2014

Universidad de Castilla la Mancha

Mehmet Canalp Külahlı

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İçindekiler1. BEARING TYPES..................................................................................................................3

1.1. Rolling Contact Bearings.............................................................................................3

1.1.1. Single-Row, Deep-Groove Ball Bearing...................................................................3

1.1.2. Double-Row, Deep-Groove Ball Bearing.................................................................4

1.1.3. Angular Contact Ball Bearing...................................................................................4

1.1.4. Cylindrical Roller Bearing........................................................................................5

1.1.5. Needle Bearing........................................................................................................5

1.1.6. Spherical Roller Bearing..........................................................................................6

1.1.7. Tapered Roller Bearing............................................................................................7

1.2. THRUST BEARINGS......................................................................................................7

2. BEARING MATERIALS.........................................................................................................7

3. BEARING TOLERANCES.......................................................................................................8

4. BEARING FITS...................................................................................................................11

4.1. The Requirement Of A Good Fit................................................................................11

4.2. Fit selection.............................................................................................................. 11

4.2.1. Tight fit and Loose fit............................................................................................11

5. BEARING SELECTION........................................................................................................13

5.1. According to the direction and magnitude of load...................................................13

6. Load Rating and Life.........................................................................................................14

6.1. Bearing life................................................................................................................14

6.2. Basic rating life and basic dynamic load rating.........................................................14

6.3. Basic static load rating..............................................................................................16

7. BEARING LOAD CALCULATION.........................................................................................18

7.1. Equivalent load.........................................................................................................18

7.1.1. Dynamic equivalent load.......................................................................................18

7.1.2. Static equivalent load............................................................................................18

7.1.3. Load calculation for angular contact ball bearings and tapered roller bearings. . .19

8. Lubrication.......................................................................................................................21

8.1. Purpose of lubrication..............................................................................................21

8.2. Lubrication methods and characteristics..................................................................22

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8.2.1. Grease lubrication.................................................................................................23

8.2.2. Oil lubrication........................................................................................................23

9. BEARING DAMAGE...........................................................................................................23

10. REFERENCES.................................................................................................................27

INTRODUCTION

Bearings are machine elements that allows or prevents moves of shafts and axles with the minimum friction.

Bearings

Bearings are formed by interlacing of an inner ring and an outer ring. One of the rings is attached to a shaft or an axle and the other is attached to pillow bed. Rings are in contact with the stable and shifting parts of the machine while rolling elements inside of the rings provide force transmission. Each of these balls being put inside of a cage for rotating freely.

Bearings can carry quite big forces and they can be narrow compared with their diameters. Because of these constructions can be made lighter and smaller. Bearings are used in many machines which are moving circular or axial.

The main task of bearings; provide the movement with least possible friction and ensure the transmission of power by sacrificing at least.

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1. BEARING TYPESWe can classify bearings with two main classes.

1-Rolling Contact Bearings 2-Thrust Bearings

1.1. Rolling Contact Bearings

1.1.1. Single-Row, Deep-Groove Ball BearingSometimes called Conrad bearings, the single-row, deep-groove ball bearing

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is what most people think of when the term ball bearing is used. The inner ring is typically pressed on the shaft at the bearing seat with a slight interference fit to ensure that it rotateswith the shaft. The spherical rolling elements, or balls, roll in a deep groove in both the innerand the outer races. The spacing of the balls is maintained by retainers or "cages." Whiledesigned primarily for radial load-carrying capacity, the deep groove allows a fairly sizablethrust load to be carried. The thrust load would be applied to one side of the inner race by ashoulder on the shaft. The load would pass across the side of the groove, through the ball, tothe opposite side of the outer race, and then to the housing. The radius of the ball is slightlysmaller than the radius of the groove to allow free rolling of the balls. The contact betweena ball and the race is theoretically at a point, but it is actually a small circular area becauseof the deformation of the elements. Because the load is carried on a small area, very high local contact stresses occur. To increase the capacity of a single-row bearing, a bearing with a greater number of balls, or larger balls operating in larger-diameter races, should be used.

