qmed bearings

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T T ABLE ABLE OF OF C C ONTENTS ONTENTS BEARINGS

1. Definitions and uses for roller bearings

2. Bearing materials and characteristics

3. How a bearing is manufactured

4. Bearing types-radial and thrust ball bearings, radial and thrust roller bearings, pillow

block bearings

5. Mitchell thrust bearings/Kingsbury thrust bearings

6. Bearing seals

7. Shafting and tolerances for roller bearings including housings

8. Mounting roller bearings

9. Bearing Monitoring including causes of vibration

10. Analyzing roller bearing failure-going forward successfully

11. Proper greasing techniques

12. Journal bearing construction and uses

13. How to determine if bearing is still good

14. Installation procedures of journal bearings

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1. DEFINITION AND USES FOR ROLLER BEARINGS1. DEFINITION AND USES FOR ROLLER BEARINGS

If this were a perfect world where everything would slide without wearing, or roll withoutwearing or building up heat, we would have no need of roller bearings. However, it is not a

perfect world and we all know how difficult it is to slide a 500 pound piece across a floor. We

also know how easy it is to move that 500 piece if it is mounted on wheels that have ball bearings. All of us have grown-up using and appreciating roller bearings. Now we want to put alittle more time in and truly understand the roller bearing.

Friction is the resistance that is the result of one object sliding against another object. We know,from the previous section, that we can vastly reduce friction by keeping a film of oil between thetwo objects. We also know that friction is reduced by having the faces of the objects be smoothrather than rough. Another way we can deal with sliding objects is to put rollers between thesurfaces. You may have used this technique at some time by putting steel pipe underneath a skidand rolling the skid across the floor. Therefore, by using balls or rollers we replace the slidingfriction with rolling motion.

Even though we are now using rolling motion we still will have metal-to-metal contact. As wedo not have full face-to-face contact the load on the balls that are in contact have to equal whatthe load on the face-to-face contact would be. This means that there is considerable loading oneach ball. The good news is that the circle configuration is very strong. And, while one ball may

be good, more surface area reduces the load and makes for a longer life.

Most roller bearings have four basic components. They are:1. Inner ring2. Outer ring3. Retainer

4. Rolling elements (balls or rollers)

FIGURE 1 - BALL BEARING WITH COMPONENTS LISTED.

The outer ring, rolling elements and inner ring all support the load. The retainer allows for positive separation between rolling elements. Without the retainer the rolling elements would

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rub each other, there would be direct contact between the rollers and there would be pounding of the rolling elements,

Because of the job we are asking the bearing to do the bearing steel must be of high quality. Thesteel must possess high strength, be wear resistant, have dimensional stability, be tough, have

excellent fatigue resistance, and have freedom from internal defects.

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2. BEARING MATERIALS AND CHARACTERISTICS2. BEARING MATERIALS AND CHARACTERISTICS

The search for better bearings has involved use of many bearing metals and alloys, several of which have proven very satisfactory.

Tin base babbitt is good for corrosion resistance and seizure resistance but is poor in fatiguestrength.

Lead base babbitt has better fatigue strength but is worse with respect to corrosion resistance.

Copper lead also has good fatigue strength and is lacking with corrosion resistance.

Leaded bronze has good fatigue strength and can carry heavy loads except on high-speed shafts.It will hold up without distorting better than copper lead and is used for main and connecting rod

bearings with a babbitt overlay.

Aluminum alloys has a good fatigue strength and good corrosion resistance and are being usedmore and more.

Steel backed aluminum alloy bearings are the most widely used, because they require lessclearance, they cost less; they have the highest fatigue strength and are good with corrosionresistance.

For standard applications the ball bearings and rings are usually made from through hardenedsteel AISI 52100. This gives a standard maximum operating range (for conventionalapplications) for the bearing from 225 0 F to 300 0 F. Higher temperatures (up to 400 0 F) can beachieved if heat stabilized 51200 steel is used.

The retainers can be made from many types of materials. Bronze, brass, steel, polymers andcomposites have all been used. Because the retainer is not load bearing the temperaturelimitations are usually higher for the retainers with a few exceptions. Certain brass, bronze and

polymer retainers may be limited to around 225 0 F.

Other parts that are sometimes included with the bearing are the seals and the shields. Shields arenormally made of either low carbon steel or stainless steel and, for the most part, are not aconsideration for temperature limitations. The seals, on the other hand, are very important toretain the grease inside the bearing and do impose temperature limitation. Seal materials canrange from felt (temperature limitation of about 212 0 F) to glass reinforced TFE Fluorocarbon

(with a temperature limit of about 5000

F).

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3. HOW A BEARING IS MANUFACTURED3. HOW A BEARING IS MANUFACTURED

The rings, inner and outer, and the rolling elements are heat-treated. This is done by very exactcontrol of temperature, pressure, and by controlling the rates of heating and cooling. If the metalis to be through hardened the parts are held above the austenizing temperature for a period of

time. This is the temperature at which ferric carbide is formed. The parts are then quenched(quickly cooled). This results in the formation of martensite. Martensite is a solid solution of iron with up to one percent carbon. To reach the final product the parts must then be tempered.The goal of this treatment is to obtain a hardness of 58-62 Rockwell C throughout a cross sectionof the material.

A second method of hardening involves treating the parts with carbon. The carbon is added in acarbon rich atmosphere within the furnace. The parts are then quenched and tempered and theend product has an exterior that is equivalent to through-hardened (58-62 Rockwell C) while thecore has a lower carbon content. The core is a 40-45 Rockwell C. The casehardened bearingscan be used where high shock loads are expected or where there can be rapid fluctuation in

temperature.

After the hardening process the parts are ground to very fine accuracies.

The balls are made of chrome steel. For applications up to 1 in diameter wire is used. For applications over 1 in diameter bar stock is used. To make the precision balls there are eightsteps taken:

1. Pressing-cold or hot to get form of ball2. Rill Filing to achieve roundness3. Tumbling- polishing and made uniform

4. Heat treating- to harden the balls uniformly through-out the cross section5. Precision Grinding-to get round with 1/10,000 of an inch6. Lapping- puts a very fine finish on the balls, done twice for very precision machines7. Gauging done electronically8. Inspection- both visual and electronically

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FIGURE 2

CLASSIFYING BEARINGS - Bearings can be generally classified either being sliding surface(friction) bearings or rolling contact (antifriction) bearings . Sliding surface bearingsmay be generally defined as bearings, which have sliding contact between their surfaces. These are lubricated bearings such as journal bearings or thrust bearings asused in engines, compressors and some low horsepower motors. Journal bearings can

be subdivided into different types such as solid bearings, half bearings, two part or split bearings. A typical solid type bearing is also known as bushing. The genre of friction bearings will be discussed a little later.

