k0ua
Epic Contributor
Lets define some terms. Lets start with the "ranges". There is an old unsynchronized standard transmission behind the hydraulic drive motor. There are either two or in most cases three gear ranges. Mine has the three. LO is a pretty low gear, with wide open throttle and full pedal depression the tractor will go from 0 about 5 miles an hour, but has boatloads of torque. You can't stop the wheels from turning no matter how big the load is or if your FEL is press up against a 3 foot diameter tree. You just can't stop them. They will always turn. If that means digging to China, they will dig to China. MED range with full throttle will go from 0 to about 10 miles an hour. I has good torque, but with loaded tires, and a very heavy ballast on the back in 4wd, it may or may not spin the tires when up against an immovable object. Mine will just barely spin them on gravel if pushing up against that 3 foot diameter tree. What happens is the pressure in the hydrostat gets so high because you are so "uphill" with you actual metal gears that the relief valve on the hydro opens up to relieve this very high pressure. Over 5000 psi by the way. The motor will bog down some, but it is not like the pure mechanical connection of crankshaft to clutch facings to manual transmission to differential to wheels/tires. In HI range, you can go from 0 to just a little over 15mph. (note all these speeds are general, each tractor is different in its gear ratio's and tire sizes) In HI, there is no way you can even begin to spin the tires up against an immovable object. The hydro relief valve will just open and squall. You don't have much torque at all. You would never want to do any pushing work in this range. This range is purely for transport of the tractor down fairly level roads. You get top speed, but not much torque. If you were pulling a loaded wagon up a hill, you might not make it up that hill if it is steep enough. The engine will bog pretty badly, but if the hill is steep enough or the wagon heavy enough the relief valve would open from very high pressure.
Now, what does the pedal do? Well the pedal/pedals are either directly linked or hydraulic assisted linked (depending on size of the tractor) to the swash plate in the hydro. You are simply changing the angle of the swash plate when you press the pedal forward or the pedal is in its neutral position which it will return to by itself with no input or if you press the reverse then the swash plate angle changes in the negative angle.
So what does changing the angle of the swash plate have to do with anything. Well the "slippers" ride on the swash plate and the pistons as they revolve around driven by the prime mover (the engine) are attached to the slippers, and they either have a longer or shorter stroke as the pistons come around to either the suction or the discharge ports.
So lets say that as the slipper/piston is climbing up the steep angle of the swash plate, it takes considerable force from the prime mover to make that piston climb up that hill to force the fluid into one of the ports we will call discharge for now. but if you back off of the swash plate angle the pistons will have an easier time of making it around and this will reduce load on the engine at the expense of peak pressure and a lowering of speed. But this will allow the engine to speed up because you have removed some of its load and if you can keep the engine in its torque band instead of being bogged down below it, you have bettered your actual progress up the hill. Of course if you knew your tractor better you would have shifted the gear transmission to MED before you started up the hill so you would have had sufficient torque and still a good speed.
When you press the reverse pedal, you move the swash plate to the other negative angle, and now the discharge/suction ports swap functions and the hydraulic motor move the opposite direction. All else remains the same, as what ever range you have selected by the manual transmission it is still in that gear. So if you need good torque and good speed, stay in MED range (gear) That way you can for instance move your bucket into the "pile" and get a full bucket, because you have good torque, and you can reverse in an instant and move fast doing it, and if you need to transport a good distance you can have good speed during transport of that full bucket.
If you need to move slower and more precise (like your pallet adventure) put it in LO, you can move at speeds that inchworms are passing you, but you have boatloads of torque (like to push over trees or rocks), and you still have a reasonable speed avaiable up to about 5MPH.
So it is just like a manual transmission, you are trading speed for torque, but you have infinite speed adjustment in each range.
Since the engine is not directly mechanically connected to the rear wheels only a fluid and pressure coupling, there is a loss of power. There is some heat generated. (there is an additional radiator on larger units or a fan on smaller units). But the amount of control, the precision of having just the exact ground speed at the amount of torque you need is just such a beautiful thing to have.
All of these explanations I have attempted here ( and probably rather badly) could be superseded by you operating my tractor for about 2 hours doing various tasks on my hills. You would have a far better understanding of all the relationships of speed torque and ranges. By the way there is no clutch on my particular tractor. Some hydro tractors have them still to engage the PTO, but that is another discussion. Also we have not even begun to explain the Hydrostat Plus of the Kubota grand L series of tractors. They have several enhancements to make operation more productive and easier. But I have "wrote a book" when you just asked a couple of questions so I will stop here.
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