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ball bearings - fewer bigger balls or more smaller balls


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Is this a bike application? Hmm, I'm not sure. Intuitively I'd think more bearings are required for higher loads due to higher number of points of contact. And more bearings = more drag. And any grease = more drag. Light oil = probably minimal increase in drag if not reduction in drag. You'd never use grease if low drag really matters. Grease is just preferred to have a long lasting lubricant.

This seems a reasonable review.  https://ridefar.info/bike/cycling-speed/mechanical-resistance/

More than you want to know from the biggest and best in the industry, SKF

https://www.skf.com/binaries/pub12/Images/0901d196807026e8-100-700_SKF_bearings_and_mounted_products_2018_tcm_12-314117.pdf

When I've had to design machine parts that need bearings, I look at the load/expected life/speeds etc and pick a bearing. Because the drag of ball bearings is pretty low already, I've never had to consider it as a factor.

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I have a choice between two 'good brand' bearing sets, both use ceramic bearings, both speced for the loads expected.  One has balls about twice the diameter of the other.

It is a relatively low speed and relatively low load application - I'm not worried about heating nor about breaking, but do want to choose the option with least drag.  I can't seem to get apples to apples numbers on drag from the mfg's.

I may be missing it but I can't see that the bicycle or skating links above address the question.  I have to admit I skimmed the skf document, but I also did not immediately see it addressed.

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Does the Number of Balls in a Bearing Matter?

Ultimately, yes. Will you ever know the difference in real life use? Probably not. Look, the discussion is simply around friction vs. durability. If you have fewer balls, each ball in the bearing must bare more pressure or load. Therefore, the fewer the bearings the more likely a ball might fail due to pressure. Conversely, the fewer the balls in the bearings the less friction the bearing has. So, supposedly you will go faster

 

it was sort of hidden in this answer

less balls = less friction

bigger = less balls ( usually )

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7 hours ago, phill_nz said:

Does the Number of Balls in a Bearing Matter?

Ultimately, yes. Will you ever know the difference in real life use? Probably not. Look, the discussion is simply around friction vs. durability. If you have fewer balls, each ball in the bearing must bare more pressure or load. Therefore, the fewer the bearings the more likely a ball might fail due to pressure. Conversely, the fewer the balls in the bearings the less friction the bearing has. So, supposedly you will go faster

 

it was sort of hidden in this answer

less balls = less friction

bigger = less balls ( usually )

Small balls gives more surface contact . More load bearing surface 

Concerning friction , think of a sheet of 36 grit sandpaper compared to a sheet of 220

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10 hours ago, phill_nz said:

Does the Number of Balls in a Bearing Matter?

Ultimately, yes. Will you ever know the difference in real life use? Probably not. Look, the discussion is simply around friction vs. durability. If you have fewer balls, each ball in the bearing must bare more pressure or load. Therefore, the fewer the bearings the more likely a ball might fail due to pressure. Conversely, the fewer the balls in the bearings the less friction the bearing has. So, supposedly you will go faster

 

it was sort of hidden in this answer

less balls = less friction

bigger = less balls ( usually )

I actually saw that bit, but was not sure exactly how to interpret it.  I can understand if the balls are the same size that fewer = higher loads but less friction (unless they deform).  And I think what they are talking about is the practice of removing a few balls from the race (some people do that with harken cars also); but was not sure if bigger + fewer = bigger total contact patches or smaller? The raceways are shaped to the bearings so there is contact around the circumference which obviously grows with diameter. 

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2 hours ago, slug zitski said:

Small balls gives more surface contact . More load bearing surface 

Concerning friction , think of a sheet of 36 grit sandpaper compared to a sheet of 220

I believe you are wrong the first point here. Most sources I have seen suggests bigger means more load carrying capability

On the 2nd, your analogy just sucks, as the sand on sandpaper does not roll, it is more like a bushing than a bearing.  Obviously a smooth bushing is better than a rough one, but it is not obvious to me that means small bearings are less friction than bigger ones.

Bigger ball bearings do seem (from youtube demonstrations) to roll down an incline (race shaped) ramp faster than smaller ball bearings - that would seem to suggest less frictional forces on the bigger bearings, but I'm not sure how momentum plays into that.

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these are the guys crazy about rotational friction...

yet not one mention of bearing choice / preference...

https://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1500&context=mesp

which leads me to conclude that you should choose the lowest number of elements that you can reasonably find that meet the load / speed requirements, and lubricate sparingly. 

 

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It doesn't answer the question, but it was news to me:

 

Bearing friction is not constant and depends on certain tribological phenomena that occur in the lubricant film between the rolling elements, raceways and cages.

Diagram 1 shows how friction changes, as a function of speed, in a bearing with a given lubricant. Four zones are distinguishable:

  • Zone 1 – Boundary lubrication condition, in which only the asperities carry the load, and so friction between the moving surfaces is high.
  • Zone 2 – Mixed lubrication condition, in which a separating oil film carries part of the load, with fewer asperities in contact, and so friction decreases.
  • Zone 3 – Full film lubrication condition, in which the lubricant film carries the load, but with increased viscous losses, and so friction increases.
  • Zone 4 – Full film lubrication with thermal and starvation effects, in which the inlet shear heating and kinematic replenishment reduction factors compensate partially for the viscous losses, and so friction evens off.

0901d19680648f47-1706-0015---17000-EN-w_

Starting torque

The starting torque of a rolling bearing is defined as the frictional moment that must be overcome by the bearing to start rotating, at an ambient temperature of 20 to 30 °C (70 to 85 °F). Therefore, only the sliding frictional moment and the frictional moment of seals, if applied, are taken into consideration.

image.thumb.png.e73d68441f7e8a5f02db6f46c50997bd.png

https://www.skf.com/us/products/rolling-bearings/principles-of-rolling-bearing-selection/bearing-selection-process/operating-temperature-and-speed/bearing-friction-power-loss-and-starting-torque

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1 hour ago, SimonGH said:

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...

 

Thermal expansion and centrifugal force affect ball bearing 

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1 hour ago, SimonGH said:

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. 

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20 minutes ago, Mizzmo said:

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

FA4BBF4D-E382-437E-B56B-E7E79D141BE0.png

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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. 

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On 1/10/2022 at 4:45 PM, DDW said:

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.  

  

On 1/9/2022 at 7:54 PM, IStream said:

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).

  

On 1/9/2022 at 11:16 PM, SCARECROW said:

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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16 minutes ago, estarzinger said:
On 1/9/2022 at 4:54 PM, IStream said:

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).

@estarzinger, sorry I never saw this so didn't respond. Sounds like you got it worked out. 

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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 ...

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1 hour ago, Feisty! said:

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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5 hours ago, estarzinger said:

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.

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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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