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Unexplained Rolling Bearing Failure

A case study by Gert Neukirch, Dipl.-Ing.

The Situation

The customer reported unpredictable failures of a rolling bearing in their conveyor system. The bearing, a 6210-2ZR, sits on the conveyor’s deflection roller and, according to the customer, runs at 250 °C and 60–80 rpm.

The customer had no idea what was causing the failures, so we arranged an on-site visit to investigate.

Analysis

Our on-site analysis showed that:

The temperature actually measured at the bearing was only 220 °C, 30 °C lower than stated.
The deep groove ball bearing in use wasn’t suitable for the operating temperature, and its load capacity was borderline for the loads involved.
The high-temperature grease in use, a polyurea grease with a PAO/ester base oil, couldn’t cope with the heat and would have needed constant relubrication, even at 220 °C.

Possible Measures

The customer could solve the problem with the following measures:

Machine modification: switch to a double-row deep groove ball bearing

A double-row deep groove ball bearing would increase load capacity. However, it would require significant structural changes that could only be carried out during an extended shutdown. For these two reasons, we ruled this option out.

Switch to a different grease with automatic daily relubrication

At the next bearing change, a new PFPE-based grease (branched or linear) could be used. It could be fed in daily via a suitable electromechanical automatic lubricator, with the relubrication quantity precisely defined by us. This measure would be expected to prevent machine failure for at least 12 months.

Machine modification: switch to a heat-stabilised 100Cr6 bearing

A heat-stabilised bearing made of 100Cr6 steel (e.g. a 6210-2Z/S1), martensitically hardened and tempered at a higher temperature (up to 62 HRC), would also help address the problem.

Our Recommendation

Provided process temperatures don’t rise significantly in future, we recommended that the customer implement measures 2 and 3.

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