What are Bearings? 

Bearing is a mechanical element that permits relative motion between two parts, such as the and the housing, with minimum friction. The functions of the bearing are as follows:

Bearings are mechanical assemblies that consist of rolling elements and usually inner and outer races which are used for rotating or linear shaft applications, and there are several different types of bearings, including ball and roller bearings, linear bearings, as well as mounted versions that may use either rolling element bearings or plain bearings. Ball bearings have spherical rolling elements and are used for lower load applications, while roller bearings use cylindrical rolling elements for heavier load carrying requirements. Linear bearings are used for linear movements along shafts and may also have rotational capabilities. Mounted bearings are assemblies in which the bearings are pre-assembled in mountings that, in turn, are bolted to frames, stanchions, etc., and are used for supporting the ends of shafts, conveyor rollers, etc. In addition to ball and roller bearings in their radial, linear, and mounted forms, bearings include those for civil engineering applications, which are called slide bearings; those used in small instruments and the like, known as jewel bearings; and very specialized bearings known collectively as frictionless bearings which includes air and magnetic varieties. Sleeve bearings, journal bearings, and other fluid-film bearings are addressed in the Bushings family.

• The bearing ensures free rotation of the shaft or the axle with minimum friction.

• The bearing supports the shaft or the axle and holds it in the correct position.

• The bearing takes up the forces that act on the shaft or the axle and transmits them to the frame or the foundation.

What is the purpose of bearing?

The main purpose of bearings is to prevent direct metal to metal contact between two elements that are in relative motion. This prevents friction, heat generation and ultimately, the wear and tear of parts. It also reduces energy consumption as sliding motion is replaced with low friction rolling.

They also transmit the load of the rotating element to the housing. This load may be radial, axial, or a combination of both. A bearing also restricts freedom of movement of moving parts to predefined directions as discussed above.

Factors to be considered while selection of bearing:

1. Installation space

When a shaft is designed, its rigidity and strength are considered essential; therefore, the shaft diameter, i.e., bore diameter, is determined at start.
For rolling bearings, since wide variety with different dimensions are available, the most suitable bearing type should be selected.

2. Load type and direction which applied

Since various types of load are applied to bearings, load magnitude, types (radial or axial) and direction of application (both directions or single direction in the case of axial load), as well as vibration and impact must be considered in order to select the proper bearing.

3. Mounting and dismounting

Cylindrical roller bearings, needle roller bearings and tapered roller bearings, with separable inner and outer rings, are recommended for applications in which mounting and dismounting is conducted frequently.

Use of sleeve eases the mounting of self-aligning ball bearings and spherical roller bearings with tapered bore.

4. Misalignment

Internal load caused by excessive misalignment damages bearings. Bearings designed to absorb such misalignment should be selected. The higher the self-aligning capability that bearings possess, the larger the angular misalignment that can be absorbed. 

5. Rigidity

In machine tool spindles and automobile final drives, bearing rigidity as well as rigidity of equipment itself must be enhanced. Elastic deformation occurs less in roller bearings than in ball bearings.

6. Running accuracy

Performance required differs depending on equipment in which bearings are installed: for instance, machine tool spindles require high running accuracy, gas turbines require high speed rotation, and control equipment requires low friction. In such cases, bearings of tolerance class 5 or higher are required.

7. Rotational speed

Since the allowable speed differs greatly depend-ing not only upon bearing type but on bearing size, cage, accuracy, load and lubrication, all factors must be considered in selecting bearings.

Different Types of Bearings:    

            

Ball Bearings:

Ball bearings are one of the most common types of bearing classes used. It consists of a row of balls as rolling elements. They are trapped between two annulus shaped metal pieces. These metal pieces are known as races. The inner race is free to rotate while the outer race is stationary.

Ball bearings provide very low friction during rolling but have limited load-carrying capacity. This is because of the small area of contact between the balls and the races. They can support axial loads in two directions besides radial loads.

Applications of Ball Bearings:

• Household Items: Bicycles, Skateboards, Sewing Machines, Washing Machines, Tumble Driers, Food Processors, Hair Dryers, DVD Players, Fishing Rods.

• Office Equipment: Photocopiers, Fax Machines, Hard-Drives, Fans, Air-Conditioners

• Industries: Elevators, Assembly Lines, Escalators, Medical and Dental Equipment, High-speed Machine Tooling Equipment, Paper Making Machinery, Chain Saws, Power Tools, Pumps / Compressors. Toy Manufacturing, Trains, Wind Turbines.

• Automotive: Engines, Steering, Driveshaft and Driveline, Electric Motors, Gear Boxes, Transmissions

Classification of Ball bearings:

1. Thrust ball 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.
Due to having ball-shaped rolling elements, the bearings concentrate all the force onto a few points of contact. This can result in early failure with high loads. use of a large number of balls results in high thrust load carrying capacity in smaller space.
This is the major advantage of thrust bearing. The ball bearing’s small point of contact also means less friction. So, there is less resistance to overcome and thus it is easier to achieve high speeds with these types of bearings.

Applications: 

• Automotive, marine, and aerospace applications.

