_BELTS FOR TRANSMITTING POWER._
The traction of belts upon pulleys, like that of locomotive wheels
upon railways, being incapable of demonstration except by actual
experience, for a long time hindered the introduction of belts as a
means of transmitting motion and power except in cases when gearing
or shafts could not be employed. Motion is named separately,
because with many kinds of machinery that are driven at high
speed--such as wood machines--the transmission of rapid movement
must be considered as well as power, and in ordinary practice it is
only by means of belts that such high speeds may be communicated
from one shaft to another.
The first principle to be pointed out in regard to belts, to
distinguish them from shafts as a means of transmitting power,
is that power is communicated by means of tensile instead of
torsional strain, the power during transmission being represented
in the difference of tension between the driving and the slack
side of belts. In the case of shafts, their length, or the
distance to which they may be extended in transmitting power,
is limited by torsional resistance; and as belts are not liable
to this condition, we may conclude that unless there are other
difficulties to be contended with, belts are more suitable than
shafts for transmitting power throughout long distances. Belts
suffer resistance from the air and from friction in the bearings of
supporting pulleys, which are necessary in long horizontal belts;
with these exceptions they are capable of moving at a very high
rate of speed, and transmitting power without appreciable loss.
Following this proposition into modern engineering examples, we
find how practice has gradually conformed to what these properties
in belts suggest. Wire and other ropes of small diameter, to avoid
air friction, and allowed to droop in low curves to avoid too many
supporting pulleys, are now in many cases employed for transmitting
power through long distances, as at Schaffhausen, in Germany.
This system has been very successfully applied in some cases for
distributing power in large manufacturing establishments. Belts,
among which are included all flexible bands, do not afford the
same facilities for taking off power at different points as shafts,
but have advantages in transmitting power to portable machinery,
when power is to be taken off at movable points, as in the case of
portable travelling cranes, machines, and so on.
An interesting example in the use of belts for communicating
power to movable machinery is furnished by the travelling cranes
of Mr Ramsbottom, in the shops of the L. & N. W. Railway, at
Crewe, England, where powerful travelling cranes receive both the
lifting and traversing power by means of a cotton rope not more
than three-fourths of an inch in diameter, which moves at a high
velocity, the motion being reduced by means of tangent wheels
and gearing to attain the force required in lifting heavy loads.
Observing the operation of this machinery, a person not familiar
with the relations between force and motion will be astonished at
the effect produced by the small rope which communicates power to
the machinery.
Considered as means for transmitting power, the contrast as to
advantages and disadvantages lies especially between belts and
gearing instead of between belts and shafts. It is true in extreme
cases, such as that cited at Crewe, or in conveying water-power
from inaccessible places, through long distances, the comparison
lies between belts and shafts; but in ordinary practice, especially
for first movers, the problem as to mechanism for conveying power
lies between belts and gear wheels. If experience in the use of
belts was thorough, as it is in the case of gearing, and if the
quality of belts did not form so important a part in the estimates,
there would be but little difficulty in determining where belts
should be employed and where gearing would be preferable. Belts are
continually taking the place of gearing even in cases where, until
quite recently, their use has been considered impracticable; one
of the largest rolling mills in Pittsburg, Pennsylvania, except
a single pair of spur wheels as the last movers at each train of
rolls, is driven by belts throughout.
Leaving out the matter of a positive relative movement between
shafts, which belts as a means of transmitting power cannot insure,
there are the following conditions that must be considered in
determining whether belts or other means should be employed in
transmitting power from one machine to another or between the parts
of machines.
1. The distance to which power is to be transmitted.
2. The speed at which the transmitting machinery must move.
3. The course or direction of transmission, whether in straight
lines or at angles.
4. The cost of construction and durability.
5. The loss of power during transmission.
6. Danger, noise, vibration, and jar.
In every case where there can be a question as to whether gearing
shafts or belts will be the best means of transmitting power,
the several conditions named will furnish a solution if they are
properly investigated and understood. Speed, noise, or angles may
become determinative conditions, and are such in a large number
of cases; first cost and loss of power are generally secondary
conditions. Applying these tests to cases where belts, shafts,
or wheels may be employed, a learner will soon find himself in
possession of knowledge to guide him in his own schemes, and enable
him to judge of the correctness of examples that come under his
notice.
It is never enough to know that any piece of work is commonly
constructed in some particular manner, or that a proposition is
generally accepted as being correct; a reason should be sought
for. Nothing is learned, in the true sense, until the reasons for
it are understood, and it is by no means sufficient to know from
observation alone that belts are best for high speeds, that gearing
is the best means of forming angles in transmitting power, or
that gearing consumes more power, and that belts produce less jar
and noise; the principles which lie at the bottom must be reached
before it can be assumed that the matter is fairly understood.
(1.) Why have belts been found better than shafts for
transmitting power through long distances?--(2.) What are the
conditions which limit the speed of belts?--(3.) Why cannot
belts be employed to communicate positive movement?--(4.) Would
a common belt transmit motion positively, if there were no slip
on the pulleys?--(5.) Name some of the circumstances to be
considered in comparing belts with gearing or shafts as a means
of transmitting power.