Mechanical - BEARING FAILURE
 


Load/Contact Patterns

As bearings rotate, the raceways of the inner ring and outer ring make contact with the rolling elements. This results in a Wear path on both the rolling elements and raceways. Running traces are useful, because they indicate the load conditions. They should be carefully observed when bearings are disassembled.

If the running traces are clearly defined, it is possible to determine whether the bearing is carrying a radial load, axial load or moment load. Additionally running traces can help determine the accuracy of bearing roundness, confirm whether unexpected loads or large mounting errors occurred, and shed light on probable causes of bearing damage. (R1)
Typical Running Traces of Deep Groove Ball Bearings (a) shows the most common running trace generated when the inner ring rotates under a radial load only. (e) through (h) show different running traces that result in a shortened life due to their adverse effect on bearings. (R2)
(a)    (b)    (c)    (d)

 Inner ring rotation
Radial load    Outer ring rotation
Radial load    Inner ring or
outer ring rotation
Axial load in one direction    Inner ring rotation
Radial and axial loads

(e)    (f)    (g)    (h)

Inner ring rotation
Axial load and
misalignment    Inner ring rotation
Moment load
(Misalignment)    Inner ring rotation
Housing bore is oval    Inner ring rotation
No radial internal clearance
(Negative operating clearance)
Typical Running Traces of Roller Bearings
(i) Shows the outer ring running trace when a radial load is properly applied to a cylindrical roller bearing which has a load on a rotating inner ring. (j) Shows the running trace in the case of shaft bending or relative inclination between the inner and outer rings. This misalignment leads to the generation of slightly shaded (dull) bands in the width direction. Traces are diagonal at the beginning and end of the loading zone. For double-row tapered roller bearings where a single load is applied to the rotating inner ring, (k) shows the running trace on the outer ring under radial load while (I) shows the running trace on the outer ring under axial load. When misalignment exists between the inner and the outer rings, then the application of a radial load causes running traces to appear on the outer ring as shown in (m). (R3)
(i)    (j)    (k)    (l)    (m)

Inner ring rotation
Radial load    Inner ring rotation
Moment load
(Misalignment)    Inner ring rotation
Radial load    Inner ring rotation
Axial load    Inner ring rotation
Radial and
moment loads
(Misalignment)

Damage Types & Causes
In general, if rolling bearings are used correctly, they will survive to their predicted fatigue life. Bearings, however, often fail prematurely due to avoidable mistakes. The causes of, this premature failure include improper mounting, mishandling, poor lubrication, entry of foreign matter or abnormal heat generation.
For example, one cause of premature failure is rib Scoring which is due to insufficient lubrication, use of improper lubricant, faulty lubrication system, entry of foreign matter, bearing mounting error, excessive deflection of the shaft or some combination of these. If all conditions are known for the times both before and after the failure, including the application, the operating conditions, and environment, then a countermeasure can be determined by studying the nature of the failure and its probable causes. A successful countermeasure will reduce similar failures or prevent them from happening again.
Examples of bearing damage and countermeasures are presented in the following sections. Please consult these sections when trying to determine the cause of bearing damage. (R4)1. Flaking (R5)
Damage Condition    Possible Causes    Countermeasures
Flaking occurs when small pieces of bearing material are split off from the smooth surface of the raceway or rolling elements due to rolling fatigue, thereby creating regions having rough and coarse texture.    Excessive load; Poor mounting (misalignment); Moment load; Entry of foreign debris, water penetration; Poor lubrication, improper lubricant; Unsuitable bearing clearance; Improper precision for shaft or housing, unevenness in housing rigidity, large shaft bending; Progression from rust, corrosion pits, Smearing, dents (brinelling)     Reconfirm the bearing application and check the load conditions
 Improve the mounting method
 Improve the sealing mechanism, prevent rusting during non-running
 Use a lubricant with a proper viscosity, improve the lubrication method
 Check the precision of shaft and housing
 Check the bearing internal clearance


Photo 1-1    Part : Inner ring of an angular contact ball bearing
Symptom : Flaking around half of the circumference of the raceway surface Cause : Poor lubrication due to entry of cutting coolant into bearing
    Part : Inner ring of an angular contact ball bearing
Symptom : Flaking diagonally along raceway
Cause : Poor alignment between shaft and housing during mounting
    Part : Inner ring of deep groove ball bearing
Symptom : Flaking of raceway at ball pitch
Cause : Dents due to shock load during mounting
    Part : Inner ring of an angular contact ball bearing
Symptom : Flaking of raceway at ball pitch
Cause : Dents due to shock load while stationary
    Part : Outer ring of Photo 1-4
Symptom : Flaking of raceway surface at ball pitch
Cause : Dents due to shock load while stationary
    Part : Balls of Photo 1-4
Symptom : Flaking of ball surface
Cause : Dents due to shock load while stationary
    Part : Inner ring of a spherical roller bearing
Symptom : Flaking of only one raceway over its entire circumference
Cause : Excessive axial load
    Part : Outer ring of Photo 1-7
Symptom : Flaking of only one raceway over its entire circumference
Cause : Excessive axial load

