Gearbox oil - am I smarter than Kubota engineers?

[Delmar]

I know Kubota recommends their hydraulic fluid for mower gearboxes, but I wonder if heavier oil isn't better. I just can't see any reason to use thinner fluid. There's no tiny tubing or high pressure in the gearbox. It seems like gear oil is the way to go.

Leaking oil seals are the culprit in every case of a Kubota gearbox failing in my experience. Thicker oil would be less likely to leak through the oil seals. At least, that's my opinion.

Anybody else have a different opinion? I know the Kubota engineers make recommendations for a reaosn, I just can't imagine what it is (other than to sell their branded fluid).

 

[Mak65]

Are you referring to a bush hog type rotary cutter? The manual for my LandPride mower calls for 80W 90 gear oil in the gearbox.

 

[Toyboy]

I use 80w90 synthetic in mine

 

[GeoHorn]

I would use what the mfr’r recommends.

From a purely theoretical perspective….thicker (heavier viscosity) oils do not necessarily have lesser leaking tendencies.
In fact, thicker oils resist “wiping” by the seals and at high operating speeds can be forced beyone seals and gaskets.
It’s also easy to “imagine” thicker oils lubricate heavily-loaded gears and bearings. But modern lubricants / additives do not need heavy oils, which also do not easily feed or re-fill into smaller spaces or recoat high-speed surfaces.
Heavier viscosity oils also do not carry heat away as readily.

 

[Hugo Habicht]

I know Kubota recommends their hydraulic fluid for mower gearboxes ....

Kubota specifies SAE90 gear oil for all G1900 mower deck gear boxes.

 

[TheOldHokie]

I know Kubota recommends their hydraulic fluid for mower gearboxes, but I wonder if heavier oil isn't better. I just can't see any reason to use thinner fluid. There's no tiny tubing or high pressure in the gearbox. It seems like gear oil is the way to go.

Leaking oil seals are the culprit in every case of a Kubota gearbox failing in my experience. Thicker oil would be less likely to leak through the oil seals. At least, that's my opinion.

Anybody else have a different opinion? I know the Kubota engineers make recommendations for a reaosn, I just can't imagine what it is (other than to sell their branded fluid).

Despite your visual perceptions, when you measure the actual viscosity of UDT it is a SAE grade 80, API GL4 gear oil. So I vote NO, you are not smarter than the engineers who know that.

Dan

 

[McMXi]

I know the Kubota engineers make recommendations for a reaosn, I just can't imagine what it is (other than to sell their branded fluid).

I think it's good to be inquisitive and look for the nuance in things. The local Kubota dealership puts 80W-90 oil in front axles of tractors when assembling them rather than UDT or UDT2 for the simple reason that they see way few leaks and therefore higher customer satisfaction.

Comparing a typical 80W-90 oil to UDT2 reveals some differences for sure.


SAE 80W-90
Viscosity @ 40°C cSt 140.8
Viscosity @ 100°C cSt 14.01
Viscosity Index (VI) 96

UDT2
Viscosity @ 40°C cSt 37.0
Viscosity @ 100°C cSt 8.1
Viscosity Index (VI) 199

cSt are units for kinematic viscosity (mm^2/second) which indicates a fluid's internal resistance to flow under the influence of gravity. It is calculated as the ratio of dynamic viscosity to the fluid's density. Viscosity index indicates how much a fluid's viscosity changes with temperature. The SAE would say that UDT2 has a high VI whereas 80W-90 has a very high VI which is apparent from the cSt value ranges.

 

[TheOldHokie]

I think it's good to be inquisitive and look for the nuance in things. The local Kubota dealership puts 80W-90 oil in front axles of tractors when assembling them rather than UDT or UDT2 for the simple reason that they see way few leaks and therefore higher customer satisfaction.

Comparing a typical 80W-90 oil to UDT2 reveals some differences for sure.


SAE 80W-90
Viscosity @ 40°C cSt 140.8
Viscosity @ 100°C cSt 14.01
Viscosity Index (VI) 96

UDT2
Viscosity @ 40°C cSt 37.0
Viscosity @ 100°C cSt 8.1
Viscosity Index (VI) 199

cSt are units for kinematic viscosity (mm^2/second) which indicates a fluid's internal resistance to flow under the influence of gravity. It is calculated as the ratio of dynamic viscosity to the fluid's density. Viscosity index indicates how much a fluid's viscosity changes with temperature. The SAE would say that UDT2 has a high VI whereas 80W-90 has a very high VI which is apparent from the cSt value ranges.

All true but it misses much of the original posters thoughts.

Also true is the viscosity of SAE 80 gear oil is [7 - <10] cSt @ 100C. That puts SUDT2 dead in the middle of SAE grade 80. Regular UDT is a bit "thicker" and both are more than adequate for wear protection.

IMO the proper way to deal with leaking seals is new seals.

Dan