Machinery Components Maintenance And Repair Episode 2 Part 5 ppt

In Figure 7-44, the three curves show the difference in the axial deflec- 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 of comparison, the deflection for Type R loose fit bearings with 100 pounds of thrust load is approximately 50 percent less while the 7000 Series is about 85 percent less. As the thrust load is increased, the abrupt rise in deflection shown for lower thrust loads is nearly eliminated. The leveling out of curves continues and, at 2,400 pounds, the deflection rate is reduced 25 percent and 58 percent, respectively. This ratio between the curves remains fairly constant for loads above this point. This comparison shows that bearings which have a low degree of contact angle (Type R standard fit) usually have the highest rate of axial deflection. 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 the same types of bearings in Figure 7-44. The amount of radial deflection for all three types is more closely grouped with small differences between each 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 in the bearing type. The Type R standard fit bearing which has an initial contact angle of approximately 10° has a lower rate of radial deflection than 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, and heavy preloads on bearings of each type. The top curve on each chart is Ball Bearing Maintenance and Replacement 415 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 heavy preload curves show reduction of axial deflection that can be obtained for bearings of each type. It is interesting to note that, in all cases, the axial deflection for preloaded types is reduced throughout the entire curve. At the low end of the applied loads, the increase is considerably less than in unpreloaded bearing types and the deflection rate levels off throughout the entire curve. 416 Machinery Component Maintenance and Repair Figure 7-45. Radial deflections. Figure 7-46. Axial deflections for type R standard fit bearings. The deflection curves presented on these charts represent calculations determined from 207-R and 7207 bearings. These specific axial and radial deflection conditions occur only in these bearings. However, in general, these curves do indicate what may occur in other bearings in the same series. Both axial and radial deflection characteristics will change in general proportion as the bearing size in the same series is increased. It is the relationship between load and axial and radial deflections that frequently makes it desirable to preload bearings. Preload refers to an initial predetermined internal thrust load incorporated into bearings for Ball Bearing Maintenance and Replacement 417 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 selec- tion of bearing type and amount of preload, axial and radial deflection rates best suited to a specific application can be obtained. When a duplex pair with preload is mounted back-to-back (Figure 7- 49) there is a gap between the two inner rings. As the two bearings are clamped together, the two inner rings come in contact to eliminate the gap. This changes the ball position to contact both inner and outer raceways under load establishing the basic contact angle of the bearings. The cen- terline on the balls shows this change. Bearings duplexed back-to-back greatly 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. In the face-to-face arrangement (Figure 7-50), the preload offset is between the outer rings. After clamping, the balls come into contact with both 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 less rigidity is given to conditions of misalignment. Preloading is accomplished by controlling very precisely the relation- ship between the inner and outer ring faces. A special grinding procedure creates an offset between the faces of the inner and outer rings of the bearing equal to the axial deflection of the bearing under the specified preload. When two bearings processed in an identical manner are clamped together, 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 contact areas between the balls and races that corresponds to the amount of preload that has been built into the bearings. Balls are forced to contact 418 Machinery Component Maintenance and Repair Figure 7-49. Duplex pair with preload mounted back-to-back. the raceways immediately upon clamping the bearings together, thus elim- inating the internal looseness. An additional load applied to the set of bear- ings will result in deflections of considerably smaller value than would be the case if the bearings were not preloaded. 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. Note the 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 offset has been greatly exaggerated to show the action that takes place. In most cases the offset is so small that it cannot be detected without the proper gauging 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 is assembled either DB or DF with a single bearing (Figure 7-51) or another tandem set of two or more bearings. Preload in these sets is the clamping force, applied across outboard sides of the set, necessary to bring all mating surfaces in contact. Ball Bearing Maintenance and Replacement 419 Figure 7-50. Face-to-face arrangement. Preload for individual tandem bearings in a set must be equal to the preload 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 as shown 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 increased proportionately to the amount of preload. Excessive preload not only causes the bearings to run hotter at a higher speed but also reduces the operating speed range. As machine tools must perform many types of work under varying conditions, the proper preload must be provided for each bearing to meet these conditions while retaining operating temperatures and 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 420 Machinery Component Maintenance and Repair 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 will function satisfactorily with the proper shaft rigidity and with no excessive operating temperature. Any change in the initial preload is generally unde- sirable and should be made only if absolutely necessary. This is especially true for machine tool spindle bearings that are made to extremely fine tol- erances. Any attempt to change the initial preload in these bearings is more likely to aggravate the faulty condition than correct it. Factors Affecting Preload There are various conditions which may adversely affect the initial preload in duplex bearings: • Inaccurate machining of parts can produce a different preload than originally intended, either increasing or decreasing it depending upon the nature of the inaccuracy • Use of spacers that are not equal in length or do not have the faces square with the reference diameter (OD or ID) can produce an improper preload • Foreign matter deposited on surfaces or lodged between abutting parts as well as nicks caused by abuse in handling may produce cocking 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 the parts are clamped together. • Make a careful check of the shaft housing shoulder faces and the end cover surfaces abutting the bearing to see that they are square with the axis of rotation • Make sure that the end surfaces of each spacer are parallel with each other and square with the spacer bore • Carefully inspect the lock nut faces for squareness • Inspect all contacting and locating surfaces to make sure they are clean and free from surface damage Preload Classifications MRC brand Type R and 