Photo 1-9    Part : Inner ring of a spherical roller bearing
Symptom : Flaking of only one row of raceway
Cause : Poor lubrication

Photo 1-10    Part : Rollers of a cylindrical roller bearing
Symptom : Premature Flaking occurs axially on the rolling surfaces
Cause : Scratches caused during improper mounting
2. Peeling (R6)
Damage Condition    Possible Causes    Countermeasures
Dull or cloudy spots appear on surface along with light Wear. From such dull spots, tiny Cracks are generated downward to a depth of 5-10 µm. Small particles fall off and minor Flaking occurs widely.    Unsuitable lubricant
Entry of debris into lubrication
Rough surface due to poor lubrication
Surface roughness of mating rolling parts    Select a proper lubricant
Improve the sealing mechanism
Improve the surface finish of the rolling mating parts

    Part : Inner ring of a spherical roller bearing
Symptom : Rounded areas of Peeling
Cause : Poor lubrication
    Part : Enlargement of Photo 2-1
    Part : Convex rollers of Photo 2-1
Symptom : Rounded areas of Peeling on the center of the rolling surfaces
Cause : Poor lubrication
    Part : Outer ring of a spherical roller bearing
Symptom : Peeling occurs near the shoulder of the raceway over the entire circumference
Cause : Poor lubrication

 3. Scoring (R7)
Damage Condition    Possible Causes    Countermeasures
Scoring is surface damage due to accumulated small seizures caused by sliding under improper lubrication or severe operating conditions. Linear damage appears circumferentially on the raceway and roller surfaces. Cycloidal shaped damage on the roller ends and Scoring on the rib surface contacting roller ends also occur.    Excessive load, excessive preload
Poor lubrication
Particles are caught in the surface
Inclination of inner and outer rings
Shaft bending
Poor precision of the shaft and housing    Check the size of the load
Adjust the preload
Improve the lubricant and the lubrication method
Check the precision of the shaft and housing
    Part : Inner ring of a spherical roller bearing
Symptom : Scoring on large rib face of inner ring
Cause : Roller slippage due to sudden acceleration and deceleration
    Part : Inner ring of a tapered roller thrust bearing
Symptom : Scoring on the face of inner ring rib
Cause : Worn particles mixed with lubricant, and breakdown of oil film due to excessive load
    Part : Inner ring of a spherical thrust roller bearing
Symptom : Scoring on the rib face of inner ring
Cause : Debris caught in surface, and excessive axial load
    Part : Cage of a deep groove ball bearing
Symptom : Scoring on the pressed-steel cage pockets
Cause : Entry of debris
    Part : Convex rollers of Photo 3-1
Symptom : Scoring on roller end faces
Cause : Roller slippage due to sudden acceleration and deceleration
    Part : Rollers of a double-row cylindrical roller bearing
Symptom : Scoring. on the roller end faces
Cause : Poor lubrication and excessive axial load
    Part : Convex rollers of Photo 3-5
Symptom : Scoring. on the roller end faces
Cause : Debris caught in surface, and excessive axial load

 4. Smearing (R8)
Damage Condition    Possible Causes    Countermeasures
Smearing is surface damage which occurs from a collection of small seizures between bearing components caused by oil film rupture and/or sliding.
Surface roughening occurs along with melting.    High speed and light load
Sudden acceleration/deceleration
Improper lubricant
Entry of water    Improve the preload
Improve the bearing clearance
Use a lubricant with good oil film formation ability
Improve the sealing mechanism
    Part : Inner ring of a cylindrical roller bearing
Symptom : Smearing around circumference of raceway surface
Cause : Roller slippage due to excessive grease quantity
    Part : Inner ring of a spherical roller bearing
Symptom : Smearing around circumference of raceway surface
Cause : Poor lubrication
    Part : Inner ring of a spherical roller bearing
Symptom : Partial Smearing around circumference of raceway surface
Cause : Poor lubrication
    Part : Convex rollers of Photo 4-5
Symptom : Smearing of rolling surfaces
Cause : Poor lubrication
    Part : Outer ring of Photo 4-1
Symptom : Smearing around circumference of raceway surface
Cause : Roller slippage due to excessive grease quantity
    Part : Outer ring of Photo 4-3
Symptom : Smearing around circumference of raceway surface
Cause : Poor lubrication
    Part : Outer ring of Photo 4-5
Symptom : Partial Smearing around circumference of raceway surface
Cause : Poor lubrication

