Question

why is plastic used in place of metals

Answer 1

They’re self-lubricating! As soon as they start moving, the inherent qualities of the plastic lubricate the bearing, reducing any delay in start-up and keeping the area clean. Microscopic bits of the bearing abrade and serve to lubricate the bearing at the outset, but there is negligible change to the bearing itself.  This also makes plastic bearings better for food applications because the FDA strictly limits the use of lubricants in food-production machinery. In addition, while dust and other particles will stick to lubricants and create a layer of grime, with plastic bearings, any particles will simply embed themselves in the bearings, having no effect of performance.