Antifriction, or rolling contact bearings are so named because their design takes advantage of thefact that less energy is required to overcome rolling friction than is required to overcome slidingfriction. Antifriction bearings can be generally defined as bearings that have rolling contact

between their surfaces. These can be subdivided as either ball or roller bearings (based on therolling element). We can further subdivide these two groups into those bearings that aredesigned for radial loads, thrust (axial) loads and a combination of the two.

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In a ball bearing the load is carried on two tiny spots that are diametrically opposed. In a roller bearing the load is carried on two narrow lines. As we have previously discussed theconstruction for this type of bearing is very exacting because of the loads that can be present inoperation.

The criteria for design, and for successful operation, of a bearing consists of the following: Available space Loads magnitude of load, direction of load, radial load, axial load, combined load Allowable misalignment Speed of rotation Precision required Need for quiet running Stiffness Axial displacement Mounting and dismounting

There are several different types and applications for antifriction bearings. The mechanic should be able to recognize the various types and know the application for which they are best suited.The most commonly used ball bearing is the single row or Conrad ball bearing. These bearingsare capable of carrying high loads in both the axial and radial directions.

The next type bearing is the single row, filling slot bearing. This type has a filling slot cutaxially through the shoulder, which allows as many balls as possible to fill the area between theinner and outer rings. The inclusion of more balls allows a greater radial loading as there is moresurface area. These bearing do not respond well to thrust and, if put into this situation, will benoisy and not perform well.

Single row angular contact bearings manufactured with a contact angle of between either 10 0to 22 0 or 22 0 to 40 0. The degree of contact angle depends on the use for the bearing. These

bearings can carry thrust in only one direction. Though nominally classified as a radial bearingthey cannot, in a stand-alone position, carry radial loads. There has to be thrust in the directionof the wide face for the bearing to work.

If you were to take two single row angular contact bearings and place them either in the face-to-face or back-to-back position you would have a bearing capable of carrying radial load, thrustload, or a combination of the loads. A tandem mounting of two or more single row angular contact bearings can be used to increase the thrust capacity. However, if used in this manner itmust be remembered that the arrangement cannot handle pure radial load; there must be thrustload.

Radial loads are best handled by double row ball bearings. The effect of double row is not twicethat of a single row but rather 1.55 times the effect of the single row bearing. This type of

bearing can also carry a large amount of thrust in either direction. In these bearings the groovesthat the balls ride in can be located in relation to each other such that the direction of loadthrough the balls forms angles with the radial plane of the bearing. Depending on the angle the

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load directions intersect either outside of the bearing (good for tilting loads) or inside the bearing(not as much sensitivity to misalignment).

This is a good place to discuss the need to check the original equipment manufacturers ( OEM ) book with regards to the bearings. Each bearing will be stamped by the bearing manufacturer.

This stamping (a series of numbers and/or letters) will designate what the bearing is capable of accomplishing. There is also a Master Bearing Book that allows the mechanic to compare one bearing manufacturer with another bearing manufacturer. All too often a mechanic will take a bearing of the same size and install that bearing without understanding what the bearing isdesigned to do. The installation and spare parts book provided by the equipment manufacturer should have the part number for the bearing. If you do not have the exact part number you cancross-reference with the Master Bearing Book. In the real world it is often the case that there is aselection of bearings available but not with OEM part numbers; if you will, a pool of bearings.The mechanic must read the book ( RTB ) and understand the function of the bearing. If you endup putting in a bearing that fits into the bearing housing, but is not suited to the job, its life will

be short, the experience a waste of time, and you may possibly damaged a perfectly good piece

of machinery. Preventative maintenance is all about staying ahead of the curve and knowing thatthe machinery can accomplish the job it was built to do. If the wrong parts are used thereliability goes down fast, the workload and costs go up, and frustration sets in. Particularly inthe realm of bearings, think of the loads, the speed, the reliability, and the potential costs beforesticking a bearing in a place where it is doomed to fail.

To read and understand a bearing the following is offered as a guide. The first number identifies the type of ball bearing. This serial number changes for each

bearing manufacturer. The second number identifies the bearing by section height within the series . The next two numbers indicate the bore size in millimeters when multiplied by 5. This is

not true for tapered roller bearings and needle roller bearings. The designators after this point are for seals (one or two), snap ring or no snap ring, cage

construction and lastly, clearance rating.

Clearance rating is a guide to let the used know what standard was used during the constructionof the rolling bearing. If a bearing is going to be used in a high temperature environment theinternal clearances should be greater than normal. They are rated as follows:

C1 has clearances less than C2 C2 has clearances less than normal C3 has clearances greater than normal C4 has clearances greater than C3 C5 has clearances greater than C4

In double row self-aligning ball bearings there is a common spherical raceway in the outer ringof the bearing. The inner raceway has separate tracks for the balls. This configuration allows the

bearing to self align. This type of bearing is very useful when there is the problem or potential(of) for misalignment. This design, because the potential non-conformity in the location of the

balls, does not lend itself to high load carry capacity.

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4. BEARING TYPES4. BEARING TYPES

There are a large number of different types of bearings used and the variety of names seemsquite complex, yet there are only three systems of classification in naming a bearing.

1. If the primary type of motion occurring between shaft and bearing is sliding, such as aconventional journal bearing, then the bearing belongs to the major class, called slidingsurface bearings.

2. If the primary type of motion is rolling, such as in roller or ball bearings, then the bearing belongs to the other major class, called rolling contact bearings.

a. A bearing required to carry a radial load is called a radial, sleeve or journal bearing depending on whether it is a rolling contact, engine, motor or machinery bearings.

b. A bearing required to carry a thrust load is called a thrust bearing.c. One, which carries both a radial and thrust load, is called a radial-thrust or a

journal thrust bearing.

3. Lastly it is based on bearing geometry. Bearings may be cylindrical, elliptical, lobed,tapers and lands, pivoted pads or pressure pockets.

Thrust ball bearings are used when the thrust load is beyond the capacity of radial bearings.Uni-directional thrust bearings cannot carry any radial load and, by definition, only carry thethrust in one direction. These bearings consist of one row of balls positioned between twogrooved washers. The smaller washer is a press fit on the shaft, which makes the bearingcentered on the shaft. The larger washer has a spherical supporting surface which seats against amatching surface. The matching surface should be at right angles to the bearing housing whichwould give a uniform loading on the balls.

