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Koyo Technical Information

12. Bearing failures and countermeasures

• Flaking
• Cracking, Chipping
• Brinelling, Nicks
• Pear skin, Discoloration
• Scratch, Scuffing
• Smearing
• Rust, Corrosion
• Electric pitting
• Wear
• Fretting
• Creeping
• Damage to Retainer
• Seizure

When a bearing is used under ideal conditions, it will only be damaged by rolling fatigue. Damage due to rolling fatigue may occur very early, however.

If damage is found on a bearing, it is important to examine the phenomenon properly and identify the causes, even if the damage is very small. In addition, it is essential to examine it comprehensively, checking not only the bearing but also the shaft, housing and lubricant.

To identify the cause of the bearing damage, it is required to have sufficient knowledge and expertise regarding bearings and lubrication, to be familiar with the characteristics of the application machine, and to be well informed of the conditions under which the bearing is installed and of the past records of the machine operation.

a. Flaking

Flaking is a phenomenon when material is removed in flakes from a surface layer of the bearing raceways or rolling elements due to rolling fatigue. This phenomenon is generally attributed to the approaching end of bearing service life. However, if flaking occurs at early stages of bearing service life, it is necessary to determine causes and adopt countermeasures.

Pitting is another type of failure caused by rolling fatigue, in which minute holes of approx.0.1 mm in depth are generated on the raceway surface.

Damages:

1) Flaking occurring at an incipient stage

2) Flaking on one side of radial bearing raceway

3) Symmetrical flaking along circumference of raceway

4) Slanted flaking on the radial ball bearing raceway

5) Flaking occurring near the edge of the raceway or rolling contact surface of roller bearings

6) Flaking on the raceway surface at the same interval as rolling element spacing

b. Cracking

Damages:

1) Cracking in outer ring or inner ring

2) Cracking on rolling elements

3) Cracking on the rib

c. Brinelling, nicks

Brinelling is a small surface indentation generated either on the raceway through plastic deformation at the contact point between the raceway and rolling elements, or on the rolling surfaces from insertion of foreign matter, when heavy load is applied while the bearing is stationary or rotating at a low rotation speed.

Nicks are those indentations produced directly by rough handling such as hammering.

Damages:

1) Brinelling on the raceway or rolling contact surface

2) Brinelling on the raceway surface at the same interval as the rolling element spacing

3) Nicks on the raceway or rolling contact surface

d. Pear skin, discoloration

Pear skin is a phenomenon in which minute brinell marks cover the entire rolling surface, caused by the insertion of contaminants. This is characterized by loss of luster and a rolling surface that is rough in appearance.
In extreme cases, this is accompanied by discoloration due to heat generation.

Discoloration is a phenomenon in which the surface color changes because of staining or heat generation during rotation.
Color change caused by rust and corrosion is generally separate from this phenomenon.

Damages:

1) Indentation similar to pear skin on the raceway and rolling contact surface.

2) Discoloration of the raceway, surface rolling contact surface, rib face, and cage riding land.

e. Scratch, scuffing

Scratches are relatively shallow marks generated by sliding contact, in the same direction as the sliding. This is not accompanied by apparent melting of material.

Scuffing refers to marks, the surfaces of which are partially melted due to higher contact pressure and therefore a greater heat effect. Generally, scuffing may be regarded as a serious case of scratches.

Damages:

1) Scratches on raceway or rolling contact surface

2) Scuffing on rib face and roller end face

f. Smearing

Smearing is a phenomenon in which clusters of minute seizures cover the rolling contact surface. Since smearing is caused by high temperature due to friction, the surface of the material usually melts partially; and the smeared surfaces appear very rough in many cases.

Damages:

1) Smearing on raceway or rolling contact surface

g. Rust, corrosion

Rust is a film of oxides, or hydroxides, or carbonates formed on a metal surface due to chemical reaction.

Corrosion is a phenomenon in which a metal surface is eroded by acid or alkali solutions through chemical reaction (electrochemical
reaction such as chemical combination and battery formation); resulting in oxidation or dissolution. It often occurs when sulfur or chloride contained in the lubricant additives is dissolved at high temperature.

Damages:

1) Rust partially or completely covering the bearing surface.

2) Rust and corrosion at the same interval as rolling element spacing

h. Electric pitting

When an electric current passes through a bearing while in operation, it can generate sparks between the raceway and rolling elements through a very thin oil film, resulting in melting of the surface metal in this area. This phenomenon appears to be pitting at first sight. (The resultant flaw is referred to as a pit.) When the pit is magnified, it appears as a hole like a crater, indicating that the material melted when it was sparking. In some cases, the rolling surface becomes corrugated by pitting.

Damages:

1) Pitting or a corrugated surface failure on raceway and rolling contact surface
The bearings must be replaced, if the corrugated texture is found by scratching the surface with a fingernail or if pitting can be observed by visual inspection.

i. Wear

Normally, wear of bearing is observed on sliding contact surfaces such as roller end faces and rib faces, cage pockets, the guide surface of cages and cage riding lands. Wear is not directly related to material fatigue. Wear caused by foreign matter and corrosion can affect not only sliding surfaces but also rolling surfaces.

Damages:

1) Wear on the contact surfaces (roller end faces, rib faces, cage pockets)

2) Wear on raceways and rolling contact surfaces

j. Fretting

Fretting occurs to bearings which are subject to vibration while in stationary condition or which are exposed to minute vibration. It is characterized by rust-colored wear particles. Since fretting on the raceways often appears similar to brinelling, it is sometimes called “false brinelling”.

Damages:

1) Rust-colored wear particles generated on the fitting surface (fretting corrosion)

2) Brinelling on the raceway surface at the same interval as rolling element spacing (false brinelling)

k. Creeping

Creeping is a phenomenon in which bearing rings move relative to the shaft or housing during operation.

Damages:

1) Wear, discoloration and scuffing, caused by slipping on the fitting surfaces

l. Damage to cage

Since cages are made of low hardness materials, external pressure and contact with other parts can easily produce flaws and distortion. In some causes, these are aggravated and become chipping and cracks. Large chipping and cracks are often accompanied by deformation, which may reduce the accuracy of the cage itself and may hinder the smooth movement of rolling elements.

Damages:

1) Flaws, distortion, chipping, cracking and excessive wear in cages. Loose or damaged rivets.

m. Seizure

A phenomenon caused by abnormal heating in bearings.

Damages:

1) Discoloration, distortion and melting together