Hydraulic fluid v.s. Gear Oil

[WVBill]

I'm moving over to this forum from the Attachments forum because it is more relevant.

I have a 197X Kubota B6100DT (Gear tranny). The operator's manual that came with it (B5100/B6100/B7100 Operator's Manual) says to use SAE 80 gear oil in the transmission/gearbox.

Kelvin's operator's manual from his later model B7100 (also a B5100/B6100/B7100 Operator's Manual) says to use hydraulic fluid (recommends Kubota UDT and several others as the first choice but also says that SAE 80 gear oil can be used.

Knowing nothing about oils and lubricants, I'm surprised that hydraulic fluid and gear oil seem to be interchangable in a transmission/gear case.

Isn't there a fundamental difference between hydraulic fluid and gear oil? I would think that Hyd Fluid is made for the transmission of pressure with any lubricating function secondary. Where as gear oil is made to lubricate gears, not necessarily under pressure and when it is used to transmit pressure (e.g. my 3 ph) it is secondary.

Can anyone shed some light or am I way off base?

Bill

 

[roysallis]

Last night I was talking with a friend tht just bought a New Holland skid steer and one of the lines busted. He was having trouble getting the dealer to come repair it so he started to get a new line and repair it himself and put in new hydraulic fluid. He thought again since it was new and waited for the dealer to come. The dealer told him that it uses gear oil and not hydraulic fluid. Hydraulic fluid would have ruined it. So there must a major difference between hydraulic fluid gear oil.

 

[TomG]

My mid-80's Ford 1710 calls for hydraulic oil in the gear transmission/reservoir as well as front axle. It took me awhile to get used to the idea since I grew up with the experience that gears and gooey oil went together. However, I think most reasonably modern tractors use hydraulic oil all around. However, I guess I have to wonder if my 1710 qualifies as reasonably modern.

On the surface, you'd think that hydraulic and transmission oil would be radically different. However, I don't know how the properties of the oils change under pressure (working gear faces create a lot of pressure). It's possible that hydraulic and gear oil have similar lubricating properties in operation. In terms of hydraulics, if it can be pumped, any oil (water for that matter) will create pressure. However, if I had a choice, I guess I rather not depend on a hydraulic pump drawing gear oil from a reservoir at -20F, but who knows.

I think one of the reasons for some seemingly odd lubrication recommendations in some manuals is that tractors are marketed worldwide. The range of oils available in North America isn't available everywhere--especially true in the case of diesel rated engine oils.

So, maybe the real question is whether a manual means an oil 'can' or an oil 'should' be used. I think a local dealer would know what is actually best to use in a particular region.

 

[art]

Hydraulic fluids are normally a 10 to 30 weight and Gear oil is a 80 weight or higher. The different uses and requirements come from pumps,seals and use. The design of the total system.

 

[Kelvin]

Bill,
My "Service Manual" covers many Kubota tractors to include the gear and hydrostat model B5100/B6100/B7100.

My "Operator's Manual", which is the one that I quoted from, only covers gear transmission models. I have used the UDT in my B7100D since I first got it during the Summer of '97; no problems so far.

The only problem that I see for you is the concern about mixing fluids. Draining all of the transmission fluid out of my B7100 is no problem; but since you have a front end loader I don't know if all of the fluid drains out or not.

Kelvin

 

[chuckp]

Gentlemen:
There is a difference between Hydraulic Oil and Hydraulic Fluid. Hydraulic Oil is somewhat like Engine Oil, whereas Hydraulic Fluid is Very light and is often red in color. These are two fundamentally different materials. They smell different, they look different and they act different in your systems. Please use the oil or fluid recommended by your manufacturer./w3tcompact/icons/smile.gif
Chuck

 

[scruffy]

Well, I personally will continue with the SAE 80# gear lube, as that is what the tractor has always had put in it from what I can find out, and what the manual calls for. Even though the hydraulics and tranny share a common resevoir, they evidently were designed to utilize the gear lube and not the UDT Hydraulic fluid. Good enough for me, that and 30 + or - years of use.

 

[Anonymous Poster]

Just make sure you use 80 wt., _not_ 80w90 EP. I read somewhere that the EP (extreme pressure) additive combined with wet brakes is a bad thing. It makes them less effective and can actually lead to the destruction of the friction material.

 

[scruffy]

That's the full intention! If I maintain what has always been used, and what the manual calls for, then I can see no problems developing from it. I am well aware of what an incorrect formula can do to materials, to include gasquets, as well as the hydraulic lines and seals.

 

[WVBill]

Well, thanks everyone.

I'm going to go with Scruffy's approach. My Father-in-law used the 80W gear oil from the time it was new. The tractor has about 1,000 hours on it and no problems so I'll continue to use the 80W gear oil even though a more recent version of the Operator's Manual for my tractor recommends Kubota UDT. "Don't mess with success."

Bill