Spalling can be an expensive and disruptive problem, so learning more about it is essential for any business counting on reliable bearing performance. It is a common and potentially serious bearing problem. By understanding the issues related to bearing failures due to spalling, better maintenance techniques can be implemented throughout industries faced with these very real problems. In this post, we’ll discuss what spalling is, why it happens, and provide suggestions for precautionary steps for dealing with bearings.
What is Spalling?
Spalling is a name for running surface fractures that develop as a result of surface or subsurface fatigue. Pieces or flakes of the material separate when the rolling parts pass over these fissures. So, you may actually know it as pitting or flaking of bearing material. You will mostly find spalling on the races and rolling elements. This damage is progressive and can be a sign that a bearing is at the end of its fatigue life.
A spall that develops at the surface usually starts as a crack at a surface flaw or debris dent. Then, it will spread into a network of cracks. This can also start at a stress riser and spread into a crack network.
The inner ring, outer ring, or balls of a bearing may all experience spalling. As a result of continued operation, this kind of failure spreads gradually after it has started. It will almost always be followed by a noticeable increase in vibration, which is a sign of abnormalities. Replacing the bearing or considering redesigning to use a bearing with a longer predicted fatigue life are two possible remedies.
Can You Prevent It?
Rolling element bearings eventually fail, even when they are functioning properly. Surface fatigue will begin as embryonic particles release from the surface of a racing or rolling element in the load zone after a varied amount of time of service. Craters caused by surface wear serve as stress concentration areas. As a result of the ongoing operation, the spalling will progress.
Surface distress, fatigue pitting, and fatigue spalling are the three main types of surface contact degradation that can happen under correct operational circumstances. You cannot prevent normal wear, but you can prevent other surface damage that is a result of poor mounting or poor operating conditions.
Spalling vs. Pitting
Oftentimes, spalling and pitting are used interchangeably, while others use them to denote various degrees of surface contact fatigue severity. Although pitting and spalling are both typical types of surface contact fatigue, spalling causes a faster decline in surface durability. Frequently, substantial secondary damage caused by spalling causes early failure. A significant spall on the contact surface, friction or heat-induced surface seizure, or total spalling over all of the contact surfaces can all cause roller or race breaking as a result of this secondary damage.
Various Types of Surface Fatigue
- Surface Distress – A smooth surface that has undergone plastic deformation in the asperity dimension. A thin work-hardened surface layer (usually less than 10 µm) is produced as a result of this plastic deformation.
- Pitting – At contact surfaces, pitting manifests as shallow craters that are no deeper than the work-hardened layer’s thickness (about 10 µm).
- Spalling – At contact surfaces, deeper voids that range in depth from 20 to 100 µm. There are currently no accepted classifications that separate spalling from pitting.
The Types of Spalling
Geometric Stress Concentration (GSC)
GSC spalling has common causes, including misalignment, deflection, and edge loading. This causes excessive stress to develop in certain areas of the bearing. GSC can happen as a result of an incorrect shaft or housing machining, or it can happen at the very margins of the race/roller paths.
Point Surface Origin (PSO)
Such damage is produced by localized stresses that are extremely high. Usually, etching, dents, nicks, debris, and hard-particle contamination in the bearing cause PSO spalling. This sort of spalling damage is the most prevalent and frequently manifests as arrowhead-shaped spalls that spread in the direction of rotation.
Inclusion Origin (IO)
A localized area of subsurface, non-metallic inclusions experiences bearing material fatigue after millions of load cycles, resulting in damage that takes the form of elliptically formed spalls. It is unlikely you will experience this form of spalling because bearing steel cleanliness has improved in recent years.
A Common Cause of Bearing Failure
Numerous factors, such as improper mounting, inadequate lubrication, and overloading, to name a few, can result in bearing degradation, and ultimately, failure. Visible characteristics, such as discoloration, wear marks, or pitting, on the rolling element and raceway surfaces are indicative of the mode of damage, or what occurred to the bearing as a result of adverse conditions. Although their causes and long-term effects might not be the same, various damage modes might look the same. Because they can offer further hints about the damage’s underlying causes, operating conditions are crucial to understanding when looking into bearing damage.