 5. Fracture (R9)
Damage Condition    Possible Causes    Countermeasures
Fracture refers to small pieces which were broken off due to excessive load or shock load acting locally on a roller corner or rib of a raceway ring.    Impact during mounting
Excessive load
Poor handling such as dropping    Improve the mounting method (shrink fit, use of proper tools)
Reconsider the load conditions
Provide enough back-up and support for the bearing rib
    Part : Inner ring of a double-row cylindrical roller bearing
Symptom : Chipping of the center rib
Cause : Excessive load during mounting
    Part : Inner ring of a spherical thrust roller bearing
Symptom : Fracture of the large rib
Cause : Repeated shock load
    Part : Inner ring of a tapered roller bearing
Symptom : Fracture of the cone back face rib
Cause : Large shock during mounting
    Part : Outer ring of a solid type needle roller bearing
Symptom : Fracture of the outer ring rib
Cause : Roller inclination due to excessive loading (Needle rollers are long compared to their diameter. Under excessive or uneven loading, rollers become inclined and push against the ribs.)

 6. Cracks (R10)
Damage Condition    Possible Causes    Countermeasures
Cracks in the raceway ring and rolling elements. Continued use under this condition leads to larger Cracks or Fractures.    Excessive interference
Excessive load, shock load
Progression of Flaking
Heat generation and Fretting caused by contact between mounting parts and raceway ring
Heat generation due to Creep
Poor taper angle of tapered shaft
Poor cylindricality of shaft
Interference with bearing chamfer due to a large shaft corner radius    Correct the interference
Check the load conditions
Improve the mounting method
Use an appropriate shaft shape

    Part : Inner ring of a spherical roller bearing
Symptom : Rounded areas of Peeling
Cause : Poor lubrication
    Part : Outer ring of a double-row cylindrical roller bearing
Symptoms : Cracks propagated outward in the axial and circumferential directions from the Flaking origin on the raceway surface
Cause : Flaking from a flaw due to shock
    Part : Raceway surface of outer ring in Photo 6-4
Symptom : Outside surface crack propagating to the raceway
    Part : Cross section of cracked inner ring in Photo 6-6
Symptom : Origin is directly beneath the raceway surface
    Part : Outer ring of a double-row cylindrical roller bearing
Symptom : Thermal Cracks on the outer ring side face
Cause : Abnormal heat generation due to contact sliding between mating part and face of outer ring
    Part : Outer ring of a double-row cylindrical roller bearing used for outer ring rolling (Outer ring rotation)
Symptom : Cracks on outside surface
Cause : Flat Wear and heat generation due to non-rotation of the outer ring
    Part : Inner ring of a spherical roller bearing
Symptom : Axial Cracks on raceway surface
Cause : Large fitting stress due to temperature difference between shaft and inner ring
    Part : Roller of a spherical roller bearing
Symptom : Axial Cracks on rolling surface


 7. Cage Damage (R11)
Damage Condition    Possible Causes    Countermeasures
Cage damage includes:
Cage deformation, Fracture and Wear Fracture of cage pillars
Deformation of side face
Wear of pocket surface
Wear of guide surface    Poor mounting (Bearing misalignment)
Poor handling
Large moment load
Shock and large vibration
Excessive rotation speed, sudden acceleration and deceleration
Poor lubrication
Temperature rise    Check the mounting method
Check the temperature, rotation and load conditions
Reduce the vibration
Use an appropriate shaft shape Select a different cage type
Select a different lubrication method and/or lubricant

    Part : Cage of a deep groove ball bearing
Symptom : Fracture of pressed-steel cage pocket
    Part : Cage of an angular contact ball bearing
Symptom : Fracture of machined high-tension brass cage
    Part : Cage of an angular contact ball bearing
Symptom : Pressed-steel cage deformation
Cause : Shock load due to poor handling
    Part : Cage of a cylindrical roller bearing
Symptom : Deformation and Wear of machined high-tension brass cage
    Part : Cage of an angular contact ball bearing
Symptom : Pocket pillar Fractures of a cast iron machined cage
Cause : Abnormal load action on cage due to misaligned mounting between inner and outer rings
    Part : Cage of a tapered roller bearing
Symptom : Pillar Fractures of pressed-steel cage
    Part : Cage of a cylindrical roller bearing
Symptom : Deformation of the side face of machined high-tension brass cage
Cause : Large shock during mounting
    Part : Cage of an angular contact ball bearing
Symptom : Stepped Wear on the outside surface and pocket surface of machined high-tension brass cage