1.1.2. Double-Row, Deep- Groove Ball Bearing

Adding a second row of balls increases the radial load-carrying capacity of the deep-groove type of bearing compared with the single-row design because more balls share the load. Thus, a greater load can be carried in the same space, or a given load can be carried in a smaller space. The greater width of double-row bearings often adversely affects the misalignment capability.

1.1.3. Angular Contact Ball BearingOne side of each race in an angular contact bearing is higher to allow the

accommodation of greater thrust loads compared with the standard single-row, deep-groove bearing. Commercially available bearings have angles of 15° to 40°.

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1.1.4. Cylindrical Roller Bearing

Replacing the spherical balls with cylindrical rollers , with corresponding changes in the design of the races, gives a greater radial load capacity. The pattern of contact between a roller and its race is theoretically a line, and it becomes a rectangular shape as the members deform under load. The resulting contact stress levels are lower than for equivalent-sized ball bearings, allowing smaller bearings to carry a given load or a given size bearing to carry a higher load. Thrust load capacity is poor because any thrust load would be applied to the side of the rollers, causing rubbing, not true rolling motion. It is recommended that no thrust load be applied. Roller bearings are often fairly wide, giving them only fair ability to accommodate angular misalignment.

1.1.5. Needle Bearing

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Needle bearings are actually roller bearings, but they have much smaller diameter rollers. A smaller radial space is typically required for needle bearings to carry a given load than for any other type of rolling contact bearing. This makes it easier to design them into many types of equipment and components such as pumps, universal joints, precision instruments, and household appliances. As with other roller bearings, thrust and misalignment capabilities are poor.

1.1.6. Spherical Roller BearingThe spherical roller bearing is one form of self-aligning bearing, so called because

there is actual relative rotation of the outer race relative to the rollers and the inner race when angular misalignments occur. This gives the excellent rating for misalignment capability while retaining virtually the same ratings on radial load capacity.

1.1.7. Tapered Roller BearingTapered roller bearings are designed to take substantial thrust loads along with high

radial loads, resulting in excellent ratings on both. They are often used in wheel bearings for vehicles and mobile equipment and in heavy-duty machinery having inherently high thrust loads.

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1.2. THRUST BEARINGSThe bearings discussed so far in this chapter have been designed to carry radial loads

or a combination of radial and thrust loads. Many machine design projects demand a bearing that resists only thrust loads, and several types of standard thrust bearings are commercially available. The same types of rolling elements are used: spherical balls, cylindrical rollers, and tapered rollers. Most thrust bearings can take little or no radial load. Then the design and the selection of such bearings are dependent only on the magnitude of the thrust load and the design life.

2. BEARING MATERIALSThe load on a rolling contact bearing is exerted on a small area. The resulting contact

stresses are quite high, regardless of the type of bearing. Contact stresses of approximately 300 000 psi are not uncommon in commercially available bearings. To withstand such high stresses, the balls, rollers, and races are made from a very hard, high-strength steel or ceramic.

The most widely used bearing material is AISI 52100 steel, which has a very high carbon content, 0.95% to 1.10%, along with 1.30% to 1.60% chromium, 0.25% to 0.45% manganese, 0.20% to 0.35% silicon, and other alloying elements with low, but controlled, amounts. Impurities are carefully minimized to obtain a very clean steel. The material is through-hardened to the range of 58-65 on the Rockwell C scale to give it the ability to resist high contact stress. Some tool steels, particularly M1 and M50, are also used. Case

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hardening by carburizing is used with such steels as AISI 3310, 4620, and 8620 to achieve the high surface hardness required while retaining a tough core. Careful control of the case depth is required because critical stresses occur in subsurface zones. Some more lightly loaded bearings and those exposed to corrosive environments use AISI 440C stainless steel elements.

Rolling elements and other components can be made from ceramic materials such as silicon nitride (Si3N4). Although their cost is higher than that of steel, ceramics offer significant advantages as shown in Table 14-2. (See Reference 6.) Their light weight, highstrength, and high temperature capability make them desirable for aerospace, engine, military, and other demanding applications.

3. BEARING TOLERANCES Bearing “tolerances” or dimensional accuracy and running accuracy, are regulated by ISO and JIS B 1514 standards (rolling bearing tolerances). For dimensional accuracy, these standards prescribe the tolerances necessary when installing bearings on shafts or in housings. Running accuracy is defined as the allowable limits for bearing runout during operation.

