Wear of rolling bearings appears in many different forms. Dr Steve Lacey describes the principle types and causes of rolling bearing wear, as well as guidance on remedial action.
Wear occurs when surfaces slide against each other and there is insufficient or no lubrication to keep them apart. If a full hydrodynamic lubricant film can be maintained at all times, wear will not occur. However, in reality, this is very rarely the case and so wear is almost always unavoidable.
Wear of rolling bearings
Unlike other causes of rolling bearing failure such as fatigue, corrosion and overloading, wear appears in many different forms that cannot all be dealt with in this article. Instead, this article will discuss the four principle types of rolling bearing wear: abrasive wear, adhesive wear, fretting wear/corrosion and false brinelling.
Abrasive wear occurs when a hard rough surface slides against a softer one, ploughing a series of grooves and removing material. This can also occur when abrasive particles are introduced between sliding surfaces, or when a part is moved through an abrasive medium.
Abrasive wear of rolling bearings, often referred to as ‘three-element’ wear, is the removal of material from three main areas of a rolling bearing: the raceways, the end faces of the rolling elements, and the ribs/cages. Abrasive wear normally takes the form of a high lustre (ie semi-glossy) finish with a mirror-like surface structure. This type of wear normally results in increased endplay or internal clearance, which can reduce fatigue life and result in misalignment in the bearing.
Abrasive wear can also affect other parts of the machine in which the bearings are used. The foreign particles may find their way into the lubricant through badly worn or defective seals. Improper initial cleaning of housings and parts, ineffective filtration or improper filter maintenance can also allow abrasive particles to accumulate.
The primary causes of abrasive wear of rolling bearings are: inadequate lubricant film formation; foreign particles (contaminants such as sand, fine metal from grinding, etc.) present in the lubricant; or insufficient lubricant. Furthermore, as the number of wear particles in the lubricant increases, this will further accelerate the bearing damage process.
Abrasive wear can be limited in several ways. Surfaces can be coated or treated to provide them with a higher hardness than the abrasive particles. Schaeffler offers an extensive modular range of coatings concepts that enable the surfaces of components and systems to be precisely matched to the specific application.
If a circulating lubricant is used, the abrasive particles can be removed by filtering. Alternatively, oil sensor systems are available, such as the FAG Wear Debris Check system from Schaeffler, an oil monitoring system that indicates damage or wear to bearings, cages and gears. The system utilises an inductive particle counter (sensor), which is able to distinguish between ferrous and non-ferrous metal particles that may be present in the lubricating oil.
The sensor provides information on the number of particulates present in the oil, and then classifies these according to their physical size. Analysing the oil in this way enables damage and wear to be detected much earlier.
Abrasive wear can also be limited by using a combination of a hard and a soft surface so that the abrasive particles can imbed themselves into the softer material.
As two surfaces slide over each other, wear occurs because of the shearing, deformation and plucking away of material at points of adhesion; these points of adhesion occur at roughness peaks. With adhesive wear, the amount of wear is generally proportional to the load and to the distance over which the surfaces have slid, and inversely proportional to the hardness of the surface on which wear occurs.
Adhesive wear of rolling bearings is often referred to as ‘smearing’ or ‘two-element’ wear. This involves the transfer of material between rolling partners and is caused by the sliding activities within the bearing, primarily due to the angular acceleration of the rolling elements upon entering the load zone. This leads to the transfer of material between the ring raceways and the rolling elements. This metal-to-metal contact can appear in the form of ‘scuffing’ or ‘scoring’ marks on the bearing surfaces.
In cases of advanced damage, abrasive wear occurs as the transferred particles become detached. There is then a risk of this being mistaken for ‘grey staining’ – a form of bearing fatigue in which tiny, very flat pits appear under a relatively low load and simultaneous slippage. These pits occur in large numbers and appear as ‘flecks’ on the bearing raceway.
In adhesive wear situations, the best results are usually obtained if the parts have hard surfaces. Hard surfaces may be obtained by manufacturing parts from hard materials or by applying the appropriate surface treatments. Some common surface treatments for the prevention of wear include chromium and nickel plating, carburising and nitriding. However, depending on the application, a number of design approaches are possible here. For example, it may be possible to change the type of bearing or the cage design or apply a black oxide coating to some of the bearing components.
Often referred to as ‘Tribo-corrosion’, fretting wear is the formation of fretting corrosion on the bearing raceways. This normally occurs on the bores, outside diameters and faces of the raceways. Typically, fretting corrosion is caused by inadequate lubrication, which leads to metallic contact and consequently oxidisation (red or black oxide of iron is usually evident) of the raceways.
The four main causes of fretting corrosion are: micro-motion or very small movements between fitted components; deviations in the form/geometrical shape of components; shaft deflection/housing deformation; and where there is no axial preloading. Most cases of fretting corrosion can be remedied by following the bearing manufacturer’s mounting instructions for appropriate fit recommendations. In addition, advice and guidance can be sought direct from Schaeffler Technology Centre (STC), a global network of local centres of technical expertise. Expert advice and support is available for all aspects of rolling bearing technology.
Fretting corrosion can also appear in the form of frictional corrosion. This can occur as a result of the dynamic load on the bearing caused by overrolling. Here, microscopic movements occur between fitted parts due to the elastic deformation of the bearing rings.
Remedial measures for fretting corrosion include providing a floating bearing function at the ring with a point load; using bearing seats that are as tight as possible; making the shaft (housing) more rigid to prevent bending; or coating the bearing seats. Expert advice and guidance on remedial measures can be sought from a local Schaeffler Technology Centre.
False brinelling is, as the name suggests, not true brinelling but is actual fretting wear caused by the slight axial movement of the rolling elements while the bearing is stationary. When the bearing isn’t turning, an oil film cannot be formed to prevent raceway wear.
Formation of wear-like indentations or grooves are worn into the race by the sliding of the rolling elements back and forth across the race. Vibration is the cause of these sliding movements.
The indentation surfaces often turn brown (corrosion) and exhibit severe hardening, particularly in the case of ball bearings. These marks can also be identified by their sharp demarcation to the surrounding surface.
There are times when false brinelling cannot be prevented, for example, when vehicles, wind turbines or other types of equipment or machines are shipped by ocean freight. The vibration present may cause sufficient movement to produce some of this false brinelling. It can be significantly reduced or eliminated by reducing the potential for relative movement and by decreasing the static weight present during shipment or storage. Other remedies include selecting a larger radial clearance for rotating loads or using lubricants that contain anti-wear additives.
Rolling bearings also exhibit false brinelling when used in positions that encounter very small reversing or angular oscillation (ie, less than one complete rotation of the rolling element).
False brinelling can be distinguished from true brinelling by examining the depression or wear area. False brinelling will actually wear away the surface texture whereas the original surface texture will remain in the depression of a true brinell.
Steve Lacey is Engineering Manager, Schaeffler (UK) Ltd, Minworth, Sutton Coldfield, West Midlands, UK.