BEARINGSFunction AND TYPESBearings 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 machinesBearings support moving parts by carrying radial and axial loads while holding theparts in alignment.Bearings reduce friction by providing either sliding contact (plain bearings) or rollingcontact (anti friction bearings). Bearings also reduce friction with a lubricant which willdissipate heat.Types of bearingsThere are two basic types of bearings: Plain bearings (bushings) Anti-friction bearings
Split Bearing Ball Bearing Roller Bearing Needle Bearing PLAIN BEARINGS ANTIFRICTION BEARINGS (BUSHINGS) PLAIN BEARINGSPlain 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, theshaft floats on a thin film of the lubricant. Some types of plain bearings
Types of plain bearings: Solid bearing or sleeve. Split bearings Thrust bearings Solid bearing Self-aligning bearingLOADS ON PLAIN BEARINGSPlain bearing can be used to withstand Loads in linear, reciprocating and rotational motions.They can carry the following loads: Radial ThrustBearing designed for radial loads onlyThrust 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 MATERIALSConstructionMonometals. Bearings made from a solid bar or tubes of an aluminum orbronze alloy have beenavailable for a number of years. They are generally used where loads arenot very high.Bimetals. A bimetal bearing has a steel back, to which is bonded a linerof Babbitt, copper-lead,or aluminum. Babbitts are soft alloys of lead or tin, with additives such ascopper, antimony, andarsenic.Trimetals. In order to achieve the desirable surface properties of aBabbitt bearing and thestrengths of harder materials, the trimetal bearing was developed forheavy-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.RequirementsEmbeddability. The ability of a material to absorb foreign particlescirculating in the oil stream isreferred to as embeddability.Conformability. The material also must be soft enough to creep or flowslightly to compensate forthe minor geometric irregularities which are present in every assembly.Fatigue Strength. This is the ability of a bearing material to withstand theloads to which it is subjectedwithout cracking.Temperature Strength. As operating temperatures increase, bearingmaterials tend to lose strength.This property indicates how well a material carries a load at elevatedtemperatures, without breakingup or flowing out of shape.
Thermal Conductivity. Shear of the oil film by the shaft generatessignificant heat, most of whichis carried away by the oil.Corrosion Resistance. Oils oxidize with use, and the products of thisdegradation 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 installationPREVENTIVE MAINTENANCE OF PLAIN BEARINGSLubricant Supply. Proper bearing design and material are necessary toachieve long service life but are not by themselves sufficient.Lubricant flow to the bearings is a function of the equipment design. Oilpressure at specified speeds should be within the limits given by theequipment 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 thesystem. This should be investigated immediately.The oil level also should be checked on a routine basis to avoid pumpcavitation and subsequent oil starvation.In non-pressurized lube systems, reservoirs should be checked on aregular schedule to ensure that adequate oil is always present.Cleanliness. Sleeve bearings simply cannot survive without adequatelubrication. Once this is assured, the next most important consideration isthe 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 andnumerous, bearing failure canLubricant Type. Ensuring an adequate flow of clean lubricant makeslong bearing life possible but does not guarantee it. The oil must be theproper 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 recommendationsbe followed.In addition, the proper combination of oil additives is necessary to preventrapid breakdown, thickening, foaming, and sludging. All these effects canlead to bearing failure, as well as to the damage of other components. A sampling of bearings, bushings, and thrust washers.
LUBRICATING PLAIN BEARINGSLubricants are used to reduce friction and wear in bearings. The less friction the less wear. Threestates of friction or lubrication may exist between sliding surfaces (As in Lubrication module in thismanual). Dry friction Boundary lubrication Full-film lubricationHow Lubricant Is Distributed To BearingsThe 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 waysTwo types of lubricants are most familiar: Oil GreaseThey 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 systemFluid Film BearingsFluid film bearings are classified according to the creation of their fluid film. Fluid film bearingsare: hydrostatic hydrodynamic squeeze film (damper) Fluid film creation is by: Externally Prusserized High speed and High frequency vibration inclination (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 BALL CYLINDRICAL SPHERICAL TAPERED NEEDLE ROLLER ROLLER ROLLER ROLLER Figure 1. Basic types of anti-friction bearingsBasic PartsMost anti-friction bearings have similar parts (Figure 2): Figure 2. Basic parts of anti-friction bearings
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 boundarydimensions have standardized boundary plans, dimensions, and tolerances according to theInternational Standards Organization (ISO). Therefore, bearings from all subscribingmanufacturers throughout the world are dimensionally interchangeable. Radial ball bearing types.
