TuffTorq Hydrostatic Transaxle Oil

Status
Not open for further replies.
Joined
Feb 28, 2006
Messages
80
Location
Lehigh Valley, PA
Hi all!

There's a post a bit below here concerning hydraulic fluid for hydrostatic transaxles. My question is similar, but different: TuffTorq, one of the largest manufacturers of self-contained hydro transaxles says to use 5W-50 synthetic motor oil for refill purposes in the Knnaa series, at least. (Ironically, new units for many tractor manufacturers apparently come with mineral oil from TT). I'm trying to understand why they'd recommend motor oil in this application. Not to say that they don't know what they're doing, but still... Seems to me that a synthetic hypoid gear oil of the same approximate viscosity as the 5W-50 would be a viable alternative, no? For what reason would they want to use something with additives designed for an internal combustion engine, in an application that's for all intents and purposes, sealed? We already know that the gear tooth pressures in the differential are typically high enough to cause pretty consistent spalling. Is it possible that motor oil performance would be better than gear oil?

Additionally, one of the classic problems that these things seem to have is viscosity change with temperature: "My tractor doesn't work when it's warmed up." Wouldn't a gear oil present better viscosity stability with temperature?

I'm confused. What could be going on here?
 
Best thing to use is a 5W-50 or 15W-50 synthetic motor oil. I believe the reason for motor oil versus gear oil as a hydrostatic trans is really a hydraulic pump with a few gears present. Wheel Horse tractors recommended 10W-30 motor oil for years and years with great longevity.
 
You are probably right. Not only does TT recommend motor oil, but so does Hydro-Gear. There must be some reason, since obviously they could tell you to use any of a wide variety of products.

I'm kind of leaning to replacing the factory fill (mineral oil according to many) with Amsoil 15W-50 Racing Motor Oil. It meets the viscosity range and has excellent pressure-wear characteristics. It looks like it's a good compromise between the best properties of motor oil and gear oil.
 
Last edited:
i'll say it, they don't know what they're doing.
I called them a couple years ago about the trans in my lawnmower, which did not have tufftorq stamped on it but it was rumored that's what it was. I called and explained, the guy had no idea, didn't care, and sent me schematics of stuff that was completely irrelevant. waste of my time, i had better luck googling hydrostatic transmissions.
a hydrostatic trans is not complex, what generally happens is the pump wears and leaks and when you run the original viscosity oil say something comparable to a 30w and it gets hot, viscosity decreases and pump leakage increases. Simple solution is to increase viscosity to combat the pump leakage. Downside of increased viscosity is when cold you get reduced flow and less power output, which may or may not be noticable. going from that if you wanted to spec an oil that's fill for life and would last the longest, go with the highest operating viscosity with lowest cold weather pumpability = 5w50. I'm sure the cost of the oil plays a big part in what's chosen to.
Whether or not a hypoid gear oil would be better would depend on the gearing... if there are hypoid gears or not. the way i look at it is it's a transmission, I use transmission fluid in mine. car and truck transmissions running dexron/mercon atf see some pretty high loads and they do fine on atf.
 
Status
Not open for further replies.
Back
Top