Petron Plus Global Inc.

Bearing Maintence

 

BEARING MAINTANCE

1.  Bearing Lubrication Basics.
2.  Notes on Oil Lubrication.
3.  Ten (10) Threats to Bearing Reliability.
4.  Ten (10) Tips for Trouble Free Bearing Operations.
5.  Preventing Bearing Lubrication Mistakes.
6.  Proper Bearing Mounting Helps Prevent Cracks, Flaking and Other Damage. 

 

BEARING

LUBRICATION BASICS

Lubrication reduces friction.  It also prevents wear and corrosion, and guards against solid and liquid contamination.  Theoretically, a properly lubricated bearing operating under ideal conditions will last forever.  This is not possible in reality, of course.  But a properly lubricated bearing has the best chance of achieving its maximum service life.

The lubricant forms a film between the bearing’s rolling and sliding surfaces, so that metal contact is minimized even under heavy load.  Rolling bearings are normally lubricated with grease or oil.  In special cases a solid lubricant is used.

Oil Versus Grease

Why one type of lubricant is used instead of another depends on bearing size, speed, type, load and operating environment.

Grease which is actually oil with a soap thickener added, is used for bearings operating under normal speed and temperature conditions.  It is simpler and less expensive to apply than oil; offers better adhesion and better protection against moisture and contaminants.  Approximately 90% of bearing applications employ grease.

Oil is used when speed and/or operating conditions make it impossible to use grease, or where heat needs to be transferred.

Lubricant Supply Systems

Oil and grease require different types of supply systems.  Several oil and grease supply systems exist that meet the needs of various bearing applications.  Oil supply systems include; oil baths, circulating oil systems, spray or mist systems, and the wick feed method.  Grease supply systems include: housings (with or without grease fittings), grease chamber lubrication, and grease quantity regulator.

Cleanliness

Lubricant cleanliness is essential, since ever minute contaminant particles can affect a bearing’s operating performance and shorten service life.

Filtering fresh oil before feeding it into the delivery system is an excellent lubrication practice.  In oil circulating systems, filters can further guard against contamination when they are suitably located in the application.

Grease should be kept in their original containers until used.  Never leave a container uncovered, since dust and other particles from the environment will quickly collect.  Wash grease guns with clean solvent and dry thoroughly before use.  Keep new bearings scrupulously clean during mounting.  Inspect and thoroughly clean bearings that have been disassembled for relubrication (see article on bearing mounting).

Lubricant properties

Viscosity is the most important lubricant property that should be considered during lubricant selection.  Viscosity is the ease with which a liquid flows, and is the guiding principle in lubricant selection.  Film-forming ability (film thickness) is related to lube type, rotational speed, temperature and viscosity.  Consistency is the degree of stiffness in a grease.

Lubricant can also have anti-oxidizing, rust inhibiting and anti-wear and anti-foaming properties.

Lubricant selection criteria differs for various bearing types and sizes.  Contact your bearing manufacturer, equipment manufacturer or lubricant supplier for detailed selection information.

Relubrication

Oil change intervals depend on operating conditions and oil quantities.  For most oil bath systems, yearly changes are acceptable.  “Closed” mist systems exist that reuse the oil; here, oil sampling can help determine acceptable frequency of change intervals.  Also, large circulating systems that filer and de-water oil are available, “make-up” oil is needed to maintain volume.

Greased bearings must be relubricated if the grease’s service life is shorter than the bearing’s expected service life.  The relubrication interval is that time period at the end of which 99% of the bearings are still reliably lubricated.

Contact your bearing manufacturer for relubrication charts covering all types of rolling bearings.

 

Although care has been taken to ensure the accuracy of this data, SKF assumes no liability for errors or omissions.

® SKF is a registered trademark of SKF

© 1998 SKF USA Inc.

The above information was supplied by the SKF USA, Inc.  The SKF Bearing Maintenance Institute recommends the above bearing maintenance tips to maximize bearing performance.  SKF USA, Inc. may be contacted at www.skfusa.com.

 

NOTES

ON OIL

LUBRICATION

 

Choosing the proper lubricant is one of the most important factors in achieving long bearing life.  Lubricants prevent contact between rolling elements and raceways in a bearing and protect against wear and overheating.  This article focuses on a common bearing lubricant- oil.

