The Main Types and Causes of Rolling Bearing Wear
Listed inMaintenance & Repair
First published in the September 2015 issue of Quarry Management as ‘Failure is Not an Option’
In this article Dr Steve Lacey, engineering manager at Schaeffler UK, describes the principle types and causes of rolling bearing wear, provides guidance on remedial action, and examines how selection of the most suitable grease can prevent premature bearing failure
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. In reality, however, this is very rarely the case and so wear is almost always unavoidable.
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 greater hardness than the abrasive particles. Schaeffler, for example, offer an extensive modular range of coating concepts that allow 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 – an oil-monitoring system that indicates damage or wear to bearings, cages and gears. The system utilizes 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 allows 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, carburizing and nitriding. However, depending on the application, various design approaches are possible. 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, oxidization (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.
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 over-rolling. 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.
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 is not turning, an oil film cannot be formed to prevent raceway wear. 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, such as, when vehicles 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.
Selecting the right grease for rolling bearings
Selecting the correct lubricant is a critical factor in ensuring the functional reliability and optimum operating life of a rolling bearing. Failure statistics show that a significant proportion of premature rolling bearing failures are directly or indirectly related to the lubricant used. The main causes of failure are unsuitable lubricants (20%), aged lubricants (20%) and insufficient lubrication (15%).
Although lubricating oils (eg mineral oils and synthetic oils) are sometimes recommended for use with rolling bearings in extreme operating conditions (eg high temperatures), most bearing manufacturers recommend the use of greases.
When selecting a suitable grease for a rolling bearing, a number of application-related factors need to be considered. These include the type of bearing, operating speed, temperature, and load. Other factors, such as mounting position, sealing, and shock and vibration, may also need to be considered.
Grease characteristics and classification
The characteristics of a grease fundamentally depend on the following three properties:
Base oil type and viscosity
The viscosity of the base oil is responsible for the formation of the lubricant film. As a base oil, mineral oils or synthetic oils are commonly used. It is important that synthetic oils are differentiated according to their type (polyalphaolefin, polyglycol, ester, fluoro oil etc), as these possess very different characteristics.
Typical thickeners used include metal soaps or metal complex soaps. Organic or polymer thickeners, such as polycarbamide, are becoming increasingly important.
All greases contain additives. A distinction is made between additives that have an effect on the oil itself (oxidation inhibitors, viscosity index improvers, detergents etc) and additives that have an effect on the bearing or the metal surface (eg anti-wear additives, corrosion inhibitors, friction value modifiers).
Greases are classified in terms of their principal components: thickener and base oil. Greases are produced in various consistencies, which are defined as NLGI grades. These are determined by the ‘worked penetration’ of the grease according to ISO 2137. The higher the NGLI grade, the harder the grease. Preferred greases for rolling bearings are those with NGLI grades of 1, 2 or 3.
Factors influencing grease selection
A distinction needs to be made between point contact (ball bearings) and line contact (needle roller bearings and cylindrical roller bearings).
In ball bearings, each over-rolling motion at the rolling contact places strain on only a relatively small volume of grease. In addition, the rolling kinematics of ball bearings exhibit only relatively small proportions of sliding motion. The specific mechanical strain placed on greases in bearings with point contact is, therefore, significantly less than in bearings with line contact. Typically, greases with a base oil viscosity ISO VG 68 to 100 are used.
In rolling bearings with line contact, higher requirements are placed on the grease. Not only is a larger grease quantity at the contact subjected to strain, but sliding and rib friction is also to be expected. This prevents the formation of a lubricant film and would therefore lead to wear. As a countermeasure, greases should be selected that exhibit a higher base oil viscosity (ISO VG 150 to 460 or higher). Anti-wear additives may also be required and consistency is normally NLGI 2.
The speed parameter of the bearing should always be a good match for the speed parameter of the grease. This depends on the type and proportion of the thickener, the base oil type and the proportion of base oil. The speed parameter of a grease is not a material parameter but depends on the bearing type and the required minimum running time.
As a general guide, for rolling bearings rotating at high speeds or with a low requisite starting torque, grease with a high-speed parameter should be selected. For rolling bearings rotating at low speeds, grease with a low-speed parameter is recommended.
