What are bearings? sliding and rolling contact bearings, materials used, different terms and designation - Mechanicaltalks

What are bearings? sliding and rolling contact bearings, materials used, different terms and designation

BEARINGS


Introduction


A bearing is a machine element which support another moving machine element  (known as journal). It permits the relative motion between the contact surfaces of the members while carrying the load. It also reduces friction between the moving parts. 

Types of bearings :

1.) Radial bearings- In radial bearings load acts perpendicular to the direction of motion of the moving element.

2.) Thrust bearings- In thrust bearing the load acts along the axis of rotation.

3.) Sliding contact bearings- In sliding contact bearings, the sliding takes place along the surfaces of contact between the moving element and the fixed element. These are also known as plain bearings.

4.) Rolling contact bearings- In rolling contact bearings the steel balls or rollers are interposed between the moving and fixed element.

Here we will discuss detailed analysis of sliding and rolling contact bearings.

Sliding contact Bearings

In sliding contact bearing the sliding takes place along the surfaces of contact between moving and fixed element. The sliding contact bearing in which the sliding action is along the circumference of a circle or an arc of a circle and carrying radial loads are known as ‘journal or sleeve bearings’. When the angle of contact of the bearing with the journal is 360°,then the bearing is called a ‘full journal bearing’. This type of bearing is commonly used in industrial machinery to accommodate bearing loads in any radial direction. When the angle of contact of the bearing with the journal is 120° then the bearing is said to be ‘partial journal bearing’. This type of bearing has less friction than full journal bearing but it can be used only where the load is always in one direction. The most common application of the partial journal bearing is found in rail road car axles. The full and partial journal bearings may be called as ‘clearance bearings, because the diameter of the journal is less than that of bearing. When a partial journal bearing has no clearance i.e. the diameters of the journal and bearing are equal, then the bearing is called a ‘fitted bearing’. 

The sliding contact bearings, according to the thickness of layer of the lubricant between the bearing and journal may be classified as follows:

1.) Thick film bearings- The thick film bearings are those in which the working surfaces are completely separated from each other by the lubricant. Such type of bearings are also called as hydrodynamic lubricated bearings.

2.) Thin film bearings- The thin film bearings are those in which, although lubricant is present, the working surfaces partially contact each other at least part of the time. Such type of bearings are also called ‘boundary lubricated bearings’.

3.) Zero film bearings- The zero film bearings are those which operate without any lubricant present.

4.) Hydrostatic or externally pressurized lubricated bearings- The hydrostatic bearings are those which can support steady loads without any relative motion between the journal and the bearing. This is achieved by forcing externally pressurized lubricant between the members.

Sliding contact bearing materials and their properties:

The materials commonly used for sliding contact bearings are babit metal (tin base and lead base babits), bronzes (gunmetal and phosphorus bronzes),cast iron, silver, carbon graphite, rubber, wood and plastics. The following properties must be considered in selecting the best material.

1.) Compressive strength- The bearing material should have high compressive strength to withstand the maximum bearing pressure so as to prevent extrusion or permanent deformation of the bearing.

2.) Fatigue strength- The bearing material should have sufficient fatigue strength so that it can withstand repeated loads without developing surface fatigue cracks.

3.) Conformability- It is the ability of a bearing material to accommodate shaft deflections and bearing inaccuracies by plastic deformation (or creep) without excessive wear and heating.

4.) Embeddability- It is the ability of a bearing material to accommodate or embed small particles of dust, grit etc. without scoring the material of the journal.

5.) Bondability- Many high capacity bearings are made by bonding one or more thin layers of a bearing material to a strength steel shell. Thus the strength of bond i.e. bondability is an important consideration in selecting bonding materials.

6.) Corrosion resistance- The bearing material should not corrode away under the action of lubricating oil.

7.) Thermal conductivity- The bearing material should be of high thermal conductivity.

8.) Thermal expansion- The bearing material should be of low coefficient of thermal expansion.

Terms used in Hydrodynamic Journal Bearing:

A hydrodynamic journal bearing is Shown in figure.

Here O is the centre of journal and O’ is the centre of bearing. The following terms used in Hydrodynamic Journal Bearing is are important;

1.) Diametral clearance- It is the difference between diameters of the bearing and the journal.

2.) Radial clearance- It is the difference between radii of the bearing and the journal.

3.) Diametral clearance ratio- It is the ratio of the diametral clearance to the diameter of the journal.

4.) Eccentricity- It is the radial distance between the centre of the bearing (O) to the displaced centre (O’) of the bearing under load.

5.) Minimum oil film thickness- It is the minimum distance between the bearing and the journal under complete lubrication condition. It is denoted by Ho and its value may be assumed as one – fourth of diametral clearance.

6.) Attitude or eccentricity ratio- It is the ratio of the eccentricity to the radial clearance.

7.) Short and long bearings- If the ratio of the length to the diameter of the journal (i.e. l/d) is less than 1, then the bearing is said to be short bearing. On the other hand if l/d is greater than 1,then the bearing is known as long bearing.

NOTES-

* When l=d, then the bearing is called square bearing.

* The value of l/d may be taken as 1 to 2 for general industrial machinery.

* In cranck shaft bearings, the ratio of l/d is frequently less than 1.

Rolling Contact Bearings:

In rolling contact bearings, the contact between the bearing surfaces is rolling. Due to the low friction offered by rolling contact bearings, these are generally called ‘antifriction bearings’. The rolling contact bearings have the following advantages and disadvantages over sliding contact bearings;

Advantages-

1.) Low starting and running friction except at very high speeds.

2.) Ability to withstand momentary shock loads.

3.) Accuracy of shaft alignment.

4.) Low cost of maintenance, as no lubrication is required while in service.

5.) Small overall dimensions and are easy to mount or erect.

Disadvantages-

1.) More noisy at high speeds.

2.) Low resistance to shock loading.

3.) More initial cost.

4.) Design of bearing housing is complicated.

The rolling contact bearings are generally of the two types;

A.) Ball bearing      B.) Roller bearing

The ball and roller bearings consists of an inner face which is mounted on the shaft or journal and an outer race which is carried by the housing or casing. In between the inner and outer race there are balls or rollers. A number of balls and rollers are used and these are held at proper distances by retainers so that they do not touch each other. The retainers are thin strips and is usually in two parts which are assembled after the balls have been properly spaced. The ball bearings are used for light loads and the roller bearings are used for heavier loads.

The bearings are designated by a number. In general the number consists of at least three digits. Additional digits or letters are used to indicate special features. The last three digits gives the series and the bore of the bearing. The last two digits from 04 onwards, when multiplied by 5,give the bore diameter in milimetres. The third from the last digit designates the series of the bearing. 

The most common Ball bearings are available in four series as follows;

a.) Extra light (100)

b.) Light (200)

c.) Medium ( 300)

d.) Heavy ( 400)

NOTES:

* If a bearing is designated by a number 305,it means that the bearing is of medium series whose bore is 05×5 i.e. 25mm.

* The extra light and light series are used where the loads are moderate and shaft sizes are comparatively large and also where available space is limited.

* The medium series has a capacity 30 to 40 percent over the light series.

* The heavy series has 20 to 30 percent capacity over the medium series. This series is not used extensively in industrial applications.

That was brief description about sliding contact and rolling contact bearings. Hope you all like this article.

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