Dimensional AccuracyDimensional accuracy constitutes the acceptable values for bore diameter, outer

diameter, assembled bearing width, and bore diameter uniformity as seen in chamferdimensions, allowable inner ring tapered bore deviation and shape error. Also included are, average bore diameter variation, outer diameter variation, average outer diameterunevenness, as well as raceway width and height variation (for thrust bearings).

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Running AccuracyRunning accuracy constitutes the acceptable values for inner and outer ring radial

runout and axial runout, inner ring side runout, and outer ring outer diameter runout.Allowable rolling bearing tolerances have been established according to precision classes. Bearing precision is stipulated as JIS class 6, class 5, class 4, or class 2, with precision rising from ordinary precision indicated by class 0.

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4. BEARING FITSFor rolling bearings, inner and outer rings are fixed on the shaft or in the housing so

that relative movement does not occur between fitting surfaces during operation or under load. This relative movement between the fitting surfaces of the bearing and the shaft or housing can occur in a radial direction, an axial direction, or in the direction of rotation. Types of fitting include tight, transition and loose fitting, which may be selected depending on whether or not there is interference.

The most effective way to fix the fitting surfaces between a bearing's raceway and shaft or housing is to apply a "tight fit." The advantage of this tight fit for thin walled bearings is that it provides uniform load support over the entire ring circumference without any loss of load carrying capacity. However, with a tight fit, ease of installation and disassembly is lost; and when using a non-separable bearing as the floating-side bearing, axial displacement is not possible. For this reason, a tight fit cannot be recommended in all cases.

4.1.The Requirement Of A Good FitIn some cases, improper fit may lead to damage and shorten bearing life, therefore it

is necessary to make a careful investigation in selecting a proper fit. Some of the bearing failure caused by improper fit are listed below.

Raceway cracking, early flaking and displacement of raceway Raceway and shaft or housing abrasion caused by creeping and fretting corrosion Seizing caused by negative internal clearances Increased noise and deteriorated rotational accuracy due to raceway groove

deformation.

4.2. Fit selectionSelection of a proper fit is dependent upon thorough analysis of bearing operating

conditions, including consideration of:

Shaft and housing material, wall thickness, finished surface accuracy, etc. Machinery operating conditions (nature and magnitude of load, rotational

speed, temperature, etc.)

4.2.1. Tight fit and Loose fitFor raceways under rotating loads, a tight fit is necessary. "Raceways under rotating

loads" refers to raceways receiving loads rotating relative to their radial direction. For raceways under static loads, on the other hand, a loose fit is sufficient.(Example) Rotating inner ring load = the direction of the radial load on the inner ring is rotating relatively

For non-separable bearings, such as deep groove ball bearings, it is generally recommended that either the inner ring or outer ring be given a loose fit.

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5. BEARING SELECTION

5.1.According to the direction and magnitude of load

RADIAL LOADS

In these situations single or double row ball bearings or needle bearings can be used.

THRUST LOADS

In these situations, depending on the size of the load, thrust ball bearings or spherical roller thrust bearings can be used. Thrust ball bearings can only withstand one-way loads, on the other hand spherical roller thrust bearing can withstand loads from both directions.7

COMBINED LOADS

If there are two kinds of loads (radial and thrust) loading on a bearing, Single-Row, Deep-Groove Ball Bearings or Cylindrical Roller Bearing

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6. Load Rating and Life

6.1.Bearing lifeEven in bearings operating under normal conditions, the surfaces of the raceway and

rolling elements are constantly being subjected to repeated compressive stresses which causes flaking of these surfaces to occur. This flaking is due to material fatigue and will eventually cause the bearings to fail. The effective life of a bearing is usually defined in terms of the total number of revolutions a bearing can undergo before flaking of either the raceway surface or the rolling element surfaces occurs.

Other causes of bearing failure are often attributed to problems such as seizing, abrasions, cracking, chipping, scuffing, rust, etc. However, these so called "causes" of bearing failure are usually themselves caused by improper installation, insufficient or improper lubrication, faulty sealing or inaccurate bearing selection. Since theabove mentioned "causes" of bearing failure can beavoided by taking the proper precautions, and are not simply caused by material fatigue, they are considered separately from the flaking aspect.

