From: Ray James <rwj4123@sigma.tamu.edu>
My thoughts on the gearcase lubrication debate--crankcase oil or gear oil?
Hi, coupla little things...
I don't know of any reason that single viscosity oil can be
"better" than multiviscosity (of similar quality) except a few
cents lower in purchase price.
The viscosity improvers which are used to make multi-viscosity oil
break down over time, particularly under extreme heat and/or
pressure. When this happens, you end up with 10W/10 instead of 10W/40
or whatever. Bad. In engines with regularly changed oil this shouldn't
be an issue. In long-service applications it might be a problem?
The oil designed for gear cases is different. No real need for
detergent (no products of combustion in there) so it isn't added.
Whether detergent can do any damage in a gear case, I don't know. The
real attribute needed in gearcase oil, though, is a "high pressure"
rating. The designations sometimes include a HP or some other
notation to indicate that the oil is formulated (with additives?) to
maintain a film under the very high pressures generated when one gear
tooth rolls over another (almost zero contact area, with a high
transmitted torque means very high pressure at the contact area).
Modern engine oil will have some extreme pressure (EP) additive
protection also, but not as much as gear oil. Good engine oil should
have enough of these additives to protect normal gears under normal
loads, as found in typical LBC transmissions. These gears do'nt really
put that much stress on oil, because the contact motion of the teeth
is a "rolling" motion - the teeth touch each other, but do not
actually slide against each other very much while they are touching.
The real issue is with hypoid gears, as found in modern rear
axles. Here, the ring and pinion teeth actually slide against each
other in normal service. This is a tradeoff of the design that lets
the pinion be below the centerline of the ring gear, which allows the
driveshaft to be lower. Serious lubrication is required to keep these
gears alive. Engine oil is not up to the job; gear oil is mandatory.
One other other thing is that the additives used in some gear oil (and
aftermarket stuff like Slick-50) are incompatable with Laycock
overdrives. This is because they literally make the oil too slippery,
and the clutches in the overdrive can't get a grip and eventually burn
up. Engine oil doesn't have this problem. I'd sort of expect that gear
oil which claims to be OK for limited-slip differentials doesn't
either, but I don't know of very much detailed information (an
exception is RedLine; their engineers seem to actually know what a
Laycock overdrive is, at least...).
cheers,
-john
John Wroclawski
jtw@lcs.mit.edu
|