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AMAZON multi-meters discounts AMAZON oscilloscope discounts Hints on Mounting Duplex Bearings If duplex bearings are mounted in any combination other than the one for which they were originally ground, the following conditions may occur: 1. Preload bearings may lose their preload or be greatly overloaded, respectively causing poor performance and premature failure. 2. If mounted DB instead of DF, the bearing won’t take care of possible misalignment. 3. If mounted DF instead of DB, the bearing won’t give the proper rigidity to the shaft. 4. If mounted DF with the outer rings floating instead of DB, the bearing may be loose and have no preload. In addition, the balls might run over the low shoulder, causing extreme localized loading and premature failure. 5. If mounted DT in the wrong direction, the bearings may support excessive thrust load against the counterbore or the low shoulder of the outer ring. Do Not Use Two Single-Row Bearings as Duplex Bearings Unless Properly Ground or Shimmed Two ordinary angular-contact or radial single-row bearings generally cannot be combined to make a duplex pair. Duplex bearings are usually produced with extreme accuracy, and the twin units are made as identical as possible. Pairing of unmatched single-row bearings will result in any one of a variety of conditions: excessive or inadequate preload, too much end play, internal looseness, etc. In any case, spindle operation will be affected and early bearing failure could occur. However, it’s permissible to separate a duplex bearing into two halves (each a single-row bearing) and use them as separate bearing supports. Fit on Shaft Duplex bearings generally have a looser fit on the shaft than other standard types of bearings. "Push" fits (finger pressure fits), are generally employed. This helps prevent a change of internal characteristics and facilitates removal and remounting of the bearings. Where heavier fits are employed, special provisions must be made internally in the bearing. Faces of Outer Rings Square with Housing Bore It’s very important that, in the back-to-back (DB) position, the faces of the outer rings be perfectly square with the housing bore. It’s possible that the units of a duplex bearing can be tilted even though preloaded, thus introducing serious inaccuracies into the assembly. The primary cause of tilting is an inadequate and/or off-square shoulder contacting the low shoulder face. This forces the outer ring to assume an incorrect position in respect to the inner ring resulting in excessive bearing misalignment. Localized overloading is caused and generally results in early failure. Foreign matter between the bearings or between the shaft and housing shoulders and faces, inaccurate threads and off-square face of nut with respect to threads also are contributing causes of misalignment that may result in premature failure. Dismounting and Remounting of Duplex Bearings Duplex bearings must be kept in pairs as removed from the spindle. Tag the bearings to indicate which end of the shaft and in which position they were so that they can be replaced in the same position when reassembling. When new bearings are to replace old ones, they should be the exact equivalent of those removed from the shaft and must be mounted in the same relationship. Even if only one old bearing in a set requires replacement, it’s recommended that all bearings be replaced at the same time. This will avoid the dangers involved when trying to match two bearings, one of which has unknown characteristics and unknown life expectancy. Preloading of Duplex Bearings Bearings are made with varying degrees of internal looseness. This allows for expansion of the inner ring and increases the capacity of a bearing when it’s subjected to a thrust load. An excessive amount of interference between the inner ring bore and the shaft seat or a higher than-anticipated temperature differential between the inner and outer rings of the bearing will reduce internal clearance in the bearing below the optimum value, creating a detrimental effect on bearing life and performance. In a bearing subjected to a load with a very small or no thrust component, an excessive amount of internal clearance may result in poor shaft stability and high heat generation due to ball skidding in the unloaded segment of the bearing. A thrust load applied to the bearing by preloading, or applying a spring load, will alleviate both of these undesirable conditions. When a load is applied to a bearing, deflection occurs in the contact area between the balls and races due to the elastic properties of steel. The relationship between load and deflection is not a straight line function. For example, if the load applied to a bearing is multiplied by a factor of two, the deflection in the contact areas between the balls and races is multi plied by a factor considerably less than two. As the load on a bearing is increased, the lack of linear relationship between the load and the deflection becomes very pronounced. A point is reached where rather large increases in load result in very insignificant increases in deflection. FIG. 44. Axial deflections. In FIG. 44, the three curves show the difference in the axial deflections among Type R bearings with standard (AFBMA Class 0) and loose (AFBMA Class 3) internal fits and the 7000 Series angular-contact bearings (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. Bearings 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. FIG. 45 illustrates the difference in radial deflection among the same types of bearings in FIG. 