Wednesday, September 10, 2008

Bearing Maintenance and Replacement

The fundamental purpose of a bearing is to reduce friction and wear between rotating parts that are in contact with one another in any mechanism. The length of time a machine will retain its original operating efficiency and accuracy will depend upon the proper selection of bearings, the care used while installing them, proper lubrication, and propcr maintenance provided during actual operation. The manufacturer of the machine is responsible for selecting the correct type and size of bearings and properly applying the bearings in the equipment. However, maintenance of the machine is the responsibility of the user. A well-planned and systematic maintenance procedure will assure extended operation of the machine. Failure to take the necessary precautions will generally lead to machine downtime. It must also be remembered that factors outside of the machine shaft may cause problems. Engineering and Interchangeability Data Rings and Bulls-The standard material used in ball bearing rings and balls is a vacuum processed high chromium steel identified as SAE 52100 or AISI-52100. Material quality for balls and bearing rings is maintained by multiple inspections at the steel mill and upon receipt at thc bcaring manufacturing plants. The 52 100 bearing steel with standard heat treatment can be operated satisfactorily at temperatures as high as 250°F (121 "C). For higher operating temperatures. a special heat treatment is required in order to give dimensional stability to the bearing parts.

Seals-Standard materials used in bearing seals are generally nitrile rubber. The material is bonded to a pressed steel core or shield. Nitrile rubber is unaffected by any type of lubricant commonly used in anti-friction bearings. These closures have a useful temperature range of -70" to +225"F (-56" to 107°C). For higher operating temperatures, special seals of high temperature materials can be supplied. Ball Cages-Ball cages are pressed from low carbon steel of SAE 1010 steel. This same material is used for bearing shields. Molded nylon cages are now available for many bearing sizes. The machined cages ordinarily supplied in super-precision ball bearings are made from laminated cotton fabric impregnated with a phenolic resin. This type of cage material has an upper temperature limit of 225°F (107°C) with grease and 250°F (121 "C) with oil for extended service. For periods of short exposure, higher temperatures can be tolerated.

Removal of Shaft and Bearings from Housing

The first step in dismantling a spindle or shaft is to remove the shaft assembly from the housing. To do this, it is generally necessary to take off the housing covers from each end.

Most machine tool spindle and API pump housings are constructed with bearing seats as an integral part of the housing. This contributes to the rigidity of the spindle. However, it makes disassembly more difficult and extreme care must be taken to avoid bearing damage. Also, it is not generally possible to remove bearings from the shaft unless the shaft assembly is first removed from the housing.

On most spindle assemblies this can be done by first placing the entire spindle in an arbor press and in alignment with the press ram. Next, carefully apply pressure to the end of the shaft making sure that there is clearance for the expulsion of the shaft assembly on the press table. As pressure is applied, the shaft is forced from the housing along with the bearing mounted on the opposite end of the shaft.

The bearing on the end where pressure is applied remains in the housing. It is removed from the housing either with hand pressure or by carefully pushing it out of the housing from the opposite side with rod tubing having a diameter slightly smaller than the housing bore. The tubing should contact the bearing outer ring and should push it from the housing with little or no pressure on the balls and inner ring. Following this procedure will help avoid brinelling of the raceways due to excessive pressure on the rolling elements and races.

Electric motor shafts are generally constructed to permit removal of one end bell, leaving the shaft and bearings exposed. The rotor or shaft assembly is then free to be removed by drawing it through the stator.