6.2.Basic rating life and basic dynamic load ratingA group of seemingly identical bearings when subjected to identical load and

operating conditions will exhibit a wide diversity in their durability.

This "life" disparity can be accounted for by the difference in the fatigue of the bearing material itself. This disparity is considered statistically when calculating bearing life, and the basic rating life is defined as follows.

The basic rating life is based on a 90% statistical model which is expressed as the total number of revolutions 90% of the bearings in an identical group of bearings subjected to identical operating conditions will attain or surpass before flaking due to material fatigue occurs. For bearings operating at fixed constant speeds, the basic rating life (90% reliability) is expressed in the total number of hours of operation.

Basic dynamic load rating expresses a rolling bearing's capacity to support a dynamic load. The basic dynamic load rating is the load under which the basic rating life of the bearing is 1 million revolutions. This is expressed as pure radial load for radial bearings and pure axial load for thrust bearings. These are referred to as "basic dynamic load rating (Cr)" and "basic dynamic axial load rating (Ca)." The basic dynamic load ratings given in the bearing tables of this catalog are for bearings constructed of NTN standard bearing materials, using standard manufacturing techniques.

The relationship between the basic rating life, the basic dynamic load rating and the bearing load is given in formula.

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6.3.Basic static load ratingWhen stationary rolling bearings are subjected to static loads, they suffer from partial

permanent deformation of the contact surfaces at the contact point between the rolling elements and the raceway. The amount of deformity increases as the load increases, and if

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this increase in load exceeds certain limits, the subsequent smooth operation of the bearings is impaired.

It has been found through experience that a permanent deformity of 0.0001 times the diameter of the rolling element, occurring at the most heavily stressed contact point between the raceway and the rolling elements, can be tolerated without any impairment in running efficiency.

The basic static load rating refers to a fixed static load limit at which a specified amount of permanent deformation occurs. It applies to pure radial loads for radial bearings and to pure axial loads for thrust bearings. The maximum applied load values for contact stress occurring at the rolling element and raceway contact points are given below.

For ball bearings 4,200 MPa {428kgf/mm2}

For self-aligning ball bearings 4,600 MPa {469kgf/mm2}

For roller bearings 4,000 MPa {408kgf/mm2}

Referred to as "basic static radial load rating" for radial bearings and "basic static axial load rating" for thrust bearings, basic static load rating is expressed as Cor or Coa

respectively and is provided in the the bearing dimensions table.

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7. BEARING LOAD CALCULATION

7.1.Equivalent load

7.1.1. Dynamic equivalent loadWhen both dynamic radial loads and dynamic axial loads act on a bearing at the same

time, the hypothetical load acting on the center of the bearing which gives the bearings the same life as if they had only a radial load or only an axial load is called the dynamic equivalent load.

For radial bearings, this load is expressed as pure radial load and is called the dynamic equivalent radial load. For thrust bearings, it is expressed as pure axial load, and is called the dynamic equivalent axial load.

(1) Dynamic equivalent radial load

The dynamic equivalent radial load is expressed by formula: Pr=XFr+YFa+…………..where,Pr: Dynamic equivalent radial load, N {kgf}Fr: Actual radial load, N {kgf}Fa: Actual axial load, N {kgf}X: Radial load factorY: Axial load factor

(2) Dynamic equivalent axial load

As a rule, standard thrust bearings with a contact angle of 90° cannot carry radial loads. However, self-aligning thrust roller bearings can accept some radial load. The dynamic equivalent axial load for these bearings is given in formula:

Pa=Fa+1.2Fr+……….where,Pa: Dynamic equivalent axial load, N {kgf}Fa: Actual axial load, N {kgf}Fr: Actual radial load, N {kgf}Provided that Fr / Fa ≤ 0.55 only.

7.1.2. Static equivalent loadThe static equivalent load is a hypothetical load which would cause the same total

permanent deformation at the most heavily stressed contact point between the rolling elements and the raceway as under actual load conditions; that is when both static radial loads and static axial loads are simultaneously applied to the bearing.

