Mariners Repository: Ball Bearing Failures

Bearing service damage terminology used

Abrasive Wear

Surface wear resulting from the lapping action of abrasive contaminants. The affected areas usually appear frosty gray, they may be highly polished if the abrasive par-ticles are extremely small.

Brinelling

Permanent deformation of the bearing surfaces where the rollers (or balls) contact the races. Brinelling results from excessive load or impact on stationary bearings. It is a form of mechanical damage in which metal is displaced or upset.

Corrosion

Rust (corrosion) is the chemical attack of the bearing metal, almost always consisting of, or accompanied by, oxidation. Covering either a large or a small area, or limited to a well-defined line, such as the line of contact between a roller and a race in a stationary bearing.

False Brinelling

False Brinelling is the result of a combination of mechanical and chemical action, occuring when a small relative motion or vibration is accompanied by some loading, in the presence of oxygen .

False Brinelling is characterized by wear and tear of the steel, and the load on the bearing far less than that required to produce the resulting impression.

Fretting, Fretting Corrosion, Friction Oxidation

service damage occurring under the same conditions as False Brinelling , with evidence of oxidation of iron(red and black oxides). Fretting occurs at the inner or outer race associated with some relative movement.

Heat Checks

Surface cracks resulting from heat generated by sliding con-tact with another part. Heat checks are oriented normal to the direction of motion. Depending on the stresses present in the component, the cracks may remain small or become nuclei for a complete fracture.

Indentations

The surface depressions caused by debris or foreign material.

Pickup

The welding and transfer of metal between rollers and race-ways during bearing operation. Pickup results from in ad-equate lubrication.

Pitting

Small, roughly circular holes or craters resulting from corro-sion, mechanical damage, or the passage of an electric current. Because these three types of pits have distinctly different causes, the word pitting should always be qualified.

Scoring, Scuffing, Seizing, Smearing

Transfer of metal from one component of a bearing to another under sliding contact. This process, which is also called galling, is caused by lack of adequate lubrication under extreme unit pressure.

Spalling

Breaking aaway of metal from the raceway or rolling element in flakes or scale-like particles. Also called flaking.

1.Excessive Loads

• Excessive loads usually cause premature fatigue. Tight fits, brinelling and improper preloading can also bring about early fatigue failure.

• The solution is to reduce the load or redesign using a bearing with greater capacity.

2.Overheating

• Symptoms are discoloration of the rings, balls, and cages from gold to blue.

• Temperature in excess of 200°C can anneal the ring and ball materials.

• The resulting loss in hardness reduces the bearing capacity causing early failure.

• In extreme cases, balls and rings will deform. The temperature rise can also degrade or destroy lubricant.

3.True Brinelling

• Brinelling occurs when loads exceed the elastic limit of the ring material.

• Brinell marks show as indentations in the raceways which increase bearing

vibration (noise).

• Any static overload or severe impact can cause brinelling.

Brinelling

close view

4.False Brinelling

• False brinelling - elliptical wear marks in an axial direction at each ball

position with a bright finish and sharp demarcation, often surrounded by a ring

of brown debris - indicates excessive external vibration.

• Corrected by isolating bearings from external vibration, and using greases

with antiwear additives

Close view

5.Normal Fatigue Failure

• Fatigue failure - usually referred to as spalling - is a fracture of the running surfaces and subsequent removal of small discrete particles of material.

• Spalling can occur on the inner ring, outer ring, or balls.

• This type of failure is progressive and once initiated will spread as a result of further operation. It will always be accompanied by a marked increase in vibration.

• The remedy is to replace the bearing or consider redesigning to use a bearing having a greater calculated fatigue life

Causes of Fatigue failure:

1. Bearing running through its life span under normal wear and tear.

2. Overloading of bearing , due to misalignment, tight fits of inner/ outer race.

Material failure due to poor quality

Squeezed outer ring

Normal bearing operation

6.Reverse Loading

• Angular contact bearings are designed to accept an axial load in one direction

only.

• When loaded in the opposite direction, the elliptical contact area on the outer

ring is truncated by the low shoulder on that side of the outer ring.

• The result is excessive stress and an increase in temperature, followed by

increased vibration and early failure.

• Corrective action is to simply install the bearing correctly

illustration

7.Contamination

• Contamination is one of the leading causes of bearing failure.

• Contamination symptoms are denting of the bearing raceways and balls

resulting in high vibration and wear.

• Clean work areas, tools, fixtures, and hands help reduce contamination

failures.

• Moisture / water ingress to be kept away , as EP additives are water soluble rust is formed as a result inside the bearing surfaces, further damaging them

Illustration

• Keep grinding operations away from bearing assembly areas andkeep bearings in their original packaging until you are

ready to install them

View of corrosive contamination

8.Lubricant Failure

• Discolored (blue/brown) ball tracks and balls are symptoms of lubricant failure. Excessive wear of balls, ring, and cages will follow, resulting in overheating and subsequent catastrophic failure.

• Ball bearings depend on the continuous presence of a very thin -millionths of an inch - film of lubricant between balls and races, and between the cage, bearing rings, and balls.

• Failures are typically caused by restricted lubricant flow or excessive temperatures that degrade the lubricant’s properties

9.Corrosion

• Red/brown areas on balls, race-way, cages, or bands of ball bearings are symptoms of corrosion.

• This condition results from exposing bearings to corrosive fluids or a corrosive atmosphere.

• In extreme cases, corrosion can initiate early fatigue failures.

• Correct by diverting corrosive fluids away from bearing areas and use integrally sealed bearings whenever possible.

10.Misalignment

• Misalignment can be detected on the raceway of the nonrotating ring by a ball wear path that is not parallel to the raceways edges.

• If misalignment exceeds 0.001 in./in you can expect an abnormal temperature rise in the bearing and/or housing and heavy wear in the cage ball-pockets.

• Appropriate corrective action includes: inspecting shafts and housings for runout of shoulders and bearing seats; use of single point-turned or ground threads on non hardened shafts and ground threads only on hardened shafts; and using precision grade locknuts

Shaft Misalignment Housing Misalignment

11.Tight Fits

• A heavy ball wear path in the bottom of the raceway around the entire circumference of the inner ring and outer ring indicates a tight fit.

• Where interference fits exceed the radial clearance at operating temperature, the balls will become excessively loaded. This will result in a rapid temperature rise accompanied by high torque.

• Continued operation can lead to rapid wear and fatigue.

• Corrective action includes a decrease in total interference

12.Loose Fits

• Loose fits can cause relative motion between mating parts. If the relative motion between mating parts is slight but continuous, fretting occurs.

• Fretting is the generation of fine metal particles which oxidize, leaving a distinctive brown color. This material is abrasive and will aggravate the looseness. If the looseness is enough to allow considerable movement of the inner or outer ring, the mounting surfaces (bore, outer diameters, faces) will wear and heat, causing noise and runout problems.

13. Electric Arcing

Cause

Static Electricity

Static electricity usually emanates from charged belts or from manufac-turing processes using calendar rolls (leather, rubber, cloth, paper). The current is carried from the belt to the pulley or sheave — from the sheave to the shaft —

through the shaft to the bearing — From

the bearing to the ground

Electric Leakage

Faulty wiring, inadequate or defective insulation, or loose rotor windings on an electric motor are all possible sources of current leakage. Either AC or DC currents will damage bearings.

Short Circuit

Wires which are crossed or contacted by a common conductor will cause a short circuit and may result in a passage of current through the bearing