Bearing Removal from Shaft

Removal of bearings from spindle shafts is a highly important part of the maintenance and service operation. In most cases, it is far more difficult to remove a bearing from the shaft than to put it on. For this reason, a bearing can be damaged unnecessarily in the process. Every precaution must be taken to avoid damage to any of the parts including the bearings. If the bearings are damaged during removal, the damage often is not noticed and may not become known until the spindle is completely reassembled. Bearing damage during removal from the shaft can occur in many ways, of which these are the most common:

The smooth, highly-polished surface of the ball raceways may be brinelled, i. e., indented, by the balls (Figure 7-4). Brinell marks on the surface of the races are usually caused when a bearing is forced off the shaft by applying excessive or uneven pressure through the rolling element complement. Any shock load, such as hammer blows on the inner or outer rings, is apt to cause brinelling. Major brinelling can sometimes be discovered on the job by applying a thrust load from each direction while rotating the inner or outer ring slowly. As the ring is turned through the brinelled area on either of the race shoulders, it can often be felt as a catch or rough spot. A brinelled bearing is unfit for further use. Never put it back into service. Ball raceways may be roughened due to dirt particles or metal chips working into the bearing. As soon as the shaft has been removed from the housing, it should be placed in a clean work area and suitably covered so that no contaminant can become lodged in the bearing prior to removal from the shaft. If contaminants enter the housing and the bearing is subsequently rotated, it is possible that they will roughen and damage the raceways.

The ball cage may be damaged if the bearing puller is used incorrectly. Use of improper tools such as a hammer or chisel to pound or pry the bearing off the shaft may result in damage to the bearing in addition to the hazard of contaminating the bearing.

Removal From Shaft

Because of operating conditions or location of the shaft, bearings are often tight and resist easy removal. This holds true even though they were originally mounted with a “push” fit, usual in most machine tool spindle applications. A “push” fit means ability to press the bearing on the shaft with hand pressure.

If these conditions occur, mechanical means such as a bearing puller (Figure 7-5) or the use of an arbor press (Figure 7-6) should be employed to effect bearing removal. The hammer and drift tube method, sometimes used to pound the bearing from the shaft, generally is not recommended, especially on machine tool spindle bearings. There is always the chance that the hammer shocks conducted through the tube will cause brinelling. For some types of bearings, electrical means of removal are possible as well. These removal methods will be described later.

Bearings are mounted on shafts or spindles in several ways so that dismounting must be accomplished by different means. Here are the most common conditions:

The bearing is free of grease and/or other parts. Place the shaft in an arbor press in line with the ram and with the inner ring of the bearing supported by a split ring having a bore slightly larger than the shaft (Figure 7-7). Press the shaft from the bearing with an even pressure, making sure it does not drop free and become damaged. If the split ring is not available, two flat bars of equal height could support the bearing (Figure 7-8).

Another means of removing a bearing from the shaft is by use of a bearing puller, several of which are shown in Figures 7-13 to 7-15. The bearing mounted with gears and/or other parts abutting it (Figure 7-9). In most cases, a bearing in this location can only be removed by a bearing puller which applies pressure on the outer ring (Figure 7-10). Extreme care must be exercised when applying pressure to make sure that the pull is steady and equal all around the outer ring. If the gears or other parts are removable, it may be possible to apply prcssure through them to force the bearing off the shaft. An arbor press may be employed to do the job if the bearing or gear can be adequately supported while pressure is applied

Applying Pressure with Bearlng PullerWhenever possible, bearings always should be moved from the shaft by square and steady pressure against the tight ring. Thus with a tight fit on the shaft, pressure should be against the inner ring; with a tight fit in the housing, pressure should be against the outer ring. If it is impractical to exert pressure against the tight ring, and the loose ring must be used, it is imperative that the same square and steady pull method be used. Pressure may be applied in either direction on bearings with shoulders of equal height (Figure 7-1 1). On counterbored bearings with one deep and one low shoulder, pressure should be applied against the deep shoulder. If pressure is applied against the low shoulder, disassembly of the bearing or serious damage may result. When the pairs of bearings on each end of the shaft are mounted in a back-to-back (DB) relationship, the counterbored outer ring is always exposed. In such cases, it will be necessary to apply the pressure against the low shoulder (counterbored ring) to effect bearing removal from the shaft even though the risk of damage to the inboard bearing is great.

