I received a lot of feedback and questions on my article discussing the concern that induction heat could cause raceway surface damage when employing a bearing with ceramic (Si3N4) rolling elements. This article is a summary of a few of my discussions with expansions into other topics.
Does Clearance change the results?
Absolutely! One of the key factors is the initial internal clearance. Clearance is the gap that is taken up during heating. It could cause the rolling elements to press into the raceway surface. In electric motor/pump repair standard clearance is C3. Using a bearing with anything less than C3 I would suggest consulting with your authorized bearing distributor.
Picture 1: Radial Internal Clearance
Does Mounting Take Out All the Internal Clearance?
There is no way to just say yes or no. In theory, bearing life is maximized in preload (or negative clearance.) Certain bearings are designed to operate in preload. For example, tapered roller bearings run preloaded. In general, the recommended fitting practice will leave a residual amount of clearance. If the bearing is designed to operate in preload the application set-up will create ideal conditions.
In electric motor and pump repair the recommended fitting normally results in a clearance. Due to uncontrollable factors it is important that we stay in the clearance range. Picture 2 shows with preload life increases. When clearance is increased life crashes downward like a roller coaster. Missing your tolerance by thousandths or two on fitting, load a little higher than expected, belts a touch too tight; grease a little worn, etc... All contribute to premature bearing failure.
Does Mounting Take Out All the Internal Clearance?
There is no way to just say yes or no. In theory, bearing life is maximized in preload (or negative clearance.) Certain bearings are designed to operate in preload. For example, tapered roller bearings run preloaded. In general, the recommended fitting practice will leave a residual amount of clearance. If the bearing is designed to operate in preload the application set-up will create ideal conditions.
In electric motor and pump repair the recommended fitting normally results in a clearance. Due to uncontrollable factors it is important that we stay in the clearance range. Picture 2 shows with preload life increases. When clearance is increased life crashes downward like a roller coaster. Missing your tolerance by thousandths or two on fitting, load a little higher than expected, belts a touch too tight; grease a little worn, etc... All contribute to premature bearing failure.
Picture 2: Fatigue vs. Operating Clearance
Table 1: Contact stress calculated results
What is the contact stress on smaller steel and ceramic sizes?
For comparison, I added some smaller sizes in both steel and ceramic ball. Additional criteria; steel rings, steel or 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.
Are you implying the only acceptable method for mounting is using an induction heater?
My statement in my introduction was: Bearing manufacturers consider induction heating the safest method to heat bearings for installation.
There are plenty of acceptable methods, with positives and negatives to all. My opinion is that using an induction heater is the best, most repeatable practice. I also believe all methods require proper training to insure all technicians are following the same practices.
Great follow-up questions!
How does Induction Heating Affect Ceramic Coated Bearings?
At this point I do not have an answer! I have asked for this to be studied and hope to have results in the near future.
Where did you get the contact stress numbers?
In my bearing engineering years we called it the Bearing Bible: Essential Concepts of Bearing Technology by Tedric Harris.
Table 9.1 from Mr. Harris' book
How big is the dent?
The resultant dent of exceeding the maximum contract stress is 0.0001 times the ball diameter. For example if the ball diameter is 3/4 inch the dent will be: 0.75 in * 0.0001 = 0.000075 in.
Final Thought:
I want to thank those who read my articles for the questions and comments! It’s a great feeling knowing that time spent in my bearing cave studying topics to discuss is read! I also need to thank our supplier’s engineering team for providing the contact stress calculations.