Re: Why use heavy oils in gearbox?

Ivan Vegvary wrote:
I am posting this so I can get educated. Just finished taking apart an automatic transaxle (1990 Nissan Maxima) for entertainment and "artsy" parts. From the bearings and races I see a welding positioner in my future!

Anyway, the planetary gears are amazing as is the differential gear setup. All of this runs in a transmission fluid bath, which makes me wonder, why we use heavy gear oils for similar applications when not unitized with an automatic tranny? Granted this unit was not as heavy duty as would be required for a rear end of a truck, but, I am sure there are trucks with front wheel drive automatic transaxles. (Maybe not?).

Educate me why heavy oils are used in gear boxes.

Most gearboxes rely on hydrodynamic lubrication. This means that the oil isn't fed into the gearbox under pressure. It's just picked up by the gears as they rotate and entrained between them. As the oil is drawn between the gears, it gets squeezed. This squeezing creates pressure in the oil, which separates the metal surfaces and reduces wear. You can think of it like an old-fashioned mangle for wringing clothes. The clothes are drawn between the rollers, and as they pass through, the clothes push the rollers apart slightly. Oil does the same thing in a gearbox.

Now if the gears rotate very fast, a lower viscosity lubricant is adequate because when it gets squeezed, it doesn't have time to flow out from between the gears. In fact, a higher viscosity lubricant would be a bad idea for gears rotating fast, as it would result in a lot of friction and consequently power loss. But for gears that rotate at a low speed, a higher viscosity lubricant is required. This is so that it takes longer to flow out from between the gears, and so prevents metal to metal contact.

The best viscosity for a lubricant also depends on how heavily loaded the gearbox is. A heavily loaded gearbox will require a more viscous lubricant than a lightly loaded gearbox. This is because in a heavily loaded gearbox the lubricant is squeezed harder, and so flows out from between the gears faster.

It's all about speed and loading.

Hope this helps!

Best wishes,

Chris

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