Most machine tool spindles employ Type R or angular-contact bearings (7000 Scrics) that do not havc scals or shields. However, it is possible that a Conrad type bearing equipped with seals or shields may be used in some applications. When using pullers for bearing removal, care must be exercised to avoid damage to the seal or shield (Figure 7- 12). If dented and then remounted, an early bearing failure during operation could result. Bearing removal damage can be caused by the selection of the wrong puller type as easily as it can with improper use of the correct puller. No matter which puller is used, remember: the bearing must be pulled off squarely under steady pressure.Identification and Handling of Removed Bearings.

As it is possible that bearings may be suitable for remounting after servicing, it is necessary to replace them in exactly the same position on the shaft. Therefore, each bearing must be specifically tagged to indicate its proper location. Duplex bearings should be tied together in their proper relationship, DB, DF, or DT and the tag should also indicate the relationship. If a spacer is used between duplex bearings, the tag should indicate its position and relationship to the bearings. On jobs where the bearing is being removed because performance has not been fully successful, it is often desirable to find out why. Be sure to preserve the bearing until it is practical to examine it. The bearing frequently contains direct evidence as to the cause of failure. It should not be permitted to rust badly and the parts should be abused as little as possible during disassemble.

If the bearing is being removed for reasons other than bearing failure, be certain that it is thoroughly cleaned and oiled immediately after removal. Otherwise there is a good chance that it will get dirty and rusty. which would prevent its reuse.Bearing PullersThere are numerous types of bearing pullers on the market, any of which would be satisfactory to use depending upon the dismounting situation encountered. A conventional claw type is used where there is sufficient space behind the bearing puller claws to apply pressure to the bearing. In the illustration (Figure 7-13), the claws are pressing against the bearing preloading spring pack which in turn will force the duplex pair of bearings and spacers from the spindle. Another type of puller (Figure 7-14) uses a split-collar puller plate (Figure 7-15), the flange of which presses against the inner ring of the bearing. The puller bolts must be carefully adjusted so that the pulling pressure is equal all around the ring. The collar must be made in two pieces so that it can be slipped behind the bearing. The collar hole should be large enough so that the two pieces may be bolted together without gripping the shaft

Most bearing companies do not manufacture bearing pullers, but many bearing distributors stock a variety of the various pullers described above.Bearing Removal Through Application of Heat

The application of heat via special devices provides a rather straightforward way of removing inner bearing rings without damaging shafts. The device shown in Figure 7-16 is initially heated by an induction heater (see Figures 7-59 through 7-61, later in this chapter). To remove the inner ring from a bearing assembly [Figure 7-17( l)], the outer race and rolling elements must first be removed [Figure 7-17(2)]. The device is then heated to approximately 450°C (813°F) and slipped over the exposed ring, Figure 7-17(3). By simultaneously twisting and pulling [Figure 7- 17(4)], the operator clamps the heated pull-off device onto the ring. Within approximately 10 seconds, thc ring will have expanded to the point of looseness [Figure 7-17(5)] and can be removed.

Cleaning and inspection of Spindle Parts

Insufficient attention is paid to small dust particles which constantly blow around in the open air. But should a particle get in one’s eye. it becomes highly irritating. In like manner, when dirt or grit works into a ball bearing, it can become detrimental and often is the cause of bearing failure.

It is so easy for foreign matter to get into the bearing that more than ordinary care must be exercised to keep the bearing clean. Dirt can be introduced into a bearing simply by exposing it to air in an unwrapped state. Within a short period of time, the bearing can collect enough contaminants to seriously affect its operation. Special care must be taken when the bearing is mounted on a shaft, a time when it is most susceptible to contamination. This cleanliness requirement also extends to the handling of spindle parts, as everything must be clean when replaced in the assembly.Cleaning the BearingDuring the process of removal from a shaft, the bearing is likely to have become contaminated. The following procedure should be used to clean the bearing for inspection purposes as well as to prepare it for possible remounting on the shaft:

1. Dip the bearing in a clean solvent and rotate it slowly under very light pressure as the solvent runs through the bearing (Figure 7- 18). Continue washing until all traces of grease and dirt have been removed. Do not force the beuring during rotation.

