BEARINGS

Function AND TYPES

Bearings do these jobs:

1. Support the moving part

2. Reduce friction by reducing metal to metal contact

3. Reduce wear

4. Provide a replaceable wear surface.

Figure 1. Types of bearing used in modern machines

Bearings support moving parts by carrying radial and axial loads while holding the parts in alignment.

Bearings reduce friction by providing either sliding contact (plain bearings) or rolling contact (anti friction bearings). Bearings also reduce friction with a lubricant which will dissipate heat.

Types of bearingsThere are two basic types of bearings:

Plain bearings (bushings)

Anti-friction bearings

PLAIN BEARINGS

Plain or sleeve bearings are designed to support shafts that rotate, oscillate, or reciprocate.

sleeve bearings are highly engineered components. sleeve bearing lubrication is hydrodynamic; that is, during operation, the shaft floats on a thin film of the lubricant.

Some types of plain bearings

Ball Bearing Roller Bearing Needle Bearing

ANTIFRICTION BEARINGS

Split Bearing

PLAIN BEARINGS

Types of plain bearings:

Solid bearing or sleeve.

Split bearings

Thrust bearings

Solid bearing

Self-aligning bearing

LOADS ON PLAIN BEARINGSPlain bearing can be used to withstand Loads in linear, reciprocating and rotational motions.

They can carry the following loads:

Radial

Thrust

Bearing designed for radial loads only

Thrust washers used with radial-load-type bearings

Some plain bearings are designed to carry both radial and thrust loads. Notice the difference between radial and thrust split bearings.

Figure 4. Bearings designed for both radial and thrust loads.

BEARING MATERIALS

Construction

Monometals. Bearings made from a solid bar or tubes of an aluminum or bronze alloy have beenavailable for a number of years. They are generally used where loads are not very high.

Bimetals. A bimetal bearing has a steel back, to which is bonded a liner of Babbitt, copper-lead,or aluminum. Babbitts are soft alloys of lead or tin, with additives such as copper, antimony, andarsenic.

Trimetals. In order to achieve the desirable surface properties of a Babbitt bearing and thestrengths of harder materials, the trimetal bearing was developed for heavy-duty applications.

MATERIALS IN PLAIN BEARINGSBearings are constructed of a variety of materials. Some of these are:

wood plastic cast iron chilled cast iron sort steel hardened steel copper brass bronze copper-lead aluminum babbit sintered metal graphite nylon.

Requirements

Embeddability. The ability of a material to absorb foreign particles circulating in the oil stream isreferred to as embeddability.

Conformability. The material also must be soft enough to creep or flow slightly to compensate forthe minor geometric irregularities which are present in every assembly.

Fatigue Strength. This is the ability of a bearing material to withstand the loads to which it is subjectedwithout cracking.

Temperature Strength. As operating temperatures increase, bearing materials tend to lose strength.This property indicates how well a material carries a load at elevated temperatures, without breakingup or flowing out of shape.

Thermal Conductivity. Shear of the oil film by the shaft generates significant heat, most of whichis carried away by the oil.

Corrosion Resistance. Oils oxidize with use, and the products of this degradation can be corrosive.

The material used in a bearing depends on :

the space available the speed the shaft turns the amount of load it carries the type of lubricant used the cost of the installation

PREVENTIVE MAINTENANCE OF PLAIN BEARINGS

Lubricant Supply. Proper bearing design and material are necessary to achieve long service life but are not by themselves sufficient. Lubricant flow to the bearings is a function of the equipment design. Oil pressure at specified speeds should be within the limits given by the equipment builder. Lower values suggest worn bearings.

In this case, replacement should be made as soon as is feasible.

Excessive pressures indicate a blockage or restriction somewhere in the system. This should be investigated immediately.

The oil level also should be checked on a routine basis to avoid pump cavitation and subsequent oil starvation.

In non-pressurized lube systems, reservoirs should be checked on a regular schedule to ensure that adequate oil is always present.

Cleanliness. Sleeve bearings simply cannot survive without adequate lubrication. Once this is assured, the next most important consideration is the cleanliness of the lubricant. Since minimum film thickness is so small, the presence of oil-borne debris can greatly accelerate the wear process.

