Fractured bearing rings: A crack can ruin your day




Your task at the cement plant today is to replace the worn-out bearings on a large roll used to crush raw material used in production. About an hour after driving up a spherical roller bearing onto the tapered shaft of the roll, you hear a sound - a sharp, metallic sound - resonate throughout the shop. You can't immediately find the source of the sound. A bit later, as you start lifting the roll to get it into position, the roll rotates, and you can see what happened: There is a crack in the bearing inner ring, straight through the hardened steel, all the way across. There's the source of that cracking sound!

Investigation: At first, you suspect material failure - the bearing steel may have had an inclusion or other steel defect below the surface. The hoop stress from mounting the bearing on the tapered seat could have found the weak spot, and caused a fracture. However, there is something else at work here. As you slide the bearing off the shaft, right under the crack location, you notice a small steel chip. The chip is about the size of a fingernail trimming, embedded in the shaft. During mounting, that chip got lodged between the bearing bore and the shaft seat. As you mounted the bearing inner ring on the tapered shaft, the stresses on the ring increased. The steel didn't fracture right away, it was sneaky. It waited until you were busy with something else and - CRACK! - that was the sound you heard.

This damage would be classified under the ISO 15243 standard as: Fracture and Cracking: Forced Fracture. The chip created a "stress riser" that focused the force of stretching the inner ring during mounting and exceeded the tensile strength of the steel. The result is a cracked ring - regrettably, a preventable damage.

Lesson learned: Preventable mistakes can have big consequences. In this case, a more thorough inspection of the shaft surface might have caught the small chip. Better yet, eliminate the source of the chip: physically separate your machinery assembly area from machine repair processes, such as grinding and machining. A minimum distance of 30 feet (about 10 meters) is recommended.

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