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Bud's Take

I was asked a very good question during a recent trade show. I would like to share my answer here.  Bearing manufacturers consider induction heating the safest method to heat bearings for installation.  The concern is will heat expansion damage the raceway surface when using a bearing with ceramic (Si3N4) rolling elements (balls.)   

Explanation of the Question: 

This was a valid and well thought out question.  We know that ceramic material is much stronger in compression than standard bearing steel (52100, SUJ2) and has a lower rate of thermal expansion.  This results in a reduction of the size of the contact ellipse.  If induction heating expands the inner ring, does the contact stress cause permanent deformation (damage)?  

Picture 1: Concept of Brinell Hardness
A brinell is the mark left in metal that is created by another piece of metal (or hard object.)  In the book Essential Concepts of Bearing Technology by Tedric Harris he states, “To cause permanent deformation (0.0001 times ball diameter) in the raceway of a ball bearing the contact stress would be greater than or equal to 609,000 psi.” 

Calculation: 

A complicated calculation is required to determine true contact stress. I asked one of our vendors with advanced computer engineering programs to run the calculations.  The criteria used were; standard bearing steel for inner ring and outer rings, ceramic balls, C3 clearance with a heating temperature of 220 degrees F.  To create worst case scenario we assumed zero expansion on the outer ring. 

Table 1: Contact stress calculated results

Induction Heater

Picture 2: Damage from bearing spinning on shaft

Example 1, C=100 lbs. 

L10 = (10^6/60*1000) (100/10^3 = 16,667 hours

Example 2, C=125 lbs. 

L10 = (10^6/60*1000) (125/10)^3 = 32,552 hours

Example 3, C=150 lbs. 

L10 = (10^6/60*1000) (150/10)^3 = 56,250 hours

P = dynamic equivalent load, we’ll use 10 lbs.       

p = constant:  ball bearing 3, roller bearing 10/3

n = rotational speed (rpm), we’ll use 1000 rpm

Conclusion: 

First, thank you to my customer for the super question. Thank you to my supplier’s engineering team for running the contact stress calculations.   

I am happy to report, based on the worst case scenario criteria put forth, the results proved that induction heating will not cause enough contact stress to result in brinell damage.  In the bearing sizes we reviewed, the resultant contact stress was more than 50% less than the maximum allowable contact stress of 609,000 psi.