Year: 1871

transmission OF Power by Beltlng.


The laws which govern the transmission
of power by means Of the friction of a belt
upon a pulley are but imperfeetly under
stood, even by mary practical machinists.
The use Of belts has been so long and un-
iyersally practiced that one would suppose
il might at this day have been reduced to
an exact science but such is very far
from being the case. There is ! vide d.
versity Of views in regard TO some Of the
most essential points in the application Of
Il belt. As the subject is one Of vital i'm
portancc in the business of every factory
and workshop, all well-estabished facts
concerning it are worthy of careful con-
sderation.


The impression prevails among a vast
humber Of mechanics, and is even embod-
led in the rules laid down by some me-
ehanical writers, that the friction of n
belt is N proportion to the amount of sur
face in contact with the puley-in other
words, to the width Of the belt. It has
been shown by repeated and careful ex-
periments, among which the tests made
by MORN are probably best known, that
the extent Of surface affects but slightly
if at all the friction and resulting power
Of the belt. Morin's experiments showed
conclusively that with The same pressure
the same amount Of power was given by
each of two belts, one of them being
twice as large as the other. In short,
the best authorities now agree in the po-
sition that it is the amount of pressure
which alone determines the amount OF
friction. There are exceptional cases,
where the pressure is very slight or very
much in excess of the ordinary degree,
where the friction is varied slightly by
Increasing or diminishing the width Of the
belt. But these cases scarcely ever oc-
cur in practice, and the rule above laid
down, that the friction Of the belt depends
entirely upon the amount Of pressure, is
therefore practically reliable.


The principle involved is very simple
one. 1t requires the same power to push
along block of wood, whether it lies on
its fiat side or on its edge. In other
words, the amount of friction is the same,
and its effect the same, whether it be
distributed over 20 square inches of sur
face or 40 square inches.


The tensile strength of belt, or the
amount of strain which it will bear, de.
termines the amout of power which it can
be made to transmit. 1t very rarely
happens, however, that the power de-
manded is beyond the tensile strength,
and this point does not require, therefore,
to be especially considered. The hexi-
bleness of the belt or the ease with which
it can be bent and made to hug the pulley,
is matter which greatly affects its work.
ing power. A broad, thin belt is there
fore more effective than a thick, stiff,
narrow belt, as there is less power wasted
in bending it to the pully. Furthermore,
as the curve is less abrupt on a large pul.
ley than on small one, the larger the
diameter Of the pulley the less will be the
resistance of the belt TO bending. and the
smaller the amount of power expended in
that way. Large pulleys and broad, thin
belts will therefore be found preferable
as regards economy Of power.


Glycerine, although an expensive arti.
Gle, is very useful iD rendering the belt
soft and flexible, and making it hug the
pulley closely. 1t has hardly a superior
for this purpose, and its cost is the only
drawback. Water is even more effective
when first applied, but it destroys the belt
by alternate drying and wetting, and its
use is therefore impracticable. Except
for making the belt pliable and giving il
closer contact with the pulley, the oiling
of the leather would be useless, as greater
friction is given by dry belt. A smooth
cast-iron pulley affords the greatest de.
gree of friction. where leather belts OF
the proper quality are used, ,--LetelS Me-
chanical NeaoS.