Machinery Components Maintenance And Repair Episode 2 Part 5 ppt

In Figure 7-44, the three curves show the difference in the axial tions among Type R bearings with standard AFBMA Class 0 and looseAFBMA Class 3 internal fits and the 7000 Series angular

In Figure 7-44, the three curves show the difference in the axial tions among Type R bearings with standard (AFBMA Class 0) and loose(AFBMA Class 3) internal fits and the 7000 Series angular-contact bear-ings (29° contact angle) Using the Type R standard fit curve as a basis ofcomparison, the deflection for Type R loose fit bearings with 100 pounds

deflec-of thrust load is approximately 50 percent less while the 7000 Series isabout 85 percent less As the thrust load is increased, the abrupt rise indeflection shown for lower thrust loads is nearly eliminated The levelingout of curves continues and, at 2,400 pounds, the deflection rate is reduced

25 percent and 58 percent, respectively This ratio between the curvesremains fairly constant for loads above this point

This comparison shows that bearings which have a low degree ofcontact angle (Type R standard fit) usually have the highest rate of axialdeflection The greatest increase takes place under low thrust load Bear-ings with a high angle of contact, such as the 7000 Series, tend to retain

a more even rate of deflection throughout the entire range of thrust loads.Figure 7-45 illustrates the difference in radial deflection among thesame types of bearings in Figure 7-44 The amount of radial deflection forall three types is more closely grouped with small differences betweeneach type for the amount of radial load applied In contrast to Figure 7-

44, radial deflection increases in relation to the degree of contact angle inthe bearing type The Type R standard fit bearing which has an initialcontact angle of approximately 10° has a lower rate of radial deflectionthan the 7000 Series bearing with an initial contact angle of 29°

Figures 7-46, 7-47, and 7-48 illustrate the effects of light, medium, andheavy preloads on bearings of each type The top curve on each chart is

Figure 7-44 Axial deflections.

for duplexed unpreloaded bearings and is the same curve used on Chart

1, for a single bearing of the same type The light, medium and heavypreload curves show reduction of axial deflection that can be obtained forbearings of each type It is interesting to note that, in all cases, the axialdeflection for preloaded types is reduced throughout the entire curve Atthe low end of the applied loads, the increase is considerably less than inunpreloaded bearing types and the deflection rate levels off throughout theentire curve

Figure 7-45 Radial deflections.

Figure 7-46 Axial deflections for type R standard fit bearings.

The deflection curves presented on these charts represent calculationsdetermined from 207-R and 7207 bearings These specific axial and radialdeflection conditions occur only in these bearings However, in general,these curves do indicate what may occur in other bearings in the sameseries Both axial and radial deflection characteristics will change ingeneral proportion as the bearing size in the same series is increased.

It is the relationship between load and axial and radial deflections thatfrequently makes it desirable to preload bearings Preload refers to aninitial predetermined internal thrust load incorporated into bearings for

Figure 7-47 Axial deflections for type R loose fit bearings.

Figure 7-48 Axial deflections for 7000 series bearings.

the purpose of obtaining greater axial and radial rigidity By careful tion of bearing type and amount of preload, axial and radial deflectionrates best suited to a specific application can be obtained.

selec-When a duplex pair with preload is mounted back-to-back (Figure 49) there is a gap between the two inner rings As the two bearings areclamped together, the two inner rings come in contact to eliminate the gap.This changes the ball position to contact both inner and outer racewaysunder load establishing the basic contact angle of the bearings The cen-terline on the balls shows this change Bearings duplexed back-to-backgreatly increase the effective shaft rigidity especially to misalignment.When equal (and square) pairs of spacers are used with these bearings,effective rigidity is increased still further

7-In the face-to-face arrangement (Figure 7-50), the preload offset isbetween the outer rings After clamping, the balls come into contact withboth inner and outer races at the basic contact angle Effective radial rigid-ity of the shaft is equal to that of the back-to-back arrangement; but lessrigidity is given to conditions of misalignment

Preloading is accomplished by controlling very precisely the ship between the inner and outer ring faces A special grinding procedurecreates an offset between the faces of the inner and outer rings of thebearing equal to the axial deflection of the bearing under the specifiedpreload

relation-When two bearings processed in an identical manner are clampedtogether, the offset is eliminated, forcing the inner and outer rings (depend-ing on where the offset occurs) to apply a thrust load on the balls and race-ways even before rotation is started This results in deflection in the contactareas between the balls and races that corresponds to the amount ofpreload that has been built into the bearings Balls are forced to contact

Figure 7-49 Duplex pair with preload mounted back-to-back.

the raceways immediately upon clamping the bearings together, thus inating the internal looseness An additional load applied to the set of bear-ings will result in deflections of considerably smaller value than would bethe case if the bearings were not preloaded.

elim-Preload Offset

The relationship of the inner and outer ring faces of DB and DF pairs

of bearings duplexed for preload is shown in Figures 7-49 and 7-50 Notethe gap between the inner rings of the DB pair and the outer rings of the

DF pair This is referred to as preload offset In the illustrations the offsethas been greatly exaggerated to show the action that takes place In mostcases the offset is so small that it cannot be detected without the propergauging equipment

DTDB and DTDF Sets

Tandem duplex bearings may be preloaded under certain conditions.These are applications where a tandem set of two or more bearings isassembled either DB or DF with a single bearing (Figure 7-51) or anothertandem set of two or more bearings Preload in these sets is the clampingforce, applied across outboard sides of the set, necessary to bring allmating surfaces in contact

Figure 7-50 Face-to-face arrangement.

