The set up

The set up
5.46mm jet delivering 0.68 l/s to the pelton which is rotating at 900 rpm and generating 135 watts into the grid.

Saturday, 4 July 2015

Post mortem on ceramic bearings

You can learn a lot from a careful inspection of a failed bearing. Yesterday I learned the story of why the trial of ceramic bearings came to an end after just 28 days: - one ball in the bearing at the Smart Drive end of the shaft appeared to have a manufacturing defect:

As the bearing rotated, this defective ball intermittently caused the inner race to 'lock up'.  When this happened, the inner race started to turn on its journal on the shaft.  Being harder than the stainless steel of the shaft, the journal got ground down:

I had noticed there was a slight 'catch' to smooth rotation of the shaft even before I had installed the housing with the new bearings, so there is reasonable certainty that this defective ball was indeed present from the outset, and did not become damaged during the 28 days of operation.

It's satisfying to have reached an explanation for this premature failure.  Just possibly, if it was a manufacturing defect, it might not mean it's totally the end of the road for using ceramics in a Powerspout.

For the moment however, the turbine continues to generate at lower flows than were possible in its first year of operation: presently putting out 223W on 0.96 litres per sec.  Nice to see the benefits being realised of the changes made to extend generation into these drier months.

The question now is: how low a flow can I usefully go to ?   All will be revealed as the dryness of summer progresses.

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