Oil lubrication is generally used when high speed or operating temperatures preclude the use of grease, when frictional or applied heat has to be transferred away from the bearing, or when adjacent machine parts are lubricated by oil.

Content and Viscosity.

Solvent-refined mineral oils are the optimal lubricants for ball and roller bearings.  Oils containing additives for improved properties such as extreme pressure behavior or oxidation resistance can be employed in special cases.  Synthetic oils, however, are used in extreme temperature conditions to lubricate rolling bearings.

Choose an oil with a viscosity that is adequate for the job.  An oil’s viscosity varies with the operating temperature, decreasing as the temperature rises.  In order for a sufficient film of oil to form between bearing rolling elements, the oil must retain a minimum viscosity at the operating temperature.  Oils with a high viscosity index of at least 85 are recommended.

The viscosity needed for adequate lubrication can vary with bearing design.  For example, tapered roller and spherical roller bearings normally have a higher operating temperature than ball and cylindrical roller bearings under the same conditions and require oil of a slightly higher viscosity.

Methods of Oil Lubrication.

The most simple method is the oil bath, in which the bottom portion of the bearing housing is filled with oil.  As the bearing rotates, the rolling elements collect the oil and distribute it within the bearing, before allowing it to drain back into the bottom of the housing.  When the bearing is stationary, the oil level should be just below the center of the lowest ball or roller.

When operating at higher speeds, temperatures rise and oil oxidation increases.  Oil circulation systems can be used to reduce the need for frequent oil changes in high-speed bearing applications.  Oil passes through the bearing, then is filtered and often cooled before being returned to the bearing.

At extremely high speeds, oil circulating inside a bearing can begin to churn, causing resistance to bearing rotation.  In these situations, an oil jet is recommended.  The oil jet directs oil under high pressure at the side of the bearing.  Small droplets penetrate the turbulent air around the bearing and get through to lubricate the rolling elements.

Oil mist lubrication is often recommended for bearings running at high speed in continuous operation applications.  Under this method, a mixture of air and atomized oil is supplied to the bearing housing under pressure.  It is important for the bearings to be wetted before they are put into actual operation to ensure that they don’t run dry at start-up.  For that reason, the oil mist system should be actuated several minutes before putting the equipment into actual operation, or a wet sump should be used.  Oil mist is very effective in reducing a bearing’s operating torque.  The system provides a continuous supply of clean, fresh oil, and the air flow prevents excessive oil from accumulating in the bearing.  Since the air under pressure in the housing escapes through the housing enclosures or vents, the system is also effective at preventing the entrance of moisture and contaminants.  Care should be taken to prevent excessive mist from escaping into the shop environment.

Another technique is called oil spot lubrication.  Here, small quantities of oil are delivered to individual bearings by compressed air and injected via a nozzle.  This minimum quantity of oil enables bearings to operate at lower temperatures or at higher speeds than other lubrication methods.  The oil spot technique is often used for grinding spindles and similar applications.

Changing Oil.

Oil change frequency depends mainly on operating conditions and oil quantity.  With an oil bath, change oil once a year- provided there is no contamination and the operating temperature does not exceed 122°F (50°C).  For operating temperatures around 240°F, change oil every three months or consider using a synthetic oil.

For circulating systems, oil change frequency depends on how often the oil is circulated and whether it is cooled.  You can determine the best frequency by carefully checking the oil for signs of contamination.

Changing oil is not necessary in oil spot lubrication, since oil circulates through the bearing only once and is not reused.

 

Although care has been taken to ensure the accuracy of this data, SKF assumes no liability for errors or omissions.

® SKF is a registered trademark of SKF

© 1998 SKF USA Inc..

 

 

TEN (10) THREATS

to BEARING

RELIABILITY

When a failed bearing causes a critical piece of manufacturing equipment to go down, it is more than a nuisance.  It is a loss to your company’s productivity.

Although bearing performance has improved markedly over the past decade, many maintenance managers still feel they are replacing bearings too often.  And they are right.

Factors that influence the operating performance of a bearing include load magnitude and load direction (radial, axial [thrust], or a combination of both); shaft speed; operating environment (temperature and contamination); lubrication (type and method); type and condition of seal; shaft alignment; mounting and dismounting technique; shaft and housing fits; running accuracy.

