My thoughts on the gearcase lubrication debate--crankcase oil or gear oil?
I used a gear oil (don't remember what viscosity right now) in my Midget
transmission
since last rebuild. I didn't even check the specs, so if it (79 1500 cc model)
calls for
motor (engine?) oil I am surprised. The primary difference in the two, in my
understanding is that engine oil has additives to make the viscosity more
nearly uniform
over a wider temperature range ie, 10W40 will behave like 10W at low
temperatures (so it
will turn over on a cold morning start, and like 40 weight at higher
temperatures (so it
won't be too "thin" on a hot day at cruise conditions). That's the reason you
should
choose a multiviscosity oil, incidentally...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. Another important additive in motor oils is a
detergent. It is
added to pick up the bits of sludge, varnish, etc., which form in the crankcase
as a
result of the combustion products getting in the oil. The detergent keeps
these bits in
suspension so they don't accumulate as "sludge". I don't see any reason *ever*
to use
non-detergent oil in an engine crankcase. I did use it one time, when the
mechanic who
rebuilt a Chevy 350 V8 for me said to use it for the first few miles while the
engine was
broken in--I don't know why he said to do that, or why I did it...As far as I
know, it
didn't do any significant harm, except that the rings in one cylinder (no 8)
has never
seated very well--I still burn oil in that cylinder and foul the plug every
10000 mi or
so. Don't know if the break-in oil could have been a factor there or not...?
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).
Presumably
motor oils are not *as good* at resisting this pressure, and the teeth may (?)
make
contact (I don't think they will do so with proper lubrication using HP oil?).
Note that
the crankcase oil *primarily* is to lubricate the journal bearings, where the
bearing
stresses are MUCH lower than in gear sets. Some gears (distributor drive from
cam) are
also lubricated by this oil, though so it will work to some extent. If a
crankcase oil is
used in the gear case, and if it isn't as good at properly lubricating
(separating?) the
gear teeth at contact point, then the contact stresses are even higher, maybe a
LOT
higher. Excessive contact stresses cause a interesting failure
mechanism--yielding and
rupture at a point a short distance below the tooth surface where the contact
takes place
(theory). The practical result should be (I have no practical experience with
such
failures) an eventual spalling of the gear teeth, that is the development of a
a worn
surface on the teeth at contact points. Once this starts, the gear set is
toast, I
suppose. There must be a significant margin of safety against this type of
failure,
though, since I do have practical experience (the transmission I just rebuilt)
with
transmissions which have been run (by the PO, who knows how long) without *any*
lubricant
in gear case. I suspect that this is a very common occurance. In my gearcase,
after this
abuse, however, I replaced only the synchromesh rings. The only damage I
observed was a
few chipped teeth (mostly on no 2 gear I think), but that was caused by
grinding it into
gear without functioning synchromesh, in my opinion. The bearing surfaces of
the teeth
appeared to me to be fine. So, if it will work with no oil for a while, it
will surely
work with good motor oil. Gearcase oil is just optimized for this service.
Summary--I always use detergent oil, multiviscosity, in crankcase; gearcase oil
in
gearcases. If there were a gearcase which shares lubricant with the crankcase,
it would
make sense to use same oil--you don't have any choice, right? In that case, I
would look
for good quality crankcase oil, and investigate to see if any make claims as HP
type oils.
I don't know what additives or characteristics lead to HP properties.
Trivia Quiz--What does the "W" in 10W40 stand for?
Motor-vs-engine. I don't think "engine" is only used to denote internal
combustion
engines: One counterexample is the steam engine. Also, the "engines" of war
developed
long before the good Prof. Diesel was a gleam in Papa Diesel's eye. This word
is much
older than gasoline. As another reader pointed out--it is used to denote the
works of a
printer, and also used to denote the guts of computer software sometimes--the
"engine" in
a database manager or an equation solver or a symbolic mathematics package.
Basically, an
engine is any "machine" which does some "work" (not necessarily in the
mechanics sense), I
think. I guess I have never heard the term engine used to describe the "simple
machines":
screw, lever, etc. In some sense, a transmission might be called an "engine",
but not a
"motor"? Different strokes for different folks--whatever turns your shaft.
>Mini transmissions, which run in the engine sump. I assume the gears,
>bearings, clearances, etc were designed and tested with motor oil in mind.
>Also, high pressure? Sprites don't have enough HP for high pressure... ;-)
-------------------------------------
Name: Ray W. James
Texas Transportation Institute
Civil Engineering Department, Texas A&M University
E-mail: Ray James <rwj4123@sigma.tamu.edu>
Date: 10/26/94
Time: 08:07:49
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