Clutches and brakes design and selection

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Clutches and brakes
design and selection

Friction Materials

Curves of the coefficient of friction as a function of load and of the speed
differential between the lining and facings and their mating surface are no
longer available from many manufacturers. Perhaps this is a consequence of
the ease with which trial lawyers in the United States can collect large financial
rewards for weak liability claims based upon often trivial, or unavoidable (due
to physical limits on manufacturing tolerances), differences between published data and a particular specimen of the manufactured product. Furthermore, differences between published and operational coefficients of friction
are beyond the control of the manufacturer because comparison of laboratory
and operational data have shown that temperature, humidity, contamination,
and utilization cycles of the machinery using these linings and facings can
cause significant changes in the effective coefficient of friction at any given
moment. Consequently, the coefficients of friction mentioned are nominal,
the following discussion is in generic terms, and all curves shown should be
understood to represent only the general character of the material under
laboratory conditions.
The value of laboratory data is twofold, even though the data should
not be used for design purposes. First, the data provides a comparison of the
performance of different lining materials under similar conditions, such as
given by the SAE 661 standard. Second, comparison of the laboratory data
with field data for a particular type of machine for several different linings
may suggest an empirical relationship that yields an approximate means of
predicting the field performance of other lining materials based upon their
laboratory data. A history of the comparison of field and laboratory data

Copyright © 2004 Marcel Dekker, Inc.


Chapter 1

may, therefore serve as a starting point in the design of the prototype of a new
machine of the same or similar type.
Field testing of a new machine by customers under the most adverse
conditions is still necessary. Users often seem to devise abuses not envisioned
by the design engineers.

The usual range of the dynamic friction coefficients for those friction
materials normally used in dry brake linings and pads is given in the Society
of Automotive Engineers (SAE) coding standard SAE 866, which lists the
code letters and friction coefficient ranges shown in Table 1 [1]. According to
this code the first letter in...
Clutches and brakes
design and selection
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