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)
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