Modern roller bearings are very good but keep rollers in place with seats, separators or a cage and use lubricants to allow slippage. Working machinery generally need new bearings every year or two.
Between concentric races my bearing uses a second ring of rollers to keep rollers in place and share the load evenly across rollers.
Roller and race size relationships are contained in the equation:
(i+o)2=(w−o)2+(a+i)2−2(w−o)(a+i) cos(II/n)
where:
While radii have been more useful in development, note that this equation may substitute diameters. Note that typical diameter measurements of bearings will have to add in the thickness of the races.
When selecting sizes for a bearing that is well hung, check that inner rollers don't touch, outer rollers don't touch or allow an opposing roller to drop through.
The limits for the inner roller gap are: 0<2(a+i)sin(II/n)−2i<2o
The limits for the outer roller gap are: 0<2(w−o)sin(II/n)−2o<2i
All rollers should be of equal length to avoid unequal wear. In assembly, final parts may need a slight rotation to seat firmly. End caps, race lips, matching groove and ridge or magnets may keep rollers aligned on the races.