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°. FIG. 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 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’s 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. FIG. 45. Radial deflections. FIG. 46. Axial deflections for type R standard fit bearings. FIG. 47. Axial deflections for type R loose fit bearings. FIG. 48. Axial deflections for 7000 series bearings. FIG. 49. Duplex pair with preload mounted back-to-back. 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’s 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 the purpose of obtaining greater axial and radial rigidity. By careful selection 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 centerline 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 ( FIG. 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 rigidity 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 (depending 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 the raceways immediately upon clamping the bearings together, thus eliminating the internal looseness. An additional load applied to the set of bearings will result in deflections of considerably smaller value than would be the case if the bearings were not preloaded. FIG. 50. Face-to-face arrangement. Preload Offset The relationship of the inner and outer ring faces of DB and DF pairs of bearings duplexed for preload is shown in FIG. 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 ( FIG. 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. 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 FIG. 51. A tandem set of two or more bearings is assembled DB or DF with single bearing. Since the deflection rate of a bearing decreases with increasing load as shown in FIG. 44 and 7-45, it’s possible, through preloading, to eliminate most of the potential deflection of a bearing under load. It’s important 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 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 undesirable and should be made only if absolutely necessary. This is especially true for machine tool spindle bearings that are made to extremely fine tolerances. 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 don’t 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. 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’s 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. FIG. 52 depicts a shaft equipped with MRC's "PumPac." The two bearings are mounted back-to-back, with the apex of the etched "V" pointing in the direction of predominant thrust. FIG. 52. Preloaded thrust bearing set with different contact angles counteracts skidding of rolling elements (from 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 TBL. 5. ====== TBL. 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. ====== FIG. 53. Check internal contact surfaces by turning outer ring slowly while holding inner ring. 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 won’t be always a deciding factor. Bearings can still be used if they are not badly pitted or brinelled on non-operating surfaces. This also applies to bearings that don’t 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 spinning the outer ring slowly while holding the inner ring ( FIG. 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 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’s always wise to consult the product engineering department of capable major bearing manufacturers. Cautions to Observe During Assembly of Bearings into Units Whether using the original bearing or replacing it with a new one, care must be taken to avoid contamination when mounting the bearing. A critical period in the life of a bearing starts when it leaves the stockroom for the assembly bench where it’s removed from its box 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. Don’t permit a bearing to lie around uncovered on work benches ( FIG. 54). 2. Don’t remove a bearing from its box and protective covering until ready for installation. 3. When handling bearings, keep hands and tools clean. 4. Don’t wash out factory-applied lubricant unless the bearing has become exposed to contamination. 5. If additional lubrication must be applied, be sure it’s absolutely clean. In addition, the instrument used for application must be clean, and chip and splinter proof. FIG. 54. Keep unboxed bearings covered until ready for mounting. 