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Why would thinner oil flowing thru the hydro motors contribute to the heating of the oil? It seems to me the reverse would be true since the motors, in that situation, would be doing less effective work and therefore not creating as much heat ???
OnTheWeb )</font>
Absolutely yes...almost all of the heat generated in a hydraulic system is because of fluid being forced past the sealing surfaces...ie...high-pressure blow-by. (A small percentage comes from the friction of the oil rubbing against the walls of the hoses and against itself as it flows through the system, but this is pretty minimal.) This generates heat, which makes the oil thinner, which in term makes it squirt through the seals easier....you get the idea, it's a downward spiral. The efficiency rating of a hydraulic wheel motor (or any other hydraulic actuator) is essentially an indication of how much oil is allowed to leak by. 90% efficiency means that about 10% of the oil squirts past the sealing surfaces. That 10% loss in ineffeciency shows up as heat, just as it does in all real-world systems that are less than 100% efficient. In theory, if you could find a "perfect" wheel motor (100% efficient), there would be no heat generated.
I definitely agree with the synopsis that, when going up an incline, the front wheels have less traction than the rears. When you carry something heavy up stairs, the person on the bottom sees about 2/3 of the weight while the person on top bears about 1/3 of the weight (assuming that the person up top can pull just as easily as they can push). On a 45 degree slope, I'd expect the rears to see 2/3 of the weight and thus have 2/3 of the traction. Since the left-front and left-rear wheel motors are in series, the rear wheel motor is essentially seeing twice as much pressure as the front wheel motor...so it certainly sounds like it's reasonable for it's efficiency to decrease (ie. more oil squirt by) under these circumstances. Even moreso true when the front wheel starts spinning...then the rear wheel is seeing probably 80-90% of the pressure! (Recall from college physics that, regardless of the surface in question, the coefficient of sliding friction is much lower than the coefficient of static friction for the same surface...in a nutshell this means that it takes much less force to keep something sliding than it does to get it started sliding in the first place. This would mean that the front wheel is seeing very little pressure when it's spinning. Think about it...this makes perfect sense.) I'd imagine that this sort of thing will create excessive wear on the wheel motors, and it's probably best to avoid it where possible. I'd definitely agree that this is either a design flaw, or it's a conscious cost vs. performance decision.
I still don't fully grasp exactly all the physics behind the parallel/series wheel motor plumbing. It's an open-center hydraulic system, so oil flows straight through and back to the tank when you're doing nothing, and there's very little pressure on the system (it doesn't take very much to push the oil straight through...maybe a couple hundred PSI.) When you, for instance, start lifting a bucket of dirt, you've diverted oil into the lift cylinders, and they now see high pressure. If you try to lift too much weight, the pressure exceeds the setting on the relief valve that is between the pump and the valve, the relief valve opens and the oils gets a path straight back to the tank. After you've successfully lifted a load and you stop lifting, the valve essentially blocks the oil's path to move between the valve and the cylinders, so those stay under high pressure, but everything behind the valve is back to a couple hundred PSI.
So, you generally can't put plumbing in parallel because if the fluid sees resistance (ie. pressure) on one of the forks, then all the fluid would flow through the other fork. In this case, though, the other fork also has wheel motors that provide resistance, so I guess that makes the difference. You can see something very similar happen when you are turning lock-to-lock and trying to raise the lift arms at the same time...the lock-to-lock turn is using all the available pressure and leaving none for the lift cylinders, so the lift arms don't raise until you stop the locked turn. I recall that MR has observed this behavior before, as have I.
Man, I haven't had time to hang out here in quite a while. I'm really enjoying seeing some of the same people around!
HTH,
Dave
Update:
Actually, I have to correct myself. The circuits that power cylinders are open-center fixed displacement. I don't actually know what you call the circuit that powers the wheel motors other than variable-displacement...it is a completely seperate circuit. When at rest, there's no oil flowing either direction in the tram pump (unlike the cylinder circuits and PTO circuit which has oil flowing through all the time)...the swash plate just spins in the oil. When you move the swashplate angle by moving the treadle, you send oil in one direction or the other through the parallel/series wheel motor circuit. Somehow (and I still haven't figured this out), I *think* the tram pump also serves as the charge pump for the gear pumps that power the cylinders and PTO, although I don't know exactly how this works since the tram pump doesn't move oil unless the swashplate is offset (perhaps providing charge pressure is a seperate secondary function built into it?). I believe this is why you have to bleed the tram pump, but you don't have to bleed the gear pumps. The gear pumps are "self-priming" but that's only when they have charge pressure from the tram pump (most pumps are "self-priming" when they're being fed by another pump!)
Enjoy the musings... /forums/images/graemlins/cool.gif
Dave