The same factors should be considered when troubleshooting an apparent problem.  Is the equipment operating beyond its original design parameters, affecting load conditions?  Is the bearing better suited to support radial load than axial load?  Or is the operating environment adversely affecting bearing life?  Ingress of solid contaminants dents raceways, disrupts lubricant film, and induces high stress.  Extreme temperature conditions can make it virtually impossible to develop adequate lubrication.

At SKF, bearing performance data have been compiled and analyzed for more than 80 years.  Guidelines have been developed on the most common causes of bearing failure.  The following 10 trouble-shooting tips should be helpful in extending bearing life.  Once you know the problem, it is much easier to prevent it from happening again.

1. Wrong Lubricant for Bearing

Lubrication should be one of the first points considered in troubleshooting a bearing failure.  Bearings can be lubricated with an oil bath, circulating system, oil mist, or grease.  Some applications, because of high speed, high temperatures or both, warrant oil lubrication systems.  However, this decision may be compromised because of the cost involved.  The result is often periodic bearing replacement.

Greases are available with varying base oils, thickeners, additives, and consistencies.  Because many options are available, lubricants are often misapplied.

Lubricants should separate the working surfaces, which prevents metal-to-metal contact, reduces friction, and helps prevent wear.  They should protect against corrosion, exclude contaminants, and dissipate heat.  The most important property of oil is viscosity, a measure of the oil’s strength, or ability to develop a film that separates the working surfaces.  Working surfaces in rolling bearings include the contacts between the rolling elements and the raceways, the rolling elements and guiding surfaces (cage pockets, flanges, guide rings), and cage piloting surfaces.

For proper lubrication of rolling element bearings, adequate viscosity at operating temperature is required.  Examine the surface of the bearing raceway and rolling elements.  Highly polished, glazed appearances or frosty, matted appearances indicate that working surfaces are and inadequate viscosity at operating temperature.  Surface roughening in contact, the result of insufficient of film, which gives surface a leathery appearance or orange peel effect, is another indication of metal-to-metal contact wear conditions.  To avoid such problems, use both the type and quantity of lubricant specified by the manufacturer, not only at the time of installation, but also during maintenance procedures.

2. Mixing Incompatible Lubricants

If two incompatible greases are mixed, lubrication failure is inevitable.  Polyurea and lithium-based greases, for example, break down rapidly when mixed.  A soupy or runny grease is an indication of the problem.  A grease that is much thicker than its original consistency may indicate incompatible mixing.  Grease color also should be observed.  For example, if the original lubricant was green but has turned brown by the time of inspection, incompatible greases may have been mixed.  Incompatible greases are a factor in many bearing failures.

3. Too Much or too Little Lubricant

A common misconception among maintenance personnel is that it is better to over-lubricate than to under lubricate bearings.  Both methods are undesirable.  Under-lubrication risk metal-to-metal contact, and over-lubrication (pushing excessive grease into the cavity) causes heat build-up and friction as the rolling elements continuously try to push extra grease out of the way.  To assure that bearings are not over or under-lubricated, follow the manufacturer’s instructions.  This recommendation applies not only to grease lubrication, but also to oil bath.  In addition to maintaining the correct oil levels, be certain to check for faulty bearing seals.  Faulty seals often allow oil to escape, which results in premature wear and the need for frequent replacement.

4. Contaminants or Corrosives in Lubricant

Although contaminants are sometimes difficult to detect, they are a common cause of lubrication failure.  Dirt, sand, and water are the most common contaminants, but acid and other corrosives also can deteriorate the bearing lubricant.  They can dilute the oil film, reducing viscosity, and they can corrode the bearing surfaces, disrupting the oil film and causing erosion, creating thousands of abrasive particles.

Solid-particle contaminants introduce the most damage and are a factor from the moment they come into contact with the internal workings of a bearing.  Depending on particle size, they may be seen or felt as grittiness in the lubricant.  It is best to perform a lubricant sample analysis, which may be available from the lubricant supplier or an independent laboratory.  The service will determine concentration levels of solid particles, identify the material, and indicate the source.