2. Blow the bearing dry with clean, dry air while holding both inner and outer rings to keep the air pressure from spinning them. This avoids possible scratching of balls and raceways if grit still remains in the bearing. A slow controlled hand rotation under light pressure is advisable.

3. After blowing dry, rotate the bearing again slowly and gently to see if dirt can still be detected. Rewash the bearing as many times as necessary to remove all the dirt.

4. When clean, coat the bearing with oil immediately. Special attention should be given to covering the racewayr and balls to ensure prevention of corrosion to the highly finished surfaces. Rotate the bearing gently to coat all rolling surfaces with oil.After cleaning, the bearing should be wrapped with lint-free material such as plastic film to protect it from exposure to all contaminants. Unless this is done, it may be necessary to repeat the cleaning procedure immediately prior to remounting. As other spindle parts are cleaned, they also should be covered to exclude contamination which could ultimately work into the bearing.Cleaning the ShaftThe shaft must be cleaned thoroughly with special attention being paid to the bearing seats and fillets. If contaminants or dirt remain, proper seating of the shaft and/or against the shaft shoulder could be impossible. Don’t overlook the cleaning of keyways, splines, and grooves.

Cleaning the Housing.
Care should be taken to remove all foreign matter from the housing (Figure 7-19). Suitable solvents should be used to remove hardened lubricants. All corrosion should be removed. After cleaning, inspect in a suitable light the bearing seats and corners for possible chips, dirt, and damage, preferably using low power magnification for better results. The most successful method to maintain absolute cleanliness inside a clean housing is to paint the nonfunctional surfaces with a heat-resisting, quick-drying engine enamel. Do not paint the bearing seats of the housing. This would reduce the housing bore limits, making it difficult, if not impossible, to mount the bearings properly.

Painting seals the housing and prevents loose particles such as core sand from contaminating the bearing lubricants and eventually the bearings. It also provides a smooth surface which helps to prevent dirt from clinging to the surfaces. The housing exterior also may be painted to cover areas where old paint is worn or chipped; but do not paint any of the locating mating surfaces.

This type of work should be done in a place outside of the spindle assembly area.Keep Spindle Parts Coated With OilAs most of the spindle parts are usually of ferrous material, they are subject to corrosion. When exposed to certain atmospheric conditions, even nonferrous parts may become corroded and unusable in the spindle. Therefore, it is important to make certain that parts are not so affected from cleaning time until they are again sealed and protected in the spindle assembly. The best protection is to keep parts coated with a light-weight oil, covered, and loosely sealed with a plastic film or foil. Such a covering will exclude contaminants such as dust and dirt. When it is necessary to handle parts for inspection, repair, transportation, or any other purpose, precautions must be taken to ensure they are recoated with oil as some may have rubbed off during handling of the part.

Inspect All Spindle Parts
After the spindle parts have been cleaned thoroughly, the various parts should be inspected visually for nicks, burrs, corrosion, and other signs of damage (Figure 7-20). This is especially important for locating surfaces such as bearing seats, shaft shoulders, faces, and corners of spacer rings if any are used in the spindle, etc.Sometimes damage may be spotted by scuff marks or bright spots on the bearing, shaft, or in the housing.

This scoring may be caused by heavy press fits or build-up of foreign matter drawn onto the mating surfaces. Bright spots may also indicate early stages of “fidgeting” or scrubbing of mating surfaces. The shaft should also be checked for outof- round and excessive waviness on both two-point and multiple point gauging or checking on centers.Bearing Seats on ShaftThe shaft seat for the inner ring of a ball bearing is quite narrow and subject to unit pressures as high as 4,000 lbs per square in.

Because of this pressure, particular attention must be paid to the shaft fit to avoid rapid deterioration of the bearing seats due to creepage under heavy load and/or “fretting.”The required fit of the inner ring on the shaft will vary with the application and service. It is dependent on various factors such as rotation of the shaft with respect to the direction of the radial load, use of lock nuts, light or heavy loads, fast or slow speeds, etc.