If the single direction thrust ball bearing is used in the horizontal position it must be mounted incombination with another bearing which will 1) make sure the thrust washers do not separate inthe absence of thrust load and 2) take up any radial load.

Cylindrical roller bearings are a type of radial roller bearing . As the name implies these bearings consist of cylindrical type rollers. This type of bearing can sustain a much higher radial load than ball bearings of the same size. A subset of the cylindrical roller bearing is theneedle roller bearing. These bearings cannot carry any thrust and they have limitation withregards to speed. The benefit is that they carry relatively high radial load for the small radialspace they require.

Tapered roller bearings have, by definition, rollers, which are tapered. This results in frictionthat is higher than in ball bearings or cylindrical roller bearings. However this type of bearingcan be constructed with different fixed contact angles. As the contact angle becomes steeper theamount of thrust load the bearing can handle increases. This type of bearing is constructed witha strength member, called a rib, to keep the rollers from being displaced under a radial load. For the most part the housings are fixed and the shafts float free axially. Again, most of the timethese bearings are used in pairs.

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FIGURE 3

Spherical roller bearings can assume considerable thrust loads and can support a radial load.These bearings consist of two rows of roller, shaped like drums. The number of rollers and their size are critical to the load carry capacity of the bearing.

Tapered roller bearings can be constructed with either the small bore washer tapered only or with both the small bore and the large bore washer tapered. With both washers tapered the

bearings are alignment sensitive but can carry high thrust loads. The bearings with the small bore and tapered only on that bore will permit displacement in the radial directions without strainon the bearing.

Pillow blocks are housings for bearings that are made to be bolted down. There are manydifferent types of pillow blocks and they can be made for either ball or roller bearing.

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FIGURE 4 TILTING PAD BEARING

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FIGURE 5 MULTISHOE TYPE THRUSET BEARING

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5. MITCHELL THRUST BEARINGS/KINGSBURY THRUST5. MITCHELL THRUST BEARINGS/KINGSBURY THRUST BEARINGSBEARINGS

In the late 1880s an Englishman, Professor Osborne Reynolds, did a lot of work with the loadcarrying capacity of oil layers. In the early 1900s and Australian, A.G.M. Mitchell, usedProfessor Reynolds work as a basis for developing a thrust bearing. Mitchells work involved asquare block sliding over a flat surface that was well lubricated. What Mitchell found was thatas the square block was slid over the surface the oil built up under the leading edge of the block causing the block to tilt. At this angle the block was subjected to less friction and had greater load capacity than if it were parallel to the flat surface. These developments led to the MitchellThrust Bearing.

The same wedge shape film of oil is the basis for the Kingsbury Thrust Bearing. It is comprisedof a plate (flat surface) with segmented shoes, which can tilt under the pressure of the wedgeshape film of oil. Both of these bearings are very common as the axial thrust bearings used for main propulsion and generator prime movers.

KINGSBURY THRUST BEARING The principle of a Kingsbury Thrust Bearing is shownin the simple sketch below. A single shaft collar pushes against several pivoted shoes,which are held in place by a stationary seat fastened to the hull of the ship. When theshaft is revolved, the shoes pivot to allow the film of oil between the collar and the shoesto take the form of a wedge. The wedge of oil can withstand tremendous pressurewithout breaking down, making it possible to operate a single collar. The entire bearingis encased in a housing, lubrication is supplied by the rotating collar dipping in the oilsump.

Kingsbury thrust bearings require much less space than horseshoe and are more efficient.

FIGURE 6 KINGSBURY THRUST BEARING

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FIGURE 7 - KINGSBURY PIVOTED-SHOE THRUST BEARING

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6. BEARING SEALS6. BEARING SEALS

There are several key elements for long bearing life; proper design, proper installation, keepingthe bearing well lubricated and keeping contaminants out of the bearing. Seals are veryimportant for the last two elements. The considerations for seal design are as follows:

Cost Environment (temperature and probable contaminants) Speed (ft/min) Qualities of lubricant Shaft surface Space available Permissible leakage Permissible shaft misalignment

A shield can be considered a type of seal. It acts to keep the lubricant in the bearing and can

easily be used when the temperature rise makes the use of elastomer contact seals not practical.The seal acts as a labyrinth. They are commonly used on electric motor bearings when they arein a dry and dust free environment.

Contact seals can be either of the outside diameter riding type (also known as O.D. riding or land type) or the chamfer contact type. Seal design and efficiency of the design can often beimproved by the use of channels or the use of one or more washers. The lubricant would fill inthe channels and be beneficial in keeping contaminants out. Washers use the labyrinth/drop in

pressure to minimize leakage of the lubricant out of the bearing.

Felt Seals are commonly used in applications where the operating temperature does not exceed212 0 F and the operating speed stays below 800 ft/min. Other types of seal include V-Ring seals,Taconite seals, and Radial Lip Ring seals. When using radial lip ring seals it is important toconsider the shaft. It should be very smooth and possibly hardened, as the seal will ride, lightly,on the shaft. It is always a good practice to closely examine the shaft prior to the installation of the bearing and seal to make sure there is high probability of success. If the surface is pock marked and/or worn the shaft should be clean and re-metaled or the shaft can be undercut and asleeve installed.

We talked earlier about the quantity of bearings and how they made our lives, in the mechanicalworld, a lot easier. We have discussed many different types of bearings that can take differentloading. We have also used the words bearing life. What we need to address next is theconcept of bearing fit. The bearing has to have a good fit between the outer ring and thehousing and have a good fit between the inner ring and the shaft. This means that the fit cannot

be too tight or too loose.

We went through the checks that are made as a bearing is being fabricated. All the bearingmanufacturers make their bearing to standard tolerances the American Bearing ManufacturersAssociation and the International Standards Organization set the standard tolerances. Themachinery designer takes into account the type, direction and amount of load, the operating

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temperature, the required accuracy of the particular machine and the expansion from heat of themetal to come up with the type of bearing and standards which have to govern the fit to makethis bearing successful.

The part of the bearing that rotates, either the inner or outer ring, will have a press fit. This will

eliminate creep or differential rolling . This means from a maintenance standpoint it is not good practice to glue a bearing, knurl the area where the bearing is going to be placed on a shaft, or to pin punch a shaft to get a fit that someone deems acceptable. By the same token, the areawhere the bearing is going to have a fit must be dimensionally round (round not oval) and nothave metal missing so there is enough gripping power (decayed metal or corrosion effects thatleave either a surface or housing suspect). In addition to possible creep, which only gets worsefrom wear, the installer also sets up the very real possibility of vibration as the alignment will not

be correct. This all leads to a short bearing life, possible collateral damage, and possibly severedamage to the machine. Both the ABMA and ISO publish the standards. For the normalmechanic there is no real need to learn the standards. It is, however, extremely important toknow that they exist and how to get them. Many bearing houses will supply their clients with a

Master Bearing Book, which allows the client to substitute one manufacturers bearing for another manufacturers bearing. Many of the standards and individual bearing manufacturersdesignators are listed in the book. For this reason this book is handy to have around as areference.