 8. Denting (R12)
Damage Condition    Possible Causes    Countermeasures
When debris such as small metallic particles are caught in the rolling contact zone, Denting occurs on the raceway surface or rolling element surface. Denting can occur at the rolling element pitch interval if there is a shock during the mounting (brinell dents).    Debris such as metallic particles are caught in the surface
Excessive load
Shock during transport or mounting    Clean the housing
Improve the sealing mechanism
Filter the lubrication oil
Improve the mounting and handling methods

    Part : Inner ring of a double-row tapered roller bearing
Symptom : Frosted raceway surface
Cause : Debris caught in the surface
    Part : Inner ring of a tapered roller bearing
Symptom : Small and large indentations occur over entire raceway surface
Cause : Debris caught in the surface
    Part : Outer ring of a double-row tapered roller bearing
Symptom : Indentations on raceway surface
Cause : Debris caught in the surface
    Part : Tapered rollers of Photo 8-3
Symptom : Small and large indentations occur over the rolling surface
Cause : Debris caught in the surface

9. Pitting (R13)
Damage Condition    Possible Causes    Countermeasures
Pitting has a dull luster and appears on the rolling element surface or raceway surface.    Debris becomes caught in the lubricant
Exposure to moisture in Poor lubrication    Improve the sealing mechanism
Filter the lubrication oil thoroughly
Use a proper lubricant


Photo 9-1    Part : Outer ring of a slewing bearing
Symptom : Pitting on the raceway surface
Cause : Rust

Photo 9-2    Part : Ball of Photo 9-1
Symptom : Pitting on the rolling element surface

10. Wear (R14)
Damage Condition    Possible Causes    Countermeasures
Wear is surface deterioration due to sliding friction at the surface of the raceway, rolling elements, roller end faces, rib face, cage pockets, etc.     Entry of debris
Progression from rust and electrical corrosion
Poor lubrication
Sliding due to irregular motion of rolling elements    Improve the sealing mechanism
Clean the housing
Filter the lubrication oil thoroughly
Check the lubricant and lubrication method
Prevent misalignment


Photo 10-1    Part : Inner ring of a cylindrical roller bearing
Symptom : Many pits occur due to electrical corrosion, and wave-shaped Wear on raceway surface
Cause : Electrical corrosion

Photo 10-3    Part : Inner ring of a double-row tapered roller bearing
Symptom : Fretting Wear of raceway and stepped Wear on the rib face
Cause : Fretting progression due to excessive load while stationary

Photo 10-2    Part : Outer ring of a spherical roller bearing
Symptom : Wear with a wavy or concave-convex texture on loaded side of raceway surface
Cause : Entry of debris under repeated vibration while stationary

Photo 10-3    Part : Tapered rollers of Photo 10-3
Symptom : Stepped Wear on the roller head end faces
Cause : Fretting progression due to excessive load while stationary

11. Fretting (R15)
Damage Condition    Possible Causes    Countermeasures
Wear occurs due to repeated sliding between the two surfaces.
Fretting occurs at fitting surface and also at contact area between raceway ring and rolling elements.
Fretting corrosion is another term used to describe the reddish brown or black worn particles.    Poor lubrication
Vibration with a small amplitude
Insufficient interference    Use a proper lubricant
Apply a preload
Check the interference fit
Apply a film of lubricant to the fitting surface


Photo 11-1    Part : Inner ring of a deep groove ball bearing
Symptom : Fretting on the bore surface
Cause : Vibration

Photo 11-3    Part : Outer ring of a double-row cylindrical roller bearing
Symptom : Fretting on the raceway surface at roller pitch intervals

Photo 11-2    Part : Inner ring of an angular contact ball bearing
Symptom : Fretting over entire circumference of bore surface
Cause : Insufficient interference fit

12. False Brinelling (R16)
Damage Condition    Possible Causes    Countermeasures
Among the different types of Fretting, false brinelling is the occurrence of hollow spots that resemble brinell dents and are due to Wear caused by vibration and swaying at the contact points between the rolling elements and raceway.    Oscillation and vibration of a stationary bearing during such times as transporting
Oscillating motion with a small amplitude
Poor lubrication    Secure the shaft and housing during transporting
Transport with the inner and outer rings packed separately
Reduce the vibration by preloading
Use a proper lubricant


Photo 12-1    Part : Inner ring of a deep groove ball bearing
Symptom : False brinelling on the raceway
Cause : Vibration from an external source while stationary