In general, the inner ring must be tight enough not to turn or creep significantly under load When the bearing has too tight a fit on the shaft, the inner race expands and reduces or eliminates the residual internal clearance between the balls and raceways. Usually bearings, as supplied for the average application, have sufficient radial clearance to compensate for this effect. However, when extremes of shaft fit are inadvertently combined with insufficient radial clearance, extreme overload is caused and may result in heating and premature bearing failure.

Tight fits in angular-contact type bearings used for machine tools may cause changes in preload and contact angle, both of which have an effect upon the operating efficiency of the machine. Finally, rings may be split by too heavy a fit.Excessive looseness under load is also very objectionable because it allows a fidgeting, creeping, or slipping of the inner ring on the rotating shaft (Figure 7-22).

This action causes the surface metal of the shaft and bearing to fret, scrub, or wear off which progressively increases the looseness. It has been noticed that, in service, this working tends to scrub off fine metal particles which oxidize quickly, producing blue-black and brown oxides on the shaft and/or the bore of the bearing.

The bearing should be tight enough on the shaft to prevent this action. If any of these conditions are noticed on a shaft that has been in service, it may be necessary to repair it to correct size and condition. If the shaft is machined for the bearing seat, it is important not to leave machining ridges, even minute ones. The load very soon flattens down the tops of these ridges and leaves a fit that is loose and will rapidly become looser.

For best results, bearing seats should be ground to limits recommended for the bearing size and application.Shaft ShouldersCorrect shoulders are important because abutment against the shoulder squares the bearing. The bearing is actually squared up when it is pushed home against the shaft shoulder and no further adjustment is necessary. If a heavy thrust load against the shaft shoulders has occurred during operation, it is possible that the load may have caused the shoulder to burr and push over. Therefore, check the shoulder to make sure that it is still in good condition and square with the bearing seat. If it is not, the condition must be corrected before the spindle assembly operations are begun.Poor machining practices may result in shaft shoulders that do not permit proper bearing seating. The shoulder in Figure 7-23A is tapered. This results in poor seating of the bearing against the corner of the inner ring.

Thc shaft shoulder in Figure 7-23B is so low that the shoulder actually contacts the bearing corner rather than the locating face of the bearing. With the condition shown in Figure 7-23C, contact between the shoulder and the bearing face is not sufficient. Under heavy thrust loads, the shoulder might break down.Figure 7-23D is exaggerated to illustrate distortion of the inner ring when forced against off-square shoulder. An off-square bearing shortens bearing life.Some of these conditions can be corrected when repairs are made on the inner ring seat of the shaft. Such work should be done away from the clean assembly area to avoid possible contamination of the bearing and spindle parts by metal chips or particles from the machining or grinding operations.The shaft shoulder should not be too high as this would obstruct easy removal of the bearing from the shaft. As described previously, a pulling tool must be placed behind the inner ring and a surface must be left for the tool. Preferably, the inner ring should project somewhat beyond the shaft shoulder to permit pulling the bearing off against this surface.

This may not be possible in the case of shielded or sealed bearings where the bearing face is srrlall.Shaft Fillets and UndercutsDuring shaft repair work, it is important to pay attention to the fillet. When it is ground, the fillet frequently becomes larger as the wheel wears? causing an oversize fillet. This in turn locates the bearing on the corner radius instead of the shaft shoulder.