As noted just above, proper mounting of a bearing is critical to long life for the bearing. This isalso a good point to refresh everyones mind as to the purpose of doing preventativemaintenance. Preventative maintenance should mean life extension or diagnosis, without killingthe patient, so corrective surgery can be done to improve the machine. In easy terms, if youseparated you shoulder and went to the doctor you would be less than impressed if the doctor was pounding your arm to get it back into the socket. Yet most people have seen mechanics

being slightly abusive to a machine and not think anything about it. You would also not behappy if the doctor had to cut off your arm, or leg, just to accomplish a diagnosis. The idea isthat something is worn out or broken and it needs to be fixed. There are also proper and improper ways to carry out maintenance.

We know that improper fit, improper alignment and/or a brand new damaged bearing will nothelp any machine. We also know that contaminants (dirt and dust) in the bearing will greatlyshorten the life expectancy. Yet we will see mechanics take an old piece of pipe, dirty and rusty,out of the scrap box, and use it, with a hammer, to drive on a new bearing. We have all probablyalso seen the mechanic, with dirty hands, go over to the grease pail (left out in the shop to collectwhatever) and fill a bearing with grease. Why then are we surprised when the bearing does notlast as long as it should?

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7. SHAFTING AND TOLERANCES FOR ROLLER BEARINGS INCLUDING HOUSINGS7. SHAFTING AND TOLERANCES FOR ROLLER BEARINGS INCLUDING HOUSINGS

SHAFT TOLERANCES FOR STANDARD INCH SIZE TAPERED ROLLER BEARINGS 1,2 SIZES (VALUES IN INCHES)

ConeBore

Shaft Seal Deviation from Minimum Cone Bore and the Resultant Fit

D Rotating Cone Stationary ConeModerate loads 3

No shock Heavy loads, or high speed or shock Heavy loads 4, or high

speed or shock Moderate loads 3, no

shock Wheel spindles

Over Incl. Tolerance Shaft sealdeviation

Resultantfit

Shaft sealdeviation

Resultantfit

Shaft sealdeviation

Resultantfit

Shaft sealdeviation

Resultantfit

Shaft sealdeviation

Resultantfit

0 3 +0.0005 +0.0015 0.0005T +0.0025 0.0010T +0.0025 0.0010T 0 0.0010L -0.0002 0.0012L0 +0.0010 0.0015T +0.0015 0.0025T +0.0015 0.0025T -0.0005 0 0.0007 0.0002L

3 12 +0.0010 +0.0025 0.0005T 0 0.0020L -0.0002 0.0022L0 +0.0015 0.0025T 0.0005/Inch 0.0005/Inch -0.0010 0 -0.0012 0.0002L

12 24 +0.0020 +0.0050 0.0010T Bearing bore Bearing bore 0 0.0040L - -0 +0.0030 0.0050T Avg. Tight Fit Avg. Tight Fit -0.0020 0 - -

24 36 +0.0030 +0.0075 0.0015T +0.0150 0.0090T +0.0150 0.0090T -0.0030 0 - -0 +0.0045 0.0075T +0.0120 0.0150T +0.0120 0.0150T -0.0030 0

1 For fitting practice for metric and J-prefix part number tapered roller bearing.2 These recommendations not applicable to tapered bore cones. For recommendations, consult your SKF representative.3 C > 8.3

P

4 C< 8.3P

C is the basic load rating P is the equivalent load.T indicates tight fit L indicates loose fit.

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HOUSING TOLERANCE FOR STANDARD INCH SIZE TAPERED ROLLER BEARINGS(SIZES AND VALUES IN INCHES)

Cup O.D Housing Seat Deviation from Minimum Cup O.D. and the Resultant FitFloating or non-adjustable Non-adjustableClamped Adjustable or in carriers or in carriers,

Sheaves-clamped sheaves-

Over Incl. Tolerance Housing seatdeviation

Resultantfit

Housing seatdeviation

Resultantfit


Resultantfit


Resultantfit


Resultanfit

0 3 +0.0010 +0.0030 0.0030L +0.0010 0.0010L -0.0005 0.0005T -0.0005 0.0005T -0.0020 0.0020T0 +0.0020 0.0010L 0 0.0010T -0.0015 0.0025T -0.0015 0.0025T -0.0030 0.0040T3 5 +0.0010 +0.0030 0.0030L +0.0010 0.0010L -0.0010 0.0010T -0.0010 0.0010T -0.0020 0.0020T

0 +0.0020 0.0010L 0 0.0010T -0.0020 0.0030T -0.0020 0.0030T -0.0030 0.0040T5 12 +0.0010 +0.0030 0.0030L +0.0020 0.0020L -0.0010 0.0010T -0.0010 0.0010T -0.0020 0.0020T

0 +0.0020 0.0010L 0 0.0010T -0.0020 0.0030T -0.0020 0.0030T -0.0030 0.0040T12 24 +0.0020 +0.0060 0.0060L +0.0030 0.0030L -0.0010 0.0010T -0.0010 0.0010T -0.0020 0.0020T

0 +0.0040 0.0020L +0.0010 0.0010T -0.0030 0.0050T -0.0030 0.0050T -0.0040 0.0060T24 36 +0.0030 +0.0090 0.0090L +0.0050 0.0050L -0.0010 0.0010T -0.0010 0.0010T - -

0 +0.0060 +0.0030L +0.0020 0.0010T -0.0040 0.0070T -0.0040 0.0070T - -

Recommended fits above are for cast iron or steel housing. For housings of light metal, tolerances are generally selected which give a slightly tighter fit than those in the table.1 For fitting practice for metric and J-prefix part number tapered roller bearings.T indicates tight fit.L indicates loose fit.

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8. MOUNTING ROLLER BEARINGS8. MOUNTING ROLLER BEARINGS

Bearings come wrapped in a heavy-duty waterproof paper. Until you are ready to mount the bearing it should stay wrapped up. If the bearing is going to be used with synthetic greases or oils it may be necessary to flush the slushing compound out of the bearing. Do not open the

package and leave it on the shop bench for a few hours or few days while the job is progressing.Make sure the shaft and the housing meet all the criteria for re-use. Be particular! Double-check the designators on the bearing that it is an exact replacement for the bearing called for in the

parts manual or from the bearing that was taken out. It is always a great idea, particularly onolder machines, to double check the book to make sure that the right part is being installed.There are then three methods of mounting bearings. They are: 1) cold mounting, 2) hotmounting (temperature mounting), and 3) hydraulic mounting.

