Investigating Bearing Failure—Spot The Clues & Uncover The Root Cause

By IBT Inc

December 16, 2018

Bearings

According to Schaeffler, 85% of bearing failures are preventable. Learn how to spot the clues to eliminate the problem from the source and maximize your bearing life.

Rolling bearings are at the heart of most machines and vehicles. When the bearing goes, the
machine or vehicle stops. However, the bearing is often the “victim” and not the root cause.
Therefore, It is important to investigate and perform a bearing failure analysis to find the root cause of the failure and prevent it from recurring.

In order to execute a proper investigation, though, you must know what clues to look for.

This investigation begins with a simple inspection of the bearing’s surfaces and raceways. During operation, changes to a bearing’s appearance will occur over time. These changes, called patterns, can provide considerable information about the operating conditions throughout the life of a bearing

Learn how to solve the mystery of bearing failure by tracing surface patterns back to root causes. »

 

Cracks + Spalling

Small cracks and subsequently micro-spalling are often the result of material fatigue. Material fatigue can occur on the surface or subsurface.

Photo Courtesy of Schaeffler

Subsurface-initiated fatigue is caused by cyclic stresses just underneath the raceway surface and ultimately results in the decay of the material. Cracks are initiated and propagate underneath the surface, and when they come to the surface, spalling occurs.

Surface-initiated fatigue, on the other hand, results from inadequate lubrication conditions. When contamination, incorrect viscosity or metal-to-metal contact occurs, the surface asperities (peaks) shear over each other and result in shear stresses at the surface. Initially, there might be a shiny surface, because the surface roughness is reduced, but as the process continues and the surface becomes dull and breaks up more and more.

 

Abnormally Dull or Shiny Surfaces

While wear is inevitable, there are two circumstances that may cause preventable wear to occur early on: abrasive and adhesive wear.

Abrasive wear is due to abrasive contamination particles in the lubricant. The abrasive particles wear out the surfaces of the raceways and rolling elements, typically resulting in dull surfaces. However, if the abrasive particles are very fine and hard, such as cement dust, a polishing effect might occur and mirror-like surfaces appear.

Adhesive wear typically occurs in contact surfaces subjected to light loads, poor lubrication conditions and speed differences. For example—the passage of a rolling element from the unloaded zone into the loaded zone. In an early stage of adhesive wear, the appearance is shiny surfaces, but quickly it turns into a dull surface with (more or less) smeared material.

 

Red/Brown Stains or Brown/Black Etching

Red/brown stains or brown/black etching in the rolling elements are more than likely the result of corrosion. There are two common causes for corrosion within a bearing—exposure to moisture or fretting.

Moisture corrosion occurs in the presence of water, corrosive liquids or moisture and can cause deep-seated rust and early bearing failure. In contrast to other damage processes, corrosion can happen fast and penetrate deep into the material. Corrosion often happens during standstills, revealing red/brown stains marks at rolling element distance.

Photo Courtesy of Schaeffler

Fretting corrosion typically occurs between two loaded surfaces, such as the bearing outside diameter and housing and/or between the bearing bore and shaft. Bearings don’t fail because of fretting, but it is an observable problem indicating inadequate fitting, shaft bending and/or imperfections in the contact surfaces. Brown/black etching marks are visible and can develop material growth and high stresses, potentially leading to premature subsurface fatigue and ring cracking.

 

Photo Courtesy of Schaeffler

Depressions in the Raceways

Raceway depressions are the product of various sources—mainly true or false brinelling, caused by impacts or vibrations, or overload, caused by static or shock loads.

True brinelling, or indentations in the raceways, are formed as a result of static overload or severe impact. Severe brinell marks can increase bearing vibration (noise) and cause premature fatigue failure.

Photo Courtesy of Schaeffler

In false brinelling, a combination of corrosion and wear occurs, creating elliptical wear marks in the raceways, often surrounded by a ring of brown. This indicates excessive external vibration.

Overload can be recognized by depressions at rolling element distance. Often, wrong mounting procedures are at the base of the problem, i.e., applying the mounting force to the wrong ring and thereby producing a shock load over the rolling elements.

 

Craters + Flutes

Photo Courtesy of Schaeffler

Craters and flutes, paired with discoloration, are telltale signs of electrical current damage.

