"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

On Tue, 8 Jun 2004 13:45:09 -0700, "Rod McInnis" wrote:


"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.

snip

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.

snip

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.

snip

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



Hi Rod.

Your "Throat Bearing" is this one:
continues on to the second bearing.


This Bearing supports ALL the load to "push" the boat in Forward.

It is usually of special shape, design or material (no tech data here).

In Reverse your Thrust Bearing is the opposite one, near the propeller.

Usually it could be of the same drawing of the opposite one, but many times it is different for performances and bore
diameter.

It supports ALL the load to "pull" the boat in Reverse.

Bevel Gearboxes with Cog Clutch have no matter to rotate Forward or Reverse.
+++

Now get a look to drawings:

Usually the second bearing (Forward Thrust one) is placed in a solid body...

meanwhile the first one (Reverse Thrust one) is placed in a removable flange...

ALL the Reverse Thrust is supported by the three or four screws or the Snap Ring fixing this flange... no matter the
size or kind of this bearing...

Your specific one could be slightly different, but this is the standard in Outboard Propeller Shafts, any Brand.
+++

Boats with on line Propeller Shaft (s) have a different Thrust Bearings but the standard one is similar to this one:

Samewhere, along the Drive Line, is fixed one HD Thrust Bearing Box ( It has to be "rigidly" fixed to the boat !).

Inside there are at least two HD Bearings (notice: NO standard "Ball Bearings" but specific ones...) sized to support
heavy loads for a long time... (for years or for presumed boat lifetime...).

Small ones are lubed by Grease, but good ones are Forced Lubed with a Gear Pump and are Forced Cooled (Lube and Case)
with heat exchangers.

Standard Vessels are "going forward" for the main time... but other ones (e.g. Clam Dredgers) are not: they operate in
Reverse!

So, standard Thrust Bearing Box could have "differently" sized Bearings for Forward and for Reverse.

Other ones have same bearings in both sides.
+++

Vessels with twin engines and twin drivelines are using engines with opposite rotation: LH & RH.

In this way ALL components of the two Drivelines are working rightly and exactly in the same way, with same components
an spare parts.

This is depending from the Marine Gearbox concept (any Brand, as standard).

Inside Gearbox there are TWO Shafts: Input & Output (plus a third small one for Reverse Idle Gear).

On Input or Output Shaft are placed TWO Hydraulically Operated Multidisc Clutches ( ...sounds familiar "Twin Disc"??)...

First one is for Forward (and adequately sized) and is "directly" coupled with Gear in Output Shaft: so Torque is
roughly transmitted to Propeller in Forward.

Notice that rotation of Propeller in Forward is always OPPOSITE to the engine one:
Two Gears engaged: LH RH
Gearbox Ratio in Forward is depending from these gears ratio.

In Reverse you need for THREE Gears: LH RH LH.

So you have to interpose a third Gear: the small idler one.

Obviously the other two cannot be anymore of the same diameters of the Forward, so Reverse Ratio is ALWAYS different
from Forward one (slightly, but different).

Now all Torque in Reverse is transmitted by means of the "small" Idler one... so...

Notice that Reverse rotation is the same one of the engine one.

Notice that standard diameter of Idle Gear is two or three times less than main ones...

.... so it rotates TWO-THREE times speedier than engine rpm ! With engine at 3.,000 rpm it could be rotating up to
6,000-9,000 rpm!

Now you acknowledge why Marine Gearboxes are using large Heat Exchanger and Heavy Forced Lube...

By the way... during the use... your Outboard Gearbox is fully "immersed" in fresh water....
+++

I hope this could help.

Bye,

WinXP