Preload for individual tandem bearings in a set must be equal to thepreload of the set divided by the number of bearings on each side.Example: Three DT bearings are matched DB with two DT bearings.The set has a 600 lb preload.

Each of these DT bearings (one side) must have preload of 600/3 or

200 lbs

Each of the two DT bearings (other side of set) must have 600/2 or

300 lbs

Importance of the Correct Amount of Preload

Since the deflection rate of a bearing decreases with increasing load asshown in Figures 7-44 and 7-45, it is possible, through preloading, to elim-inate most of the potential deflection of a bearing under load It is impor-tant to provide the correct amount of preload in each set of duplex bearings

to impart the proper rigidity to the shaft However, rigidity is not increasedproportionately to the amount of preload Excessive preload not onlycauses the bearings to run hotter at a higher speed but also reduces theoperating speed range As machine tools must perform many types of workunder varying conditions, the proper preload must be provided for eachbearing to meet these conditions while retaining operating temperaturesand speed ranges to which the bearings are subjected

Duplex bearings are generally manufactured so that the proper amount

of preload is obtained when the inner and outer rings are simply clamped

Figure 7-51 A tandem set of two or more bearings is assembled DB or DF with single

bearing.

together If the duplex bearing has the correct preload, the machine willfunction satisfactorily with the proper shaft rigidity and with no excessiveoperating temperature Any change in the initial preload is generally unde-sirable and should be made only if absolutely necessary This is especiallytrue for machine tool spindle bearings that are made to extremely fine tol-erances Any attempt to change the initial preload in these bearings is morelikely to aggravate the faulty condition than correct it.

Factors Affecting Preload

There are various conditions which may adversely affect the initialpreload in duplex bearings:

• Inaccurate machining of parts can produce a different preload thanoriginally intended, either increasing or decreasing it depending uponthe nature of the inaccuracy

• Use of spacers that are not equal in length or do not have the facessquare with the reference diameter (OD or ID) can produce animproper preload

• Foreign matter deposited on surfaces or lodged between abuttingparts as well as nicks caused by abuse in handling may producecocking of the bearing and misalignment Either condition can result

in a variation of the preload or binding in the bearing

The following precautions should be taken to avoid distortion when theparts are clamped together

• Make a careful check of the shaft housing shoulder faces and the endcover surfaces abutting the bearing to see that they are square withthe axis of rotation

• Make sure that the end surfaces of each spacer are parallel with eachother and square with the spacer bore

• Carefully inspect the lock nut faces for squareness

• Inspect all contacting and locating surfaces to make sure they areclean and free from surface damage

Preload Classifications

MRC brand Type R and 7000 Series angular-contact ball bearings areavailable with any of three classes of preloads—light, medium, or heavy.The magnitude of the preload depends upon the speed of the spindle andrequired operating temperatures and rigidity requirements

Preloaded Replacement Bearings

Normally replacement duplex bearings will be supplied universallyground with predetermined light preload These are designated as “DS”bearings If preload recommendations are desired when ordering bearings,all data possible, such as the equipment in which the spindle is used,spindle speeds, loads, and lubrication, should be supplied

Preloaded Bearings with Different Contact Angles

Less than 5 percent of all pump bearings reach their calculated life

Compared to the average calculated thrust bearing life of 15 to 20 years,

actual application life for pump bearings in the hydrocarbon processingindustry (HPI) is only 38 months or less based on 2004 data

Preloaded bearings with different contact angles can significantlyincrease the service life of bearings in many pump applications The key

to their superior performance lies in the system’s directionally dissimilaryet interactive spring rates One such bearing system, MRC’s “PumPac,”consists of a matched set of 40° and 15° angular contact ball bearings withcomputer-optimized internal design It is designed to interact as a system,with each component performing a specific function

By using this special set of bearings, ball skidding and shuttling are virtually eliminated The result: lower operating temperatures, stable oilviscosity, consistent film thickness, and longer service life

Figure 7-52 depicts a shaft equipped with MRC’s “PumPac.” The twobearings are mounted back-to-back, with the apex of the etched “V” point-ing in the direction of predominant thrust

Figure 7-52 Preloaded thrust bearing set with different contact angles counteracts skidding

of rolling elements (courtesy MRC Bearings, Jamestown, New York).