6. If subassemblies are left for any length of time, they should be lightly covered with clean, lintless material ( FIG. 55). Some other precautions to be exercised during assembly were discussed under "Cleanliness and Working Conditions" earlier. If there is any chance that 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 manufacturer to make sure that the bearings are delivered in a clean condition. In a few seconds, carelessness can destroy the protective measures of the manufacturer... shorten the life of the bearing... jeopardize the reputation of the organization for which you work. It pays to do everything possible to prevent abrasive action caused by dirt in a bearing. But assembly precautions don’t stop here. The user must resist his inclination to "clean" a bearing by removing the preservative coating applied by the bearing manufacturer. 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 ABEC-5, ABEC 7, or ABEC-9, it’s essential to orient them on the shaft with reference to the "high point of eccentricity." Super-precision bearings are usually marked to indicate this detail. FIG. 55. Cover subassemblies, especially those with mounted bearings, with plastic material while waiting to assemble into housing. The high point of eccentricity of the outer ring is the highest reading obtained when measuring its radial runout. It’s found by placing the bearing on a stationary arbor and applying an indicator directly over the ball path on the outside diameter of the outer ring. When the outer ring is rotated, the difference between the highest and lowest reading is the amount of radial runout of the outer ring. The high point of eccentricity of the inner ring is determined in the same manner except that the inner ring is rotated. To indicate the high point of eccentricity, a dot is burnished on both inner and outer rings ( FIG. 56) on Type R and 7000 Series angular-contact bearings of ABEC-5 or higher tolerance grades. The burnished dots are applied to the rings so that the bearings can be mounted to reduce or cancel the effects of shaft seat runouts. When mounted, the dots should be 180° from the high point of eccentricity of the bearing seat on each end of the shaft. The high point of eccentricity of the shaft also should be determined and marked when the shaft is on centers (or V-blocks). This method of mounting will help keep radial runout of the spindle assembly to a minimum. This is important especially in high speed applications. Matching the burnish marks in duplexed bearings will reduce internal fight between bearings. FIG. 56. Burnished dots show high point of eccentricity. Thrust Here The words "Thrust Here" are stamped on the back of the outer ring of all MRC brand Type R and angular-contact bearings. This serves as a guide when mounting the bearing so that the shaft thrust carries through the bearing ( FIG. 57). Note that in the "right" method of mounting, the thrust is along the shaft, through the inner ring along the angle of contact of the balls, through the heavy shoulder of the outer ring (stamped "Thrust Here") to the shoulder of the housing. If the bearing position were reversed as shown in the "wrong" method, the shaft thrust would follow the angle of contact through the low shoulder side of the outer ring. As the outer ring won’t carry loads of any magnitude, it’s likely that the thrust would then force the balls to ride the edge of the low shoulder. This could cause early failure due to concentrated loads at the race-shoulder intersection and possibly even cause cracking of the balls. FIG. 57. "THRUST HERE" on outer ring shows the side of the ring to which shaft thrust is to be imposed. Improper mounting may force balls to ride the edge of the low shoulder. FIG. 58. Shaft is held in a vise when mounting bearing with a push fit. Cover vise jaws with wood or soft metal. Mount Bearings with Push Fit Precision bearings used in the machine tool industry normally are mounted on the shaft with a push fit, that is, pressing the bearing in place ( FIG. 58) with hand pressure. In some cases, the original bearings may be used again. No difficulty should be encountered while mounting them. However, if a new bearing is to be used, the proper tolerance grade bearing must be selected so that a push fit results. An application of light oil on the shaft will increase ease of mounting. FIG. 59. Typical induction heater for mounting rolling element bearings (courtesy Prüftechnik A. G., Ismaning, Germany). FIG. 60. Large induction heater used for bearing assembly on machinery shafts (courtesy Prüftechnik A. G., Ismaning, Germany). FIG. 61. Compact bearing induction heater (courtesy Prüftechnik A. G., 8045 Ismaning, Germany). Mounting with Heat If a bearing is to be mounted with a tighter than "push" fit, a convenient and acceptable method of mounting is to expand the rings by moderate heating. To make sure the bearing is not overheated, a thermo statically controlled heat source