"JAXAshby" wrote in message
...


what happens is this. if the driven gear (the prop gear) in the lower
unit is
in the foward part of the unit it has a large, easy to use surface to be
put to
use as the thurst bearing.

I am not sure I am following: are you saying that the thrust is transfered
from the output shaft to the gear, and then from the gear to the housing?


I am having a hard time picturing that design. I have only had one lower
unit (a Volvo Penta from about 30 years ago) completely apart enough to
become intimately familiar with it, but the exploded views of other lower
units, including my 2000 Mercury outboards, appear to be very similar.

On the lower units I am familiar with, the prop shaft enters the lower unit
housing through an oil seal and fairly good sized bearing. This bearing is
also the thrust bearing for forward. I had thought that it was also the
thrust bearing for reverse, but I would not swear to that (memory is a
little foggy).

The shaft continues into the housing, passing through a beveled gear. This
gear uses the prop shaft as a bearing surface, it does NOT attach to or
drive the shaft directly. Continuing on the shaft passes through the "dog
clutch", which IS splined to the shaft. The clutch can slide easily forward
and back but turns with the shaft. Continuing on the shaft passes through
the other gear (which spins on the shaft, just like the first gear) then
continues on to the second bearing.

The two gears MUST be the same size, at least in diameter and number of
teeth because they both have the same center (the prop shaft) and both must
engage the same gear. Therefore, forward and reverse ratios are always the
same.

When the engine is running, the three bevel gears (one on the main drive
shaft, two on the prop shaft) are all turning. When in neutral, the clutch
is positioned between the two bevel gears, not engaging either one. To
engage a gear, the clutch is moved either forward or aft, engaging one of
the bevel gears that is rotating about the prop shaft, forcing the prop
shaft to turn in that direction.

Ideally, I would think that the two bevel gears would be totally "floating"
on the prop shaft so that there would be no movement at all with respect to
the driving gear. If the thrust was transferred through the shaft to the
forward gear when the boat was in "forward" then there would be some amount
of movement of the gear forward. As the thrust surface wore, this gear
would move farther and farther forward. As it moves it will change the
engagement with the gear on the driveshaft and before long you would end up
with a stripped gear.

As I said, I can't say if all outdrives are built this way, but the Volvo
Penta I/O I had 30 years ago was, and I am pretty sure my Mercury outboard
is like this as well.

Rod