Damage from excessive voltage takes place when an electric current passes through a bearing. As a result, the contact surfaces the material is heated to temperatures ranging from tempering to melting levels, which leads to the appearance of discolored areas, varying in size, where the material has been tempered, re-hardened or melted. Craters (varying approximately from 0.1 mm to 0.5 mm) are formed where the material has melted.

Damage from current leakage results from stray electric currents that pass through a bearing, often caused by frequency variations.

Photo Courtesy of Schaeffler

The main visual damage is discoloration and flutes, also described as a washboard pattern. These flutes have the shape of the contact ellipse in ball bearings and contact lines in roller bearings. Because the current passes through a larger area, the current intensity is smaller and the damaging temperature is lower than excessive voltage damage. Therefore the main visual damage is a tempering effect, i.e., a softening of the steel. However, looking at the damage with high magnification shows that there are craters present, as well.

 

Discoloration

If the raceways or rolling elements reveal changes in color, typically blue, the bearing could be overheating as a result of insufficient lubricant or a variety of other culprits.  

Overheating can result in gold or blue discoloration of the rings, balls and cages. Temperatures in excess of 400°F 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. Common culprits are heavy electrical heat loads, inadequate heat paths, and insufficient lubricant or cooling or when loads and speeds are excessive.

 

Abnormal Wear Path

An abnormal ball wear path can indicate misalignment. The maximum allowable misalignment varies greatly with different applications, decreasing, for example, with speed.

Misalignment can be detected on the raceways of the nonrotating ring by a ball wear path that is not parallel to the raceway edges. Other symptoms can include spalling, an unusual temperature rise or heavy wear in the cage ball-pockets. Common causes of misalignment are: bent shafts, burrs or dirt on the shaft or housing shoulders, shaft threads that are not square with shaft seats and locking nuts with faces that are not square to the thread axis.

 

Dents + Deformation

Dents and deformation are characterized by damage to the geometry of the raceway at the location of the dent and are oftentimes associated with a disruption of lubrication.

Indentation from debris results from foreign particles (contaminants) that have gained entry into the bearing and are rolled into the raceways by the rolling element. The size and shape of the dents depend on the nature of the particles.

Photo Courtesy of Schaeffler

 

Fracture

Fracture occurs when the ultimate tensile strength of the material is exceeded. There are various explanations to consider when a fracture is spotted in a bearing.

For example—fracture, as a result of force, can result from rough treatment or mishandling of a bearing. Any impact to the ring may cause the formation of fine cracks that results with pieces of the ring breaking off during operation. Always handle bearings with care because even the smallest dent can make it unserviceable. Forced fracture can also be a result of excessive drive-up on a tapered seat or sleeve, increasing tensile stresses in the rings and producing cracks or fractures during operation.

Fatigue fracture, on the other hand, can occur when a tight fit has been used, leading to high hoop stresses. What begins as a crack will then propagate, ultimately fracturing the whole ring or cage.

Finally, thermal fracture can occur when two surfaces slide heavily against each other. The frictional heat that is developed causes cracks, generally at right angles to the sliding direction.

 

85% of Bearing Failures Are Preventable —According to Schaeffler

According to Schaeffler, about 85% of bearing failures are preventable. In order to solve the mystery of the failure, an in-depth investigation is required and begins with decoding the physical evidence left on the bearing surface. This will, ultimately, reveal the bearing’s role as the victim, as well as uncover the larger “culprit” that is causing the failure. By taking the proper action, you can eliminate the issue from the source and minimize bearing failure going forward.

IBT is proud to partner with The Schaeffler Group, leading, worldwide automotive and industrial supplier, who stands for the highest quality, outstanding technology and distinctive innovative ability. In all aspects, Schaeffler strives for continuous improvement with the aim of achieving “zero defects“ by implementing measures to ensure defect prevention and the minimization of risk. Learn more about Schaeffler.

 

Do Your Bearings Require Further Investigation? We’re Here To Help

The Industrial Maintenance Technologies department uses state-of-the-art diagnostic equipment to analyze machine dynamics and performance, identifying misalignment and imbalance problems that cause vibration and excess wear and tear on rotating equipment.

To request a bearing failure analysis, or for additional questions regarding your bearings’ performance, our industrial maintenance experts are just a phone call away. Give us a call today at 913-677-3151!

 


Information and photos courtesy of

 

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