6 ball versus 7 ball ceramic bearing design 1

[toolingguy]

6 ball ceramic versus 7 ball design in a "608" size. The application would be in skateboards. I have been asked this question and feel that maybe 6 balls would generate too much in the way of sympathetic harmonics in the cage leading to vibration and that 7 balls would help to cancel this out. I'm not a bearing expert but you guys are. Any help would be appreciated.

 

[diskullman]

A skateboard? I don't think the design change would cause much problem until you reach a 120 MPH, at which point, you may want to roll up the windows. You may want to recall that the earliest skateboard models used unground garage door wheels.

Russell Giuliano

 

[dimjim]

I would expect less amplitude with more balls
but more frequencies of the impulse loads.

 

[Heckler]

I guess one advantage of going with 6 balls is you can use larger balls then a 608 with the 7th ball. I think you greatest challenge will be what lubrication to use and how you will keep those balls clean.

 

[Tmoose]

How do conventional bearings fail?
Are the HC bearings being touted as a speed secret due to "reduced friction?"

HC bearings generally have the curious characteristic of higher race contact stress (due to stiffer ceramic material, picture riding a narrow tired racing street bicycle on beach sand) but longer life (for tribological reasons). Resistance to denting under impact is lower, size for size. A dented bearing sings a very unhappy song.

If the housing supporting the bearing is not at least 2X outer race thickness, and made of steel, then the race and housing will likely ripple like an anaconda swallowing a pig as each ball passes thru the load zone.

 

[toolingguy]

"Are the HC bearings being touted as a speed secret due to "reduced friction?"

Yep. Exactly. I know loading is a problem as ceramic isn't exactly "load bearing material"

Some people just want too much.


thanks for your replys!

 

[volpegrigia]

Many skeptics about ceramic balls. I was one of them until I visited Norton (now Cerbec). I was given a hammer, a ceramic ball and some plastiline. I placed the ball on an anvil with the help pf the plastline and whecked it with the hammer. The ball stayed intact butb the hammer had a huge brinell mark on it (and so did the anvil). Ceramic balls are used in high speed bearinsg for two reasons.
a: the lower mass reduces the centrifugal foece on the outer ring
b: the thermal properties prevent ball failure due to heat accumulation.
Steel balls have limited possibilities to shed heat through conduction and rely on large flow of fluid to maintain the temperture within acceptable range (as to maintain the properties of the material). By accumulating too much heat, the balls could reach the temperature where the material properties change and the strength beins to decay with the ultimate result of complete bearing failure.
In the samll bearing you are using, the ball mass is rather small and the material selection is probably attributable to fear of heat accumulation under continuous use at high speed.