The best safeguard against contaminants is a clean, dry operating environment.  If operating circumstances do not permit this environment, select bearings with seals or shields to keep contaminants out.  In addition, replace worn housing seals.  If humidity is a problem, consider selecting a lubricant with a good rust inhibitor.  Harsh environments can sometimes be overcome by increasing the frequency of relubrication or oil changes.  These procedures, however, add to the risk of over-lubrication.  Proper grease levels can be maintained by opening purge plugs to allow excess grease to escape or by periodically removing the existing grease and repacking with the appropriate fresh clean grease.  Oil systems lubrication provides a continuous supply of cool, clean oil that flushes away contaminants.

5. Misalignment of Components

One of the great causes of premature bearing failure is misalignment between the equipment shaft and bearing housing bore.  Some, but not all, bearings can tolerate minor misalignment.  Serious misalignments introduce excessive vibration and loads.

In the case of belt-driven motors or equipment, never over-tighten belts, because over-tightening introduces unnecessary loads.  Use just enough tension to prevent belt slippage.

To Correct bearing alignment problems, shim the housings as necessary.  Shafts should be coupled in a straight line, especially when three or more bearings operate on a shaft.

6. Distorted Housing Bore

If a housing bore is out-of-round or if it is not the right geometric shape, excessive loads and wear result.  This problem arises when the housing is mounted to a pedestal that is not flat.  For example, if the housing is bolted to a crowned surface, the housing will become distorted, which in turn elongated the bore.  If an out-of-round housing bore is suspected, correct the mounting surface before installing a new bearing.

The same principle also applies when aligning pillow block bearings.  When adjusting height, even a fraction of an inch, fully support the complete pillow block base with shims.  If the center portion of the block is unsupported, the housing bore can become distorted, leading to another failure.

7. Inadequate Internal Clearance

Internally, there is a small clearance to accommodate the thermal expansion of the bearing components.  With the bearing in a radial position, subjected to a load acting radically downward, the internal clearance is the total distance between the outer raceway and the topmost rolling element.  Rolling bearings are manufactured to specified clearance categories and marked accordingly, for example, suffix C3.  If the internal clearance is inadequate, excessive heat buildup results.  Temperature affects viscosity, leading to a lubrication problem.  Or worse, internal friction becomes so great that the bearing locks up.  Replacement bearings must satisfy the internal clearance specification the same as the original bearing.

Shaft interference directly influences bearing internal clearance.  If the bearing shaft seat is oversized, the internal clearance may be completely removed before the equipment operates.

8.        Bearing Arrangement

The bearing arrangement usually consists of a two-bearing systems that provides radial support.  The bearings are supported in housings, one provided with shoulders to axially locate or position the shaft assembly, and the other designed so the bearing is unrestrained, or axially free, to accommodate thermal shaft expansion.

Terms associated with the two bearings in this arrangement are held and free, fixed and floating, locating and expansion, and thrust and radial.  If the arrangement does not provide for adequate shaft expansion, parasitic thrust loads and increased operating temperatures can result.  The held bearing carries both radial and axial load capabilities vary among bearing types and may also be influenced by operating speed and lubrication method.

9. Distorted Seals and Shields

Integral bearing seals and shields are sometimes inadvertently pushed in and damaged as bearings are mounted.  Distorted seals or shields can interfere with the functioning of the bearing cage or rolling elements and will permit contaminants to enter the bearing.  To prevent seal damage, always follow the prescribed mounting techniques when installing bearings.

Sealed or shielded bearings are not intended to be modified.  Normally, sealed and shielded bearings are considered sealed for life and do not require lubrication.  To satisfy particular conditions, seals or shields are sometimes removed from bearings, presenting the risk of physical damage to the bearing components, especially the cage, during the removal process.  If the removal is successful, the bearing should be treated as an open bearing, which requires a greater amount of grease.  If grease is not added, or if an incompatible grease is added, the bearing will fail early because of lubrication failure.  The best practice is to use the correct bearing, as specified.

10. Undersized Shaft Diameter

In most applications, the shaft rotates while the housing is stationary.  The bearing inner ring normally has an interference fir on the shaft, and the outer ring has a loose fir in the housing.

