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I'm very curious now that I know somewhat how hydrostatic systems work. On various machines I occasionally run into an overheat situation and the hydrostatic transmission either loses almost all power or just starts to slow down. If it's a piston pump how does this happen? Is there a certain amount of gap that when the fluid becomes really hot and loses viscosity it just flows through the pump and can't build pressure? Or is it somthing on the side of the motors? Built in safety mech like a mechanical overheat bypass? I'm just curious what actually causes a hydrostatic transmission to lose power when it gets to hot. Thanks!
 

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Its a viscosity problem.

Above a certain temperature the oil thins out too much and additives start to break down. If pushed to far and too hot the oil will not recover when it is cooled.

The end result is that the pump still pumps the fluid but the fluid is too thin to work properly in the hydraulic motor that drives the wheels. The thin fluid just passes thru the system. The viscosity (resistance to shear) of the fluid is what allows the fluid to convert pressure into rotational torque inside the hydraulic mototr.
 

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CJademec explained it very well.

This is a fairly common issue with the smaller L&G tractors having no active cooling feature. As you get into the larger sub compact and compact tractors they have active cooling for just this reason. A cooling radiator plumbed into the system with the engine fan pulling air over it keeps the fluid cool or at least at a fairly constant temperature.
 

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Larger system issue as well

Years ago we ran a tomato harvester that was pretty much hydraulic everything. Of course during harvest it was hot anyway and we'd have dirt and tomato juice coating things and the bottom line was 2-3 times a day we'd have to take a high pressure hose to everything to cool it down. First the conveyors would stop, then the rotating cutters and finally the drive wheels. I guess that was the relative strain vs. motor capacity of the various systems. Once the oil got too hot it took quite a bit to cool it down as the reservoir was so large.

Treefarmer
 

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It's important to distinguish what's exactly happening. As CJadamec touched on, viscocity, shear, and heat play a key factor here.

In a normally operating transaxle, under normal circumstances, using the correct oil, etc, the TA should not overheat in the first place. It is crucial to avoid damage by ensuring the TA is always CLEAN and you do not allow mud and debris to cover the unit, which in turn diminishes it's ability to dissipate the heat and cool itself. Heat is enemy #1. Under normal conditions, you should be able to put your hand on the unit, and linger a bit, meaning 150 to 170 max is acceptable.

The issue is, over time when the TA overheats, it tends to stretch the pump cylinder bores, and the oil as it gets hotter, and hotter, tends to slip past the pistons, which in turn reduces the output. This shear, in turn accelerates the heating of the oil as well, so it becomes a vicious circle causing more damage by overheating the metal cylinders stretching them even more.

So typically, with a light load, you will notice the tractor running fine at start up, but slowly as the tractor warms up - still under a light load, it will start slipping more and more. For example going up hills, etc. As the damage worsens, the bypass and shear is super heating the oil even at a very light load, and the performance continues to deteriorate. Eventually this overheating will render the unit inoperable. At that point, a total rebuild or a new (or good used) replacement is required. I've rebuilt a good number of these TT transaxles, and when dealing with these kinds of symptoms, once you crack them open, the cylinders are so bad, they are visually egg shaped.

So again, it's important to maintain correct operating temps, keep the unit CLEAN, and when testing a new machine, run it at least 1/2 hour to make sure it's fully up to temp and no slippage is noticed. Also, be aware that for many machines, slippage may occur externally if the gears on the drive output pulley are stripping, or even the belt as well. The difference is, belt and pulley issues will manifest themselves right from the get-go with a cold start, whereas with true TA issues, they usually run pretty good until they warm up, then get progressively worse.
 
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