Well, it was either write this or work...
Normal bevel gears are designed so that, as much as possible, the
teeth which are actually transferring power at a given instant are
"rolling" against each other rather than actually sliding against each
other. Since there is very little relative motion of the teeth when
they're under pressure, the lubrication requirement is modest.
Hypoid gears, found in the rear axle of modern cars, are different in
that, because the pinion gear meshes with the ring gear below the
centerline of the ring gear, a substantial amount of "wiping" or
sliding occurs between meshed teeth of the ring and pinion. This is
why the alignment of ring and pinion is so critical. It also
complicates the lubrication requirements considerably.
Oils designed for hypoid gears deal with this with two types of
additives; extreme pressure (ep) agents and friction reducers. EP
agents keep the thin film of oil present on meshed gear teeth from
breaking down under the high load and shear stress caused by the
wiping action of hypoid gears. The lubrication agent itself is still
the oil. Friction reducers, like teflon, serve as a lower friction
lubricant which is held to the gear teeth by the oil.
Now then. Laycock overdrives, as several folks have pointed out,
depend on friction clutches for correct operation (actually, I am
convinced they depend on elves, but that's another story). They also
use a planetary gear train. This produces two separate lubrication
problems.
First, any oil with friction reducers is a disaster, because it will
reduce the effectiveness of the friction clutches. At best you will
instantly notice that something is wrong because the O/D stops
working. More likely the clutches will just slip some until the
linings all wear off. The worst case failure is that, while the car is
in overdrive, slippage in the clutch burns the lubricant and glues the
clutch pieces together. Then, you put the car in reverse, the O/D
stays on, and a little internal roller clutch is destroyed.
Fortunately, yer standard 'murrican limited-slip differential is also
based on friction clutches, and has the same problems. Most gear oil
manufacturers specifically label products which do not contain
friction reducers as safe for limited-slip diffs. I'd personally avoid
anything that didn't say this.
The other problem comes in because ep additives in high concentration
tend to come out of solution and turn into, um, goop. The planet gears
in an overdrive help this along because they spin quite fast and act
as little centrifuges. The laycock service manuals specifically state
that ep gear oils should not be used under any circumstances for this
reason. I imagine that additives have gotten a bit better since that
time, but since centrifugal separation isn't a big concern for normal
transmission designs, I doubt anyone worried about it much.
Note that premium engine oils have a good selection of ep additives,
too, though not as much as hypoid gear oils. Engine oil blenders are
more worried about additive separation than gear oil blenders, but it
might still be a problem. I don't know. Interestingly, a synthetic
base reduces the need for ep additives quite a bit.
I guess my own conclusions are that
a) Any gear oil that doesn't say it's safe for limited-slip diffs is
definitely not safe for O/D transmissions. Same for additives that
claim to cut friction or reduce wear.
b) I'd stay away from -any- heavy duty or extreme service gear oil.
c) a medium service gear oil with a synthetic base is probably OK.
Oh, this bit with detergent oil and spridget transmissions is
interesting. There's no particularly good reason to put detergent,
specifically, in a gear oil (it's in engine oil to deal with
combustion byproducts). So if it's really the detergent that eats the
coatings, gear oil shouldn't be a problem.
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