Why use heavy oils in gearbox?

Gear oil film is maintained under extreme pressure. No metal-to-metal contact despite high pressure at gear contact areas.

You'll probably get better answers to this, but you may be interested that my '87 Mazda 626 with manual gearbox specified *either* a heavy gear oil

*or* ATF for the gearbox. That one really threw me when I read it in the factory shop manual. I used both at different times and the gearbox definitely was quieter with the heavy lube. However, the synchro was also much slower to work in cold weather.

That transmission, BTW, was one of the worst corruptions the Japanese ever built. It was just that year -- 1987. I had it rebuilt twice, and I'm easy on gears.

For any automobile gears, you need a high-pressure lubricant. I don't know how they got away with ATF for that job. For hypoid and spiral-bevel gears, as in a rear-axle diff, you also need a lubricant with high shear strength. That's what heavy rear-end oil is supposed to have.

-- Ed Huntress

"Ivan Vegvary" wrote" (clip) I am sure there are trucks with

^^^^^^^^^^^^^^^^^^^^ The VW Rabbit pickup truck comes to mind. (I don't know whether it came with an automatic, though.) Not a good idea, because the load in the truck bed makes the front end lift. Picture trying to get traction going up hillon a wet dirt road.

My '85 Volvo DL also had ATF in the manual tranny....very nice on those -35 F mornings.

Wolfgang

My 4 liter V6 '92 Ranger 5spd used ATF in the gearbox and the transfer case- it was a mazda trans. They improved a bit I guess, I got just shy 250,000 miles out of the trans before a bearing started making noise. But, had it lasted longer, I'd still have it.

Dave

Another answer is that heavy oils aren't so heavy. An oil with a viscosity of 150 cSt at 100 F could be an SAE 40 engine oil or an SAE 90 gear oil. Confusing, isn't it.

As Ed says, the hypoid gears typically used in a diff need EP (extreme pressure) additives which form a chemical film on the metal at the high temperatures and pressures generated locally in the contact.

Also, the viscosity of oils drops rapidly as temperature increases (even with the additives used in engine oils). Stick a thicker than normal oil in a gearbox working at reasonable speeds and it will heat up and get thinner.

Mineral oil is remarkably effective on concentrated contacts without too much sliding, like rolling element bearings or simpler gears. Under the high pressure, the viscosity goes up very high and the metal parts deform elastically, giving a contact perhaps a millimetre wide but a lubricant film only a micron ( 40 microinches) thick. Look up EHL for more details.

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

Part of the reason I bought that car is that people told me they made really nice transmissions and solid engines. I saw inside of my gearbox when my friendly shop owner had it cracked open and I couldn't believe what I saw. They slimmed down the case and made the walls so thin that there was hardly any flange to support the bearings. The bearings would flex, and there would go the transmission.

In '88, I'm told, they fixed that. Too late for me.

-- Ed Huntress

Christopher Tidy wrote in news: snipped-for-privacy@cantabgold.net:

Just to add, the oil in an automatic _is_ pressurized, which means it can be forced where it needs to be.

If that's the case, it would probably explain the difference Ivan was referring to. I've never taken an automatic transmission apart myself.

Best wishes,

Chris

I've always thought that ATF is used now to bump up the EPA Cafe numbers. Like how cars that used to use 10w-30 are now spec'd for 5w-30.

If you think walls are thin now wait until they try to meet the 2012 35mpg CAFE standards.

Listening to the j*ck*sses in Washington claim how the new energy bill is going to save each citizen lots of money just about made me want to puke. My next car will likely rust out before I finish paying for it.