The temperature range of the grease must correspond to the range of possible operating temperatures in the rolling bearing. The operating temperature range is dependent on the type and proportion of thickener, the type and proportion of base oil, and the production quality and production process. The stability of the grease at high temperatures also depends primarily on the production quality and production process.
In order to achieve reliable lubrication and an acceptable grease operating life, it is generally recommended that greases should be selected according to the bearing temperature that normally occurs in the standard operating range.
Other factors to consider include the upper operating temperature of the grease, the dropping point (ie the temperature at which slowly heated grease passes from a semi-solid to a liquid state and the first drop of grease falls from the standardized dropping-point nipple), and the lower operating temperature.
For a load ratio C/P <10 or P/C > 0.1, greases are recommended that have higher base oil viscosity and anti-wear additives. These additives form a reaction layer on the metal surface that provides protection against wear. These greases are also recommended for bearings with an increased proportion of sliding motion (including slow running) or line contact, as well as under combined radial and axial loads.
Water and moisture
If the application is in a damp environment, moisture can enter the bearing. Water may condense within the bearing if there are rapid temperature fluctuations between warm and cold. This is a particular problem if large cavities exist in the bearing or housing.
Water can cause severe damage to the grease or bearing and is often due to ageing or hydrolysis, interruption of the lubricant film and corrosion. Barium- and calcium-complex soap greases have proved favourable in these conditions as they provide good water resistance and act to repel water. The anti-corrosion effect of a grease is also influenced by additives.
Oscillations, shocks and vibrations
Oscillation loads can have a considerable effect on the structure of thickeners in greases. If mechanical stability is not sufficient, changes in consistency may occur. This leads to softening, de-oiling on an isolated basis, but also hardening of the grease with a corresponding reduction in lubrication capability. It is recommended, therefore, that a grease should be selected whose mechanical stability has been tested accordingly. Options here include the expanded worked penetration, the Shell Roller Test in accordance with ASTM D 1831, and a test run on the FAG AN42 test rig.
If hard contaminant particles penetrate the bearing, this will not only lead to increased noise, but also to wear. Appropriate sealing of the bearing should prevent this. The grease can assist this sealing effect by forming a stable collar on the seal. In this case, more solid-type greases are more suitable, as greases that are too soft tend to favour the escape of grease.
Mounting position and adjacent components
Even where an axis of rotation is vertical or inclined, lubricant must remain at the lubrication point. In addition to appropriate seals, flowing away of the grease can be prevented by using a more viscous grease. If several lubrication points are located close together, unintentional contact can occur. Attention must therefore be paid to compatibility of the lubricants with each other. However, where possible, the optimum solution is to use only one grease, which should also be compatible with the cage and seal material.
A new publication is now available from precision bearing manufacturers Schaeffler UK. ‘Lubrication of Rolling Bearings’ contains a wealth of useful information for engineers who want to learn more about the principles, methods, selection and testing of lubricants for rolling bearings.
The comprehensive 200-page publication begins by looking at the main principles and theories of rolling bearing lubrication, as well as the key design considerations such as viscosity ratio, fatigue theory, lubricant film thickness, load-carrying capacity, calculation of the rating life, and the effects of friction, speed and operating temperature on lubricant performance.
A chapter on ‘Lubrication Methods’ includes grease and oil lubrication, as well as advice and guidance on the selection of the most suitable method. This chapter concludes by providing examples of both individual (single bearing) and centralized lubrication methods (multiple bearings).
‘Lubricant Selection’ provides in-depth information on how to select the most suitable grease (or lubricating oil) for rolling bearings. This includes information on the influencing factors such as speed, temperature, load, water and moisture, shock and vibration, vacuum conditions, mounting position, bearing type, as well as legal and environmental regulations that may also need to be considered.
Other chapters include ‘The Supply of Lubricant to the Bearings’, including the miscibility of lubricants and the various types of lubrication supply systems and condition-monitoring methods for lubricants.
A section on ‘Contaminants’ deals with solid foreign matter, liquid contaminants, gaseous contaminants and the cleaning of contaminated rolling bearings.
Other sections include lubricant testing; the storage and handling of rolling bearings; dry running and media lubrication; and coatings for rolling bearings, including protection against wear, friction and slippage.
For more information or a free copy of ‘Lubrication of Rolling Bearings’ (publication TPI 176), contact Schaeffler UK’s Marketing Department: [email protected]
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