Cold Mounting-This method can be used with bearings up to a 4 outside diameter. A sleeve,which fits perfectly on the inner ring or perfectly on both rings (if the press fit is required both onthe inside and the outside), can be used with either a hammer or a press. The sleeve should be

clean, parallel and flat and the hammer should be in good shape and of solid material. Soft metalhammers (heads) are unacceptable as the metal may fly off and lodge in the bearing. If the shaftis put in a vice, and it probably will, the shaft must be protected to prevent damage. With thehammer or the press the bearing can be driven into place. When using a hammer do not hit oneside and then the other side to drive the bearing into place. The force should be put in themiddle. If there is a direction to the bearing make sure it is correct. When finished make surethat the bearing is properly seated and everything is clean.

Hot (temperature) Mounting This method can be done one of three ways;1. Heat one part (most commonly done)2. Cool one part (least common)

3. A combination of the two heat one part while cooling the other part.

Methods for heating the bearings include hot oil bath, induction heaters, ovens and hot plates. Ahot oil bath can never be used with bearings that have seals as the seals may become damaged.Maximum temperature for heating is 250 0 F. Thirty years ago the oil bath was a very common

procedure. As induction heaters have dropped in price, and shown how easy they are to use, theyare much more prevalent today. If you use a hot oil bath you must use clean oil that will notcarbonize. Also make sure the bearing is not on the bottom of the container. If you use a torch asthe heat source do not go for the quick, high temperature, big flame solution. The bearing must

be heated up uniformly. Just prior to installation make sure the bearing is wiped clean. Note: if heat is used make sure the bearing is locked in place until it cools to prevent the bearing from

creeping away from the shoulder.

Hydraulic Mounting - This method is not normally used except in some shops that do a lot of mounting and have the equipment and can make it pay for itself. This method is also known asoil-injection mounting. This is a form of cold mounting as no heat is applied. A lightweight oil,20-30 SAE, is pumped under pressure of up to 10,000 psi through a hole in the shaft that goesinto the inner diameter of the bearing. This pressure forces the bearing to spread and the bearingcan be brought into proper position with minimal force.

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When an operator or mechanic is making his rounds he will be looking for leaks, unusual smellsand unusual noises. Most good mechanics/operators will put their hands on rotating machineryto feel for vibrations. Over time they will develop an appreciation for the normal vibration of that machine. If there is a change in the characteristics of the vibration they will stop and spenda little more time by that machine to understand what is causing the change in vibration. As

people spend more time around machinery they come to learn that a slight change is not, byitself, catastrophic. It means that there is a change and that something is causing that change.

Vibration can be defined as rapid periodic oscillation of an elastic body. The oscillations are brought about by a force being put upon the elastic body. When this force is applied the bodychanges its position to compensate for the force. However, the body stores the energy thatcomes with the change and uses this energy to restore itself to the original position when theforce is removed. However, the energy pushes the elastic body beyond the normal position.Until the energy is consumed the body continues to oscillate. To picture this imagine a rubber

band secured in two locations which you then pull slightly in one direction and then you let it go.The band swings back and forth until the inputted energy is dissipated.

We all have a hard time thinking about a steel shaft as being elastic. Think in terms of a verylong shaft. Now it is easier to think of the shaft bending.

The frequency is defined as the rate at which the vibrations occur and this is normally done invibrations per second. Even though the bearings are made very well we know that there is a

practical limit to precision grinding and lapping because of cost considerations. Therefore, allmachines will have some small vibration built in to the machine. This is known as the naturalvibration of the machine. When an outside (read not normal) force acts on the machine this isknown as a forced vibration . This outside force could be loose windings on a generator, a worn

bearing, or an impeller that is worn. If the forced vibration and the natural vibrations occur at thesame frequency, or they occur at a multiple of the natural vibrations the vibrations will bereinforced. As an example, if the frequency is 30 cps a multiple or harmonic would be 60 cps or 90 cps. When the natural vibrations are reinforced the condition is known as resonance . Under this condition the end vibrations will be magnified and can possibly produce stresses that the

piece was not built to withstand. It is the reason that soldiers never march in cadence over a bridge.

With respect to bearings, the vibrations will increase when there is wear on any of the rollingelements. This is because the support is no longer uniform and that allows the body to distort.Different vibrations can also be caused by creep. So, the mechanic, by placing his hand on themachine will build a history of how the machine should feel. When that feel changes themechanic needs to know what has changed.

As there may be two or more mechanics involved with any machine it may pay to have avibration monitoring system in place. By having the system there would be a record of thevibrations. The placement of the probe and how the readings are taken will affect the readings.Therefore, just having the machinery to accomplish the job is not enough. There needs to betraining on each system until all the people who can take readings are competent. There are threetypes of vibration probes. They measure the following:

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Acceleration measures the rate of change in velocity Displacement the distance of an object relative to its reference point (eddy probe) Velocity the rate at which the displacement changes

Each of these types has advantages and disadvantage. Large motors and machines would

probably do better with displacement monitoring. Additionally, this type of measurement isrecommended to track for imbalance, misalignment, bowed shafts, and fluid film bearinginstability. If the rotor-to housing weight ratios are close the velocity probe or acceleration probeis more than sufficient to detect an aberrant condition.

Once the readings are recorded someone has to study the results. Questions should be asked tomake sure that other conditions have not changed that will be reflected on the machine beingtested. What you are looking for are trends in the vibrations. For pumps an unbalance should bein the range of 5 to 90 cps. When a failure is just beginning to manifest itself there will be achange in the signature of the machine. The next reading will reflect another change. As thereadings get worse the machine is accelerating toward failure. At this point the machine should

be scheduled for an overhaul. When taking the machine apart the mechanic should be looking toverify the vibration information. Great care should be taken in disassembly. In essence, youknow the answer; now you have to verify the parameters that gave you that answer. After careful re-assembly with new parts a vibration signature should again be taken and recorded. Onthe preventative maintenance sheets vibration reading should be consistent with historical data.If something was not done correctly the vibration reading should point this out right away. If the overhaul was done correctly the vibration signature should reflect what was found in the past.

Part of the machinery history should include the expected bearing life. If the expected bearinglife was shorter than expected there should be an examination to determine the reason why. Thisis an autopsy of a shaft and a bearing. The lubricant, condition of lubricant, condition of shaft,

condition of bearing, ambient temperature, ambient conditions and temperature should all belooked at. In technical terms this is called a root cause analysis. If something has changed in the baseline conditions this should be noted and the bearing may have to be up-graded. Part of condition monitoring is completing the work and having a record of the conditions found.

