Some months agon, Barney Gaylord posted a detailed analysis of this subject.
I can only find part 2 of this in the archives (http://www.listquest.com),
but since this includes the concusions (read for yourself), I have attached
it:
Date: 17-Dec-1998 12:16:57
From:
Subject: Tranny oil flow - Part 2
[continued from part 1]
Besides being submerged in oil, the layshaft also has a form
of forced oil
flow to the bearings. There are holes drilled in the front
of the main
case, and channels cast into the front cover plate, such
that oil can get
to the front end of the layshaft. The layshaft has an axial
hole and one
or more radial holes to allow oil to flow into the bearing
cavity inside of
the laygear. Rotational motion of the laygear causes some
centrifugal
force (oops, sorry for the big word) in the oil between the
laygear and the
ends of the main case. This force moves oil away from the
layshaft at the
ends, effectively dragging oil through the bearing cavity
inside the
laygear, and creating oil flow around those needle roller
bearings.
Without this flow metal particles could accumulate inside of
the laygear
and accellerate the wear on the layshaft. With this flow
any particles
there will be flushed out and (hopefully) settle out in the
bottom the the
gearcase.
And while we're on the subject, in light of this last
statement, how many
of you know that you should change the oil in the gearbox
and rear axle
about every second oil change? More inportantly, how many
of you actually
do it? Better check the maintainance schedule in the shop
manual. For
most people this may nearly match the interval for flushing
the brake
fluid, another sorely neglected point.
Now there's one more bearing in the gearbox, the one at the
far rear of the
mainshaft where the driveshaft connects. MGA 1500s have a
bronze bushing
here with a sliding spline connector for the driveshaft
yoke. Very late
MGA 1500s and all later MGA and all MGB gearboxes have a
ball bearing here,
and a flanged coupling for the driveshaft. In the case of
quick
acceleration a generous splash of oil may find its way back
there. At
steady cruising this will not happen, but this action is not
left to
chance. Along the inside of the rear case, about half way
up on the right
side, is cast another oil catch trough. Whenever the output
shaft is
turning oil is splashed up here by the speedo drive gears.
This oil runs
to the back of the rear case where it can enter a cross
drilled hole and
end up just in front of the rear bearing. There is a large
flange in the
casting here surrounding the mainshaft, so as to encourage
the oil to flow
farther back and to flow through the rear bearing. Behind
the rear bearing
there is a groove inside the bottom of the case to allow oil
to flow
forwards below the rear bearing and through a hole back into
the forward
part of the rear case. As such, oil will flow here
constantly and bathe
the rear bearing whenever the mainshaft is turning.
Now as to the question of whether you can tow one of these
things with the
driveshaft connected without damage, the answer is a
resounding "it
depends", and what it depends on is the type of gearbox.
The real parts in
question here are the bronze bushings inside of 2nd and 3rd
gears (and
sometimes 1st), and also the needle roller bearing at the
front of the
mainshaft. At this time it is my opinion that the oil pump
being driven on
the output shaft will NOT supply forced oil to the mainshaft
while being
towed in the 3-syncro gearbox, but that it will work in the
4-synchro
gearbox. This is because the oil supply to the pump is from
splash in the
top of the main gearcase.
In the case of the 4-syncro box, the large reverse gear on
the mainshaft
will always be turning with the mainshaft, and being
unshielded will throw
plenty of oil up into the catch trough on top. So if the
oil is at the
correct level the entire gearbox would be properly
lubricated by the pump
while towing. So as a challenge at this time, I would like
to know if
anyone has ever towed a car with a 4-syncro gearbox with the
driveshaft
connected and incurred any internal damage to the gearbox?
I think this
would not be a problem, and 4-syncro units can be towed with
the driveshaft
connected without worry.
In the 3-synchro box 1st gear (shared by reverse) is always
turning with
the output shaft, but it looks to me that the shape of the
1-2 shift fork
shields the top of that gear and will prevent oil splash in
the direction
of the catch trough up top. I do not know this for sure,
but I may soon
install a clear plastic side cover on a 3-syncro box and
spin the output
shaft in an attempt to find out. This feature does not
however necessarily
preclude towing these cars with the driveshaft connected.
For starters, I don't think that would be a good idea for a
3-syncro car
that has been sitting for some years without running, as
there may be
little or no liquid oil in the mainshaft bushings. It would
be a great
advantage if the car could be driven for a short distance
before towing to
distribute oil throughout the gearbox. I would guess that
running the
engine and idling it in neutral would NOT do the trick.
If the 3-syncro car can be driven some first, then I think
it would be no
problem towing it for a short to moderate distance with the
driveshaft
connected. I myself have towed an MGA more than 100 miles
on the rear
wheels without disconnecting the driveshaft, with no
appearant problem, and
no noticable wear on the 2nd or 3rd gear bushings when
disassembled many
years and many hard miles later. Once there is a proper oil
film in the
mainshaft bushings it will remain there for quite a long
time, as the gears
in question are simply idling stationary with their own
weight on the
bushings as the mainshaft turns. However, if the 3-syncro
car is towed for
more than a few hours with the driveshaft connected, the oil
film in those
bearings may ooze out or break down and solidify from the
heat of friction,
and this could cause some damage. So in this case, tavel at
your own risk.
Any comments are welcome, as I think I will soon be posting
this on my web
site. And once again, PLEASE do NOT copy this entire memo
when responding
to one or two of the finer points, but just copy the
pertinent parts you
wish to mention.
Cheers,
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
|