Rolling Contact Bearings and their Applications

In this tutorial, you will learn why bearings are used in the industry and what rolling contact bearings are. You will become acquainted with the different types of bearings, learn about their labeling, and understand the selection process which precedes the selection of the bearing.

Contents:

1. What are Bearings?
2. Types of Rolling Contact Bearings
3. Selection of Bearings depending on Application
4. Load Carrying Capacity of Bearings
5. What is Bearing Life?
6. What are Bearing Numbers
7. Bearing Manufacturer’s Catalogue
8. Causes of Bearing Failures

What are Bearings?

A Bearing was a mechanical element that allows relative motion between two parts such as shafts and their housing, with minimum friction. They ensure free rotation of the shaft, hold them in the correct position and take up forces that act on the shaft or the axle.
The following figure shows a rolling contact bearing.

• A radial bearing supports the load which is perpendicular to the axis of the shaft. A thrust bearing supports the load which acts along the axis of the shaft.
• Rolling-element bearings are made by all major bearing manufacturers worldwide to standard dimensions defined by the Anti-Friction Bearing Manufacturers Association (AFBMA) or the International Standards Organization (ISO), and they are interchangeable.
• Rolling-element bearings can be grouped into two broad categories, ball, and roller bearings, both of which have many variants within these divisions.
• Ball Bearings capture several hardened and ground steel spheres between two raceways, an inner and outer race for radial bearings, or top and bottom races for thrust bearings. A retainer (also called a cage or separator) is used to keep the balls properly spaced around the raceways.
• Ball bearings are most suitable for small, high-speed applications. For large, heavily loaded systems, roller bearings are preferred.
• Roller Bearings use straight, tapered, or contoured rollers running between raceways. Roller bearings can support larger static and dynamic loads than ball bearings because of their line contact and are less expensive for larger sizes and heavier loads.

Types of Rolling Contact Bearings

A rolling contact bearing consists of four parts: inner and outer races, a rolling element like a ball, roller, or needle, and a cage that holds the rolling elements together and spaces them evenly around the periphery of the shaft. The following figure shows some common types of rolling contact bearings.

• The most frequently used bearing is the deep groove ball bearing. The balls and the races may roll freely without any sliding. The deep groove ball bearing has a high load-carrying capacity. Due to point contact between the balls and races, frictional loss and the resultant temperature rise are less in this bearing.
• When maximum load-carrying capacity is required in each space, the point contact in the ball bearing is replaced by the line contact of roller bearing. A cylindrical roller bearing consists of relatively short rollers that are positioned and guided by the cage. Due to line contact between rollers and races, the radial load-carrying capacity of the cylindrical roller bearing is very high.
• In angular contact bearing, the grooves in inner and outer races are so shaped that the line of reaction at the contact between balls and races makes an angle with the axis of the bearing. This reaction has two components: radial and axial. Therefore, angular contact bearing can take radial and thrust loads.
• There are two types of self-aligning rolling contact bearings, self-aligning ball bearing and spherical roller bearing. Both types of self-aligning bearing permit minor angular misalignment of the shaft relative to the housing.
• The taper roller bearing consists of rolling elements in the form of a frustum of a cone. They are arranged in such a way that the axes of individual rolling elements intersect in a common apex point on the axis of the bearing.
• A thrust ball bearing consists of a row of balls running between two rings: the shaft ring and the housing ring. Thrust ball bearing carries thrust load in only one direction and cannot carry any radial load.

Selection of Bearings depending on Application

The selection of the type of bearing in a particular application depends upon the requirement of the application and the characteristics of different types of bearings. The guidelines for selecting a proper type of bearing are as follows:

• For low and medium radial loads, ball bearings are used, whereas, for heavy loads and large shaft diameters, roller bearings are selected.
• Self-aligning ball bearings and spherical roller bearings are used in applications where misalignment between the axes of the shaft and housing is 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.
• Deep groove ball bearings, angular contact bearings, and spherical roller bearings are suitable in applications where the load acting on the bearing consists of two components: 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.
• Double row cylindrical roller bearings or taper roller bearings are used under rigidity conditions. The line of contact in these bearings, as compared with the point of contact in ball bearings, improves the rigidity of the system.
• Noise becomes the criterion of selection in applications like household appliances. For such applications, deep groove ball bearings are recommended.