should be used. An inexpensive type of household oven will usually serve the purpose satisfactorily. The oven also protects the bearing from contamination while being heated. Before heating, the bearing must be removed from its plastic packing bag or other wrapping as the temperature reached may melt the material. If gloves are used when handling the bearing, they should be made of a lint-free material such as nylon or neoprene. Set the thermostat to a temperature between 175°F and 200°F (80° to 94°C). In most cases, this will be adequate to expand the bearing without overheating it. Heat the bearing a sufficient amount of time to allow for ring expansion. Upon removal, slip the bearing on the shaft immediately with full regard for direction of thrust as well as orientation of the high points of eccentricity of both bearing and shaft. Certain bearings not employing cages, seals, or lubricants susceptible to damage at the higher temperature may be heated to 275°F (136°C). Induction heaters ( FIG. 59 through 61) offer several different heating programs to suit user requirements in a variety of situations. The heaters are suitable for continuous use, and their compact temperature probes ensure exceptionally exact temperature control. Most importantly, properly engineered, state-of-the-art induction heaters completely demagnetize the bearing automatically at the end of the heating cycle, because, otherwise, the magnetized bearing would literally act as a trash collector for ferritic particles, leading to an untimely demise of the bearing. Large induction heaters include features such as swivel-arm crossbar design and a pedal-operated crossbar lift, which allows one-man operation even when mounting extremely large workpieces, while the heavy, welded-steel carriage provides on-site portability. Further features might include auto demagnetization, automatic temperature probe recognition, dynamic heating power regulation, and suitability for continuous operation. Typical technical data of large units are as follows: Power consumption: 14 kVA max. Heating capacity: approx. 400 kg (880 lb) Heating duration: 10 sec.-1hr Precision, Time: +1 sec. Temperature: +2°C (3.6°F) More compact and still robust, smaller units may offer such standard features as microprocessor-controlled heating by time or temperature, auto demagnetization, automatic temperature probe recognition, and dynamic heating power regulation. Technical data of the more compact units are typically as follows: Power consumption: 3.5 kVA max. Heating capacity: approx. 15 kg (33 lb) Heating duration: 10 sec.-1hr Precision: better than 3°C (5.4°F) Time: +1 sec. Temperature: +2°C (3.6°F) FIG. 62. A variation of the infrared lamp method suited for large size bearings. Take care not to overheat the bearing. FIG. 63. Arbor press and hollow tube method used to mount bearings on shafts when press fits are involved. Other Mounting Methods A variation of the dry heat method to expand the bearing involves the use of infrared lamps inside a foil-lined enclosure. The lamps should be focused on the inner ring. Care must be taken to keep the temperature below 200°F (94°C), except as noted previously ( FIG. 62). It also is possible to use dry ice to cool the shaft which will then contract sufficiently to permit mounting of the bearing with the proper finger pressure. In this method, special precautions should be used to prevent resulting condensation from producing corrosion on the bearing components. An arbor press and a hollow tube ( FIG. 63) are frequently employed to mount bearings on shafts in those cases where press fits are involved. When doing so, the press and the tube should be completely clean to avoid possible contamination of the bearing. The tube must contact the inner ring when pressing the bearing on the shaft ( FIG. 64). This will avoid possible brinelling of the bearing which might occur if pressure were applied on the outer ring. The press ram, tube, and bearing axis should be in good alignment. If abnormal pressures are required, the alignment probably is not good enough. In this case, alternate application of pressure and relief may help ease the pressure required. Exercise Caution When Starting Bearing On Shaft To start the bearing, the shaft should have a lead and the bearing face should be square with the shaft ( FIG. 64). Pressure should be in line with the shaft and must be uniform against the face when pres sing the bearing into place. If excessive binding occurs, it generally indicates an off-square condition, a burr, high spots, a tapered shaft, dirt or chips wedged under the bearing. When sticking or binding occurs, the bearing should not be forced on the shaft as the hard inner ring is apt to cut the softer metal of the shaft and raise a ridge or burr ( Figure 65). Remove the bearing from the shaft to determine and correct the problem. Checking Bearings and Shaft After Installation After the bearings have been assembled on the shaft, a number of points should be checked to make certain the bearings have been correctly installed. These include visual checks and the use of various gauges to determine the accuracy of the mounting. It also may be necessary to balance the shaft assembly before insertion into the housing. FIG. 64. Bearing may be pressed on shaft using a tube and arbor press. Check for Internal Clearance The outer rings should rotate freely without binding except in unusual cases where a tight fit has been specified or where preloaded duplexed bearing sets are used. Residual internal clearance can be felt by holding the outer ring between the thumb and forefinger ( FIG. 