Gear Lube Question

[Steve Frazier]

I'm servicing my Kubota mower and am changing the oil in the gear boxes for the drive axles. What these gear boxes do is take the input power from the hydrostatic motor and transfer it to the drive wheel, reducing it at the same time. The gears are straight cut and the machine weighs 1200 pounds. My service manual says to use either 80/90 gear oil or hydrostatic oil. I've got a 5 gallon pail of hydrostatic oil, but it sure seems to me that gear oil is a whole lot different than hydro oil. I'm curious how both can be acceptable.

I think what is in the gear box now is gear oil, it smells about the same as the hydro oil but seems a little thicker. As I mentioned, I've got 5 gallons of hydro oil on hand which would be enough to do the whole machine, but I'd have to buy a gallon of gear oil to fill both gear cases. My market is real sour so I'm looking to save any spending I can at this point, so if it's safe to use the hydro oil I'd like to.

Can any of our oil specialists offer their opinions here?

 

[willie59]

What the heck...I'll take a stab at it. Is it possible, that Kubota is simply saying the gearbox's are not "particular" to a specific type of oil? That either "will" in fact provide adequate lubrication? After all, they use Dexron in auto trans gearbox's now, which I still haven't figured out. Seems kinda odd for these days though, when most mfg's make statements like "Use only OEM ACME super fortified synthetic space shuttle research oil ONLY!" Maybe, just maybe, Kubota is refreshingly saying, "it's a gearbox, guys, put some oil in it!" Could it really be that simple? :tong

 

[bill onthehill]

I have an old Satoh tractor made by Mitsubishi and it lists the same thing for the tranny. Most guys run gear oil if they don't have a loader but the hytran will work fine. My kubota RTV uses a hytran type of oil for Tranny and hydraulic lift.

 

[Steve Frazier]

LOL! For some reason that commercial with the old guy driving the smoking car popped in my head....."Motor oil is motor oil..."

So I guess if they are recommending the hydro fluid, I ought to be safe, just wanted a little reassurance. Thanks guys

 

[bill onthehill]

Steve, I'm not that old! The only thing I drive that smokes is my GMC dump with a detroit in it.

 

[Johnsoils]

Viscosity and Gear Oils

Steve,

I would use either oil recommended by the OEM. The key is viscosity and whether or not there is shock-loading of the gear box. GL-5 gear oils have an EP (extreme pressure) additive to protect the gears from shock loading. The sulfur based EP additives provide shock load protection, and these additives are what gives the gear oil a rotten egg or sulfur smell. A GL-4 gear oil does not contain EP additives and are typically used in gear boxes that contain yellow metals such as bronze or brass bushings or synchronizers.

I typically recommend AMSOIL 5W-30 tractor transmission oil or our 10W-30/SAE30 small engine oil in hydrostatic transmissions. These are 30W oils as where a 80W-90 gear oil has the viscosity equivalent of 50W engine oil. There are a lot of automobiles that call for a 5W-30 fluid in the manual transmission, GM pickups come to mind. These vehicles weigh in at over 5,000+ pounds and have the capibility to tow 10,000+ pounds. Over the road trucks use a 50W engine oil in the transmissions, and have tow capacities of 80,000+ pounds.

So go ahead and use your hydrostat oil and you should be fine. I have attached a copy of a viscosity comparison chart for your reference and others to use. It clears up a lot of confusion when comparing the viscosity of different oil types. As you can see on the chart a 80W-90 gear oil have a viscosity range of a 20W to 50W engine oil.