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

 

[McMXi]

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

I simply presented data for the OP to consider. What he does with that data is up to him.

 

[WI_Hedgehog]

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.

How does that apply to front axle use? If my front axle is 100° Communist there's an issue.

The way I see it the extremes are around:
Winter: -10F = -23C
Summer: 120F = 49C (given the engine could heat the front axle, maybe, if driving upside-down in a ravine)

It could get colder, but I'm probably not using the tractor at that point. Probably. If the snow is all powdery because it fell when it was cold out instead of a block of ice stretching from the door to the road because it was soft then froze, then sure, I'll probably wander outside, start the tractor, wander inside, get involved in something else, eventually hear an odd noise and just about fart myself when realizing it's the tractor, get dressed and blow some powder.

 

[TheOldHokie]

How does that apply to front axle use? If my front axle is 100° Communist there's an issue.

It applies because at the contact points where gear oil does its work the localized oil temps are well over 100C.

Dan

 

[WI_Hedgehog]

It applies because at the contact points where gear oil does its work the localized oil temps are well over 100C.

Dan

Good point, thank you.

 

[N3BP]

I run 80W-90 gear oil in my MMM gearbox (mower is 41 years old). I occasionally check gearbox temps with the laser thermometer. It ranges anywhere between 180-195F depending on outside ambient temperatures.....

 

[skeets]

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.

Thats a lot of good info, now, ya wana tell me what it means in real world uses,,

 

[TheOldHokie]

Thats a lot of good info, now, ya wana tell me what it means in real world uses,,

In simple words it means UDT is "thinner" in the pail than conventional sae 80 gear oil but thins out slower as it warms up. By the time borh oils reach 100C they both have the same viscosity.

It also means that at even higher temps the UDT is actually thicker.

Simple enough for you?

Dan