A tight fit forms a bond between the bearing and shaft so they operate as an integral assembly.  Dimensional interference develops a fit pressure, or a gripping action of the inner ring on the shaft.  The holding power of the fit depends on the amount of interference, the surface area in contact, and the friction between the mating parts.

The bearing needs to be fitted to an accurately sized shaft.  If the shaft is oversized, the internal clearance is reduced.  If the shaft is undersized, the bearing creeps on the shaft, promoting wear of the shaft and the bearing bore.  Friction and heat are then created, increasing the operating temperature and generating particle debris that acts as a contaminant.  This condition can be corrected only by restoring the shaft diameter to the correct size and shape.

In the case of an adapter sleeve mounting or a tapered bore bearing on a tapered shaft, looseness might indicate that the bearing was not properly mounted.  Do not arbitrarily retighten the bearing.  If the shaft is worn, restore the shaft to the specified size and shape, and properly mount the replacement bearing with adequate holding power.

Other Causes of Bearing Failure

These 10 troubleshooting tips cover most failures, but there are literally hundreds of reasons bearings go bad.  For example, a technician might have inadvertently installed a held bearing when a floating bearing was needed on a shaft.

Other common errors include installing bearings with inadequate load capacity, or using rigid bearings in place of-self-aligning bearings.

Although most of these problems can be spotted quickly, others are quite bewildering.  For example, if a technician installs a bearing in or near a high-velocity fan, airflow over the bearing creates a pressure imbalance and may pull oil from the housing.  To prevent a failure, install a baffle which will divert air away from the affected bearing.

The majority of bearing problems can be solved by focusing on the 10 trouble spots discussed here.  At SKF, we recommend an ongoing multi-parameter condition monitoring program, which includes periodic measurement of temperature, machine vibration, bearing condition, and lubricant oil sample analysis.  The extra effort of a monitoring program pays off dramatically in reducing equipment downtime and production losses.

 

Although care has been taken to ensure the accuracy of this data, SKF assumes no liability for errors or omissions.

© SKF is a registered trademark of SKF

® 1998 SKF USA Inc.

 

 TEN (10) TIPS

for TROUBLE FREE

BEARING OPERATION


1.
Handle with care.

Always handle bearings as the precision components they are.  Crack and nicks too fine for the human eye will lead to poor bearing performance and eventual failure.  Avoid bringing sharp objects into contact with the bearing and never pound directly on a bearing or a ring.  If a bearing is dropped, it is best not to install it.  Store bearings horizontally in a dry place in their original unopened package and never place bearing on a dirty surface.  Avoid leaving a bearing exposed to air-borne contaminants.  A speck of dirt in a raceway can lead to premature failure.

2. Inspect shaft and housing.

Always inspect the shaft and housing for size and physical condition before mounting a bearing.  Check for damage, remove nicks and burrs with emery paper, and wipe clean with a soft cloth.  Replace or repair shafts and housings showing obvious signs or wear or damage.  A shaft placed in a vise for mounting should be protected from vise jaws with sheets of brass, copper or other soft metal.

3. Avoid overheating.

During heat-mounting operations, never bring a flame in direct contact with the bearing and never heat beyond 250°F.  Also, immediately hold a heated bearing in place against the shaft until it cools and locks in place.  Otherwise, the bearing may creep away from the proper position.

4. Use the right tool.

Induction heaters, oil injection kits, and hydraulic nuts are among the specialized tools available for mounting and dismounting.  Their use lowers the possibility of damaging bearings and speeds the mounting/dismounting process.

5. Same replacement bearing.

Replacement bearings should be identical to the bearing they replace.  Bearing identification usually appears on the bearing and/or the box in which it was originally packaged.  Contact the manufacturer for part number verification if there is any uncertainty about the replacement.

6. Watch bearing press fit.

In bearing press mounting operations, pressure should be applied only to the ring with the press fit, which is usually the ring that rotates after the bearing is installed.  Pressure to the ring without the press fit will damage the raceways.

7. Don’t wash new bearings.

Bearings manufacturers take great care to package and ship bearings that are dirt-free and ready for specific bearing requirements.  There’s usually no need to wash them or remove the protective slushing compound.  Exceptions arise when the operating lubricant to be used is a polyurea grease or synthetic.  Use of these lubricants may require removal of the slushing compound.