Life expectancy for a bearing is calculated from the following formula:p

L 10 =

P

C

Where: L10 - number of millions of revolutions that 90% of a group of bearings will equal or

exceed before the first sign of fatigue on any part C - Dynamic capacity of a bearing (load in pounds given by manufacturer that will give

1,000,000 revolutions of service) P - Equivalent dynamic load on a rotating radial bearing if applied to a bearing with a

rotating inner ring and stationary outer ring which would give the same life as that whichunder constant or variable conditions of load and rotation

p - 3 for ball bearings, 10/3 for roller bearings

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There are also nomograms available to determine the service life. Both the nomogram and theformula are based on the bearing being new and undamaged, being properly installed withcomponent parts made to correct specifications, that there are no inherent defects in the bearingand that the bearing was properly lubricated.

From a common sense perspective most premature bearing failures are the result of dirt andcontaminants in the lubrication medium, improper installation, vibration, the wrong kind of lubrication (ineffective or wrong viscosity), the wrong bearing installed or corrosion. There aremany more specific types of failures but most of them fall in to these categories. As previouslynoted, great care must be taken when the machine is taken apart as well as when the machinegoes together. Valuable information can be lost if the work is not done systematically. Picturesand notes are easy to take at the time the work is done. Look closely at the condition of theshaft. Check for spalling and cracks. Carefully measure the shaft and double check that the

bearing being installed is the correct bearing for the job.

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9. BEARING MONITORING INCLUDING CAUSES OF9. BEARING MONITORING INCLUDING CAUSES OF VIBRATIONVIBRATION

Machinery Maintenance personnel regularly make visual and hands-on inspections of machinery,looking for conditions such as leaking or dirty oil, excessive vibration, and unusual noises. Theseinspections provide some assistance in monitoring machinery operation. However, there is no

precise way to determine exactly when to make a repair in order to prevent equipment breakdown.

PREVENTIVE STEPS - Bearing monitoring or predictive maintenance is the process of determining the condition of machinery as it operates. If a problem is indicated, bearingmonitoring provides information useful in determining what the problem is and, moreimportantly, what caused the problem. Bearing monitoring is beneficial because it:

1. Minimizes machine downtime and production loss through improved scheduling of spare parts procurement and repair activities.

2. Minimizes repair costs by prevention of catastrophic failures that may result in extensivemachine damage.

3. Prevents product quality deterioration in cases where the precision of a productionmachine affects product quality.

It is not sufficient just to replace components such as bearings and gears that are beginning tofail. It is also necessary to ensure that the manner in which they are installed does not contributeto early failure. Vibration monitoring of machinery after repair or rebuild may expose the needfor improved design and manufacture of machine components or more careful control of assembly procedures.

For example, vibration measurement after component replacement or machine rebuild may provide indications of misalignment (bearing housing to housing or shaft coupling shaft), faultyhousing or shaft geometry (taper, out-of-round, or excessively loose or tight fits), imbalance or eccentricity of rotating components, brinell damage of bearings, dirt in bearings or lubricant, or aweak machine assembly (loose mounting bolts or structural defects).

CAUSES OF VIBRATION - Any machine, regardless of how meticulously it has beendesigned and built, will vibrate to some degree because there are loads and resultingmotions (deflection, velocity, acceleration) that vary cyclically within rolling bearingsand on gears, fan blades, pump impellers, and motor stators and rotors. The cyclic(dynamic) forces are a result of the fact that loads and forces are not uniformlysupported but are carried by a discrete number of moving components.

In addition, there are imperfections or abnormalities, however slight, which contribute tovibration. The task of vibration monitoring is to determine if imperfections and abnormalities areso large or numerous that machine failure is imminent.

IMBALANCE - If the mass of a rotating member is not symmetrically distributed about the axisof rotation, then it is said to be imbalanced. The more something is imbalanced themore vibration will occur.

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10. ANALYZING ROLLER BEARING FAILURE GOING10. ANALYZING ROLLER BEARING FAILURE GOING FORWARD SUCCESSFULLYFORWARD SUCCESSFULLY

In order to be able to fix why a roller bearing had a failure so that it will not happen again. Youwill need to understand how bearings fail and why. This will be explained more in class.

Below is a general list of reason why bearings fail:

Excessive load thrust occurs when there is an axial overload from the machine or the bearinghas been improperly mounted.

Internally preloaded occurs when there is not internal clearance in a mounted bearing becausethe rolling elements are pressing tightly on the raceways.

Pinched outer ring an outer ring that has been forced out of round during installation.

Outer ring misaligned this happens when it is not perpendicular to the centerline of the shaft.

Inner ring misaligned this happens when the outer ring is misaligned and not perpendicular tothe centerline of the shaft.

Defective bearing seat on shaft when the inner ring does not have a sufficient press on a shaft(when there is inner ring rotation), the inner ring will turn.

Defective bearing seat in housing when the inner ring does not have a sufficient press fit on theshaft (when there is inner ring rotation), creeping of the inner ring around the shaft will occur.The end face of the inner ring will show rub marks.

Misalignment self-explanatory

Improper mounting self-explanatory

False brinelling - when there is vibration without rotation, false brinelling will occur.

Vibration when the mass of a rotating member is not symmetrically distributed about the axisof rotation.

Brinelling the bearing was press mounted on a shaft by pressing on the end face of the outer ring.

Electric arcing occurs when a current passes through a bearing to ground.

Contamination ineffective seals allowed hard contaminants to enter the bearing.

Ineffective lubrication occurs when the viscosity of the oil or the viscosity of the oil in thegrease is not adequate to separate the rolling elements from the raceway.

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Corrosion occurs when there is an ineffective seal that allows water/corrosive chemicals toenter a bearing where there is no corrosion inhibitor on the bearing.

Excessive thrust load occurs if the shaft was not allowed to expand or the machine wasoverloaded.

Spalling occurs when the metal reaches its fatigue limit.

Ineffective seals occurs when shaft alignment is wrong.

Manufacturing defects self-explanatory

The mechanism of action, the cause and the solution will be explained in class with some physical examples.