• Main and tail rotor blade grips of RC (radio controlled) helicopters.

• Forward gears in modern car gearboxes, radio antenna masts to reduce the load on an antenna rotator.

• Automobile the clutch. 

2. Self-aligning ball bearing:

In a self-aligning bearing, the inner ring has two raceways and the outer ring has a single spherical raceway with its centre of curvature coincident with the bearing axis. This allows the axis of the inner ring, balls and cage to deflect around the bearing centre to automatically correct misalignment caused by housing and shaft machining or installation error. The self-aligning ball bearing consists of two rows of balls, which roll on a common spherical surface in the outer race. In this case, the assembly of the shaft, the inner race and the balls with cage can freely roll and adjust itself to the angular misalignment of the shaft.

 

Applications:

• Agriculture machinery

• Ventilators

• Railway axial boxer 

• Material handling

• Food and beverages

• Pulp and Paper Industry                                                                                                                

3. Deep Groove type

The most widely used roller bearing type in the world due to their versatility and overall performance. They are characterized by having deep raceway grooves in which the inner and outer rings have circular arcs of slightly larger radius than that of the balls. They also have non-separable rings.

They can operate at high speeds and can carry radial and (limited) axial loads. because they can carry radial and axial loads, they have a wider range of applications for many industries.

Applications:

• Electric motors

• Compressors

• Fans

• Conveyors.

4. Angular contact ball bearing.

Angular contact ball bearings can withstand high radial-axial loads and reach high speeds. They are asymmetrical for manufacturing reasons and can withstand unidirectional axial loads only. Angular bearings are usually mounted in a group of two or more opposed preloaded units with rigid or elastic spacers.
Angular contact ball bearings have inner and outer ring raceways that are displaced relative to each other in the direction of the bearing axis. This means that these bearings are designed to accommodate combined loads, i.e. simultaneously acting radial and axial loads.
The axial load carrying capacity of angular contact ball bearings increases as the contact angle increases. The contact angle is defined as the angle between the line joining the points of contact of the ball and the raceways in the radial plane, along which the combined load is transmitted from one raceway to another, and a line perpendicular to the bearing axis.

Applications:

• Industrial pump

• Industrial gearboxes.

• Trucks trailers and buses.

• Industrial generators.

Roller Bearings:

Roller Bearings are mechanical assemblies that consist of cylindrical or tapered rolling elements usually captured between inner and outer races.
They provide a means of supporting rotating shafts and minimizing friction between shafts and stationary machine members.
Roller bearings are used primarily in machinery with rotating shafts that require the support of heavier loads than ball bearings provide.

Tapered roller bearings are often used to accommodate higher thrust loads in addition to the radial loads. Types range from cylindrical to spherical rollers. 

Classification of Roller bearing:

Roller bearings on the other hand are classified according to the shape of the rollers: cylindrical, needle, tapered and spherical. Rolling bearings can be further classified according to the direction in which the load is applied; radial bearings carry radial loads and thrust bearings carry axial loads.

1. Cylindrical roller bearing:

When maximum load carrying capacity is required in a given space, the point contact in 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 cag. Due to line contact between rollers and races, the radial load carrying capacity of the cylindrical roller bearing is very high.
Cylindrical roller bearing is more rigid than ball bearing.  The coefficient of friction is low and frictional loss is less in high-speed applications.
The main difference between cylindrical roller bearings and other bearing styles is in the name – they use cylinders as the rolling elements as opposed to the ball you’d find in ball bearings. The cylinders are slightly greater in length than diameter. Compared to ball bearings, cylindrical rollers have a greater radial load capacity. The cylindrical roller design also allows these parts to accept relatively faster speeds than other styles of roller bearings.
There are multiple types of cylindrical roller bearings. Single row cylindrical rollers are the most popular and are separable, which makes for easier mounting and dismounting.

Applications:

• Mining, petroleum production, power generation, power transmission, cement processing, aggregate crushing, and metal recycling

• Briquetting machines, rubber mixing equipment, rolling mills, rotary dryers, or pulp and paper machinery 

• Construction equipment, crushers, electric motors, blowers and fans,

• Gears and drives, plastics machinery, machine tools and traction motors and pumps.

2. Tapered roller bearing.

The taper roller bearing consists of rolling elements in the form of a frustum of 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.
In kinematics’ analysis, this is the essential requirement for pure rolling motion between conical surfaces. In taper roller bearing, the line of resultant reaction through the rolling elements makes an angle with the axis of the bearing. Therefore, taper roller bearing can carry both radial and axial loads.
Therefore, taper roller bearings are always used in pairs to balance the thrust component. Taper roller bearing has separable construction. The outer ring is called ‘cup’ and the inner ring is called ‘cone’. The cup is separable from the remainder assembly of the bearing elements including the rollers, cage and the cones. Tapered angles allow the bearings to efficiently control a combination of radial and thrust loads. The steeper the outer ring angle, the greater ability the bearing has to handle thrust loads. To provide a true rolling motion of the rollers on the raceways, the extensions of the raceways and the tapered surfaces of the rollers come together at a common point, the apex, on the axis of rotation.

