When can you use roller bearings?

Roller bearings can handle heavier radial loads than ball bearings. However, roller bearings don't function well under any significant axial loads. What kind of systems don't have significant axial loads? For example, a hub of a bicycle bottom bracket must have ball bearings because there is significant axial load from the peddling motion. What about Rotor Cranks (www.rotorusa.com)? There are other movable parts as part of a complex crank design. Could the hub connecting two of these other parts use roller bearings?

Roller bearings can be used wherever there are minimal or no axial loads. Sometimes they are used paired with ball bearings which take care of the axial seperately.
Roller bearings can take more radial load true but it is not as much more than ball bearings as you might think. Ball bearings too, have full line contact at full load although the friction is slightly higher than a roller.

a turntable will have sig radial load and minimum axial loads. as will a vertical motor and usually will have tapered brngs. Some ball bearings are designed to have a sig axial load with a large radial load. Everything depends on aplication. The brng mfgrs usually have an application guide on their website

There are many types of bearings and many arrangements of bearings to cope with all types of loads. For instance; tapered roller bearings are commonly used as this arrangement can accommodate both axial and radial loads.