Load Carrying Capacity of Bearings

There are two load capacity specifications for recirculating linear guides and ball screws: static load capacity and dynamic load capacity. Most engineers are familiar with dynamic load capacity since this is the value used to calculate bearing life. But it’s also important to consider the component’s static load capacity to avoid premature bearing failure.

• The static load is defined as the load acting on the bearing when the shaft is stationary. It produces permanent deformation in balls and races, which increases with increasing load.
• The static load carrying capacity of a bearing is defined as the static load which corresponds to a total permanent deformation of balls and races, at the most heavily stressed point of contact, equal to 0.0001 of the ball diameters.
• The life of a ball bearing is limited by the fatigue failure at the surfaces of balls and races. The dynamic load-carrying capacity of the bearing is, therefore, based on the fatigue life of the bearing.
• The dynamic load-carrying capacity of a bearing is defined as the radial load in radial bearings that can be carried for a minimum life of one million revolutions.

What is Bearing Life?

Bearing life is essentially the length of time a bearing can be expected to perform as required in predefined operating conditions. It is based primarily on the probable number of rotations a bearing can complete before it starts showing symptoms of fatigue, such as spalling or cracking due to stress. The relationship between the dynamic load-carrying capacity, the equivalent dynamic load, and the bearing life is given by,
L₁₀ = \((\frac{C}{P})^P\)
Where,
L₁₀ = rated bearing life
C = dynamic load capacity (N)
p = 3 (for ball bearings) and 10/3 (for roller bearings)
P = Dynamic Load Capacity

What are Bearing Numbers

Bearings are identified by their numbers. The number can be of multiple digits, but it has three major parts. One set of numbers denote the bearing type in the material catalogue. For example, 6 represents a deep groove ball bearing. One set of numbers identify the load capacity and working conditions. The last set represents the shaft or bore diameter the bearing is made for. Usually, the number is multiplied by five to get the shaft diameter. For example, Bearing 6015 identifies a Deep Groove Ball bearing for light applications with a shaft diameter of 75 mm while Bearing 32320 identifies a Taper Roller bearing of shaft diameter 100 mm.

Bearing Manufacturer’s Catalogue

The Bearing Manufacturer’s Catalogue provides a listing of all bearing sizes and types available commercially in the market along with their characteristics. As engineers, we can select a bearing for our application by defining the required characteristics of our application and selecting an appropriate bearing from the catalogue. The basic procedure for the selection of a bearing from the manufacturer’s catalogue consists of the following steps:

• Calculate the radial and axial forces on the bearing.
• Select type of bearing based on application.
• Determine radial and thrust factors for that bearing series.
• Calculate equivalent dynamic load.
• Calculate bearing life in million rotations.
• Calculate dynamic load capacity.
• Check whether the selected bearing satisfies this load capacity. If no, select the next bearing series and reiterate the steps.

Causes of Bearing Failures

The fracture in the outer race of the ball bearing occurs due to overload. When the bearing is misaligned, the load acting on some balls or rollers sharply increases and may even crush them. The complete breakage of the parts of the ball bearing can be avoided by selecting the correct ball bearing. In general, the failure of antifriction bearing occurs not due to breakage of parts but due to damage of working surfaces of their parts.

• Abrasive wear occurs when the bearing is made to operate in an environment contaminated with dust, foreign particles, rust or spatter. Remedies against this type of wear are the provision of oil seals, increasing surface hardness, and the use of high viscosity oils.
• The corrosion of the surfaces of bearing parts is caused by the entry of water or moisture in the bearing.
• Pitting is the main cause of the failure of antifriction bearings. Pitting is a surface fatigue failure that occurs when the load on the bearing part exceeds the surface endurance strength of the material.
• Scoring is a stick-slip phenomenon, in which alternate welding and shearing take place rapidly at high spots. Here, the rate of wear is faster.