Pick a bearing by matching it to the load, speed, precision and space you have — then check it survives long enough. Get the type right first; the size follows.
Type by load direction
Deep-groove ball bearings are the default: cheap, low-friction, take radial and some axial load. Angular-contact balls take combined radial + thrust and are used in pairs. Tapered roller bearings carry heavy combined loads (wheel hubs, gearboxes). Cylindrical roller take heavy radial load but little thrust. Thrust bearings take pure axial load. Plain (bushing) bearings suit low speed, high load, dirt or no lubrication.
Speed, precision and friction
Balls spin faster and cooler than rollers; plain bearings are speed-limited by the PV (pressure × velocity) limit. For spindles and instruments, precision (ABEC/ISO class) and preload matter. Higher speed usually means grease → oil → oil-mist lubrication as you go up.
Life and sizing
Rolling bearings are sized by L10 rating life: L10 = (C/P)^p million revolutions
(p = 3 for balls, 10/3 for rollers). Run your numbers in the Bearing Life tool. Double the
load and life drops ~8×, so a small load reduction buys a lot of life. Add a service factor for shock loads.
Mounting matters as much
One bearing is usually located (takes thrust, axially fixed) and the other floats to allow thermal growth — clamp both and you'll preload the shaft to death when it heats. Respect the fits (see choosing a fit): rotating ring gets an interference fit, stationary ring a close clearance. Seal it against the actual environment.