4WD final drive, gear oil instead of "hydraulic" fluid?

[montejw]

I've got an AC 5020, 4WD that uses tractor hydraulic fluid for the trans, hydraulics, front differential, and the 2 front wheel final drives. The final drives at each wheel consist of 13t and 29t straight toothed gears.

I've got both sides torn down for another repair and have found the gears on one side to be pitted. The final drive housing on that side has been repaired in the past, the gears probably took some pieces of the housing and ground them up whenever the housing broke.

I'm thinking of running some gear oil at least in that one final drive to give it a little more lubrication. At least I think it would, compared to the hydraulic fluid otherwise used.

There shouldn't be a problem doing this, should there? It will help the gears, providing a little "better" lubrication, right? The final drives are sealed from the rest of the front end so the fluids won't mix.

In fact, what would be the problem with running gear oil in the front differential also?

Monte

 

[Doc_Bob]

I've got an AC 5020, 4WD that uses tractor hydraulic fluid for the trans, hydraulics, front differential, and the 2 front wheel final drives. The final drives at each wheel consist of 13t and 29t straight toothed gears.

Monte

My New Holland TN70A is the same. I would only use the suggested fluid. I trust the engineers who built it!
Bob

 

[john_bud]

On the old equipment you can switch from 80w90 to the hy-trans fluid. But you can't switch from hy-trans to 80w90! The clearances are too small for adequate fluid flow and parts can be oil starved. Talk to folks that repair modern manual transmissions. They use ATF in them, switching to 80w90 gear oil for more "performance" has kept repair shops in business for a while now.

jb

 

[davemhughes]

Just a thought.......I am a huge believer in Synthetics......Amsoil makes a Tractor Hydraulic oil.

 

[DieselPower]

On the old equipment you can switch from 80w90 to the hy-trans fluid. But you can't switch from hy-trans to 80w90! The clearances are too small for adequate fluid flow and parts can be oil starved. Talk to folks that repair modern manual transmissions. They use ATF in them, switching to 80w90 gear oil for more "performance" has kept repair shops in business for a while now.

jb

I have switched out a couple of manual transmissions over the years that were factory filled with ATF to a 50W synthetic medium/heavy duty truck transmission gear oil. Worked great and my wear metal counts dropped drastically in every one when compared to UOA's done when they had ATF in them. In 8 or 9 years of use in two of my own pickups I changed they are failure free and actually run cooler by about 20 degrees F. I agree a 80 or 90 weight would be pushing it though.

 

[montejw]

Thanks. It sounds like everyone thinks the gear oil might be too thick, too viscous, etc. This tractor is almost 27 years old, granted it only shows 800 hours on the hour meter, but I find it sort of hard to believe that the tolerances are that tight. 4 unshielded ball bearings, 2 straight toothed gears, a double universal joint, and an unshielded thrust bearing with 105 balls. One gear case has already blown up, I found a repair when I tore it down to fix something else. The gears on that side look like they ground up parts of the cast iron case, which they most likely did. Badly pitted, tops of teeth chipped, the drive shaft from that universal gouged from a bearing seizing, and that bearing was still being used. It was rough, but it still turned, a little.

Unfortunately they didn't make these tractors very long, only about 8000 of them. Of course now I know, too late. Parts are very hard to find. I've been researching these now, it sounds like there's a history of this type of damage. One salvage place I talked with had 2 of these tractors, both had the same damage as mine, on the same side. Maybe I'll try a synthetic, and be very diligent about the fluid level.

Thanks again,
Monte

 

[SPYDERLK]

I would go to Mobil 1 75W 90 gear oil. This will be about the same viscosity as the hyd oil in cool temps and a little thicker in hot, while giving the added lubricity of a synthetic. Its weird, but gear oils have #s running on a different scale than engine oils.
Larry

 

[DieselPower]

SPYDERLK is correct about the viscosity. Just take a quick look at the Kinematic viscosity centistokes @ 40 C chart. However I would not use a multi viscosity gear oil. I would stick with a lower weight/single weight gear oil. At the high temp end of the gear oil the 90W gear oil would be pushing a ISO 220 hydraulic oil so in hot operation it would be way to thick.

 

[SPYDERLK]

DieselPower, multis do not get thicker as they heat. They just dont thin as fast. If an oils viscosity is ok cold and warm it will never be too thick when hot. The oil being thicker than standard fluid at hi temp only ensures better protection. The only problem would be a tiny loss in effeciency with the fluid remaining thicker because its alittle harder for the gears to push it around. Also, drive gears dont run as hot as 100C unless there is something wrong.
Larry

 

[DieselPower]

That's true. However, many hydraulic systems are engineered to operate at the higher temperatures with a thiner fluid. The use of a multi viscosity oil will not thin as much at the higher operating temps. I have seen this happen in industrial hydraulic systems before. Company uses a multi viscosity oil in place of a straight weight hydraulic oil and the equipment is slugish at high temps. This is why I suggested using a straight weight in my original post. This is also the major reason hydraulic systems come with a straight weight oil as factory fill and not a multi viscosity.

Now with that said, I agree after thinking about the original post that this is for a gear system, you are correct. A multi viscosity would probably be fine. Had the darn word hydraulic stuck in my head.