels - the connection passes thru fluid and fluid pressure is limited by a relief valve. So the torque to the wheels is limited by the combination of the relief setting and the displacement of the hyd motor being driven and then the final drive [geared] ratio to the wheels. This is the ratio you select when you change ranges.
With an HST, regardless of the relief setting the engine cannot be stalled without abusive operation of the pedal because you are varying the displacement of the hyd pump.
Without the relief, infinite forces would be available in any range just by adjusting the pedal, [and thereby pump displacement], so that the HP driving the pump would be able to deliver whatever pressure required to give the torque needed at the wheels. Of course the pressures would have to be proportionately higher in the higher ranges to get the torque needed at the wheels. Well,
darn it anyway, we cant just keep kicking the forces up thruout the powertrain like that. We have to limit each part according to its ability. We limit the pump/motor with a protective relief valve, and we make the gears in the ranges amply strong to take the [thereby limited] hyd motor torque. We adjust the range gear ratios to give usable speed and wheel torque combinations. This part of the hst tractor is exactly analogous to a gear tractor if you recognize that each gear ratio is a range.
>>Not surprisingly, on modern gear tractors that usually have at least 8gears, the ratios are much more closely spaced thruout than they are with hst which has just 2 or 3 gears/ranges to cover the full range of conventional speeds. So you dont find a lo range
gear ratio on an HST as low as 1st on an 8 speed. The hst will go faster in 1 of 3 than the gear in 1 of 8. That means the HST is effectively in a higher gear. And that means that the HST cannot deliver as much torque to the wheels if the relief valve is set safely. To realize this you have to take a close look at the interaction of the engine, pump, relief valve, and motor. Since the pump is variable from 0-100% the engine will be capable of driving it to deliver HUGE pressure at low pedal. Losses by leakage and other friction will limit the theoretical
infinite pressure capability. However not to a low enuf level that destruction would not be inevitable if the wheels didnt turn. The relief has to be set at a level low enuf so that it will relieve if the tractor is stopped dead in a pull. If it didnt something requiring repair would give.
The wheels would spin? In lo range maybe, but not in hi. If the relief doesnt give and the wheels dont spin the engine stops dead. This kind of impulse must be prevented.
,,,For maximum and "safe" power delivery it is done by setting the relief at a pressure that will stall the full engine power gracefully [say, about 1second] at full pedal. Somewhat more than full engine torque is being delivered to the pump in this situation, and an approx 2 or 3x multiple of engine HP is flushing fluid to the sump. The hyd pressure seen here is as high as it can go in the protected transmission. This pressure defines the max torque available from the motor that is driving the final ratio. Changing the flow with the pedal has only a tiny effect on this torque as the engine bogs. Because of the relief, backing off the pedal to let the engine work easy does
only that - the pressure and therefore torque limit at the motor remains the same. That motor is working against a higher speed gear than 1st on the gear tractor. From all this, it follows directly that the HST cannot deliver as much torque to the wheels as the gear. ... For example - if the
pressure protected HST goes 2mph at full pedal and rpm in lo range, and 1st on the gear goes 1mph topped out, the gear will deliver about twice the wheel torque. [And yes, a gear can be overloaded to failure] It is essentially an HST set for 1MPH and having no relief. But this only occurs in 1st on the gear. This setting is possible in any range on the HST. Whereas the gear would bog and stall while its clutch slipped a few revolutions, the HST operator could just lift the pedal and apply destructive hydraulic pressure without even bogging the engine. This is and must be prevented with the relief valve.
,,,With a gear, the higher overtorque limit allowed while the operator has real feedback foretelling the approaching limit lets him know where he begins to go into the overdesign safety factor, and know breakage is a multiple of the experienced lugdown torque. Theres lots to play with above the lugdown point and where the clutch will slip by design. This indication is not there with a HST and demands an automatic safety device. This causes an INHERENT deficiency in force capability of the HST. You want to turn up the relief? Fine, but realize the presence of the automated limit is part of the design.
Since it is a highly repeatable "instant acting" limit the designers have set it much closer to the ultimate system strength.
>>Suitable design for geared however, demands
high overtorque support with the
hard linked system because that engine is going to experience a hard stall in a pull, perhaps many times. This torsions the geartrain and buckles the tire sidewalls and maybe induces a few revolutions of clutch slip. The gear was not designed to do this.
It was designed to be able to survive it. You will never see such forces in proportion to its design goal in a system with an automated protective device. Turn your HST up 50%, and at load requiring that pressure you are operating beyond its safety limit with no indication. Turn it up 100% to make it able to deliver anywhere near the force induced in a gear stall and it might do it once ... Engine singing happily at lo pedal. But it made a new sound and now it doesnt seem the same.
..:confused3:
,,,larry