7000 Series angular-contact ball bearings are available with any of three classes of preloads—light, medium, or heavy. The magnitude of the preload depends upon the speed of the spindle and required operating temperatures and rigidity requirements. Ball Bearing Maintenance and Replacement 421 Preloaded Replacement Bearings Normally replacement duplex bearings will be supplied universally ground 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 processing industry (HPI) is only 38 months or less based on 2004 data. Preloaded bearings with different contact angles can significantly increase the service life of bearings in many pump applications. The key to their superior performance lies in the system’s directionally dissimilar yet interactive spring rates. One such bearing system, MRC’s “PumPac,” consists of a matched set of 40° and 15° angular contact ball bearings with computer-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 oil viscosity, consistent film thickness, and longer service life. Figure 7-52 depicts a shaft equipped with MRC’s “PumPac.” The two bearings are mounted back-to-back, with the apex of the etched “V” point- ing in the direction of predominant thrust. 422 Machinery Component Maintenance and Repair 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 bearings removed from the spindle or to replace them with new bearings. The choice probably will be self-evident, especially after the visual inspection mentioned in item #5 on the checklist in Table 7-5. If the bearing has defects that will affect its operation, it must be replaced with a new bearing of the same size and tolerance grade. Experience will be a guide in determining if the bearing is to be replaced. 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 or brinelled on nonoperating surfaces. This also applies to bearings that do not show excessive wear or signs of overheating. There are some instances where the boundary dimensions may have been affected by operation. Where possible, they should be checked to determine if they are within the desired tolerances. Often a simple check on a bearing’s internal contact surfaces can be made by spinning the bearing by hand. This may be done after the bearing has been thoroughly cleaned to eliminate possible harmful grit inside it. If the bearing has some imperfect contact surface, this can be felt when Ball Bearing Maintenance and Replacement 423 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 has been in service for a long time and, according to the records, is nearing the end of its natural life, it should be replaced with a new bearing. If a longer life can be expected, then an evaluation must be made comparing the cost of a replacement bearing against the remaining life of the old bearing 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 that dimensional interchangeability does not necessarily guarantee functional interchangeability. In certain applications, there are other characteristics 424 Machinery Component Maintenance and Repair Figure 7-53. Check internal contact surfaces by turning outer ring slowly while holding inner ring. [...]... unless the bearing has become exposed to contamination 5 If additional lubrication must be applied, be sure it is absolutely clean 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  426 Machinery Component Maintenance and Repair Figure 7 -55 Cover subassemblies, especially those with mounted bearings,... recognition, and dynamic 4 32 Machinery Component Maintenance and Repair heating power regulation Technical data of the more compact units are typically as follows: Power consumption: Heating capacity: Heating duration: Precision: Time: Temperature: 3 .5 kVA max approx 15 kg (33 lb) 10 sec.-1 hr better than 3°C (5. 4°F) +1 sec +2 C (3.6°F) Other Mounting Methods A variation of the dry heat method to expand the... temperature control Most importantly, 430 Machinery Component Maintenance and Repair Figure 7 -59 Typical induction heater for mounting rolling element bearings (courtesy Prüftechnik A G., Ismaning, Germany) Figure 7-60 Large induction heater used for bearing assembly on machinery shafts (courtesy Prüftechnik A G., Ismaning, Germany) Ball Bearing Maintenance and Replacement 431 Figure 7-61 Compact bearing... bearings This serves as a guide when mounting the bearing so that the shaft thrust carries through the bearing (Figure 7 -57 ) Note that in the “right” method of mounting, Figure 7 -56 Burnished dots show high point of eccentricity  428 Machinery Component Maintenance and Repair Figure 7 -57 “THRUST HERE” on outer ring shows the side of the ring to which shaft thrust is to be imposed Improper mounting may... hard inner ring is apt to cut the softer metal of the shaft and raise a ridge or burr (Figure 7 65) Remove the bearing from the shaft to determine and correct the problem 434 Machinery Component Maintenance and Repair Figure 7-64 Bearing may be pressed on shaft using a tube and arbor press Checking Bearings and Shaft After Installation After the bearings have been assembled on the shaft, a number of... preferable to drill 438 Machinery Component Maintenance and Repair Figure 7-68 Dynamics of balancing spindle assembly This equipment is used by TRW’s Spindle Maintenance Department, but other types are available which will balance assemblies as accurately as necessary a hole of the correct size and depth in the shaft Metal generally is removed from the shaft in the area of the largest diameter and at the greatest... and protective covering This critical period continues until the bearing passes its first full-load test after assembly Here are a few rules that should be observed during 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 until ready for installation 3 When handling bearings, keep hands and. ..Ball Bearing Maintenance and Replacement 4 25 such as internal fit, type and material of cage, lubricant, etc., that are of vital importance If you have questions about the selection of the correct ball bearing replacement, it is always wise to consult the product engineering department of capable major bearing manufactuers Cautions to Observe During... and forth bearing sets are used Residual internal clearance can be felt by holding the outer ring between the thumb and forefinger (Figure 7-66) and rocking it back and forth If the bearing is free, the ring will have a slight axial freedom of movement or “rock.” This applies to all single-row bearings except a single bearing of the angular-contact type which is very loose 436 Machinery Component Maintenance. .. Ball Bearing Maintenance and Replacement 437 Foreign matter between the bearings and shaft shoulder, fillet interference, a nick on the shaft shoulder, raised metal from the bearing seat, a nick on the spacer rings, and many other causes will produce this offsquare condition Under these circumstances, remove the bearing from the shaft to determine the actual cause and make the necessary repairs Balancing . the etched “V” point- ing in the direction of predominant thrust. 422 Machinery Component Maintenance and Repair Figure 7- 52 . Preloaded thrust bearing set with different contact angles counteracts. be felt when Ball Bearing Maintenance and Replacement 423 Table 7 -5 Spindle Servicing Checklist At this point, all cleaning and repair work on the shaft and spindle parts should have been completed types and the deflection rate levels off throughout the entire curve. 416 Machinery Component Maintenance and Repair Figure 7- 45. Radial deflections. Figure 7-46. Axial deflections for type R standard