Can I use a 6305ZZ bearing

[davidblythe]

on a 60's 2 stroke lambretta the mains bearing is a 6305 which is lubricated by oil splash from the gearbox, clutch and transmission chain, whilst the crank side has a hallite gasket to seal a retaining plate which contains an oil seal in order to stop this oil passing through to the crank

people are now using a 6305RS bearing for the benefit of an additional oil seal crank side, however the limiting speed of the bearings is only 7500rpm, power outputs are now 3 times as standard from the 60's and 8000rpm is common, meaning the bearings are failing quickly from this and also contimation from the gearbox oil

reading on the internet the 6305ZZ bearing has a much higher limiting speed, could i use this bearing or am I missing some obvious point, I guess the idea of 2 metal shields is making me nervous

any help or advice appreciated

 

[tbuelna]

davidblythe,

At the dN your 6305 bearing sees at 8000 rpm, a lip seal would have a rubbing speed in excess of 5000 fpm, which is way too high. So you would likely have better luck with a (non-contacting) metal shield, even though it has some small leakage. And a single shield would be highly preferable to two shields, since a continuous and free flow of oil through the bearing is absolutely necessary to control the bearing temps.

Rolling element bearings typically fail due to surface spalls in the raceway. These surface spalls are the result of high contact stresses causing fatigue failures. The surface spalls initiate at surface pits (due to debris passing thru the bearing) or at subsurface fractures (due to material shear failures). The surface pits can be prevented by keeping the lubricant flow through the bearing free of debris. The subsurface fractures can be minimized, in part, by keeping the bearing race temperatures within the allowables for your bearing material and heat treatment. The typical E52100 alloy bearing material begins to lose strength above about 300degF.

The basic dynamic load rating on a typical off-the-shelf single row, deep groove ball bearing like your 6305, is about 5000 lbs. Once you apply all of the life adjustment factors for things like temperature, speed, lubrication, misalignment, material quality, surface finish, operating lambda, etc., your calculated bearing L10 life becomes amazingly short.

Even though the stock Lambretta main bearing may have had a 6305 dimensional cross section, it quite possibly was different internally than a commercial 6305 ball bearing. Bearing life can be significantly affected by slightly larger ball diameters, material and heat treat quality, and raceway osculation curvatures. If you can't squeeze a larger OD bearing (ie. a 6405) into your existing crankcases, give your local bearing supplier a call and ask if they can supply a 6305 bearing with higher grade materials and a better precision class. It won't be cheap, but it will give you better bearing performance.

Good luck and stand on the gas.
Terry