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. FIG. 65. Result of starting bearing off-square. FIG. 66. Check a mounted bearing for internal clearance by rocking outer ring back and forth. Make Visual Check of Bearing Be sure that the bearing is flush against the shaft shoulder all the way around. The best simple check for this is to hold the shaft in front of a light source such as a window or an electric light. If no light shows between the inner ring face and the shaft shoulder, the bearing may be considered in proper position. This check should be made all the way around the shaft. If light is visible at any point, carefully remove the bearing and recheck the shoulder and fillet for burrs, out-of-round or too large a radius on the fillet. Check the height of the shoulder against the height of the inner ring. In general, it should be a minimum of about half the width of the inner ring face. If heavy thrust pressures are involved, the shoulder should be higher. Check for Bearing Squareness on the Shaft Check the face of the outer ring for squareness of the inner ring with the shaft using a suitable indicator ( FIG. 67). The assembled front and rear bearings or sets of bearings should be placed in V-blocks with an indicator point contacting the face of the outer ring. When the shaft and inner rings are turning, any off-square condition is transmitted through the balls to the outer ring. This will cause the outer ring to rock or tilt and shows up as a variation on the indicator reading. Readings in excess of the bearing tolerances indicate an effective misalignment, resulting from an off-square condition. FIG. 67. Checking for squareness of bearing on shaft. 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 off square condition. Under these circumstances, remove the bearing from the shaft to determine the actual cause and make the necessary repairs. Balancing the Shaft Assembly After the bearings and other units such as pulleys, etc., are properly seated on the shaft, the assembly should be balanced, preferably dynamically, to obtain a smooth-running spindle ( FIG. 68). All parts that rotate with the assembled spindle should be included in the balancing operation with whatever is applied to retain the bearings on the shaft. Common Causes of Unbalance in Shaft Assemblies Unbalance is commonly introduced through an eccentricity in some portion of the shaft assembly that has not been properly finished. Eccentricities may be present in the components affixed to the shaft. Ground bearing seats may not be concentric with turned portions of the assembly. Strains may develop in a shaft that has been heat-treated, causing warp in the shaft that may create misalignment of the bearings. In other cases, a thread may not be true to the shaft center. If the bore of the bearing spacer is too loose when used, it may not be properly centered with respect to the shaft axis. Causes of unbalance could include other factors in addition to these items. FIG. 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. Correction of Unbalanced Shaft Assemblies There are several methods to correct unbalance in either hardened or soft shafts. In general, balancing consists of removing material from the heavy side of the shaft or adding material to the light side of the shaft. Any balancing operations should be conducted in an area removed from the clean assembly area. To balance a hardened shaft, sufficient material usually is removed to create proper balance by grinding on the heavy side of the shaft nearest the end which needs to be balanced. The grinding is usually done on a portion of the shaft where the largest diameter occurs. Several methods may be used to bring a soft shaft into balance. It’s possible to apply a correct amount of weight. It may be preferable to drill 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 possible distance from the center of the shaft toward the end which must be balanced. Extreme care must be taken to prevent the removed metal from being introduced into the bearings mounted on the shaft. Protect Bearings and Shaft Assembly from Contamination After a ball bearing has been mounted on a shaft, often there is a time interval, possibly overnight, before final installation in the housing can be started. In such cases, it’s advisable to wrap the bearings and shaft in plastic film to protect