MechChem Africa September-October 2021

Correct lubrication for enhanced bearing performance Wayne Holton, BMG’s business unit manager for the Bearings, Seals and Gaskets division, looks at how friction, wear and lubrication affect bearing performance.

Easylube single point lubricators from BMG are Electro-Mechanical, which allows for accurate automatic lubrication.

T o achieve optimum bearing per- formance and extended life, cor- rect 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 such as speed, ambient temperature, load, vibration and environmental conditions need to be considered. Fr ict ion i s the force of res i stance between two bodies moving over one another. It is measured as the force parallel to the relative direction of movement of the surfaces that is required to overcome the resistance and cause the bodies to start

sliding. Frictionwastes power. It is a genera- tor of unnecessary heat, the cause of wear and tear damage, and it reduces the mechanical efficiency of plant and machinery. The undesired effects of friction are effectively reduced by using an appropriate lubricant and lubricating system, purpose designed for each specific application. Damage to moving parts can manifest itself in different forms, including surface damage and wear. Whenmaterials of differ- ent hardness rub together, damage occurs mainly on the softer material. Damage is most severe whenmaterials of similar hard- ness 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 re- moval of material from at least one surface. Modes of wear include 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 thematerials. Abrasivewear oc- curs when a harder surface gouges a softer one, or when hard particles are introduced between the surfaces, resulting in the tear- ing of the materials. Pitting wear occurs as a result of sur- face 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 op- timum 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 re- action forces. This implies that, in the case of journal bearings, the shaft will, at low speeds and high loading, be eccentric. This eccentricitywill reducewith increased shaft speed and a 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

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.

6 ¦ MechChem Africa • September-October 2021