8. Proper lube is critical.

Bearing manufacturers evaluate several factors before determining the type of lubricant required for specific bearing requirements.  Be sure to follow their recommendations.  Check lubrication levels frequently and change the lubricant at least once a year.  Temperature and contamination conditions will influence the frequency of lubrication changes.

9. Rotate idle bearing.

Bearings installed in equipment that is subject to vibration while the shafts are stationary may incur false brinelling damage.  False brinelling also occurs when equipment is not properly protected to prevent movement of bearing during shipment.  It can also appear as bright polished depressions on the inner and/or outer races, as well as on the rolling elements.  Guard against this by periodically rotating the bearings or briefly operating the equipment in which they are installed.

10.
Look for danger signs.

Keep alert for these sure signs of improper bearing operations: excessive noise and increase in vibration and/or temperature.  Bearing exhibiting any of these symptoms should be checked frequently for deterioration and possible removal.  Troubleshooting instruments like hand held vibration pens, digital thermometers, and electronic stethoscopes help spot bearing in poor operating condition.

 

Although care has been taken to ensure the accuracy of this data, SKF assumes no liability for errors or omissions.

® SKF is a registered trademark of SKF

© 1998 SKF USA Inc.

The above information was supplied by the SKF USA, Inc.  The SKF Bearing Maintenance Institute recommends the above bearing maintenance tips to maximize bearing performance.  SKF USA, Inc. may be contacted at www.skfusa.com.

 

 

PREVENTING BEARING

LUBRICATION MISTAKES

 During operation, balls or rollers in a rolling bearing are separated from the bearing raceway by a microscopically thin film, the bearing lubricant.  Proper lubrication reduces friction and prevents metal-to-metal contact between moving components.  It also protects the bearing rings and rolling elements from corrosion.

 Although the fundamentals of bearing lubrication are not difficult to learn, maintenance technicians often overlook the basics.  Errors in lubricants selection and application are commonplace.  Luckily, many such errors can be easily corrected at little expense to the bearing user.  Here are ten of the most common errors, and how to correct or prevent them:

1. Using a Lubricant with the Wrong Viscosity.

 Viscosity is a measurement of the thickness, or ability to flow, of a lubricating oil.  It is one of the most important factors to consider when selecting a bearing lubricant.  In the United States, a common unit of measurement for viscosity is Saybolt Universal Seconds (SUS).  The recommended minimum viscosity varies according to bearing type, ranging from 70 SUS for a ball bearing to 100 SUS at the operating temperature for a spherical roller bearing, and 150 SUS for a spherical thrust bearing.

 Many plants continue to use a single all-purpose lubricant for all bearing applications.  A generic lubricant may be acceptable for the low-viscosity needs of the typical ball bearing, but the same lubricant may be inadequate for the high-viscosity requirements of a spherical roller bearing.  A lubricant’s viscosity declines rapidly as the temperature rises, so it’s important to know the approximate operating temperature of a bearing application and choose your lubricant accordingly.

2. Misuse of Grease and Oil.

 In bearing applications, the actual lubricating fluid is generally a mineral oil or a synthetic oil.  This oil can be used alone, or in conjunction with a thickener to form a lubricating grease.  The thickener (in most cases, lithium, calcium, or sodium soap) forms a lattice which carries the oil between its fibers.

 Bearing users sometimes use and oil where a grease is called for, or grease where oil is indicated.  For example, in food processing applications, oil dripping from housings can contaminate the food product; grease is generally the preferred lubricant.  Grease may also be called for in situations where the bearing housing is inaccessible and an oil slight gauge can’t be routinely checked.  On the other hand, oil is generally used when other components in the arrangement are oil-lubricated.

3.        Over-Lubrication of Bearings.

 Too much oil or grease in a bearing or its housing causes an effect called churning resulting in a sharp temperature rise and often premature lubricant and/or bearing failure.  On start-up, grease lubricated bearings expel grease into the housing.  To prevent churning there must be sufficient empty space in the housing to accommodate this grease.  Therefore, when using grease, fill the bearing completely, but fill only one-third of the bearing housing.

4. Using an Ineffective Lubrication System.

 When oil is the preferred lubricant, there are several methods of delivering oil to the bearings, including static oil, circulating oil and the spray-mist system.  In Static oil or oil bath systems, the bath should be filled to a level just below the middle of the bottom rolling element.

