MRO Magazine

The strange Hollow Ball Phenomenon

Problem: Your pump has failed. You disassemble it and lo and behold, you find broken, hollow bearing balls amongst the wreckage.


Machinery and Equipment Maintenance

April 1, 2013
By BY CARROLL MCCORMICK

Problem: Your pump has failed. You disassemble it and lo and behold, you find broken, hollow bearing balls amongst the wreckage.

Solution: Don’t chew out your supplier for selling you faulty bearings. You are looking at something that occasionally happens when a pump, typically a process pump operating at high RPM, fails, according to Jennifer Moritz, training manager, SKF Canada.

“This is not a quality control problem. Bearing steel has oxygen inclusions. For some unrelated reason, the bearing begins to fail. The temperature gets so high, as high as 350°C, that the oxygen [inside the bearing] expands and the ball diameter increases. The ball material laps on the raceway, the inclusion grows, the ball grows and laps some more, until the wall of the hollow ball becomes so thin that it bursts. Upon inspection, the ball is seen to be hollow,” Moritz explains.

In the absence of manufacturers’ steel specifications that might allow end-users to compare the number of oxygen inclusions and their maximum size, the best bet is to buy from one of the bearing companies that have high requirements for their bearing steel, Moritz advises. “We have really tight control on the allowable number of inclusions and their maximum size.”

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Mr. O thanks Jennifer Moritz of SKF Canada for this tip!



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1 Comment » for The strange Hollow Ball Phenomenon
  1. Gordon Snieder says:

    That there is gaseous oxygen in bearing steel is news to me. Oxides and silicates probably but oxygen? Certainly not enough to turn a ball bearing into a steel balloon.

    Then why don’t we see high pressure steam lines spontaneously hollowing out and exploding? The temperature there gets much higher than 350°C.

    I suggest you investigate ‘country of origin’ as a possible cause; i.e., the steel is junk.

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