5. Choosing a bearing type
The type is decided by the load direction and by how much misalignment the housing will give it — usually before any number is computed.
| Type | p | Axial | Where it belongs |
|---|---|---|---|
| Deep groove ball | 3 | moderate, both ways | the default; cheap, fast, forgiving |
| Angular contact ball | 3 | one way, high | stiff and fast — in pairs, preloaded |
| Self-aligning ball | 3 | little | when the shaft will not be straight |
| Cylindrical roller | 10/3 | none | heavy radial load, and it must float axially |
| Tapered roller | 10/3 | one way, high | heavy combined load — always in pairs |
| Spherical roller | 10/3 | moderate | heavy load plus misalignment |
| Crossed roller | 10/3 | both, plus moment | one bearing per robot joint |
Located and non-located
A shaft needs exactly one bearing to fix it axially. The other must be free to slide, or thermal expansion will load both of them until something gives. The classic arrangement is a deep groove ball bearing located, and a cylindrical roller floating — which is precisely why a cylindrical roller takes no axial load: that is its job.
Why a robot joint uses a crossed roller
A joint has one bearing, not two, and that bearing has to take a bending moment as well as radial and axial force. A crossed roller ring does all three because its rollers alternate direction around the race. The moment appears as an equivalent radial load over its own pitch circle:
which is why slew bearings are large in diameter and thin in section: the diameter is what carries the moment, not the amount of steel.