Applications:

• Agriculture, construction and mining equipment. 

• Sports robot combat, axle systems, gearbox, engine motors and reducers, propeller shaft.

• Railroad axle-box, differential, wind turbines.

3. Spherical roller bearing.

Heavy radial and axial loads can be a greater challenge when the shaft is prone to misalignment.
This situation can be handled very well by spherical roller bearings. They have high load carrying capacities and can manage misalignment between the shaft and housing. This reduces maintenance cost and improves service life.
Spherical roller bearing raceways are inclined at an angle to the bearing axis. Instead of straight sides, the rollers have spherical sides that fit onto the spherical raceways and accommodate small misalignments. A spherical roller bearing is a rolling-element bearing that permits rotation with low friction and permits angular misalignment. Typically, these bearings support a rotating shaft in the bore of the inner ring that may be misaligned with respect to the outer ring. The misalignment is possible due to the spherical internal shape of the outer ring and spherical rollers. Despite what their name may imply, spherical roller bearings are not truly spherical in shape. The rolling elements of spherical roller bearings are mainly cylindrical in shape but have a profile that makes them appear like cylinders that have been slightly over-inflated.

Applications:

• Gearboxes, wind turbines, continuous casting machines, material handling, pumps, mechanical fans and blowers

• Mining and construction equipment, pulp and paper processing equipment, marine propulsion and offshore drilling, off-road vehicles.

4. Needle roller bearing:

Needle roller bearing is a special type of roller bearings that has cylindrical rollers that resemble needles because of their small diameter.
Normally, the length of rollers in roller bearings is only slightly more than its diameter. When it comes to needle bearing, the length of rollers exceeds their diameter by at least four times.
As needle bearings have a smaller diameter, more rollers can be fit in the same space which increases the surface area in contact with the races. Thus, they are capable of handling high loads. The small size can also prove helpful in applications where space is limited as they require smaller clearances between the axle and the housing.

A needle roller bearing is a special type of roller bearing which uses long, thin cylindrical rollers resembling needles. Ordinary roller bearings rollers are only slightly longer than their diameter, but needle bearings typically have rollers that are at least four times longer than their diameter.

Applications:

• Needle bearings are heavily used in automobile components such as rocker arm pivots, pumps, gearboxes

• Also used in automotive power transmission systems, two and four stroke engines, planetary gear sets and air compressors.

Sliding Contact Bearing:
A plain bearing is the simplest type of bearing. It usually only consists of a bearing surface. There are no rolling elements. The bearing is basically a sleeve mounted on the shaft and fit into the bore. Plain bearings are inexpensive, compact and lightweight. They have high load carrying capacity. Plain bearings are used for rotational, sliding, reciprocating or oscillatory motion. The bearing remains fixed while the journal slides on the bearing’s inner surface. To facilitate smooth movement, material pairs with low coefficients of friction are selected. The sliding contact bearings in which the sliding action is guided in a straight line and carrying radial loads, called as slipper or guide bearings. The sliding contact bearings 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.

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

Thick film bearings: The thick film bearings are those in which the working surfaces are completely separated from each other by the lubricant.

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.

Zero film bearings: The zero film bearings are those which operate without any lubricant present.

 

Specialized Bearings:

1. Magnetic Bearings:

 

A magnetic bearing is a type of bearing that supports a load using magnetic levitation. Magnetic bearings support moving parts without physical contact. For instance, they are able to levitate a rotating shaft and permit relative motion with very low friction and no mechanical wear. Magnetic bearings support the highest speeds of all kinds of bearing and have no maximum relative speed.

Applications: Magnetic bearings are used in several industrial applications such as electrical power generation, petroleum refinement, machine tool operation and natural gas handling. They are also used in the Zipp type centrifuge, for uranium enrichment and in turbomolecular pumps, where oil-lubricated bearings would be a source of contamination.

2. Jewel Bearings:

Jewel Bearing is a plain bearing in which a metal spindle turns in a jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. The jewel material is usually synthetic sapphire or ruby (corundum). Jewel bearings are used in precision instruments where low friction, long life, and dimensional accuracy are important. 

Application: Largest use is in mechanical watches.

3. Fluid Bearings:

 

Fluid Bearings are bearings in which the load is supported by a thin layer of rapidly moving pressurized liquid or gas between the bearing surfaces. Since there is no contact between the moving parts, there is no sliding friction, allowing fluid bearings to have lower friction, wear and vibration than many other types of bearings.

Applications: Heavy-duty rotating equipment, including in hydroelectric plants to support turbines and generators, heavy machinery such as marine propeller shafts.

4. Flexure Bearings:

Flexure Bearings are engineered to be compliant in one or more angular degrees of freedom. Flexure bearings are often part of compliant mechanisms. Flexure bearings serve much of the same function as conventional bearings or hinges in applications which require angular compliance. However, flexures require no lubrication and exhibit very low or no friction.

Application: Door Hinges, Lids for Pez dispensers, Flip-top covers, etc.

 

Bearing Selection:

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