If foreign materials such as metal chips and abrasives are large and numerous, bearing failure can

Lubricant Type. Ensuring an adequate flow of clean lubricant makes long bearing life possible but does not guarantee it. The oil must be the proper one for the application. From a bearing performance viewpoint, lubricant viscosity is the most important parameter. Lower-viscosity (i.e., thinner) oils reduce oil film thickness. This increases the wear rate and can possibly lead to failure.

It is critical that the equipment manufacturer’s lubricant recommendations be followed.

In addition, the proper combination of oil additives is necessary to prevent rapid breakdown, thickening, foaming, and sludging. All these effects can lead to bearing failure, as well as to the damage of other components.

A sampling of bearings, bushings, and thrust washers.

Bearing nomenclature.

Flange bearing nomenclature.

LUBRICATING PLAIN BEARINGSLubricants are used to reduce friction and wear in bearings. The less friction the less wear. Three

states of friction or lubrication may exist between sliding surfaces (As in Lubrication module in this

manual).

Dry friction

Boundary lubrication

Full-film lubrication

How Lubricant Is Distributed To Bearings

The bearing must have these characteristics in order to perform well:

1. It must have a means of distributing the lubricant, (oil grooves).

2. Oil clearance must be correct to fit the shaft well.

3. The bearing surface must be compatible with the shaft, the lubricant, and the speed of

rotation.

STATES OF LUBRICATION: Surfaces in contact

Surfaces separated by lubricant

Surfaces separated by hydrodynamic pressure (Hydrodynamic Journal Bearing)

METHODS OF LUBRICATIONLubricants may be applied to plain bearings in many different ways

Two types of lubricants are most familiar:

Oil

Grease

They are applied by oil can and grease gun. The oil can used in different ways:

Apply lubricant directly on the parts

An oil cup attached to the bearing

Oil bath splash system

Fluid Film Bearings

Fluid film bearings are classified according to the creation of their fluid film. Fluid film bearings

are:

hydrostatic

hydrodynamic

squeeze film (damper)

Fluid film creation is by:Externally Prusserized High speed and

inclinationHigh frequency vibration

(a) Hydrostatic Bearing (b) Hydrodynamic Bearing ( c) Squeeze film bearing

Types of fluid film bearings

ANTI-FRICTION BEARINGSWe are concerned with three basic types of anti-friction bearings (Figure 1).

Ball bearings Roller bearings (cylindrical) Needle bearings Spherical Tapered

Figure 1. Basic types of anti-friction bearings

Basic Parts

Most anti-friction bearings have similar parts (Figure 2):

Figure 2. Basic parts of anti-friction bearings

BALL CYLINDRICAL SPHERICAL TAPERED NEEDLE

1. Two hardened-steel rings called races.

2. Hardened-steel balls, rollers or needles which roll between the races.

3. Optional separators or cages (made of steel, bronze or plastic) which space the rolling

elements around the races.

BOUNDARY DIMENSIONSIn general, most ball, spherical roller, and cylindrical roller bearings made to metric boundary dimensions have standardized boundary plans, dimensions, and tolerances according to the International Standards Organization (ISO). Therefore, bearings from all subscribing manufacturers throughout the world are dimensionally interchangeable.

Radial ball bearing types.

Radial roller bearing types.

Roller bearing types for radial and axial loads combined.

Thrust bearings.

Tapered roller bearings, single-row, multiple-row, and thrust.

BEARING SERIES

For any given bore size, all types of metric rolling bearings are manufactured in several series each for different severity of service. For instance, most ball bearings are made in three series: light, medium, and heavy duty. These are designated as the 2-, 3-, and 4-diameter series according to the boundary plan shown in Fig. 2.4. Spherical roller bearings are normally available in eight different series, as shown inFig. 2.5. Taper roller bearings, both inch- and metric-dimensioned, have a larger number of series or duty classifications, but all series are not necessarily available for every bore size (see Fig. 2.6).

Metric rolling-bearing boundary dimension plan.

Spherical roller bearings of different diameter series with common bore size.

Metric tapered roller-bearing diameter series with common bore size.

LOAD RATINGS

Basic rating life is the number of revolutions (or the number of operating hours at a given constant speed) which the bearing is capable of enduring before the first sign of fatigue occurs in one of its rings or rolling elements.There are several other “bearing lives,” including service life and design or specification life. Service life is the actual life achieved by a specific bearing before it becomes unserviceable. Failure is not generally due to fatigue, but due to wear, corrosion, contamination, seal failure, etc. The service life of a bearing depends to a large extent on operating conditions, but the procedures used to mount and maintain it are equally important. Despite all recommended precautions, a bearing can still experience premature failure. In this case it is vital that the bearing be examined carefully to determine a reason for failure so that preventive action can be taken. The service life can either be longer or shorter than the basic rating life. Specification life is the required life specified by the equipment builder and is based on the hypotheticalload and speed data supplied by the builder and to which the bearing was selected. Many times this required life is based on previous field or historical experiences.