them from contamination ( FIG. 69). If the bearings are left exposed on the shaft, or even when installed in the housing, dust and/or other contaminants may enter the bearing. If installed in the housing, always be sure to cover the open end with film until all assembly work is completed and the housing is completely closed. FIG. 69. Protect bearings and shaft from contamination until assembled and completely sealed in housing. Use plastic film to cover spindle. Assembly of Shaft and Bearings into Housing After the bearings have been assembled on the shaft, checked and balanced, the entire assembly is ready for insertion into the housing ( FIG. 70) in accordance with the methods outlined in the manufacturer's manual. During this operation, care must be taken to start the bearings into the housing seats squarely to avoid damage to the bearings or housing. Any force exerted on the shaft passes through the bearings and, if excessive, can cause bearing damage. The outer ring generally should have a slightly loose fit in the housing. This is necessary to permit the bearing axial movement to assume its normal operating position regardless of temperatures which occur during operation. If the bearing is too tight, axial movement is prevented and violent overloads could result in non-fixed radial positioning of the shaft. This causes noisy operation, excessive fretting and pounding out of the housing seat and, occasionally, excessive spinning in the housing. Testing of Finished Spindle When the spindle has been completely assembled, a final check of the eccentricity should be made as follows. Place an indicator point against the center of the shaft extension on the work end of the spindle and rotate the spindle by hand ( FIG. 71). Where possible, the opposite end of the spindle also should be checked. Sometimes it’s possible to detect roughness or vibration in the spindle when turning the shaft by hand. Roughness may be felt as a hitch or click. Don’t attempt to run the spindle if these conditions are major. Dismantle it and find the cause of the roughness. The cause may be the application of excessively dirty lubricant or dirt that has worked its way into the bearing during assembly. Under conditions of slow rotation, necessary cage looseness also may create some binding which disappears when running to speed and under load. Vibration is usually detected when the spindle reaches its normal operating speed. Causes of vibration include excessive runout of the pulley, a loose cap on the spindle assembly, bearings damaged in assembly, or possibly loose bearing spacers. In any case, when vibration is detected, spindle run-in should be discontinued to investigate the cause. FIG. 70. Start bearings into housing seat squarely to avoid damage to either bearings or housing. A "push" fit should be used. During run-in, a close check should be kept on the temperature attained, especially in the first part of the run. This is particularly true for spindles which are grease-packed. If the heat becomes excessive, over 140° to 150°F (60° to 66°C), it’s usually advisable to stop the spindle and permit it to cool off. This type of excessive heat is commonly caused by insufficient channeling of the lubricant in the spindle. Stopping the spindle will allow the temperature to equalize, reducing the risk of radial or axial preloading. When restarted, the spindle usually will run at temperatures within the recommended range unless excessive quantities of grease are in the housing or if overloads are present. If the spindle continues to heat excessively, checks should be made to determine the cause. Maintain Service Records on All Spindles The maintenance department in any company should keep records regarding the history of each shaft or spindle serviced ( FIG. 72). All particulars should be recorded from the day the spindle was placed in operation until retirement. Such a record may be as complete and detailed as possible. It may simply record dates when the spindle or shaft was checked to correct certain conditions. In either case, it’s recommended that the record state clearly the corrective actions taken to place the spindle in proper operating condition. FIG. 72. Form provides space to record complete history of spindle from date of purchase. FIG. 71. Check eccentricity of assembled bearing with indicator gage on shaft extension. Shop records also will enable the maintenance department to keep a close periodic check on spindles throughout the plant. It’s possible to establish a regular inspection procedure which will help assure continued spindle operation. This will reduce machine downtime, always a factor in maintaining a low cost operation. Shaft and Housing Shoulder Diameters TBL. 4 shows minimum shaft shoulder diameters for general purpose installations. TBL. 6 Bearing Maintenance Checklist Bearing Maintenance Checklist Finally, we direct your attention to TBL. 6, which summarizes all necessary bearing maintenance steps in checklist form. |