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SAFETY NOTICE
SAFETY
NOTICE
After
working on and reading about a number of catastrophic gearbox failures
in late model Guzzis I began doing some research on a bearing change
that MG made that I had some concerns about. For years MG used a 3205 A
double row angular ball bearing for the input (front of clutch shaft)
and output (rear of layshaft) bearings. It is an open bearing having no
seals and the bearings are held in place be a metal cage. At some
point, which would be very nice to determine, the 3205 A bearing was
discontinued and replaced by the 3205 ATN9. This is the same dimension
as the earlier bearing, but the TN9 suffix refers to a cage change. The
new cage material is called polyamide 6.6 and is a glass fiber
reinforced cage. As you can see in the table below left, these cages
are not happy running in gear lubes with EP additives (extreme
pressure) or synthetic oils. They also do not react well to prolonged
high operating temperatures. It is easy to imagine that a MG gearbox
could reach the 176 °F limit, quoted for one of the
synthetics,
at high speeds on a very hot day. Also, it is not clear to me that they
tested with both EP and Synthetic Oil together which might make the
situation more dire.
Another disconcerting aspect of
this bearing change is the open style of this bearing. The open design
allows the lubricant to effectively cool the bearing, but a MG gearbox
is a very hostile enviornment filled with brass, bronze, aluminum and
steel swarf. The old steel caged bearing lived quite through all of
this. I'm not sure that a plastic cage can.
There are other factors that may
be causing these gearbox failures. Guzzi switched to a three engagement
dog arrangement on their gears and sleeves to allow quicker, smoother
shifts. This results in an increase in driveline lash. On bikes without
a rear wheel cush drive the increased shocks that the gearbox sees may
be a factor. The bikes with straight cut gears seem to have the problem
first. The boxes with helical gears last a bit longer. There was also a
redesign of the internal cush drive on the clutch shaft where the old
coil spring which absorbed shocks was replaced by bellville washers.
This may be a stiffer arrangement than the coil spring shock absorber.
There's more. The output bearing (this one fails first) is quite a bit
above the oil level in the box and relies on splash to cool itself. If
the oil level drops very far or the oil is not changed as frequently as
recommended, that would aggravate things further. Like we needed
more......this type of bearing does not deal well with misalignment,
nor do gears. It is fifth gear that fails first when the bearing goes,
often stripping all the teeth off of fifth on the mainshaft and the
layshaft and occasionally blowing the back out of the box. I came into
possession of one of these a couple years ago that had 5 or 6 gears
stripped to the root. Not a tooth left.
While I cannot say for certain
that these bearings are the cause of the late model failures, I wil
tell you that I won't work on a box without replacing them. I have
found an identical replacement for the original 3205 A. If anyone can
find out when this new bearing was first introduced, it would be a good
thing for all of us.
I highly recommend that anyone
with the new bearings seriously consider having them replaced by me, a
qualified MG dealer, a qualified mechanic or themselves if possible.
The entire box has to be stripped so be sure you have the proper tools
to do the swap without causing more damage.
NOTE: Whatever the cause,
this is not the fault of the bearing manufacturer. It is the OEMs
responsibility to spec the correct bearing for a particular
application. (if it is in fact a bearing failure causing the gearbox
blow ups) Read up on it. Decide for yourself. Below is information
supplied by one of the bearing manufacturers. Moto Guzzi specs Agip
Rotra MP SAE 80W/90 and when I checked the specs on this gear oil it
does have EP additives in it.
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Table 18: |
Permissible
operating
temperatures for cages of glass fibre |
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reinforced
polyamide 6,6 with
various bearing lubricants |
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Lubricant |
Permissible
operating |
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temperature1)
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Mineral
oils |
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Oils
without EP additives, e.g. |
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machine
or hydraulic oils |
120
°C 248 °F
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Oils
with EP additives, e.g.
industrial |
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and
automotive gearbox oils |
110
°C 230 °F
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Oils
with EP additives, e.g.
automotive |
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rear
axle and differential gear
oils |
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(automotive),
hypoid gear oils |
100
°C 212 °F
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Synthetic
oils |
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Polyglycols,
poly-alpha-olefins |
120
°C 248 °F
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Diesters,
silicones |
110
°C 230 °F
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Phosphate
esters |
80
°C 176 °F
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Greases |
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Lithium
base greases |
120
°C 248 °F
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Polyure,
bentonite, calcium
complex greases |
120
°C 248 °F
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For
sodium and calcium base
greases and |
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other
greases with a maximum
operating |
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temperature
below 120 °C, the
maximum |
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temperature
for the polyamide
cage is the |
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same
as the maximum operating
temperature |
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for
the grease. |
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1)Measured
on the
outside surface of the outer ring |
Polyamide 6,6
For the majority of injection
moulded cages polyamide 6,6 is
used. This material, with glass fibre reinforcement or without, is
characterized by a favourable combination of strength and elasticity.
The mechanical properties like strength and elasticity of polymeric
materials are temperature dependent and subject to permanent changes
under operating conditions, called ageing. The most important factors
that play a role in this ageing behaviour are temperature, time and the
medium (lubricant) to which the polymer is exposed. The relationship
between these factors for glass fibre reinforced polyamide 6,6 is
illustrated in diagram 1.
It appears that the cage life decreases with increasing temperature and
the aggressiveness of the lubricant.
Therefore, whether polyamide cages are
suitable for a specific application depends on the operating conditions
and life requirements. In table 18
the classification of lubricants into "aggressive" and "mild" is
reflected by the "permissible operating temperature" for the use of
cages made from glass fibre reinforced polyamide 6,6 in various
lubricants. The permissible operating temperature in this table is
defined as the temperature, which gives a cage ageing life of at least
10 000 operating hours.
Towards the low operating temperature side,
also a limit can be set since polyamide loses its elasticity which can
result in cage failures. Cages made from glass fibre reinforced
polyamide 6,6 should for this reason not be applied at a
continuous
operating temperature below –40 °C.

Note
Bearings with polyamide
6,6 cages can be operated at
temperatures up to +120 °C. The lubricants generally
used for
rolling
bearings do not have a detrimental effect on cage properties, with the
exception of a few synthetic oils and greases with a synthetic oil
base, and lubricants containing a high proportion of EP additives when
used at high temperatures.
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