ball bearings - fewer bigger balls or more smaller balls

Mizzmo

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the ultimate case is something like a turbo tubine spindle - most have no elements at all and rely only on the oil film as the bearing medium...
Thats common on high load, low speed applications as well. Ship thrust and line bearings also completely rely on the oil film. 

 

slug zitski

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Thats common on high load, low speed applications as well. Ship thrust and line bearings also completely rely on the oil film. 
Those oil bath thrust and roller bearings are also common on yachts

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DDW

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Oil film bearings are high load, but not generally low friction. They are used in places you might not expect (like disc drives and precision lathes) due to more predicable motion characteristics. An air bearing is a film bearing, air replaces the oil and being a lot less viscous has a lot less friction. One of the main reason sleeve oil film bearings are used in turbochargers is to accommodate the imbalance of the rotating assembly - the axle is allowed to do the hula in the bearing (and the bearing in the housing). 

Evans, if you have the two bearings it is pretty easy to test the rotating friction: wrap a string round the shaft and hang a weight. You have to get it started to overcome static friction, then pick a weight which results in constant velocity. From there it is easy to calculate friction. I have some machine tools for which this is the prescribed procedure to set bearing preload. 

 

estarzinger

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Evans, if you have the two bearings it is pretty easy to test the rotating friction
yea, I was trying to avoid buying both . . . but I concluded that if I wanted the answer I would have to . . so I have.  The one with the bigger bearings won . . . but I am still not sure it was the bearing size that made it win as there are other differences between the two.  In any case I know which to use.  

How fast is it supposed to spin? What kind of static and dynamic loads does it bear?
As I indicated above - relatively low rpms (ballpark 100) and relatively low loads (ballpark several 100kgs).

how clean are the bearings going to be kept?  Go the bigger size if they're going to be exposed to a lot of crap and cleaned sporadically.
This was a useful practical comment - thanks.  I think it will be a relatively clean environment but It is not going to get a lot of maintenance. 

 
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Seems to me that the majority of friction will be due to the scheme for keeping the balls properly spaced (or not: full complement). I've seen cages, spacers, counter-rotating-slightly-smaller-balls, etc. All of these have some sort of drag, especially when used with lubricant and the associated seals/shields necessary to keep said lubricant captured/clean. The only practical way of comparing bearings would seem to be testing.

That said, the original question of larger or smaller balls may have a difference when it come to acceleration/deceleration which is mathematically beyond me any more ...

 

estarzinger

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Seems to me that the majority of friction will be due to the scheme for keeping the balls properly spaced (or not: full complement). 

That said, the original question of larger or smaller balls may have a difference when it come to acceleration/deceleration which is mathematically beyond me any more ...
I agree with both points.  The bearing that tested better had lighter pressure on the ball capture and a better raceway coating, and I have no way to prove it but I believe those two factors contributed more to it winning on drag than the ball size.

And the pure ball size question seems to be difficult and I suspect relatively small effect - it is interesting that @SimonGH link is a pretty exhaustive lower frictional search and yet does not mention this issue at all - making me think it is like a 3rd order effect.

 
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SCARECROW

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I agree with both points.  The bearing that tested better had lighter pressure on the ball capture and a better raceway coating, and I have no way to prove it but I believe those two factors contributed more to it winning on drag than the ball size.

And the pure ball size question seems to be difficult and I suspect relatively small effect - it is interesting that @SimonGH link is a pretty exhaustive lower frictional search and yet does not mention this issue at all - making me think it is like a 3rd order effect.
this is supported by research I did when racing Solar cars in the late 90s.  All the data suggested that self aligning bearings had lower friction than standard bearings when everything else was equal.

 

nota

Anarchist
REAL RACE QUALITY TURBO'S USE BB not oil but cost more

may not last as long as oil bearings  as they spin faster

there are sealed BB but small seal drag added many better bikes used sealed

how do needle  rollers  effect drag they have more load area

 
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