SHAFT AND HOUSING FITSIt is a basic rule of design that one ring of a rolling-element bearing must be mounted on its mating shaft or in its housing with an interference fit, since it is virtually impossible to prevent rotation by clamping the ring axially. Generally, it is the rotating ring that is tight, but more correctly stated, it is the ring that rotates relative to the load. In some special cases this is not the rotating ring; for instance, in a vibrating unit where vibration is produced by eccentric weights, the load rotates with the rotating ring, and it is best to have the stationary ring have the tight fit. Except for special cases as mentioned above, the stationary ring normally can be assembled with

the mating shaft or housing with a slip or loose fit. The magnitude of interference fit will vary with the severity of duty, type of bearing, and different shaft and housing materials. Ball bearings under normal load conditions will have approximately 0.00025 in interference per inch of shaft when the inner ring is the tight fit. Roller bearings will have fits of approximately 0.0005 in per inch of shaft. Fits will be increased for heavy-duty service and decreased for light duty. In general, when the outer ring is the tight fit, the interference is less than a corresponding shaft fit. All bearing manufacturers show recommended fitting practices for their bearings in their general catalogs. With the exception of inch-taper roller bearings, the recommendations are normally expressed in ISO standards. ISO standards define the fit tolerance between the bearing outsidediameter and the housing and utilize a designation system using a capital letter and a number such as H7, J6, P6, and so on. Fit tolerances between the shaft and bore of the bearing are designated by a lowercase letter and number such as g6, m5, r7, and so on. In the ISO system, the letter indicates the class or type of fit, and the number indicates the tolerance range. The diagram in Fig. 2.7 shows the relationship between the nominal diameters and the tolerance grades. The crosshatched areas indicate the bearing bore diameter variation and the outside diameter variation, respectively. The blackened rectangles show the range of tolerances for shafts (lower half) and housings (upper half).

ISO fit tolerances. Uppercase letters refer to housings; lowercase letters refer to shafts

Types of Bearing Races

The type of races in the bearing controls the load-carrying capacity, especially in ball bearings. Ball bearings have a variety of races. Basically, ball bearings have four race designs.

1. Conrad bearing2. Full bearing3. Split race bearing4. Angular contact bearing

anti-friction bearings are normally used where:

space is available

the location is not readily accessible

loads are heavy

speeds are high

less service is required

cost is not the critical factor

Single Row

Radial

Single Row

Radial & Thrust

Double Row

Radial & Thrust

Double Row Self Aligning

Radial

Figure 4. Types of ball bearings

Single Direction Double Direction Double Direction

Thrust Bearing Thrust Bearing Angular Contact

Thrust Bearing

Figure 5. Thrust load ball bearings

ROLLER BEARINGS:Where great loads are present, roller bearings are used instead of ball bearings,

Figure 6. Roller bearings are also designed for loads and applications similar to those of ball

bearings. As with ball bearings, roller bearings are precision made and must be handled with

care and kept clean.

The five basic types of roller bearings are:

1. Radial load, straight roller

2. Radial and thrust load, tapered roller

3. Self aligning, radial and thrust load, spherical roller

4. Self aligning, radial and thrust load, concave roller

5. Thrust load

Cylindrical Tapped Spherical Double Cylindrical

Radial Radial & Thrust Radial & Thrust Radial & Thrust

Figure 6. Basic types of roller bearings

Cylindrical Thrust Bearing Tapped Thrust Beraing

Figure 7. Thrust load roller bearings

NEEDLE BEARINGSNeedle bearings have very small diameter rollers or needles, but high load carrying capacity

(Figure 8). They are most often used where space is limited, but they do not perform at high

speeds as well as roller or ball bearings. Also, they will not tolerate much misalignment.

Needle Radial Self Aligning Needle Needle Thrust

with Inner Race Radial without Inner Race

Figure 8. Basic types of needle bearings

There are two main types of needle bearings:

1. Radial load bearings

2. Thrust load bearings