“The effects of friction and the resulting wear of moving components is reduced by effective lubrication. To achieve optimum bearing performance and extended life, correct lubrication is as important as the appropriate selection of each bearing. Although a general multipurpose grease is adequate in many applications, more arduous operating conditions demand the careful choice of the correct lubricant and lubricating system. In selecting the right lubricant for a specific application, factors to be considered are speed, ambient temperature, load, vibration and environmental conditions.” This is the view of Wayne Holton, BMG’s business unit manager, Bearings, Seals and Gaskets division, who gives us insight and an in-depth look at how friction, wear and lubrication affect bearing performance.
Friction is the force of resistance of two bodies moving over each other. It is measured as the force (parallel to the relative direction of movement of the surfaces) required to over-come resistance and cause sliding. Friction wastes power, it is a generator of unnecessary heat, the cause of wear and tear and reduces mechanical efficiency of plant and machinery. The undesired effects of friction are effectively reduced by the use of an appropriate lubricant and lubricating system, designed for each specific application.
Damage to moving parts can manifest itself in different forms, including surface damage and wear. When materials of different hardness rub together, damage occurs mainly on the softer material. Damage is most severe when materials of similar hardness are rubbed together and when clean and smooth surfaces are rubbed together. In these instances, friction is high and the two surfaces are likely to seize. Wear is a consequence of friction and involves the removal of material from at least one surface.
Different modes of wear are adhesive and abrasive wear. Adhesive wear, which is caused by local welding of contact points and the subsequent ripping of material, increases with load and reduces with the hardness of the materials.
Abrasive wear occurs when a harder surface gouges a softer one, or when hard particles are introduced between the surfaces, resulting in the tearing of the materials.
Pitting wear occurs as a result of surface fatigue of the material due to contact stresses exceeding the fatigue strength. This type of wear occurs when the surfaces are being ‘run in’ and reduce when any high spots are flattened sufficiently to distribute the load uniformly over the total area of the mating surfaces. When excessive loading occurs, pitting continues and will eventually destroy the surfaces.
The effects of friction and the resulting wear of moving components is reduced by effective lubrication.The purpose of any lubricant, which may take the form of an oil, a grease or a solid, is to separate the mating surfaces and thereby reduce friction and wear. Correct lubrication is critical for optimum bearing performance and extended service life.
Hydrodynamic lubrication has, as its basis, the presence of a ‘wedge’ of lubricant separating the two running surfaces and supporting the load by hydrodynamic reaction forces. This implies that in the case of journal bearings, the shaft will, at low speeds and high loading, be eccentric. The eccentricity will reduce with the increase in shaft speed and reduction in loading.
Ball and roller bearings are designed to be lubricated using boundary lubrication, thin film lubrication or full hydrodynamic lubrication systems. The specific duty will determine the most appropriate form of lubrication for the application. Extreme pressure additives may be necessary and for bearings operating at high temperatures, a lubricant with a high viscosity index is often required.
Oil lubricants may be applied in a number of ways:
Oil bath. The bearing is submerged in oil. This method is not suitable for high speed applications.
External pressure. A pump is used to supply the bearing with the lubricant. The circulating oil cools the bearing and removes any particles of dirt.
Mist spray lubrication. The lubricant is sprayed onto the bearing under pressure, in the form of a mist.
Greases, which are a combination of mineral oil and a thickening agent (metallic soap) are difficult to pump continuously into bearings and are usually applied periodically, providing only boundary lubrication. Special devices are available however, to continuously supply bearings with the appropriate amount of grease.
The main advantage of grease lubrication is that because this material is a semi-solid, it also acts as a good sealant to prevent the ingress of dirt particles. However, working temperatures may limit the use of grease in some applications.
BMG’s lubricants, oils and greases
BMG’s range of lubricants, oils and greases has been developed by leading suppliers, with the latest formulations and technologies, to meet the demands of all sectors, including mining, automotive, marine, general industry and consumer applications.
“Included in this range are mining lubricants and allied chemicals, that offer solutions for the most difficult lubrication problems,” says Wayne Holton, BMG’s business unit manager, Bearings, Seals and Gaskets division.
BMG’s range of industrial products includes anti-seize compounds and penetrants, assembly and disassembly products, chain lubricants, transmission oils, cleaners and degreasers, electrical maintenance products, plastic moulding and cutting compounds, as well as engineering and fabrication materials. Also available are a number of greasing solutions, including open gear lubricants and wire rope dressings, bearing and synthetic bearing greases, as well as other general grease products.
BMG has developed a bearing course for companies, which includes technical training on correct bearing selection, handling, installation and maintenance. It also includes relevant information on the importance of correct shaft and bore tolerances, proper fitting tools and techniques, advances in lubrication technology and the basics of condition monitoring and failure investigation.