[13th Installment]
"Lubrication and Viscosity:"
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This column explores familiar topics which are closely related to our life, society and industry. The topic of this column is "lubrication". When we think of "lubrication", one may recall the phrase, lubricant of human relationships". In such usage, lubricants refer to the tools and materials which help to smooth relationships between individuals. "Lubrication" in the context of rheology also expresses a similar meaning and we shall explore this subject a little further in this column.
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The mechanism of avalanches - the power of friction |
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Lubrication is a field of academics which deals with the movement of surfaces of physical bodies which are in contact with each other and the relative motion of these interacting surfaces. Before delving into some technical aspects of lubrication, we may perhaps gain an easier understanding of our assumptions about lubrication and the mechanisms involved by looking at the example of snow avalanches.
Year 2007 saw a record warm winter in Japan, but there were fears of avalanches occurring in regions where snowfalls usually occur. The previous year was one of record snowfall but there were snow avalanche warnings then as well. Avalanches which occur during warm winters with infrequent snowfall and those that occur in severe winters with a lot of snowfall are of different types. Snow avalanches are phenomena where snow which has accumulated on a surface rapidly slides off an incline. Entire snow pack or slab avalanches involve the ground surface (undersurface of the accumulated snow), while surface layer avalanches are characterized by snow accumulations sliding off a basal layer which has been formed within the snow pack. Although slab and surface layer avalanches differ in this manner, the basic mechanism involved is the same.
Regarding the dynamics of avalanches, one may wonder why snow does not begin to slide off before an avalanche occurs. In simple terms, accumulated snow does not slide off the underlayer because equilibrium is maintained between gravitational force (motive force) exerted in the direction of the incline and static frictional force (sustaining force). Avalanches are triggered when the motive force overcomes the threshold of the sustaining force. With the warm winter experienced this year, it was easier for melting snow, ice, and rain to percolate through the snow cover toward the ground surface to form sliding surfaces. A sliding underlayer is one where sustaining forces are very weak The sustaining force which is equivalent to the force which supports the accumulation of snow is the frictional force between the ground surface and the bottom surface of the snow pack.
Hypothetically, by minimizing the frictional force of the surface of the roof of a house, snow would not accumulate on such sloped surface (although the snow may tend to slough off the roof onto the eaves) and this may free the inhabitant from the arduous task of roof snow removal. Research on lubrication considers examples such as these. Control of the movement of physical objects which are subjected to relative motion or more tangibly, on the relationship of frictional forces and supplementary forces, the impact that lubricants have on such relationship, and the control of the wear and abrasion caused by friction have been the subjects of such research.
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Machines would not work without lubrication |
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"Tribology" is a term associated with the academic field of lubrication and has been in use for 40 years. Although ÅgtribologyÅh is considered nearly synonymous with lubrication, the use of the term was spurred by the rapid growth in automation and higher performance machinery and the impact such machinery has had on industrial manufacturing. In order for machines to provide work output similar to man, moving components called actuators must be employed. Actuators are put into motion by integral gears, bearings, cylinders, pistons and other elements and this requires precise control over friction, wear, seizures, and other problems.
Lubricants are without a doubt one of the keys to controlling friction. The properties of lubricants greatly determine how the parts assembled in a machine perform. The piston of an engine is perhaps a clear example which demonstrates how important lubricants are. Reciprocating engines incorporate pistons which shuttle straight back and forth repeatedly in a cylinder. As the cylinder is a combustion chamber, suctioned fuel gas must not leak out. However if you design your cylinder-piston assembly to deal with this problem in such way that the outer diameter of the piston is of the same dimension as the inner diameter of the cylinder - for practical purposes, the friction caused by contact of the cylinder wall and piston surface would lock these parts together and prevent any motion. Any attempt to apply large force to forcibly move the piston will merely result in severe wearing of the contacting surfaces of the cylinder and piston. Neither would the solution be to increase the gap between the inner and outer diameters of these parts, because as mentioned, this would allow gas to escape and cause improper combustion.
Lubricants are thus critical for these types of applications. Lubricants fill the gap between the piston and cylinder walls with a thin film of oil. This reduces friction, retards wear caused by parts rubbing against each other and from the heat that is generated while at the same time preventing leakage of fuel and combustion gases.
The use of lubricants will not completely eliminate friction however. In addition, small particles of metal which are formed during normal wear mix into the lubricant which exacerbates the wearing process. Thus important functions which lubricants must possess are their ability to reduce friction, increase airtightness, and to the extent possible, prevent wear from occurring.
Viscosity is a critical factor which governs the performance of lubricants. |
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Viscosity - the determiner of lubricant performance |
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As we have frequently indicated, the viscosity of fluids with nonlinear properties vary greatly with changes in temperature. Generally, viscosity falls as the temperature of the fluid rises. Machinery parts involved in movement which encompass a wide range of speeds from low to high are especially predisposed to extreme temperature changes. With normal lubricating oils, viscosity would vary in a corresponding manner. However such changes in oil viscosity relative to operating conditions may not be desirable from the standpoint of machine design.
In machinery applications, you need to be able to control the range of viscosity variation in the formulation of lubricating oils to obtain properties which are as stable as possible. A Ågviscosity indexÅh is used for this purpose as a gauge which expresses the degree of variation in lubrication oil viscosity relative to temperature changes. Small decreases in viscosity even at high temperatures are indicated by a high viscosity index. Most common lubricating oils in use today have viscosity indexes which exceed 100.
In our example of reciprocating engines, oil viscosity is high when the engine is started up but falls once the engine is warmed up. In cold environments, higher lubricating oil viscosities and greater viscous resistance make it difficult to crank start the engine (turning the engine over with the starter). Also even after the engine is warmed up, viscosity of the oil tends to remain high and this results in high frictional forces and loss in transmission efficiency. Thus engine oils of lower viscosities are an important focus in the development of lubricants.
Such development however is simply not just a matter of lowering the viscosity of lubricants. Engine oils tailored for low viscosities are prone to breakdown in oil film formation at high temperatures - and this leads to wear and seizure problems. Also with low viscosity oils, there is accelerated vaporization of the base oil and increased oil consumption becomes an issue. Design and development efforts relative to low viscosity oils thus must devise ways to address numerous related factors such as the maintenance of high viscosity indexes and the inclusion of anti-wear additives to compensate for performance degradation.
Efficient and high accuracy viscosity measurement is indispensible in the development of lubricants. Toki SangyoÅfs expertise which spans a diversity of rheological substances, including lubricating oils, supports research and development of new and advanced fluid products which contribute to enhancement of our society and environment. |
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