Photo 12-3    Part : Outer ring of a thrust ball bearing
Symptom : False brinelling of raceway surface at ball pitch
Cause : Repeated vibration with a small oscillating angle

Photo 12-2    Part : Outer ring of Photo 12-1
Symptom : False brinelling on the raceway
Cause : Vibration from an external source while stationaryt

Photo 12-4    Part : Rollers of a cylindrical roller bearing
Symptom : False brinelling on rolling surface
Cause : Vibration from an external source while stationary


13. Creep (R17)
Damage Condition    Possible Causes    Countermeasures
Creep is the phenomenon in bearings where relative slippage occurs between fitting surfaces and thereby creates a clearance between them surface. Creep causes a shiny appearance, occasionally with Scoring or Wear.    Insufficient interference or loose fit
Insufficient sleeve tightening    Check the interference, and prevent rotation
Correct the sleeve tightening
Study the shaft and housing precision
Preload in the axial direction
Tighten the raceway ring side face
Apply adhesive to the fitting surface
Apply a film of lubricant to the fitting surface


Photo 13-1    Part : Inner ring of a spherical roller bearing
Symptom : Creep accompanied by Scoring of bore surface
Cause : Insufficient interference

Photo 13-2    Part : Outer ring of a spherical roller bearing
Symptom : Creep over entire circumference of outside surface
Cause : Loose fit between outer ring and housing

 14. Seizure (R18)
Damage Condition    Possible Causes    Countermeasures
When sudden overheating occurs during rotation, the bearing becomes discolored. Then, the raceway rings, rolling elements, and cage will soften, melt and deform as damage accumulates.    Poor lubrication
Excessive load (Excessive preload)
Excessive rotational speed
Excessively small internal clearance
Entry of water and debris
Poor precision of shaft and housing Excessive shaft bending    Study the lubricant and lubrication method
Reinvestigate the suitability of the bearing type selected
Study the preload, bearing clearance, and fitting
Improve the sealing mechanism
Check the precision of the shaft and housing
Improve the mounting method


Photo 14-1    Part : Inner ring of a spherical roller bearing
Symptom : Raceway is discolored and melted. Worn particles from the cage were rolled and attached to the raceway.
Cause : Insufficient lubrication

Photo 14-3    Part : Inner ring of an angular contact ball bearing
Symptom : Raceway Discoloration, melting at ball pitch intervals
Cause : Excessive preload

Photo 14-3    Part : Balls and cage of Photo 14-3
Symptom : Cage is damaged by melting, balls discolored and melted
Cause : Excessive preload

Photo 14-2    Part : Convex rollers of Photo 14-1
Symptom : Discoloration and melting of roller rolling surface, adhesion of worn particles from cage
Cause : Insufficient lubrication

Photo 14-4    Part : Outer ring in Photo 14-3
Symptom : Raceway Discoloration, melting at ball pitch intervals
Cause : Excessive preload

15. Electrical Corrosion (R19)
Damage Condition    Possible Causes    Countermeasures
When electric current passes through a bearing, arcing and burning occur through the thin oil film at points of contact between the raceway and rolling elements. The points of contact are melted locally to form "fluting" or groove-like corrugations which can be seen by the naked eye.
Magnification of these grooves reveals crater-like depressions which indicate melting by arcing.    Electric current passing through a bearing    Design electric circuits which prevent current flow through the bearings
Insulate the bearing


Photo 15-1    Part : Inner ring of a tapered roller bearing
Symptom : Striped pattern of corrosion occurs on the raceway surface

Photo 15-3    Part : Inner ring of a cylindrical roller bearing
Symptom : Belt pattern of electrical corrosion accompanied by pits on the raceway surface

Photo 15-2    Part : Tapered rollers in Photo 15-1
Symptom : Striped pattern of corrosion occurs on the rolling surface

Photo 15-4    Part : Balls of a deep groove ball bearing
Symptom : Dark color covering the entire ball surfaces
16. Rust and Corrosion (R20)
Damage Condition    Possible Causes    Countermeasures
Bearing rust and corrosion are pits on the surface of rings and rolling elements and may occur at the rolling element pitch on the rings or over the entire bearing surfaces.    Entry of corrosive gas or water
Improper lubricant
Formation of water droplets due to condensation of moisture
High temperature and high humidity while stationary
Poor rust-preventive treatment during transporting
Improper storage conditions
Improper handling    Improve the sealing mechanism
Study the lubrication method
Anti-rust treatment during periods of non-running
Improve the storage methods
Improve the handling method


Photo 16-1    Part : Outer ring of a cylindrical roller bearing
Symptom : Rust on the rib face and raceway surface
Cause : Water entry

