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.
Bearing Life Temperature Chart
     
Table 18: Permissible operating temperatures for cages of glass fibre
  reinforced polyamide 6,6 with various bearing lubricants
   
 
Lubricant Permissible operating
  temperature1)
Mineral oils  
Oils without EP additives, e.g.  
machine or hydraulic oils 120 °C   248 °F
 
Oils with EP additives, e.g. industrial  
and automotive gearbox oils 110 °C   230 °F
 
Oils with EP additives, e.g. automotive  
rear axle and differential gear oils  
(automotive), hypoid gear oils 100 °C   212  °F
 
Synthetic oils  
Polyglycols, poly-alpha-olefins 120 °C   248  °F
Diesters, silicones 110 °C   230  °F
Phosphate esters 80 °C     176  °F
 
Greases  
Lithium base greases 120 °C   248  °F
Polyure, bentonite, calcium complex greases 120 °C   248  °F
 
For sodium and calcium base greases and  
other greases with a maximum operating  
temperature below 120 °C, the maximum  
temperature for the polyamide cage is the  
same as the maximum operating temperature  
for the grease.  
 
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 Â°C.

Note
Bearings with polyamide 6,6 cages can be operated at temperatures up to +120 Â°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.