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38827_Ward's World+MGH Friction

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4 up (a so-called negative characteristic), frictional vibrations may be produced. At slow sliding speeds the vibrations take the form of relaxation oscillations (creaking, chattering, and groaning noises), while at high speeds they are harmonic oscil- lations (squeaking). As a general rule, unlubricated and poorly lubricated surfaces give a negative characteristic and show a tendency toward oscillation, while well-lubricated surfaces give a positive characteristic and are immune to oscillations. Frictional oscillations are generally referred to as "stick-slip," but strictly speaking this term applies only to relaxation oscilla- tions, because during part of each oscillation cycle the sliding surfaces actually stick and have zero relative velocity. There is always some interfacial slip during harmonic oscillations. Frictional oscillations can generally be avoided by changing one or more of the parameters. Helpful changes include reduc- ing the normal load, increasing the sliding speed, introducing damping into the system, stiffening all components acting as springs, changing materials, or introducing good lubricants. Frictional oscillations can be useful. They are the mechanism of music production in bowed stringed instruments, and they have been used occasionally to detect the entry of intruders into a home. More significantly, in many sliding systems they act as a warning signal that the lubricant has disappeared, that excessive wear damage is imminent, and that relubrication is in order. Lubrication In prehistoric and early historic times, humans' main inter- est in friction was to reduce the friction coefficient, to reduce the labor involved in dragging heavy objects. This led to the invention of lubricants, the first of which were animal fats and vegetable oils. A great breakthrough was the use of rolling action — in the form of rolling logs and then wheels — to take advantage of the lower friction coefficients of rolling systems. In modern engineering practice available materials and lubricants reduce friction to acceptable values. In special circumstances when energy is critical, determined efforts to minimize friction are undertaken. Friction problems of practi- cal importance are those of getting constant friction in brakes and clutches, so that jerky motion is avoided, and avoiding low friction in special circumstances, such as when driving a car on ice or on a very wet road. Also, there is considerable interest in developing new bearing materials and new lubricants that will produce low friction even at high interfacial temperatures and maintain these properties for long periods of time, thus reducing maintenance expenses. Perhaps the most persistent problem is that of avoiding frictional oscillations, a constant cause of noise pollution of the environment. Friction (continued) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com This article was originally published by McGraw Hill's AccessScience. Click here to view and find more articles like this.

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