Not much holdin' that roller, is there?
In this picture you can see the axle has a polished end. This is because the bushing doesn't run the entire length of the roller and the axle is turned a bit thinner in the center to reduce the area making contact with the bushing. In fact, there are two bushings, one at each end of the roller. I suppose if you must use bushings that this is the way to do it, but if I was going to use this sander for actual sanding I can see how this would become a maintenance nightmare. The axle was quite oily and I'm sure it would be a dust magnet.