INTRODUCTION TO TRIBOLOGY
Many fields of human endevour have for long been plagued by energy & profit sapping problems resulting from wear that arise from friction. Since ancient times, it was known that rubbing certain materials like animal fats on moving surfaces, such as the wheels of chariots, made them run more easily. Also, the wheels and axles did not get hot or wear out so fast. As the industrial revolution began it was found that the machines needed constant maintenance and more and more fats (from animal & vegetable origin) to keep them moving. Eventually these natural lubricants could not keep up with increased powers & loads. Early engineers solved many such problems with empirical solutions born out of practical experience. Over time theories regarding friction, wear and lubrication developed.
Tribology is the study of friction, wear and lubrication. The word was coined in 1967 and derives from the greek ‘tribos’, meaning rubbing or sliding.
The drive to minimize wear & friction, by analyzing & predicting mechanisms that occur between interacting surfaces in relative motion, that began in the late 19th & early 20th century have evolved into this dynamic science. From studies made from the 1960’s it became clear that the cost of tribological losses & the potential savings that could result from applying tribological principles were immense.
Wear is the major cause of material wastage and loss of mechanical performance. Friction is the principal cause of wear and energy dissipation. Lubrication is an effective means of controlling wear, reducing friction and minimizing energy loss. It is estimated that 1/3rd of the world’s energy resources in present use is needed to overcome friction in one form or another. The cost of wear has been estimated at 2/3 of the cost of energy.
As machines began exceeding previous limitations – high performance gas turbines, nuclear plants, jet engines, booster rockets – the need for more sophisticated lubrication, friction and wear technology accelerated. Modern industries faced critical problems with ultra high speed bearings,, gears, seals, operating at high temperatures and in extremely difficult environments, that could not be solved by conventional lubrication, bearings, or materials. As a result advanced research conducted for these systems have led to new understandings, new lubricants, new designs, and new materials that have vastly expanded the scope of tribology.
Tribology affects our lives to a much greater extent than is commonly realized. Human beings and animals have been instinctively modifying friction and wear as it affects their own bodies. For instance, human skin becomes sweaty as a response to stress or fear. It is now understood that sweating on the palms or soles of feet of humans and dogs, but not rabbits, has the ability to raise friction between palms or feet and a solid surface. In other words, when an animal or human senses danger, sweating occurs to promote either rapid flight from the scene of danger, or else the ability to firmly hold a weapon or climb the nearest tree.
|Interacting surfaces in relative motion’ (which essentially means rolling, sliding, normal approach or separation of surfaces) dictates or controls the functioning of virtually every device developed by man. Everything that man makes wears out, almost always as a result of relative motion between surfaces. Analysis of machine break-downs show that a majority of failures and stoppages are associated with interacting moving parts such as gears, bearings, couplings, sealings, cams, clutchs, etc. The majority of problems are tribological. Our human body also contains interacting surfaces, e.g. joints, which are subjected to lubrication and wear.
Film formation between any pair of sliding objects is a natural phenomenon which can occur without human intervention. Film formation might be the fundamental mechanism preventing extremely high shear ratrs at the interface between two rigid sliding objects. Even non-mechanical sliding systems provide examples of film formation. Studies of the movement of adjacent geological ‘tectonic’ plates on the surface of the earth reveal that a thin layer of fragmented rock and water forms between opposing rock masses. Chemical reactions between rock and water initiated by prevailing high temperatures ( ≈ 600° C) and pressures ( ≈100 bar) are believed to improve the lubricating function of this layer. Although the thickness of the intervening layer of fragmented rock is believed to range between 1 – 100 m, this thickness is insignificant when compared to the extent of geological plates, and these layers can be classified as ‘films’. There is a fundamental similarity between sliding on such a geological scale and sliding between blood cells and capillaries. A possible reason is that thin is inherently mechanically stable. It is not difficult to squeeze out some of the film, but its complete removal is virtually impossible. Although sliding is destructive to these films, it also facilitates their replenishment by entrainment of a ‘lubricant’ or else by the formation of fresh film material from wear particles.
It might appear that objective of tribology is minimizing the two principal disadvantages of solid to solid contact : friction and wear, but this is not always the case. Reduction of wear but not friction is desirable in brakes, clutches, tyres, clamps. Reduction of friction but not wear is desirable in pencils. Increase in friction and wear is desirable in erasers
A variety of machine elements depend upon tribological principles and techniques to achieve acceptable performance levels and service life. Among these are
• Journal & Thrust Bearings
• Rolling Element Bearings
• Traction drives
• Hypoid Differentials
• Oscillating bearings
• Face Seals
which are incorporated as vital components in internal combustion engines, turbines, compressors, gearboxes, transmissions, etc that are found in industrial machinery, transportation and aerospace vehicles and energy related equipment. Failure of any of these elements can be catastrophic.
What is tribology
|To illustrate some causes of failures we may consider rolling element bearings. Bearings are the most common elements in mechanical equipment and should be very long-lasting components. The bearings should have very low wear rate, and they very rarely fail due to material faults. But most bearings do fail far earlier than their design life. Analysis indicate that most bearings fail by accident (96 %) and approximately 10% of these are due to fatigue. A fair number fail due to lubrication problems, seal problems, and improper mounting. Some of the bearings are already damaged due to mishandling before they are assembled on the machine (e.g.fretting damage during transportation). Typical causes of damage to rolling element bearings are shown in adjoining figure.
Contact between sliding and rolling surfaces creates highly complex reactions that can only be understood by drawing from a range of disciplines. Tribology is a muliti-disciplinary science based on fluid mechanics, machine dynamics, metallurgy, physical & surface chemistry, heat transfer and stress analysis. Briefly, Tribology is the study of
• The study of Friction, Wear & Lubrication of interacting surfaces in relative motion;
• The characteristics of films of intervening materials between contacting bodies and;
• The consequences of either film failure or absence of film which are usually manifested by severe friction and wear.
(Readers may also refer to the TRIBOLOGY & LUBRICATION TIMELINE in the CHARTS & TABLES Section of KNOWLEDGE CENTER)