Wes

Wes: We are driving a 2003 car that if it ever gets below 40mpg at 80mph, I'm going to start checking it over. Recent trip to Oklahoma and return was done with the speedo showing 80 in all the 75mph zones even in head winds. The worst we got was 47mpg. However you can't buy a new one of these in California. The VW Jetta TDI would have to be degraded to get down to

35mpg. We have a few J*ck*asses >

Many constant mesh standard transmissions run automatic trans fluid, engine oil, or specifically synthetic engine oil. Some constant mesh transmissions also run GL oil. Most sliding gear standard transmissions (Non constant mesh, syncromesh) use GL rated (heavier) oils. Virtually all Hypoid differentials use a GL rated heavy gear oil. Worm gear drives use a specific worm gear lubricant - extremely heavy "lard like" lubricant

Newshound. You are the first person I've heard talk that knows SAE 40 and SAE 90 can have the same viscosity. I ran into this in a Bearing Handbook. I started asking all of my friends who have been in the car/motorcycle/aircraft business about which had the higher viscosity. No one knew about this and assumed as I did the SAE 90 was more viscous.

Ford used the Mazda R2 5-speed box in Rangers and F-150s. Mine has 106K on it and shifts fine. My friend has 300K on his and it shifts fine. Both trucks have the 300 cube six which belts out 292 lbft of torque. The R2 uses ATF with a change interval of 60K. A lot of stick shift transmissions now have needle bearings, which are better lubricated by lighter oils (like ATF or even 10W-30 motor oil) than good old 80W/90.

-Carl

But diesels are BAAAADDD in the USA, good in Europe. I'd drive a decent afordable small diesel in a heart beat. As it is, I'm driving a Saturn SL which does get 35-36 mpg (summer) using gasoline.

Simple economics will drive people to more fuel efficent cars.

Wes

That may be, but the trend toward lighter oils also contributes to longer engine life, they tell me. I don't know about transmission life.

My 2.3 liter Ford Focus is lubed with factory-recommended 5W - 20, semi-synthetic. After 40,000 miles it doesn't burn a drop, and my mechanic tells me the engine life with these new oils (and the new cylinder machining, and the new coatings) is incredible.

After my first car it was 21 years before I owned another car (not counting two full-size vans) that got *worse* than 32 mpg. So you're talking to the wrong guy. I'm the one who thinks that any car that weighs more than 2,500 pounds should be taxed out of existence.

I've owned four cars that weighed 1,650 pounds dry or less; one was 1,350. The biggest engine in any of those was 1,300 cc, but one of them did 125 mph -- although it was a little rough running below 4,000 rpm. d8-)

Think how much lighter it will be when the quarter panels and door panels rust off. It might not be such a bad thing.

-- Ed Huntress

If you compare the size of the gear faces in the auto trans to an old-style manual trans, for instance, you will likely find the auto trans has a much larger load-bearing face, so as to reduce the cross-sectional loading. This can also be accomplished by making the diameter of the gears larger.

Different gear tooth profiles can also reduce the wiping action of the gear teeth, reducing or increasing the wear. The optimum amount of sliding action on the tooth face keeps the hydrodynamic film working to keep the metal apart.

Auto transmissions use a light oil because the clutches and bands need to slip freely when not engaged. Also, the torque converter works efficiently with a lighter working fluid.

Jon

It is going to be really hard to meet the 35 MPG standard without major changes in the drive train. Lightening just isn't going to get much further. The last gasp was lock-up torque converters. Now, they are going to have to get serious. Combining start/stop technology with something like stratified charge or some other scheme to get rid of the throttle and run the engine at full cylinder pressure all the time (like Diesel) is going to be required. C'mon, Detroit, we know you've had stratified charge systems running since the 1970's.

Electric steering is already coming in, saves a Hp on the steering pump. Next is something in the auto trans to reduce the demand of the hydraulic pump. They may already have gone to variable-displacement pumps instead of fixed-displacement and a pressure regulator in many newer transmissions. I think the start/stop systems may use an electrical pump so the engine doesn't need to pump up hydraulic pressure before the transmission can engage.

But, these are all small tweaks. The Otto cycle needs to go, and something more efficient at mid-throttle needs to replace it. Diesel, of course, has been there for decades.

Jon