 With a circulating oil system, oil is pumped from a separate reservoir, where it cools down and lubricates simultaneously.  The heated oil is then returned to the reservoir, where it cools down again.  Filters in the system remove contaminants from the oil as it circulates.  A circulating oil system can often greatly increase bearing life expectancy.

 Circulating oil, however, should not be used in high-speed applications, such as machines tool spindles, because of unacceptable friction losses as the lubricant moves through the bearing.  In these applications, only a small amount of oil is needed, and a spray-mist system is generally preferred.

5. Under-Lubrication of Bearings.

 Even with the right lubricant system, under-lubrication can occur.  The consequences are excessive heat and eventually metal-to-metal contact between bearing components.  The reasons for underlubrication vary.  When grease or static oil system is used, a small amount of lubricant can evaporate over time, leading to underlubrication.  But underlubrication can also happen with circulating oil systems.

 Recently, at a paper mill, several bearings failed because there was insufficient oil in circulation.  Maintenance technicians had decreased the rate of oil flow because at a higher rate oil had run out of the housing and damaged the paper.  The problem was eventually diagnosed as poor oil drainage from the housing.  The solution involved installing a larger drainage pipe in the system so that oil would drain more quickly from the housing instead of leaking.

6. Failure to Prevent Lubricant Contamination.

Lubricant contamination is a leading cause of bearing failure.  Dirt particles and other contaminants can damage bearings by leaving dents in rotating elements and raceways.  Contamination can occur due to improper handling of the bearing from it packaging until ready for mounting, and clean dirt and debris from the housing.  During installation, minimize the amount of time the bearing lubricant is exposed to the air.

7.        Using the Wrong Seal.

 

 

 Improper sealing can also cause contamination.  Labyrinth seals are effective in many applications.  But mine, steel mills, brick mills and other harsh environments, ambient dust may work its way past these seals and into the bearings, causing contamination and wear.  This problem can be prevented by using a higher-grade seal, such as a rubber seal, garter spring seal or a taconite seal.

8. Failure to Relubricate Bearings.

 Even without exposure to contaminates, lubricant quality can deteriorate over time.  It’s often impossible to determine grease quality in a sealed bearing by visual inspection.  Relubricating every month with a small amount of fresh grease will replace the lost grease and refill the bearing.  SKF recommends changing the grease completely every year, even sooner in some extreme cases.  Usually, the grease is injected via a grease gun, automatically expelling the used grease and any contaminants it may carry.

 In oil lubrication systems, visual inspection can be useful.  Monitoring sight gauges and oil reservoirs can help determine relubrication intervals.  The oil in oil-bath systems is generally changed once a year.  More frequent oil changes may be necessary in applications with high temperatures or heavy contamination.

9. Mixing Incompatible Lubricants.

 During bearing relubrication, maintenance technicians may sometimes inject a different lubricant into a bearing than the one used originally, inadvertently mixing the two lubricants.  If the two lubricants are incompatible, there will be a deterioration in lubricating capability.

 Never mix greases with different thickeners, such as a lithium-based grease with a sodium-based grease, before checking with the suppliers.  Never mix a mineral oil with a synthetic oil.  Some lubricants are compatible, but assessing the compatibility of two lubricants can be difficult.  As a general rule, always relubricate a bearing with the same lubricant used originally.  If a different lubricant must be used, carefully remove all traces of the old lubricant from the bearing and housing before relubricating.

10. Failure to Provide Relubrication Training.

Maintenance technicians commonly receive training or bearing selection and installation, but not lubrication.  Plant supervisors should make sure that technicians are thoroughly trained in lubrication fundamentals.

 There are many opportunities for training.  Some bearing manufacturers offer seminars that cover lubrication as part of course curriculum.  Sales representatives of bearing companies and lubricant suppliers can often provide technical training at customer sites.




Although care has been taken to ensure the accuracy of this data, SKF assumes no liability for errors or omissions.

 ® SKF is a registered trademark of SKF

© 1998 SKF USA Inc.

 The above information was supplied by the SKF USA, Inc.  The SKF Bearing Maintenance Institute recommends the above bearing maintenance tips to maximize bearing performance.  SKF USA, Inc. may be contacted at www.skfusa.com.

