This is a great thread! Excellent detailed discussion.
I have an old Power-trac, which is hydraulic drive. This machine is articulated (steers by bending in the middle), so you can probably guess that designing a 4WD articulated tractor with a mechanical tranny would be quite complicated. Power-trac chose to make it completely hydraulic, which is a good choice, but also has some disadvantages. Essentially, there's a variable-displacement swash-plate type piston pump that is turned by the engine. It's fed by the reservoir with a filter just ahead of the pump input...there's no charge pump, so you have to bleed this HST pump each time you change the hydraulic filter (not difficult, just has to be done). There are then hoses in a series/parallel configuration that go to hydraulic wheel motors at each wheel. This is a pretty cool setup...this configuration gives essentially a hydraulic differential, so it's really easy on turf (particularly when paired with articulated steering) but still provides good traction and full-time 4WD. The wheel motors are simple vane-type hydraulic motors, if I'm not mistaken. I *believe* the sides are in series (front and back), then each side in parallel....this would make sense with the articulation, since at least one wheel on each side would always be in contact with the ground. Direction and speed are controlled by a foot treadle, which is connected to the swash-plate lever on the HST pump by something called a "hydro-back", which is essentially a souped-up throttle cable...
There are a few disadvantages to this skid-steer like hydraulic-only transmission setup though. Since there's no gearbox, so there's only one speed range...0-8 mph. There's no way to put it in "neutral" short of connecting a jumper hose between two ports on the HST pump (and my old machine doesn't have those jumper ports...made for some difficult loading once when I had an ignition system problem and had to get it to a shop!).
So, this is very similar to the HST drive on a more traditional CUT except that there's no gearbox and mechanical differential, and the components are actually seperate rather than all in one housing. Or put more accurately, the hydraulic parts of it are very similar.
So, to throw some thoughts at the original question, let me try to describe my understanding of what this variable-displacement pump looks like on the inside. There's a plate (the swash plate) connected to the input shaft. Several pistons are connected to this plate by swivel joints, and the pistons sit in a cylinder block which also spins with the plate. The plate itself can swivel one direction or the other. When the plate and cylinder block are spinning perpendicular to the shaft, the pistons aren't moving in and out of the cylinders and thus don't move any oil. When you change the angle of the plate, then the pistons do slide in and out of the cylinders and they do move oil...the greater the angle, the greater the volume of oil they move, thus the continuously variable speed. Reverse the angle of the swash plate, and the oil flows in the opposite direction.
So, to my understanding, these variable-displacement pumps (and any piston pump for that matter) are very sensitive to having air in the suction hose. If the pistons pump air rather than oil, they don't get lubrication and cooling...this is called "cavitation" although I'm not sure where exactly that name came from, and according to Power-trac will burn these pumps up in under 60 seconds at high rpms. That would be what we've called the "catastrophic failure".
No hydraulic pump ever operates at 100% efficiency...I think most operate at around 85-90% when new, maybe some get as high as 95%. So, even on new systems, there will be some inevitable wear and tear, and energy lost as heat. That's the trade-off versus mechanical. That said, my Power-trac is 15 years old, and judging by it's condition when I bought it, it had the crap beat out of it and was poorly maintained. (I had to replace one of the wheel motors, and the guy who attempted to rebuild it said that it was scored internally as if a piece of metal had been pumped through it!) Surprisingly, the HST drive pump and the other gear pump that runs cylinders all seem to be in great shape. I've pulled a 5x10 trailer loaded with 2000+ lbs of sand or gravel into my backyard with this 1200 lb machine on a couple of occasions. Either all 4 wheels will spin or the engine will bog down if I try to do something that's just too much for it. It sure seems like that HST pump is holding up well, so that's really encouraging, both for me and for folks with more traditional CUT's with similar HST pumps. /forums/images/graemlins/grin.gif
Dave