Radial roller bearing types.Roller bearing types for radial and axial loads combined.
Tapered roller bearings, single-row, multiple-row, and thrust.
BEARING SERIESFor any given bore size, all types of metric rolling bearings are manufactured in several serieseach 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 seriesaccording to the boundary plan shown in Fig. 2.4. Spherical roller bearings are normallyavailable in eight different series, as shown inFig. 2.5. Taper roller bearings, both inch- and metric-dimensioned, have a larger number ofseries or duty classifications, but all series are not necessarily available for every bore size (seeFig. 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 RATINGSBasic rating life is the number of revolutions (or the number of operating hours at a givenconstant speed) which the bearing is capable of enduring before the first sign of fatigue occursin 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 theprocedures used to mount and maintain it are equally important. Despite all recommendedprecautions, a bearing can still experience premature failure. In this case it is vital that thebearing be examined carefully to determine a reason for failure so that preventive action can betaken. The service life can either be longer or shorter than the basic rating life. Specificationlife 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 timesthis 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 itsmating shaft or in its housing with an interference fit, since it is virtually impossible to preventrotation by clamping the ring axially. Generally, it is the rotating ring that is tight, but morecorrectly stated, it is the ring that rotates relative to the load. In some special cases this is notthe rotating ring; for instance, in a vibrating unit where vibration is produced by eccentricweights, the load rotates with the rotating ring, and it is best to have the stationary ring havethe tight fit. Except for special cases as mentioned above, the stationary ring normally can beassembled withthe mating shaft or housing with a slip or loose fit. The magnitude of interference fit will varywith the severity of duty, type of bearing, and different shaft and housing materials. Ballbearings under normal load conditions will have approximately 0.00025 in interference perinch of shaft when the inner ring is the tight fit. Roller bearings will have fits of approximately0.0005 in per inch of shaft. Fits will be increased for heavy-duty service and decreased for lightduty. In general, when the outer ring is the tight fit, the interference is less than acorresponding shaft fit. All bearing manufacturers show recommended fitting practices for
their bearings in their general catalogs. With the exception of inch-taper roller bearings, therecommendations are normally expressed in ISO standards. ISO standards define the fittolerance between the bearing outsidediameter and the housing and utilize a designation system using a capital letter and a numbersuch as H7, J6, P6, and so on. Fit tolerances between the shaft and bore of the bearing aredesignated 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. Thediagram in Fig. 2.7 shows the relationship between the nominal diameters and the tolerancegrades. The crosshatched areas indicate the bearing bore diameter variation and the outsidediameter variation, respectively. The blackened rectangles show the range of tolerances forshafts (lower half) and housings (upper half). ISO fit tolerances. Uppercase letters refer to housings; lowercase letters refer to shafts
Types of Bearing RacesThe type of races in the bearing controls the load-carrying capacity, especially in ballbearings. Ball bearings have a variety of races. Basically, ball bearings have fourrace designs. 1. Conrad bearing 2. Full bearing 3. Split race bearing 4. Angular contact bearinganti-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 factorSingle Row Single Row Double Row Double Row Self Aligning Radial Radial Radial & Thrust Radial & Thrust 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 bearingsROLLER 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 ballbearings. As with ball bearings, roller bearings are precision made and must be handled withcare 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 bearingsNEEDLE 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 highspeeds 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 bearingsThere are two main types of needle bearings: 1. Radial load bearings 2. Thrust load bearings