Brinelling is a type of surface damage that occurs in rolling element bearings when excessive force creates permanent indentations in the metal raceways. The term comes from the Brinell hardness test, which measures material hardness by pressing a ball into a surface and measuring the resulting dent. In bearings, brinelling produces similar-looking depressions where the rolling elements (balls or rollers) sit against the races. These indentations cause increased vibration, noise, and premature bearing failure. Engineers distinguish between two forms: true brinelling and false brinelling, which look similar but arise from completely different mechanisms.
How True Brinelling Happens
True brinelling is straightforward plastic deformation. When the contact stress between a rolling element and its raceway exceeds the material’s elastic limit, the metal permanently deforms, creating a dent. This can result from a single heavy impact, a static overload, or repeated overload events during operation. The bearing doesn’t need to be moving for this to happen.
Common real-world causes include mishandling during transport or installation, excessive preloading, and static overloads when equipment is idle. In wind turbines, for example, a locked rotor during maintenance or insufficient wind speed to turn the blades can leave the full weight of the rotor pressing down on a stationary bearing. If the bearing wasn’t designed for that load, the rolling elements indent the raceway. Shock loads during operation, like a sudden impact or jolt, can produce the same result even while the bearing is spinning.
One telltale sign of true brinelling is that the original machining marks on the raceway surface remain visible inside the indentation. The metal has been pushed down, not worn away, so those fine grooves from manufacturing survive despite the deformation. You may also see small bulges of displaced material at the edges of each dent.
How False Brinelling Differs
False brinelling produces depressions that look like true brinelling at first glance, but the mechanism is entirely different. It’s a wear process, not a deformation process. When a stationary or near-stationary bearing experiences tiny vibrations or oscillations, the rolling elements rock back and forth against the raceways by microscopic amounts. Over time, this repeated micro-movement wears shallow craters into the surface.
Because the damage comes from friction rather than force, false brinelling marks have a distinctly polished, shiny appearance. The original machining marks are rubbed away completely. The shape of the depressions is also less precise than the clean indentations of true brinelling. In their uncleaned state, false brinelling marks often show dark discoloration, ranging from reddish-brown to black, caused by wear particles mixing with the grease. Engineers sometimes describe lubricated bearings with false brinelling as “bleeding” because of this discolored grease. In unlubricated contacts, the wear debris takes on a powdery, cocoa-like appearance.
The craters from false brinelling are evenly spaced around the raceway, matching the spacing of the rolling elements. The wear particles consist mainly of iron oxide. In lubricated contacts, this is mostly magnetite, a relatively mild form. Without lubrication, the debris shifts to hematite, which is more abrasive and accelerates further damage through fretting corrosion.
Where False Brinelling Is Most Common
False brinelling is essentially fretting wear that occurs specifically in rolling element bearings. It shows up most often when bearings are subjected to external vibrations while not rotating, or rotating only through very small angles. This makes it a particular problem during shipping and storage. Bearings mounted in equipment that gets transported by rail or sea are exposed to constant low-level vibration for days or weeks. The rolling elements never complete a full revolution, so the lubricant film can’t replenish itself, and the same tiny contact patches get worn repeatedly.
It also affects equipment that sits idle while nearby machinery runs. A motor that vibrates a mounting frame, a pump that transmits vibration through a shared base, or any situation where a stationary bearing absorbs ambient vibration can lead to false brinelling over time. The radial load on the bearing and the amplitude of the oscillation are the two primary factors that determine how severe the damage becomes.
Effects on Bearing Performance
Both forms of brinelling create uneven surfaces on the raceways that rolling elements must pass over during operation. Each indentation acts like a small bump in the road, generating a spike of vibration and noise every time a ball or roller crosses it. Because the dents are evenly spaced, these spikes create a rhythmic pattern that increases overall bearing noise and can be picked up by vibration monitoring equipment.
More critically, the indentations concentrate stress at their edges. Each time a rolling element drops into and climbs out of a depression, it generates localized forces higher than the bearing was designed for. Over thousands of revolutions, this accelerates fatigue cracking and spalling, where chips of metal break away from the raceway surface. A bearing that might have lasted years under normal conditions can fail in months once brinelling damage is established.
Preventing True Brinelling
Since true brinelling comes from excessive static or impact loads, prevention focuses on load management. Properly sizing the bearing for all expected loads, including the static weight of components when the machine is idle, is the first step. During installation, avoiding impacts from hammering or dropping components protects the raceways from shock loads. Transport restraints that prevent heavy rotors or shafts from resting their full weight on stationary bearings also help. Correct preload settings, following the manufacturer’s specification rather than overtightening, eliminate another common source of damage.
Preventing False Brinelling
False brinelling is trickier to prevent because you can’t always eliminate the vibration that causes it. Lubricant selection plays a significant role. The conventional wisdom has been that greases with high oil-bleed properties are best, since they readily release oil to keep the micro-contact zones lubricated. Research shows this only partially explains the protection. Grease formula matters in more complex ways.
Greases containing certain anti-wear additives (specifically zinc-based compounds that form a protective film on the metal surface) have shown the strongest resistance to false brinelling. In laboratory testing, bearings lubricated with these greases showed no visible wear marks on the raceways at all. The protective film reduces friction and limits oxygen access to the contact zone, both of which slow the wear process. For greases without these specialized additives, formulations with more abundant, less viscous oil that bleeds easily from an open-structured thickener reduced false brinelling wear by 97% compared to less favorable grease types.
Oil viscosity matters differently depending on how much the rolling elements are moving. Lower-viscosity oils work better when oscillation amplitudes are extremely small (under about 9 micrometers). At larger amplitudes above 60 micrometers, viscosity has much less impact on wear reduction. For equipment being shipped by rail or sea, selecting a grease specifically formulated for vibration resistance can make the difference between a bearing that arrives ready to run and one that’s already damaged before the machine is ever turned on.
Other practical steps include rotating shafts periodically during long storage to redistribute the lubricant and change the contact points, isolating equipment from external vibration sources when possible, and using transport fixtures that lock rotating components in place to prevent micro-movement at the bearing contacts.