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11. PROPER GREASING TECHNIQUES11. PROPER GREASING TECHNIQUES

Bearing housings equipped with pressure fittings and relief plugs may be cleaned and re-lubricated without ever removing the bearings from the housings. When this method of cleaningis used, both the bearing and housing are simultaneously purged of old grease. The cleaning

agent may be either a light mineral oil heated to about 75C or an approved safety solvent. If thelatter is used, the bearing should be rinsed with a light mineral oil to remove all traces of solvent

before re-greasing. Figure 1 a, b, c, and d illustrate the recommended method for cleaninghorizontal motors that are equipped with pressure fittings and relief plugs. The bearing housingsand relief fittings should be wiped clean to prevent the entry of dirt. The grease fittings and

pressure relief plugs, located respectively at the top and bottom of the bearing housings, should be removed, and a screwdriver should be used to free the openings of hardened grease. Thenwith the motor running, an approved type of grease solvent should be injected into the bearinghousing. This is easily done with a syringe, injecting the solvent into the bearing through the tophole. As the solvent thins the grease, it drains out through the relief hole. Solvent should beadded in small quantities until it drains out reasonably clear. The relief plug should then be

replaced, and a small amount of solvent should be added and allowed to churn for a few minutes.The relief plug may then be removed, and the solvent allowed to drain. If the drainage is notclear, replace the relief plug, add more solvent, and allow it to churn a few minutes more. Thisshould be repeated until the solvent drains clear. The bearing must then be flushed out with lightmineral oil to remove all traces of solvent. The cleaning of vertical machines or machines notequipped with pressure-relief systems must be done by disassembling the bearing housing or completely disassembling the machine.

FIGURE 8 CLEANING A BALL-BEARING MOTOR

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The recommended procedure for re-greasing machines that are equipped with pressure fittingsand relief plugs is illustrated in Fig. 2 e, f, g, and h. The bearing housings, pressure plugs, relief fittings, and grease gun should be wiped clean. The relief plugs should be removed, and theopenings cleaned of hardened grease with a small screwdriver. This permits the expulsion of oldgrease and prevents the buildup of excessive pressure in the bearing housing that might rupture

the bearing seals. Then with the motor running, grease should be added with a hand-operatedgrease gun until it begins to flow from the relief hole. The motor should be allowed to run longenough to permit the bearing to expel all excess grease from the housing. The machine shouldthen be stopped, the relief plug replaced, and the housing wiped clean. If it proves dangerous tolubricate the motor while it is running, follow the same procedure with the motor at standstill.Then start the machine, and allow it to run until all excess grease is expelled. Only greaserecommended for electric motors should be used.

FIGURE 9 - LUBRICATING A BALL-BEARING MOTOR

The greasing of vertical machines or machines that are not equipped with pressure-relief systems

may be done by disassembling the bearing housings. The bearing and housing should be washedwith an approved safety solvent, rinsed with light mineral oil, and then packed with grease. Onlythe lower half of the bearing housing and the space between the balls should be packed withgrease. Do not fill the entire housing with grease; it may overheat and build up excessive

pressure, thus forcing the grease into the motor housing.

A recommended re-greasing schedule for electric motors is given in Table 1.

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Ball bearings are precision made and are adversely affected by dirt. Hence ball bearings shouldnot be unnecessarily removed from the shaft and housing. New bearings should not be removedfrom their original wrapper until ready for immediate installation. A defective bearing should bereplaced with the same size and type as the original. The defective bearing should be removedfrom the shaft with a bearing puller or arbor press by applying pressure against the inner race.

The replacement bearing should be pressed on or tapped on, using a hammer and a clean metaltube or pipe that fits evenly against the inner race. Do not tap the outer race, because it maydamage the bearing.

If preheated to 200F, the bearing will expand sufficiently to slip on the shaft with little or nodriving. Heating of bearings is done best in a temperature-controlled oven. If an oven is notavailable, bearings may be heated in a hot oil bath. However, heating in hot oil may causecontamination and deterioration of the grease

TABLE 1 RECOMMENDED RE-GREASING SCHEDULE

Type of service Typical examples

Horsepower

- 7 10-40 50-200Easy Motor operating infrequently(1 hr/day) 10 yr. 7 yr. 5 yr.

Standard Machine tools, fans, pumps, textilemachinery 7 yr. 5 yr. 3 yr.

SevereMotors for continuous operations in keylocations subject to severe vibration; steel-mill service - coal and, mining machinery

4 yr. 2 yr. 1 yr.

Very severe Dirty and vibrating applications where endof shaft is hot; high ambient

9 mo. 4 mo. 4 mo.

SOURCE: GENERAL ELECTRIC CO

Note: Tapping or pressing on the outer race is done only when the bearing is being pressed into ahousing.

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12. JOURNAL BEARING CONSTRUCTION AND USES12. JOURNAL BEARING CONSTRUCTION AND USES

JOURNAL BEARINGS can be divided into two major groups: bearings that handle a variableload such as a crankpin bearing, and bearings that handle a steady load such as a bearing on anelectric motor. For the most part, in todays world, journal bearings are precision bearings. This

means that they are manufactured to a specification to be a certain size and to handle a certainload. Bearings used in high output diesel engines are usually made with a steel backing. Theycan also be made from bronze or cast iron. Bronze, because of problem with being brittle israrely used any more as a backing material. The second layer is a type of bronze (here it is verygood because of its bonding capability) and the top layer is normally a white metal (babbitt) thatis sometimes referred to as a running-in metal. Another top layer metal would be aluminum.

Precision bearings do not require fitting to the journal-they are ready to install. There are still afew older style bearings around that need to be scraped. These bearings had a heavier layer of

babbitt and they were hand fitted to the journals. The mechanic would blue the shaft and find thehigh spots on the bearing. The scrapers looked like long, heavy duty knives with a twist in them.

After a little babbitt was scraped off the mechanic would again blue the shaft to determine fit.This would continue until a good fit was found. Precision bearings can come as bushings , whichmean that they are circular and one piece or split. Some smaller precision bearings, such asthose found in air compressors can be made of only bronze.

For journal bearing to work properly they must be properly lubricated. When the bearing isremoved, for inspection, you should expect to see some wear on the top layer. A list of causesfor failures of journal bearings in diesel engines is as follows:

Dirt 44 % Incorrect assembly 14 % Lack of lubrication 13 % Incorrect alignment 12 % Overload 9 % Corrosion and miscellaneous 8 %

We can see from this list that dirt is the overwhelming problem with these bearings. You willsee this problem by the rings formed on the bearing surface. You will also find out that a lot of the dirt is introduced when the bearing is installed by people with either dirty hands or using oilthat has been exposed to grit in a machine shop. Clearly, if the dirt can be eliminated the

potential for problems drops drastically. We come back to installation. Make sure everything isspotlessly clean. Make sure the oil being put on the bearings is very clean and contaminate free.Once the machine is run make sure that the lube oil filters are meticulously clean and welloperated. As a rule of thumb, the micron size (filter size) for the lube oil filter will be smaller than any particle that could do any damage or will pass though the engine.

