APPLICATIONS OF THE VISCOMETRIC METHOD 283
while another cord, apparently no better as judged by the weight,
tensile strength, method of fabrication and length of staple will
last perhaps one hundred times as long. It is evident that oxi-
dation or decay cannot play an important part because the fail-
ure may be brought about in a few hours. It is not due to
friction of the pulley as the pulley in all cases is running free. The
surprising thing about it is that the cord often wears out on the
side which is away from the pulley, or the center of the cord may
become completely pulverized while the outside is apparently
sound.
An analysis of what happens when a belt moves over a pulley
shows that the outside of the belt moves along a longer arc and
therefore tends to get ahead of the inside of the belt. There is
consequently a shearing stress set up within the belt. Since the
individual fibers are comparatively weak, it is of the utmost
importance that the individual fibers "be protected from undue
strains. In order to obtain relief where the strains are greatest,
a lubricant between the fibers and plies should always be pro-
vided. A rosined bow adheres to a violin string and in the pro-
duction of sweet sound accumulates stresses advantageously,
but the workman who gets rosin on a machine belt with the idea
of gaining greater traction, may quickly bring about the destruc-
tion of the belt. A certain amount of slipping of a belt and
particularly in the belt is necessary and desirable.
Lard and certain fixed oils are used to "stuff" leather, and a
good leather belt will practically never wear out if well-used and
dressed with lubricant occasionally. Window cords are often
lubricated with a soft paraffin. The paraffin has a tendency to
work out in use and since it becomes hard at low temperatures,
it then tends to make the cord stiff. Pitch and its congeners
is unsuitable for use on textile belting due to its having a high
temperature coefficient of fluidity. What is needed as a lubricant
is a substance which adheres strongly to the material, lubricates
the fibers, and has a small or negligible temperature coefficient of
fluidity. Oils which serve well with leather will not fill the
coarser pores of textile belting, hence rubber, balata, and semi-
drying oils are often used. In ordinary fabrics a certain amount
of oil present will add to their life. Even a wire rope will last
longer if there is lubricant between the strands.