For radial bearings this hypothetical load refers to pure radial loads, and for thrust bearings it refers to pure centric axial loads. These loads are designated static equivalent radial loads and static equivalent axial loads respectively.

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(1) Static equivalent radial load

For radial bearings the static equivalent radial load can be found by using these formulas. The greater of the two resultant values is always taken for Por.

Por=Xo.Fr+Yo.Fa….OrPor=Fr…….where,Por: Static equivalent radial load, N {kgf}Fr: Actual radial load, N {kgf}Fa: Actual axial load, N {kgf}Xo: Static radial load factorYo: Static axial load factor

(2) Static equivalent axial load

For spherical thrust roller bearings the static equivalentaxial load is expressed by formula.

Poa=Fa+2.7Fr……where,Poa: Static equivalent axial load, N {kgf}Fa: Actual axial load, N {kgf}Fr: Actual radial load, N {kgf}Provided that Fr / Fa ≤ 0.55 only.

7.1.3. Load calculation for angular contact ball bearings and tapered roller bearings

For angular contact ball bearings and tapered roller bearings the pressure cone apex (load center) is located as shown in Figure.

When radial loads act on these types of bearings the component force is induced in the axial direction. For this reason, these bearings are used in pairs. For load calculation this component force must be taken into consideration and is expressed by formula.

Fa=0.5FrY

where,Fa: Axial component force, N {kgf}Fr: Radial load, N {kgf}Y: Axial load factorThe dynamic equivalent radial loads for these bearing pairs are given in Table.

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8. Lubrication

8.1.Purpose of lubricationThe purpose of bearing lubrication is to prevent direct metallic contact between the

various rolling and sliding elements. This is accomplished through the formation of a thin oil (or grease) film on the contact surfaces. However, for rolling bearings, lubrication has the following advantages:

(1) Reduction of friction and wear(2) Dissipation of friction heat(3) Prolonged bearing life(4) Prevention of rust(5) Protection against harmful elements

In order to exhibit these effects, a lubrication method that matches service conditions. In addition to this, a quality lubricant must be selected, the proper amount of lubricant must be used and the bearing must be designed to prevent foreign matter from getting in or lubricant from leaking out.

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Figure shows the relationship between oil volume, friction loss, and bearing temperature. Table detailsthe characteristics of this relationship.

8.2.Lubrication methods and characteristicsLubrication method for bearings can be roughly divided into grease and oil

lubrication. Each of these has its own features, so the lubrication method that best offers the required function must be selected.

The characteristic are shown in table.

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8.2.1. Grease lubricationGrease lubricants are relatively easy to handle and require only the simplest sealing

devices. For these reasons, grease is the most widely used lubricant for rolling bearings. It is used a bearing that is pre-sealed with grease (sealed/shield bearing), or if using an unsealed bearing, fill the bearing and housing with the proper amount of grease, and replenish or change the grease regularly.

8.2.2. Oil lubricationOil lubrication is suitable for applications requiring that bearing-generated heat or

heat applied to the bearing from other sources be carried away from the bearing anddissipated to the outside.

9. BEARING DAMAGE

Rolling bearing damage is generally detected by unusual operational behaviour of the bearing arrangement. Uneven running and uncommon running noise usually indicate flaked running surfaces due to material fatigue or an alteration of the radial clearance due to wear. High friction, i.e. resistance to smooth running, can indicate detrimental preload, poor lubrication or damaged rolling contact surfaces. Higher than normal temperatures are a sign of an increase in friction. Breakdown of the lubrication may be responsible for a sudden rise in operating temperature occurring without a change in the operating conditions. In the case of machine tools, wear and other bearing damage are evidenced by the deterioration in quality of the workpieces.

These operational characteristics of a damaged bearing can be exploited to monitor the bearing arrangement, perhaps using temperature and vibration measuring instruments. At the first signs of premature bearing damage, the causes and effects should be analysed so that steps to avoid further damage can be taken. To this end, it is often necessary to dismount and examine the bearing.

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10. REFERENCES Machine Elements In Mechanical Design – Robert L. MOTT Ball and Roller Bearings – NTN Catalogue Makine Elemanları – Kadir Çavdarlı www.engineeringtoolbox.com