Assembly of Bearings on Shaft Bearing Salvage vs Replacement Considerations

The final decision now must be made whether to reuse the bearingsremoved from the spindle or to replace them with new bearings Thechoice probably will be self-evident, especially after the visual inspectionmentioned in item #5 on the checklist in Table 7-5

If the bearing has defects that will affect its operation, it must bereplaced with a new bearing of the same size and tolerance grade.Experience will be a guide in determining if the bearing is to bereplaced The apparent condition of a bearing will not be always a decid-ing factor Bearings can still be used if they are not badly pitted orbrinelled on nonoperating surfaces This also applies to bearings that donot show excessive wear or signs of overheating There are some instanceswhere the boundary dimensions may have been affected by operation.Where possible, they should be checked to determine if they are withinthe desired tolerances

Often a simple check on a bearing’s internal contact surfaces can bemade by spinning the bearing by hand This may be done after the bearinghas been thoroughly cleaned to eliminate possible harmful grit inside it

If the bearing has some imperfect contact surface, this can be felt when

Table 7-5 Spindle Servicing Checklist

At this point, all cleaning and repair work on the shaft and spindle parts should have been completed A review of all steps taken in the servicing of a spindle are listed here for checking purposes.

1 Remove shaft and bearings from the housing.

2 Dismount bearing from shaft using arbor press or bearing puller.

3 Tag bearings and spacers (if any) for identification and proper location when remounting on the shaft.

4 Clean bearings and spindle parts.

5 Make visual inspection of all spindle parts for nicks, burrs, corrosion, other signs of damage.

6 Prepare shaft for remounting of bearings Make any repairs necessary on bearing seat, shaft shoulders, fillets, etc.

7 Prepare housings by making any required repairs on machine mounting surfaces, Paint non-functional surfaces as necessary.

8 Check shaft and housing measurements for bearing seat out-of-round, off-square shoulders, housing bore, etc.

spinning the outer ring slowly while holding the inner ring (Figure 7-53).This test should be made under both lubricated and dry conditions.However, when dry, extreme care must be taken when spinning the bearing

as the rolling surfaces of the balls and raceways are even more sensitive

to possible scratching by grit

Another point to consider is anticipated bearing life If a bearing hasbeen in service for a long time and, according to the records, is nearingthe end of its natural life, it should be replaced with a new bearing If alonger life can be expected, then an evaluation must be made comparingthe cost of a replacement bearing against the remaining life of the oldbearing and its later replacement Also, the evaluation should take into

account the possibility of new bearings in certain services having a statistically provable higher failure rate than bearings that have been in

successful short time service

If a replacement bearing is to be used, it should be understood thatdimensional interchangeability does not necessarily guarantee functionalinterchangeability In certain applications, there are other characteristics

Figure 7-53 Check internal contact surfaces by turning outer ring slowly while holding inner

ring.

such as internal fit, type and material of cage, lubricant, etc., that are ofvital importance If you have questions about the selection of the correctball bearing replacement, it is always wise to consult the product engi-neering department of capable major bearing manufactuers.

Cautions to Observe During Assembly of Bearings into Units

Whether using the original bearing or replacing it with a new one, caremust be taken to avoid contamination when mounting the bearing A critical period in the life of a bearing starts when it leaves the stockroomfor the assembly bench where it is removed from its box and protectivecovering This critical period continues until the bearing passes its firstfull-load test after assembly Here are a few rules that should be observedduring this crucial period

1 Do not permit a bearing to lie around uncovered on work benches(Figure 7-54)

2 Do not remove a bearing from its box and protective covering untilready for installation

3 When handling bearings, keep hands and tools clean

4 Do not wash out factory-applied lubricant unless the bearing hasbecome exposed to contamination

5 If additional lubrication must be applied, be sure it is absolutelyclean In addition, the instrument used for application must be clean,and chip and splinter proof

Figure 7-54 Keep unboxed bearings covered until ready for mounting.

6 If subassemblies are left for any length of time, they should be lightlycovered with clean, lintless material (Figure 7-55).

Some other precautions to be exercised during assembly were discussedunder “Cleanliness and Working Conditions” earlier If there is any chancethat the bearing may have become contaminated, don’t take any chances—wash the bearing again following the procedure outlined in that section

In summary, many precautions have been taken by the bearing facturer to make sure that the bearings are delivered in a clean condition

manu-In a few seconds, carelessness can destroy the protective measures of themanufacturer shorten the life of the bearing jeopardize the reputa-tion of the organization for which you work It pays to do everything pos-sible to prevent abrasive action caused by dirt in a bearing But assemblyprecautions do not stop here The user must resist his inclination to “clean”

a bearing by removing the preservative coating applied by the bearingmanufacturer Prelubrication is not usually necessary and extreme vulnerability can he introduced by precoating certain rolling element bearings with extreme light viscosity or inferior quality oils

High Point of Eccentricity

When remounting bearings with tolerance grades of 5,

ABEC-7, or ABEC-9, it is essential to orient them on the shaft with reference tothe “high point of eccentricity.” Super-precision bearings are usuallymarked to indicate this detail

Figure 7-55 Cover subassemblies, especially those with mounted bearings, with plastic

material while waiting to assemble into housing.