 

 PROPER BEARING MOUNTING

HELPS PREVENT CRACKS,

FLAKING AND OTHER DAMAGE

 How rolling bearings are mounted has a direct impact on their service life.  If you use correct mounting methods and observe a few simple precautions, your bearings are far more likely to deliver maximum performance.  Mount bearings improperly.  And you may experience premature bearing failure.

 

Cleanliness:  The First Rule

 Cleanliness is the first rule of proper bearing handling.  Whenever possible, keep the working environment clean of sand, sawdust, cement, corrosive substance and, wherever possible, high humidity.  Prior to installing a bearing, clean the shaft and housing with a solvent and dry thoroughly using a clean, lint-free cloth.  Cover exposed area with plastic sheeting or lint-free cloth when you are not working with the bearing.

 New bearings should be keep in their packaging until just before mounting.  Do not wash new bearings or remove the rust-inhibiting compound, except from the bore and the outer ring’s surface.  Wash these with a petroleum-based solvent and dry with a clean cloth.

 Thoroughly inspect used bearings for damage prior to installation, and clean them with solvent or hot cleaning oil (not more than 250°F).

 Lubricant should be clean and fresh.  Keep lubricant containers closed until needed.  Use the right type and quantity- contaminated lubricants, or incorrect lubricant amounts, can cause early bearing failure (See the following articles for more information about lubrication).

 

Check Shaft Preparation

 Prior to mounting a bearing, check to ensure that the shaft size is within its specified tolerances.  The bearing seat should be perfectly round, and not tapered, the shaft should be clean and free form nicks and burrs.  Inspect the shaft for damage that may have occurred when the previous bearing was dismounted.

 

Use Different Mounting Methods

 There are four basic methods for mounting bearings: mechanical, heating, hydraulic and oil injection.  The correct method depends on bearing type and size.

Mechanical Mounting

Mechanical mounting is generally suitable for small (4” or less O.D.) bearings.  Mounting force can be applied to the bearing placing a fitting tool sleeve against the inner ring and using a press or hammer to advance the bearing to its proper location on the shaft.  Be sure to use the correct size sleeve from a bearing fitting tool kit, or a pipe sleeve of the proper dimensions.  The bearing should be exactly at right angles to the shaft before beginning, and the shaft lightly lubricated.

 Do not apply a sleeve to the outer raceway when mounting on a shaft, or to the inner raceway when mounting in a housing- and NEVER mount by striking the bearing directly with a hammer.

 A spanner wrench or impact spanner and lock nut can be used to cold-mount small and medium-size bearings with tapered seatings, such as adapter sleeve arrangements.

Heat Mounting

Heat mounting is suitable for all medium and large size bearings, and for small bearings with cylindrical seating arrangements.  Normally a bearing temperature of 150°F above shaft temperature (not to exceed 250°F) provides sufficient expansion for mounting.  As the bearing cools, it contracts and tightly grips the shaft.  It’s important to regulate heat accurately, since excess heat destroys a bearing’s metallurgical properties, softening the bearing.

 Heat mounting tools include induction heaters, ovens, heating rings and electric plates with thermostats.  Never heat a bearing using an open flame, such as a blowtorch.

Hydraulic Mounting

Hydraulic mounting uses hydraulic pressure instead of mechanical force.  A hydraulic ram will mount small bearings with tapered bores; a hydraulic nut is recommended for bearings with bore diameters over 50 mm.

Oil Injection

The oil injection method, which is sometimes used in conjunction with hydraulic bearing mounting tools, is suitable for all bearings having tapered seating arrangements and for large bearings with withdrawal or adapter sleeves.  Oil injection forces oil under high pressure through a passage between the shaft and the bearing inner ring to separate the two, which makes mounting easier by reducing friction to almost zero.

 

Although care has been taken to ensure the accuracy of this data, SKF assumes no liability for errors or omissions.

® SKF is a registered trademark of SKF,

© 1998 SKF USA Inc.

The above information was supplied by the SKF USA, Inc.  The SKF Bearing Maintenance Institute recommends the above bearing maintenance tips to maximize bearing performance.  SKF USA, Inc. may be contacted at www.skfusa.com.



 




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