If a bearing is extracted from an engine and the wear metal is only slightly worn and there are nodeep scratches the bearing can possibly be reused. Depending on the location of the bearingwithin the engine you have to check the location of any wear. If the sides are showing wear butthe center of the bearing is not showing wear it indicates that the shaft has a bow in it. By thesame token if all the wear is only in the center of the bearing then the shaft has a sag. The

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location of the wear is as important as the amount of wear. There should be no problem for a bearing to last 40,000 hours in a diesel engine.

As noted above great care needs to be exercised when installing bearings. The journal should beextremely clean. The back of the journal bearing should be dry and clean. The bearing housing

should also be clean and dry. The hands of the installer should also be clean. The oil, put on the bearing for use during the overhaul, should also be clean and unused. When the bearing isinstalled the locating tabs should line up with the slots. Once the bearing is assembledclearances should be taken to insure that the journal is free to spin. After the overhaul iscomplete the engine should be run for 5 minutes and then the bearing should be checked for hotrunning by placing your hand on the bearing. A heat detection gun can also be used. If all iswell the engine is again run for a period of up to 15 minutes and again the bearing is checked.The last run-in may be for up to one hour. At each step of the way the bearing is checked for hotrunning. If imbedded RTDs (resistance temperature device) are located in the bearing halvesthe readouts should be monitored closely for several days. If there is a problem it will usually

present itself quite quickly.

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13. HOW TO DETERMINE IF A BEARING IS STILL GOOD13. HOW TO DETERMINE IF A BEARING IS STILL GOOD

If a bearings vibration falls within proper operating parameters along with the temperature, theneverything should work fine. Baring any failures a bearing should last until the metal justfatigues due to old age. Below is a chart that shows how many hours of service you can expect to

get.

Class of Machine Hours of ServiceDomestic machines, agricultural machines, instruments, technicalapparatus for medical use.

300 to 3,000

Machines used for short periods or intermittently: electric hand tools,lifting tackle in workshops, construction machines

3,000 to 8,000

Machines working intermittently with high reliability: hoists, workshopcranes, auxiliary machinery in power stations, domestic heating andrefrigerating appliances, machine tools infrequently used

8,000 to 12,000

Machines for use 8 hours per day and not always fully utilized: general- purpose gear drives, electric motors.

10,000 to 25,000

Machines used for 8 hours per day and fully utilized: machine tools,wood processing machinery, machines for he engineering industry,cranes for bulk materials, ventilating fans, conveyors, printingequipment, centrifuges

20,000 to 30,000

Machines for continuous use 24 hours per day: rolling mil gear drives,compressors, pumps, mine hoists, stationary electric machines, textilemachinery

40,000 to 50,000

Water works machinery, rotary furnaces, cable stranding machines, propulsion machinery for ocean going vessels

60,000 to 100,000

Pulp and papermaking industry, large electric motors, power station plant, mine pumps, mine ventilating fans, tunnel shaft bearings.

>100,000

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14. INSTALLATION PROCEDURES OF JOURNAL BEARINGS14. INSTALLATION PROCEDURES OF JOURNAL BEARINGS

The primary cause of bearing failure is dirt. The next leading cause of failure only amounts toless than one third of the failures due to dirt. Therefore, it would seem to be obvious thatsomeone installing a bearing be it a journal bearing or a rolling bearing, would be concerned

about the implications of dirt or contamination in the vicinity of the bearing.

For many mechanics an overhaul is a dirty job. Also, among mechanics there are a lot of jokesabout the pictures in the maintenance books showing the person in a white suit handling a

bearing insert. The bottom line is that when the mechanic is going to install a bearing the areaand the mechanic should be spotless.

Dirt, or contamination, can be introduced in many ways. The mechanics hands can be dirty, the bearing shell (also known as an insert) may have been left lying around the work area and hadsome particulates land on it, the oil to be used to lubricate the bearing may not be clean, and the

bearing housing or cap may also not have been properly cleaned. In order to accomplish the job

correctly the oil lines, the bearing, the cap and saddle and the area around the bearing should all be cleaned. The mechanic should wash his hands and the oil that is to be used should come froma known clean source. Just prior to installing the bearing it should be wiped by a clean rag. Themechanic should then run his clean hands over the outside and inside of the bearing shell. Theoutside should be clean and dry! Oil should not be placed on the outside of the shell. It will

prevent the bearing from being installed properly and the oil will eventually burn off leaving aresidue that can lead to failure of the bearing from the outside.

Readings of the shell should have been taken with a special micrometer, with rollers on the ends,to verify the thickness. Other readings which should be taken are the outside diameter of the

bearing shell and the inside diameter of the housing. This is done to confirm that the bearing

will seat properly. Too much clearance and the bearing will be able to vibrate or move while it isin the seat causing damage and ultimate failure. Too little clearance and the bearing will run hotand also may suffer ultimate failure.

Most precision bearings today have tangs that align the bearing shell in the housings. A new bearing should snap into place when you press it into the housing. The precision bearings aremanufactured to a little more than one half of the circumference to each half. This amount,known as crush, is there to stress the bearing backs into the housings and caps when the bearingis properly torqued. This seats the bearing and prevents circumferential motion of the bearingwhile it is in service. If the crush is not present the bearing can begin to rotate, causing frettingand potentially failure.

The mechanic also has to be very aware of the location of the oil hole. The bearing must line upwith this hole in order to have proper lubrication.

Finally, the bearing cap has to be torqued properly. When all of this is done a check on the bearing for fit and motion should be performed.

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After an overhaul the bearing should be checked for temperature during running. On engines,and some air compressors, it is a very good idea to run the machine for a few minutes and tocheck the temperature of the bearing. Several runs should be accomplished and the bearingconstantly checked to make sure it is not running hot. If the bearing is running hot the causeshould be ascertained and corrected.

One thing a good mechanic should do is to be very observant. When the old bearing is comingout of a machine the mechanic should clean the bearing and check out the wear pattern. Thewear pattern will tell the mechanic is the machine is running true or has some alignment issuesthat need to be resolved. In todays world it is very possible to take pictures of the bearings.This can be done either electronically or with film. Line all the bearings up and indicatedirection. Make sure the photos come out clearly before the bearings are discarded. This willgive a good historical record. Also, if there is a problem, do not just replace the bearings andexpect the machine to run properly. Use the old bearings as a way to diagnose the problem so itcan be taken care of to ensure that the bearings will run for their expected life.

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