Hydrostatic transmissions....

15 years ago

ok guys...I'm about to ask ANOTHER stupid question. I have a tractor with a hydro (GR2100). I am concerned about over taxing the tranny. Lets take a hypothetical situation. I would not do this, but for illustration purposes....if I were to take a chain and secure my tractor to a tree, take up the slack slowly and then when it was tight, push the pedal to the floor, what is happening internally? Assuming no wheel slippage, are there clutches inside that are slipping, or is it a pure fluid drive??? IF anyone knows, I would appreciate them enlightening me : ) Thanks!

Chris

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Comments (8)

musli
15 years ago
It's a fluid drive. If you attach it to an immovable object and push the pedal to the floor, pressure-relief valves should operate to protect the transmission.
steve2ski
15 years ago
It may stall the engine with the wheels locked. Lock the brakes and and engage the hydro - Let us know the results. A short duration test should not hurt the tractor. Most hydro equip I operated will stall the engine - both my Craftsman DLT2K and JD GT275 will stall the engines, I have also rented other equip with hydro's, same thing. I have test drove a Bota RTV900 same thing there also.
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heh heh rusty... Sometimes that is best way as used Sears tractors are sitting around every other corner. But to answer the OP's post, it isn't clear if you are dealing with a seeping axle seal, a no drive situation or what. If you pull the t/a, clean it externally to keep the inside from becoming contaminated. Throw the HG service # into google and get an IPL so you can see what you are up against. If the t/a was starting to get a little lazy, but still drove, wasn't too groany- then maybe, just maybe you just need to refresh the lube, purge it and you're good to go.
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Tackled the monster today. Surprising results. Took an old belt along to fabricate friction washers, also a ..."cloth"... anyway those woven things we all wore in the military as it had good thickness and I ain't going to be using a 26" belt in the near future....:-) Disassembled the unit. Consisted of a coil spring, hollow metal tube with a 9/16" (nut size) bolt through it, and a thick plastic washer against a thick ...waxed cardboard....(like the stuff gaskets in carbs etc. are made of) washer. The metal tube was obviously to control how much the spring could be tightened. The plastic and cardboard washers were pristine, no wear, no deformation that I could see. HOWEVER, someone had oiled them previously. CUDOS TO "JUSTALURKER" who suspected this. I cleaned them up, per his suggestion. I then took the metal tube to my grinder and removed about 1/16" from each end. I did this as I could easily compress the spring against the tube and there was a lot of space between the coils. Also, since the unit is 20+ years old there has got to be wear that I couldn't see. Reassembled and tried it, works fine, motion control lever stays where it's put now. Still have some more to tighten if I need to. So happy ending to the story, thanks for all the good input. Left the belts in the barn as I have another need for a friction washer. Thanks again, Ev
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Have you done any work on the drive wheels over the winter? I ask because it has happened a few times where someone removed a drive wheel to have a tire fixed, and the axle key fell out unnoticed. later when the wheel was put back on, without the axle key, the axle shaft would turn inside the wheel hub but the tractor would not move. Have you checked the oil level in the trans? Have you tried doing the purging procedure? Usually it is only necessary to purge air after the trans has had the oil drained and refilled, but it may be possible that the pump has somehow "lost its prime" and become filled with air that needs to be purged. If the tractor was parked over winter at something other than a level angle, plus having a low oil level, that scenario might have conspired to allow the pump to empty itself of oil and fill with air. This post was edited by mownie on Wed, May 7, 14 at 1:14
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To answer the part about "less strain" going down hill. Yes, there will be less strain because gravity is giving you a boost instead of the engine and transmission having to fight to overcome gravity as when you go uphill. Would it be worth the trouble to always mow down hill? I really doubt it would make much difference in the long run. Heat and worn out oil are really the killers of hydrostats when they are overworked and undercooled. In theory, you would think maybe avoiding an uphill run would/could result in longer life for the trans, but if you are doubling the driving distance in order to always mow downhill, the extra driving distance probably cancels out any benefit you could get from always mowing down hill. If you want to improve the life of the hydro, make sure the transmission case is always clean and free of grass or other debris that insulates and blocks cooling air from the fan. Always run the engine at about 75% to 100% of the governed RPM. At low RPM, the fan is not turning fast, so it does not wash much air across the trans case. At low RPM, the oil flow inside the trans pump and motor is not very energetic, and oil is not splashed/thrown/sprayed on the trans case as much as at high RPM, so more heat stays in the oil instead of being drawn into the case casting metal to be dissipated to the air from the fan. Change the hydro oil at least every 3 years. Even if it requires removing the transmission assembly for an upside down drain, it is worth the effort.
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chris_va
Original Author
15 years ago
I have had the machine in this postion before. I have Jonny Bucket and if I am pushing into something that stops the machine the front wheels sometimes spin, but the backs never do. The whine from the hydro gets louder which I assume is the bypass opening, and the engine does not bog down at all...not even a little bit. I do not see how a hydro could stall the engine anyway...seems to me that as the engine speed droped so would the speed of the hydro pump and thus the pressure inside the pump. I would think that the entire system would reach some point of eqlibrium where the engine slowed down due to the hydro pressure but as that happened the pressure inside the hydro woulkd also drop easing the load on teh engine. Does that make any sense???? Now I've confused myself...damn!
fix_it
15 years ago
While not a true hydrostat, I have used a Case 222. It is a hydraulic pump connected to a hydraulic motor through a spool valve. It will stall the engine. I have seen a similar situation on a ditch whitch machine. It was hydraulic drive and would stall if you hit a tree root. On the other hand, most small lawn tractors will have a pressure relief valve. These will not stall the engine.
engine_tech
15 years ago
Actually, most small hydros do not have relief valves in them. Spinning the drive tires is the fuse or relief. That's why you don't ever want to over ballast the drive tires on a hydro, or abuse one on pavement.
chris_va
Original Author
15 years ago
So....I either have a very weak hydro, or a relief valve?
engine_tech
15 years ago
chris_va, I'm sure you are fine. It is possible that there is a relief for the rear but not for the front drive of the GR. Isn't the front end hydraulically driven, or is it shaft driven?
chris_va
Original Author
15 years ago
engine_tech, The fronts are shaft driven, as is the mower deck. The front wheels do not always spin...depends on the load on them and the type of surface they are on.

Hydrostatic transmissions....

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