I've always thought it was from improper pre-load! Getting the pre-load right is finicky & usually requires several iterations to get right, something not done by the factory.? What you want is no measurable axial play & minimal bearing heating at full speed.
A reasonable upgrade is to replace the bearings with 7206, an angular contact bearing that's the same size as the 6206 deep groove bearings originally installed. They're available sealed,? to keep crud out. They can handle a higher axial load than their deep groove counterparts without the hassle of converting to unsealed taper? roller bearings.
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Roy (retired M.E.)
