Types of bearings #
Plain bearings are also called bushings, journal bearings, or sleeve bearings.
A ball bearing uses spheres; a roller bearing uses cylinders; and a needle bearing uses real skinny cylinders.
How ball bearings are made #
For a small, cheap ball bearing like those used in skateboard wheels, we start with an outer ring and an inner ring.
(picture of outer ring and inner ring)
We load 7 loose balls between the rings. We would like to fit more, but we can’t fit them in.
(picture with 7 balls loaded in)
Then we arrange the balls evenly around the inner ring and weld in a ball cage to keep them spread out.
(picture with cage in place)
Finally, we add a shield or seal. Both increase friction, but it can be worth it to keep dust out and grease in.
Side note: for larger, fancier bearings, we can make a small groove in the side of the outer race to allow more balls to be added. Then we weld, grind, and polish that groove away. The added balls let the bearing take high loads with less deformation, which probably leads to longer life.
Preloading ball bearings #
When ball bearings are manufactured, the rings are sized so that they are a little loose on the balls. To make the bearings run smoothly, you want to preload them. This means pushing the inner ring sideways slightly compared to the outer ring to take up the slack. You preload the bearings with a slightly compliant nut or spacer so that when the bearings heat up as they run, they can push against the preload and still run smoothly.
(picture of face-to-face and back-to-back bearing preloading)
Skateboard bearings dimensions #
Inner diameter: 8 mm Outer diameter: 22 mm Width: 7 mm Number of balls: 7