I am agreeing with jcc on this one. It is straight out of the Machine Design textbook.

Best way I can describe it is, the surface where the wheel center bottoms on the axle flange is EXACTLY like a clutch disk and flywheel. The axle studs are not supposed to see any shearing. If your wheels were contacting the studs and wearing on them, you have a "clutch disk" slipping because the "pressure plate" isn't clamping hard enough.

The 5/8" studs can be torqued tighter and that will provide the clamping force necessary.

The wheel centering rings are for mounting the wheel, getting it centered.

R.