Photo 16-3    Part : Inner ring of a spherical roller bearing
Symptom : Rust on raceway surface at roller pitch intervals
Cause : Entry of water into lubricant

Photo 16-2    Part : Outer ring of a slewing ring
Symptom : Rust on raceway surface at ball pitch intervals
Cause : Moisture condensation during stationary periods

Photo 16-4    Part : Rollers of a spherical roller bearing
Symptom : Pit-shaped rust on rolling contact surface. Corroded portions.
Cause : Moisture condensation during storage

17. Mounting Flaws (R21)
Damage Condition    Possible Causes    Countermeasures
Straight line scratches on surface of raceways or rolling elements caused during mounting or dismounting of bearing.    Inclination of inner and outer rings during mounting or dismounting
Shock load during mounting or dismounting    Use appropriate jig and tools
Avoid shock loads by using a press machine
Center the relative mating parts during mounting


Photo 17-1    Part : Inner ring of a cylindrical roller bearing
Symptom : Axial scratches on raceway surface
Cause : Inclination of inner and outer rings during mounting

Photo 17-3    Part : Rollers of a cylindrical roller bearing
Symptom : Axial scratches on rolling surface
Cause : Inclination of inner and outer rings during mounting

Photo 17-2    Part : Outer ring of a double-row cylindrical roller bearing
Symptom : Axial scratches at roller pitch intervals on raceway surface
Cause : Inclination of inner and outer rings during mounting


 18. Discoloration (R22)
Damage Condition    Possible Causes    Countermeasures
Discoloration of cages, rolling elements and raceway rings occurs due to their reacting with lubricant at high temperature    Poor lubrication
Oil stain due to a reaction with lubricant
High temperature    Improve the lubrication method


Photo 18-1    Part: Inner ring of an angular contact ball bearing
Symptom: Bluish or purplish Discoloration on raceway surface
Cause: Heat generation due to poor lubrication

Photo 18-2    Part : Inner ring of a 4-point contact ball bearing
Symptom : Bluish or purplish Discoloration on raceway surface
Cause : Heat generation due to poor lubrication

 Bearing Fits (26)
1. Interference
For rolling bearings the bearing rings are fixed on the shaft or in the housing so that slip or movement does not occur between the mated surface during operation or under load. This relative movement (sometimes called creep) between the fitted surfaces of the bearing and the shaft or housing can occur in a radial direction, or in an axial direction, or in the direction of rotation. This creeping movement under load causes damage to the bearing rings, shaft or housing in the form of abrasive wear, fretting corrosion or friction crack. This, in turn, can also lead to abrasive particles getting into the bearing, which can cause vibration, excessive heat, and lowered rotational efficiency. To ensure that slip does not occur between the fitted surfaces of the bearing rings and the shaft or housing, the bearing is usually installed with an interference fit. The most effective interference fit is called a tight fit (or shrink fit). The advantage of this “tight fit” for thin walled bearings is that it provides uniform load support over the entire ring circumference without any loss in load carrying capacity. However, with a tight interference fit, ease of mounting and dismounting the bearings is lost; and when using a non-separable bearing as a non-fixing bearing, axial displacement is impossible.
2. Calculation of interference
1) Load and interference
The minimum required amount of interference for inner rings mounted on solid shafts when acted on by radial loads, is found by formula (7.1) and (7.2).
2) Temperature rise and interference
To prevent loosening of the inner ring on steel shafts due to temperature increases (difference between bearing temperature and ambient temperature) caused by bearing rotation, an interference fit must be given. The required amount of interference can be found by formula (7.3).

3) Effective interference and apparent interference
The effective interference (the actual interference after fitting) is different from the apparent interference derived from the dimensions measured value. This difference is due to the roughness or slight variations of the mating surfaces, and this slight flattening of the uneven surfaces at the time of fitting is taken into consideration. The relation between the effective and apparent interference, which varies according to the finish given to the mating surfaces, is expressed by formula (7.4).

