Knowledge of the design characteristics of different types of bearings and proper appreciation of the needs of an application enables the designer to select a proper type of bearing.
The proper selection of bearing is important for the application it is being used. The characteristics of the bearing should match the requirements of the application.
Bearings are selected with the help of the following two criterion
- According to the type of application
- From the Manufacturer’s catalog
In order to choose the bearing, it is important to know the type of application. This is then further used for the selection of bearing from the manufacturer’s catalog.
Selection of Bearing According to Type of the Applications
For low and medium loads, ball bearings are used whereas for heavy loads and large shaft diameters, roller bearings are used.
Deep groove ball bearings mainly take the radial load and a limited amount of thrust load. Angular contact bearings and spherical roller bearings are suitable in applications where the load acting on the bearing consists of two components i.e. radial and thrust.
The maximum permissible speed of the shaft depends upon the temperature rise in the bearing. For high-speed applications, deep groove ball bearings, angular contact bearings, and cylindrical roller bearings are recommended.
Self-aligning bearings and spherical roller bearings are used in applications where misalignment between the axes if the shaft and housing are likely to exist.
Thrust ball bearings are used for medium thrust loads whereas, for heavy thrust loads, cylindrical roller thrust bearings are recommended. Double-acting thrust bearings can carry the thrust load in either direction.
Rigidity controls the selection of the bearing in certain applications like machine tool spindles. Double row cylindrical roller bearings or taper roller bearings are used under these conditions. The line contact in these bearings as compared with the point contact in ball bearings improves the rigidity of the system.
Noise becomes the criterion of selection in applications like household appliances. From noise considerations, point contact creates less noise than line contact. For such applications, deep groove ball bearings are recommended.
Read also: Types of Rolling Contact Bearing
Selection of Bearing from Manufacturer’s Catalogue
The selection of bearings from the manufacturer’s catalog consists of the following steps:
- Calculate the radial forces (Fr) and axis forces (Fa) acting on the bearing.
- Find out the diameter of the shaft on which bearing is to be fitted.
- Calculate the values of radial factors (X) and thrust factors (Y), from the catalog shown below.
- These values of radial factor and thrust factor mainly depends upon two ratios, (Fa/Fr) and (Fa/C0)
where Co is the static load capacity.
- Value of Co is found out with the help of the catalog. Here is the link to the catalog – Dimensions and load-carrying capacities of ball bearings.
- Hence, the selection of the bearing is done by a trial and error method.
- Equivalent dynamic load is calculated by equation, P= XFr+YFa
- A decision about expected bearing life is taken and is expressed in million revolutions. The selection of bearing life is discussed further in this post.
- Dynamic load capacity is calculated by the equation, C = P(L10)^a
For ball bearings, a=3
For roller bearings, a=10/3
- Check whether the selected bearing of the series has the required dynamic capacity.
If not, select the bearing of the next series and go back to step number 3, and calculate again.
This procedure is used for the selection of all types of bearings.
Flowchart for the selection of bearing from the manufacturer’s catalog is shown below.
Bearing Life and its selection
What is Bearing life?
Bearing life can be defined as the time length of a bearing that is expected to perform as required under predefined conditions.
Bearing life plays an important role while selecting the bearing for a particular application. Hence for a design engineer, it is necessary to know the life of a bearing which should match the type of the application.
Selection of Bearing life
While selecting the proper size of the bearing, it is necessary to specify the expected life of the bearing for a given application.
The information regarding life expectancy is generally unclear and the values used are based on past experiences.
For the applications like vehicles, the speed of the rotation is not constant and hence, the desired life is expressed in terms of millions of revolutions.
|Applications||Life (Million revolutions)|
In other applications where the rotational speed is constant, the desired life of the bearing is expected in terms of hours of services.
|Applications||Hours of Service|
|Intermittently used machines such as lifting tackle, hand tools, and household appliances||4000 – 8000 hr|
|Machines used for eight hours of service per day, such as electric motors and gear drives||12000 – 20000 hr|
|Machines used for continuous operation such as pumps, compressors, and conveyors.||40000 – 60000 hr|
These values given in the table are only for general guidance. For a particular application, the designer should consider the past experience, the availability of bearings, maintenance, and replacement of bearings, and economics of breakdown costs.