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KT0403: Concepts, terms, and terminology related to bushes and bearings
Bearing loads
The bearings that support the shafts of motors and pulleys are subject to a radial load.
Bearings typically face two kinds of loading: radial and thrust. Depending on where the bearing is being used, it may see all radial loading, all thrust loading, or a combination of both. The bearings in the electric motor and the pulley pictured above face only a radial load. In this case, most of the load comes from the tension in the belt connecting the two pulleys.
The bearings in this stool are subject to a thrust load.
The bearing above is like the one in a barstool. It is loaded purely in thrust, and the entire load comes from the weight of the person sitting on the stool.
The bearings in a vehicle wheel are subject to both thrust and radial loads
The bearing above is like the one in the hub of your vehicle wheel. This bearing has to support both a radial load and a thrust load. The radial load comes from the weight of the vehicle; the thrust load comes from the cornering forces when you go around a turn.
Bearing terminology
Bearing components
Bearings make the rotation of objects smooth. Bearings are made up of components that allow two surfaces to roll over each other while also reducing friction. Balls or rollers act as rolling elements between races of the bearing assembly. Because of their round shape, bearings support radial (forces from above and below), axial (lateral), combined (radial and axial), and moment (tilting) forces.
A typical rolling bearing consists of the following components:
A cage.
An inner ring.
An outer ring.
Balls or rollers, as rolling elements.
There are several different bearing types. Bearings capped on both sides are factory-filled with grease. Compared to external sealing, they provide an economical and space-saving solution.
Internal clearance
Bearing internal clearance is defined as the total distance through which one bearing ring can be moved relative to the other in the radial direction (radial internal clearance) or the axial direction (axial internal clearance).
In almost all applications, the initial clearance in a bearing is more significant than its operating clearance. Two effects mainly cause the difference:
- Bearings are typically mounted with an interference fit on the shaft or in the housing. The expansion of the inner ring or the compression of the outer ring reduces the internal clearance.
- Bearings generate heat in operation. Differential thermal expansion of the bearing and mating components influences the internal clearance.
Sufficient internal clearance in a bearing during operation is essential. Preload (clearance below zero) is possible for certain bearing types. Bearings are available in different clearance classes to enable the selection of the appropriate initial internal clearance to achieve the desired operational internal clearance. ISO has established five clearance classes for many bearing types.
| ISO clearance class | Internal clearance |
| Group 2 | Smaller than normal |
| Group N | Normal |
| Group 3 | Greater than normal |
| Group 4 | Greater than Group 3 |
| Group 5 | Greater than Group 4 |
Heat and surface treatment
Rolling bearing rings and rolling elements must:
- Be hard enough to cope with fatigue and plastic deformations.
- Be tough enough to cope with applied loads.
- Be sufficiently stable to experience only limited changes of dimensions over time.
This is achieved through heat and surface treatments.
a. Hardening
Three typical hardening methods may be applied to bearing components:
- Through-hardening—This is the standard method for most bearings and provides good fatigue and wear resistance, as hardening is applied over the entire cross-section.
- Induction-hardening – Surface induction-hardening is used to selectively harden a component’s raceway to limit rolling contact fatigue, leaving the remainder of the component unaffected to maintain structural strength.
- Case hardening – Case-hardening provides hardness to the surface. It is used, for example, where bearing rings are subjected to high-impact loads causing structural deformations.
b. Dimensional stability
Heat treatment is used to limit dimensional changes caused by metallurgical effects at extreme temperatures. There is a standardised classification system for dimensional stability. The various SKF bearing types are stabilised to different classes as standard.
c. Surface treatment and coatings
Coating is a well-established method for providing bearings with additional functional benefits to accommodate specific application conditions. Widely used coatings are zinc chromate and black oxide.
Two other methods developed by SKF have proven successful in many applications:
- Insocoat bearings are standard bearings with the external surfaces of their inner or outer ring coated with an aluminum oxide layer. This coating increases the bearing’s resistance to electric current.
- Nowear enhances the wear resistance of the raceway or rolling element surfaces. It can help the bearing withstand long periods of operation under poor lubrication conditions and reduce the risk of low-load damage.
Boundary dimensions
Boundary dimensions are the main dimensions of a bearing. They comprise of:
- The bore diameter (d).
- The outside diameter (D).
- The width or height (B, C, T or H).
- The chamfer dimensions (r).
Boundary dimensions radial bearing
Boundary dimension thrust bearing
Plain bearing/bushing components
A bushing or plain bearing is a cylindrical part used to reduce friction between components. Unlike a bearing, a bushing, often referred to as a sleeve, is a single piece. Though bushings and bearings are in similar categories, they’re not interchangeable, and they don’t move in the same way. Bushings slide while bearings roll. Bushings are ideal for distributing the weight of heavy or high load-bearing tolerances. They act as shock absorbers and decrease noise and wear. Many are self-lubricating, saving labour and time. Reservoirs on the surface hold grease or oil and spread it automatically when used. In the food and textile industries, factories use non-lubricating or dry bushings.
1. Outer ring
2. Sliding contact surfaces
3. Seal
4. Inner ring
5. Lubrication hole
6. Lubrication groove
Spherical plain bearing (Bushing)
1. Spherical Plain Bearing
2. Rod end
2a. Rod end housing
2b. Rod end shank, with an external (male) thread. Shanks are also available with an internal (female) thread or with a welding shank
3. Grease fitting
Rod end
HEM – PM-10: DIAGNOSE AND REPAIR HEAVY EQUIPMENT ELECTRICAL SYSTEMS
HEM – PM-09: DIAGNOSE AND REPAIR HEAVY EQUIPMENT MECHANICAL SYSTEMS
HEM – PM-08: REMOVE, TEST, REPAIR AND REFIT HEAVY EQUIPMENT COMPONENTS AND PARTS
HEM – PM-07: DISMANTLE, ASSESS AND REASSEMBLE HEAVY EQUIPMENT POWER TRAIN SYSTEMS
HEM – PM-04: WORK WITH ELECTRICAL AND FLUID POWER COMPONENTS
HEM – PM-03: REMOVE NAD INSTALL MECHANICAL COMPONENTS (GASKETS, SEALS, BEARINGS, AND LOCKING DEVICES
HEM – PM-02: USE TOOLS AND EQUIPMENT
HEM – PM-01: WORK SAFELY
HEM – KM-08: PROBLEM-SOLVING AND ENGINE OPTIMISATION
HEM – KM-07: ADVANCED VEHICLE AND EQUIPMENT SYSTEMS
HEM – KM-06: ELECTRICAL SYSTEMS AND BASIC ELECTRONICS
HEM – KM-05: VEHICLE, EQUIPMENT AND PROPULSION SYSTEMS
HEM – KM-04: BASIC ENGINE SYSTEMS
HEM – KM-03-KT02: FLUIDS, LUBRICANTS, MATERIALS AND METALS APPLICABLE TO THE TRADE
HEM – KM-03-KT01: FUNDAMENTALS OF ELECTRICITY
HEM – KM-03: VEHICLE AND EQUIPMENT FUNDAMENTALS
HEM – KM-02: FOUNDATIONAL CONCEPTS FOR MECHANICS
HEM – KM-01: WORKPLACE FUNDAMENTALS
HEM – KM-01-KT02: ORGANISATION OF WORK
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