4) Maximum interference
When bearing rings are installed with interference fit on shafts or housings, the tension or compression stress may occur. If the interference is too large, it may cause damage to the bearing rings and reduce the fatigue life of the bearing. For these reasons, the maximum amount of interference should be less than 1/1000 of the shaft diameter, or outside diameter.
3. Fit selection
Selection of the proper fit is generally based on the following factors: 1) the direction and nature of the bearing load, 2) whether the inner ring or outer ring rotates, 3) whether the load on the inner or outer ring rotates or not, 4) whether there is static load or direction indeterminate load or not. For bearings under rotating loads or direction indeterminate loads, a tight fit is recommended; but for static loads, a transition fit or loose fit should be sufficient (see Table 2).
The interference should be tighter for heavy bearing loads or vibration and shock load conditions. Also, a tighter than normal fit should be given when the bearing is installed on hollow shafts or in housings with thin walls, or housings made of light allows or plastic.
In applications where high rotational accuracy must be maintained, high precision bearings and high tolerance shafts and housings should be employed instead of a tighter interference fit to ensure bearing stability. High interference fits should be avoided if possible as they cause shaft or housing deformities to be induced into the bearing rings, and thus reduce bearing rotational accuracy.
Because mounting and dismounting become very difficult when both the inner ring and outer ring of a non-separable bearing (for example a deep groove ball bearing) are given tight interference fits, one or the other rings should be given a loose fit.
Lubrication (R25)
Lubrication of rolling bearings
The purpose of bearing lubrication is to prevent direct metallic contact between the various rolling and sliding elements. This is accomplished through the formation of a thin oil (or grease) film on the contact surfaces. However, for rolling bearings, lubrication has the following advantages.
(1) Friction and wear reduction
(2) Friction heat dissipation
(3) Prolonged bearing life
(4) Prevention of rust
(5) Protection against harmful elements
In order to achieve the above effects, the most effective lubrication method for the operating conditions must be selected. Also, a good quality, reliable lubricant must be selected. In addition, an effectively designed sealing system prevents the intrusion of damaging elements (dust, water, etc.) into the bearing interior, removes dust and other impurities from the lubricant, and prevents the lubricant from leaking from the bearing.
Almost all rolling bearings use either grease or oil lubrication methods, but in some special applications, a solid lubricant such as molybdenum disulfide or graphite may be used.
Grease lubrication
Grease type lubricants are relatively easy to handle and require only the simplest sealing devices—for these reasons, grease is the most widely used lubricant for rolling bearings.
Type and characteristics of grease
Lubricating grease is composed of either a mineral oil base or a synthetic oil base. To this base a thickener and other additives are added. The properties of all greases are mainly determined by the kind of base oil used by the combination of thickening agent and various additives.
Standard greases and their characteristics are listed in (Table 3). As performance characteristics of even the same type of grease will vary widely from brand to brand, it is best to check the manufacturers’ data when selecting grease.
 Replenishment
As the lubricating efficiency of grease declines with the passage of time, fresh grease must be re-supplied at proper intervals.
The replenishment time interval depends on the type of bearing, dimensions, bearing’s rotating speed, bearing temperature, and type of grease.  An easy reference chart for calculating grease replenishment intervals is shown in Fig. 1
This chart indicates the replenishment interval for standard rolling bearing grease when used under normal operating conditions. As operating temperatures increase, the grease re-supply interval should be shortened accordingly. Generally, for every 10°C increase in bearing temperature above 80°C, the relubrication period is reduced by exponent “1/1.5”.
(Example)
Find the grease relubrication time limit for deep groove ball bearing 6206, with a radial load of 2.0 kN operating at 3,600 r/min.
Cr/Pr=19.5/2.0 kN=9.8, from Fig. 2 the adjusted load, fL, is
0.96.
From the bearing tables, the allowable speed for bearing 6206 is 11,000 r/min and the numbers of revolutions permissible at a radial load of 2.0 KN are
therefore,

Using the chart in Fig. 1, find the point corresponding to bore diameter d=30 (from bearing table) on the vertical line for radial ball bearings. Draw a straight horizontal line to vertical line I. Then, draw a straight line from that point (A in example) to the point on line II which corresponds to the no/n value (2.93 in example). The point, C, where this line intersects vertical line III indicates the relubrication interval h. In this case the life of the grease is approximately 5,500 hours.
 Fig. 1 Diagram for relubrication interval of greasing


Fig. 2 Value of adjustment factor fL depends on bearing load (R27)
 Oil lubrication
Generally, oil lubrication is better suited for high speed and high temperature applications than grease lubrication. Oil lubrication is especially effective for those application requiring the bearing generated heat (or heat applied to the bearing from other sources) to be carried away from the bearing and dissipated to the outside.
Lubricating oil
Under normal operating conditions, spindle oil, machine oil, turbine oil and other minerals are widely used for the lubrication of rolling bearings. However, for temperatures above 150°C or below –30°C, synthetic oils such as diester, silicone and fluorosilicone are used.
For lubricating oils, viscosity of the oil is one of the most important properties and determines the oil’s lubricating efficiency. If the viscosity is too low, the oil film will not be sufficiently formed, and it will damage the load carrying surface of the bearing. On the other hand, if the viscosity is too high, the viscosity resistance will also be high and cause temperature increases and friction loss. In general, for higher speed, lower viscosity oil should be used, and for heavy loads, higher viscosity oil should be used.
In regard to operating temperature and bearing lubrication, Table 4 lists the minimum required viscosity for various bearings. Fig. 3 is a lubricating oil viscosity-temperature comparison chart is used in the selection of lubricating oil.
It shows which oil would have the appropriate viscosity at a given temperature. For lubricating oil viscosity selection standards relating to bearing operating conditions, see Table 5.

