Bearings are machine elements that support shafts, wheels, gears, and rotating parts while reducing friction and wear. They let parts move smoothly while carrying forces from weight, torque transmission, belts, gears, or external loads. Choosing the right bearing type matters because it affects efficiency, heat, noise, alignment, service life, and failure risk.
In engineering design, bearings are selected by load direction, speed, accuracy, environment, lubrication, and available space.
Ball bearings use small rolling balls to handle moderate radial loads and some axial loads with low friction at high speed. Roller bearings use cylinders, needles, tapers, or spherical rollers to spread contact over a larger area, so they can carry higher loads than ball bearings of similar size. Thrust bearings are arranged to carry axial loads along the shaft, while plain bearings use sliding contact and a lubricating film instead of rolling elements.
Lubrication separates surfaces, removes heat, prevents corrosion, and strongly affects bearing life, which is often estimated using L10 life calculations.
Key Facts
- Radial load acts perpendicular to the shaft axis, while axial load acts parallel to the shaft axis.
- Rolling bearings reduce friction by replacing sliding contact with rolling contact between races and rolling elements.
- Ball bearings are good for high speed and moderate loads, while roller bearings are better for heavier radial loads.
- Basic bearing life relation: L10 = (C/P)^p, where p = 3 for ball bearings and p = 10/3 for roller bearings.
- Friction power loss can be estimated by P = Tω, where T is friction torque and ω is angular speed in rad/s.
- A lubricant film reduces metal to metal contact, lowers wear, carries heat away, and helps keep contaminants out.
Vocabulary
- Ball bearing
- A rolling element bearing that uses balls between inner and outer races to reduce friction and support mainly radial loads with some axial load capacity.
- Roller bearing
- A bearing that uses cylindrical, needle, tapered, or spherical rollers to carry larger loads by spreading contact over a longer line.
- Thrust bearing
- A bearing designed mainly to support axial force along the direction of a shaft.
- Plain bearing
- A sliding contact bearing in which a shaft moves against a sleeve, bushing, or surface separated by lubricant.
- L10 life
- The estimated life at which 90 percent of a group of identical bearings are expected to still operate without fatigue failure.
Common Mistakes to Avoid
- Confusing radial and axial loads: radial loads push across the shaft, while axial loads push along the shaft, so the bearing arrangement must match the real force direction.
- Choosing only by shaft diameter: a bearing that fits the shaft may still fail if its load rating, speed limit, lubrication, or alignment tolerance is not suitable.
- Assuming more grease is always better: overfilling can cause churning, heat buildup, and seal damage, so lubrication amount and type must match the application.
- Ignoring misalignment: many bearings need accurate alignment, and a small angular error can create edge loading, vibration, overheating, and early fatigue.
Practice Questions
- 1 A shaft is supported by two identical bearings and carries a 1200 N pulley load midway between them. If the load is vertical and centered, what radial load does each bearing support?
- 2 A ball bearing has a dynamic load rating C = 9000 N and an equivalent load P = 3000 N. Using L10 = (C/P)^3 million revolutions, calculate the L10 life in million revolutions.
- 3 A vertical shaft supports a heavy rotating turntable. The main load is the weight of the turntable acting along the shaft axis, while a belt also pulls sideways on the shaft. Explain which bearing types or bearing arrangement could support both loads and why.