Selecting a bearing is not simply a matter of choosing the same number from a catalogue that looks close to your shaft diameter. A bearing is part of the machine’s load path, and its selection affects life, noise, temperature, alignment, maintenance, and reliability.
A practical bearing selection process starts with four questions: What loads act on the bearing? How fast does it rotate? How long must it last? How will it be mounted?
Radial and axial loads
Radial load acts perpendicular to the shaft. Axial load acts along the shaft. Some applications have mostly radial loading, while others have a significant combination.
Deep-groove ball bearings can handle radial load and a degree of axial load. Other bearing families may be better suited to high axial loads, heavy radial loads, misalignment, or very high stiffness.
Calculate the actual bearing load
Do not use the machine motor rating as a substitute for bearing load. Calculate the forces generated by gears, belts, chains, pulleys, couplings, gravity, and external equipment.
Then use the shaft free-body diagram to determine the reaction at each bearing.
Bearing life
Rolling bearings are commonly selected using a rating-life relationship involving dynamic load rating and equivalent dynamic bearing load. The exact exponent depends on bearing type.
The important concept is that a small change in load can have a large effect on calculated life. This is one reason reducing unnecessary bearing load can be more valuable than simply buying a larger bearing.
Speed and temperature
A bearing rotating at high speed generates heat. Lubrication type, internal clearance, cage design, sealing, and operating temperature all become important.
Do not assume a bearing rated for a static load will automatically work at a high rotational speed.
Fits matter
The inner ring fit on a rotating shaft and the outer ring fit in a housing depend on which ring experiences rotating load, load magnitude, temperature, and mounting requirements.
A fit that is too loose can allow creep. A fit that is too tight can reduce internal clearance and increase friction or heat.
Internal clearance
Bearing internal clearance is the total movement possible between the rings before mounting and operating conditions are considered. The selected clearance class should account for fit, temperature, speed, and application.
This is one reason bearing selection cannot be separated from shaft and housing design.
Lubrication
Grease is common because it is simple and provides sealing benefits. Oil may be preferred for high-speed or high-temperature applications and for systems where heat removal is important.
Use the bearing manufacturer’s lubrication recommendations rather than guessing based only on the grease already used elsewhere in the machine.
Alignment
If the shaft and housing are not aligned, the bearing may experience additional loading. Self-aligning bearing types can help in suitable applications, but they should not be used to hide poor structural design.
Practical bearing-selection workflow
- Calculate radial and axial reactions.
- Determine operating speed.
- Estimate required life.
- Check static load capacity for severe or shock loads.
- Select a suitable bearing family.
- Calculate equivalent dynamic load and life.
- Check shaft and housing fits.
- Check internal clearance and operating temperature.
- Choose lubrication and sealing.
- Confirm mounting, maintenance, and replacement access.
Think about the complete assembly
A bearing cannot compensate for a weak shaft, distorted housing, poor shoulder geometry, or excessive belt tension. During design review, check the complete load path.
Final takeaway
Bearing selection is an engineering decision, not a catalogue shortcut. Load, speed, life, fit, clearance, lubrication, alignment, and maintenance all need to agree.
If you select the bearing only from the bore diameter, you have selected only one part of the problem.