Table 4 Minimum viscosity of lubricating oil for bearings
 
Fig. 3 Relation between viscosity and temperature


Table 5 Selection standards for lubricating oils
Oil quality
In forced oil lubrication systems, the heat radiated away by housing and surrounding parts plus the heat carried away by the lubricating oil is approximately equal to the amount of heat generated by the bearing and other sources.
For standard housing applications, the quantity of oil required can be found by formula (11.1).
where,
Q : Quantity of oil for one bearing cm3/min
K : Allowable oil temperature rise factor (Table 6)
q : Minimum oil quantity cm3/min (From chart)
Because the amount of heat radiated will vary according to the shape of the housing, for actual operation it is advisable that the quantity of oil calculated by formula (11.1) be multiplied by a factor of 1.5 to 2.0. Then, the amount of oil can be adjusted to correspond to the actual machine operating conditions. If it is assumed for calculation purposes that no heat is radiated by the housing and that all bearing heat is carried away by the oil, then the value for shaft diameter, d, (second vertical line from right in Fig. 4) becomes zero, regardless of the actual shaft diameter.

Table 6 Factor K

(Example)
For tapered roller bearing 30220U mounted on a flywheel shaft with a radial load of 9.5 kN, operating at 1,800 rpm; what is the amount of lubricating oil required to keep the bearing temperature rise below 15°C?
d=100 mm, dn=100´1,800=18´104 mm r/min from Fig. (4), q=180 cm3/min.
Assume the bearing temperature is approximately equal to the outlet oil temperature, from Table (6), since K=1, Q=1´180=180 cm3/min.

Fig. 4 Guidance for oil quantity


Sealing Devices (R24)
Bearing seals have two main functions: 1) to prevent lubricant from leaking out and 2) to prevent dust, water and other contaminants from entering the bearing. When selecting a seal the following factors need to be taken into consideration: the type of lubricant (oil or grease), seal sliding speed, shaft fitting errors, space limitations, seal friction and resultant heat, and cost.
Sealing devices for rolling bearings fall into two main classifications: contact and non-contact types.
Non-contact seals
Non-contact seals utilize a small clearance between the seal and the sealing surface; therefore, there is no wear, and friction is negligible.
Consequently, very little frictional heat is generated making non-contact seals very suitable for high speed applications. As shown in   (Fig. 5), non-contact seals can have the simplest of designs. With its small radial clearance, these types of seal are best suited for grease lubrication, and for use in dry, relatively dust free environments.
When several concentric oil grooves (Fig. 6) are provided on the shaft or housing, the sealing effect can be greatly improved. If grease is filled in the grooves, the intrusion of dust, etc. can be prevented.
For oil lubrication, if helical concentric oil grooves are provided in the direction opposite to the shaft rotation (horizontal shafts only), lubricating oil that flows out along the shaft can be returned to the inside of the housing (see Fig. 7). The same sealing effect can be achieved by providing helical grooves on the circumference of the shaft.
Labyrinth seals employ a multistage labyrinth design which elongates the passage, thus improving the sealing effectiveness. Labyrinth seals are used mainly for grease lubrication, and if grease is filled in the labyrinth, protection efficiency (or capacity) against the entrance of dust and water into the bearing can be enhanced.
The axial labyrinth passage seal shown in (Fig. 8) is used on one-piece housings and the radial seal shown in (Fig. 9) is for use with split housings.
In applications where the shaft is set inclined, the labyrinth passage is slanted so as to prevent contact between the shaft and housing projections of the seal (Fig. 10).
Contact seals
Contact seals accomplish their sealing action through the constant pressure of a resilient part of the seal on the sealing surface. Contact seals are generally far superior to non-contact seals in sealing efficiency, although their friction torque and temperature rise coefficients are somewhat higher.
The simplest of all contact seals are felt seals. Used primarily for grease lubrication (Fig. 11), felt seals work very well for keeping out fine dust, but are subject to oil permeation and leakage to some extent. Therefore, the Z type rubber seal shown in (Fig. 12) and GS type shown in (Fig. 13), have been used more widely.