ball bearings - fewer bigger balls or more smaller balls

E

estarzinger

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What are the pros and cons?

I am primarily interested in reduced friction/drag.

Does the drag from even super light lub/grease way overwhelm any differences in bearing size?

The axle size is fixed.

 
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IStream

IStream

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How fast is it supposed to spin? What kind of static and dynamic loads does it bear?

 
Zonker

Zonker

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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.

 
E

estarzinger

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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.

 
phill_nz

phill_nz

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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 )

 
slug zitski

slug zitski

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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 )
Click to expand...
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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E

estarzinger

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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 )
Click to expand...
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. 

 
E

estarzinger

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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
Click to expand...
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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slug zitski

slug zitski

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To reduce friction , high speed bearing races only use 4 ball bearings 

The sandpaper analogy is the more particle surface contact the more friction 

4F9C446C-A509-4338-91DA-53A30A0692A9.png

 
D

DDW

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If you after the lowest friction, why not an air bearing? In low speed operation they are nearly frictionless.

 
W

weightless

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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_tcm_12-297279.png



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.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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SimonGH

SimonGH

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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...

 
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