Learning topic
Bearings and Couplings
Plain and rolling bearings, shaft support arrangements, couplings, alignment and basic life calculations.
Bearings locate and support shafts while permitting relative motion. Couplings connect shafts to transmit torque and may compensate for misalignment, damp vibration, or protect a drive from overload.
Bearing systems
Plain bearings carry load through sliding surfaces and, where possible, a lubricating film. Rolling-element bearings use balls or rollers between raceways. A shaft system must define axial location without thermal overconstraint; a common arrangement combines one locating support with one non-locating support.
Loads and selection
Reactions are determined from the complete shaft free-body diagram. Bearing selection depends on radial and axial loads, speed, life, stiffness, accuracy, temperature, lubrication, contamination, installation, and available space.
For rolling bearings, basic rating life is expressed as
$$L_{10}=\left(\frac{C}{P}\right)^p10^6\ \text{revolutions},$$
where \(C\) is dynamic load rating, \(P\) the equivalent dynamic load, and \(p=3\) for ball bearings or \(10/3\) for roller bearings.
Couplings
Rigid couplings require accurate alignment. Flexible couplings accommodate limited radial, angular, or axial misalignment and can modify torsional dynamics. Safety and overrunning couplings add overload protection or directional function. The nominal torque is increased by a service factor:
$$T_d=K_AT.$$
The complete assembly must also be checked for keys or splines, hub and shaft stresses, fasteners, fatigue, balance, heat, and guarding.
System approach
Bearings and couplings cannot be selected independently of the shaft and housing. Bearing arrangement controls axial location and thermal expansion; coupling stiffness and misalignment forces affect bearing reactions; shaft deflection changes bearing load distribution and seal performance.
Locating and non-locating supports
A locating support restrains the shaft axially in both directions. A non-locating support permits thermal displacement either inside the bearing or through a sliding ring fit. Two opposed angular-contact or tapered-roller bearings may form a locating set, but their preload and thermal response must be controlled.
Static and dynamic bearing load
Static capacity protects against permanent indentations under peak or stationary load. Dynamic capacity relates to rolling-contact fatigue under repeated revolutions. Shock, oscillation, contamination, misalignment, and poor lubrication can shorten real life far below the basic rating-life prediction.
Fits and assembly
The ring experiencing rotating load relative to its seat generally requires interference to prevent creep. Interference, temperature difference, and tight fits reduce internal clearance. Assembly force must pass through the fitted ring, never through rolling elements. Shoulders, nuts, sleeves, spacers, and end covers must provide reliable axial retention.
Coupling influence
A coupling generates reaction forces when displaced from its neutral position. Manufacturer stiffness values can be used to estimate these forces and include them in the shaft-bearing model. Flexible elements also affect torsional natural frequencies and transient peak torque.
Selection sequence
- Determine shaft reactions, axial location, speed, duty, and environment.
- Select bearing types and the locating arrangement.
- Calculate equivalent loads, life, and static safety.
- Choose internal clearance, preload, fits, lubricant, and seals.
- Select the coupling from design torque, speed, bores, misalignment, and dynamics.
- Verify the complete assembly for heat, stiffness, mounting, inspection, and maintenance.