In other cases, the corner fillet is not properly blended with the bearing seat or shaft shoulder. This too may produce incorrect axial location of the bearing. The bearing corner radius originally may be a true 90" segment in the turning. but when the bores, OD's, and faces are ground off, it becomes a portion of a circle less than 90" while the shaft fillet may be a true radius (Figure 7-24A).Shaft fillet radius specifications are shown in bearing dimension tables with the heading “Radius in Inches” or “Corner Radius.” This dimension is not the actual corner radius of the bearing but is the maximum shaft fillet radius which the bearing will clear when mounted.
The radius should not exceed this dimension. The actual bearing corner is controlled so that the above mentioned maximum shaft fillet will always yield a slight clearance. Figure 7-24B illustrates the conventional fillet construction at the shaft shoulder. Where the shaft has adequate strength, an undercut or relief may bc preferred to a fillet. Various types are shown in Figure 7-24 C, D, and E. Where both shaft shoulder and bearing seat are ground, the angled type of undercut is preferred.
Break Corners to Prevent BurrsWhen the shaft shoulder or bearing seat is repaired by regrinding, it is desirable to break the corner on the shaft. This will help prevent burrs and nicks which may interfere with the proper seating of the inner ring face against the shaft shoulder (Figure 7-25). If left sharp, shoulder corners are easily nicked, producing raised portions which, in turn, may create an off-square condition in bearing location. The usual procedure to break a corner is to use a file or an abrasive stone.
This should be done while the shaft is still in grind position on the machine after regrinding the bearing seat and shoulders. The corner at the end of the bearing seat also should be broken, thus providing a lead to facilitate starting the bearing on the shaft.If nicks or burrs are found during an inspection and no other work is necessary on the shaft, they can be removed by careful use of a file or stone (Figure 7-26).
This work should be done elsewhere than in the clean assembly area. Any abrasive material should be removed from the part before returning it to the assembly area.Check Spindle Housing SurfacesIn many cases, housings will require as much preparatory attention as the shaft and other parts of the spindle. Check the surfaces which mate with the machine mount.
Frequently burrs and nicks will be evident and they must be removed before remounting the bearings. Failure to do so may cause a distortion in the bearing, resulting in poor operation and reduced life. These precautions apply to both bearing seats and shoulders.Shaft and Housing Shoulder

DiametersRecommended shaft and housing shoulders (Figure 7-27) for various sizes of bearings are shown in Table 74.Checking Shaft and Housing MeasurementsAfter all repair work on the shaft has been completed, shafts should be given a final check to make sure the repairs are accurate and within the recommended tolerances.
This work may be done with suitable gauging equipment such as an air gauge, ten-thousandths dial indicator, electronic comparator, an accurate micrometer, and other instruments as necessary. Accuracies of readings depend on the quality of equipment used, its precision, amplification; and the ability and care exercised by the operator. It is usually advisable to use a good set of centers which will hold the shaft and permit accurate rotation. The center points should be examined to make sure they are not scored and should be kept lubricated at all times to prevent possible corrosion.
Center holes of the shaft must also be of sufficient size, clean and smooth, and free from nicks. Be sure to remove particles of foreign matter that could change the centering of the shaft on the points.V-blocks will also be helpful to hold the shaft while making various checks. It is important that the V-blocks are clean on the area where the shaft contacts the blocks. Foreign matter and nicks will change the position of the shaft in the blocks and affect any measurements taken.Check Bearing Seat for Out-of-RoundA simple check may be made with a hand gauge on the bearing seat (Figure 7-28).
This will provide a reading at two points on the shaft 180" apart. However, it does not indicate how those points are related to other points on the shaft.For a more accurate reading on out-of-round (radial runout) of a bearing seat, mount the shaft between centers and place a suitable indicator in a position perpendicular to the axis of the shaft and contacing the bearing seat. On rotating the shaft slowly by hand, a check is obtained on all points of the shaft which the indicator contacts (Figure 7-29). Another method of measuring out-of-round is the three-point method using a set of V-blocks and a dial type indicator (Figure 7-30). The shaft should lay in the V-blocks and be rotated slowly while the indicator is centrally located between the points of shaft contact with the V-blocks and perpendicular to these lines of contact. This method will reveal outof-round which would not have been found by the two-point method of gauging. Therefore, if the equipment is available, it is desirable to check bearing seats using centers or V-blocks as well as two-point gauging. In all of these checks, the gauge should be placed in different locations on the bearing seat. This will give assurance that the seat is within the recommended tolerances in all areas. While the spindle is mounted on centers, the high point of eccentricity of the bearing seat should be located. Using a dial type indicator, find the point and mark it with a crayon so that it can be easily located when the bearing is to be remounted. The high point of eccentricity is covered in more detail later.