_MACHINERY FOR TRANSMITTING AND DISTRIBUTING POWER._
To construe the term "transmission of power" in its full sense,
it will, when applied to machinery, include nearly all that has
motion; for with the exception of the last movers, or where power
passes off and is expended upon work that is performed, all
machinery of whatever kind may be called machinery of transmission.
Custom has, however, confined the use of the term to such devices
as are employed to convey power from one place to another, without
including organised machines through which power is directly
applied to the performance of work. Power is transmitted by means
of shafts, belts, friction wheels, gearing, and in some cases by
water or air, as various conditions of the work to be performed may
require. Sometimes such machinery is employed as the conditions do
not require, because there is, perhaps, nothing of equal importance
connected with mechanical engineering of which there exists a
greater diversity of opinion, or in which there is a greater
diversity of practice, than in devices for transmitting power.
I do not refer to questions of mechanical construction, although
the remark might be true if applied in this sense, but to the kind
of devices that may be best employed in certain cases.
It is not proposed at this time to treat of the construction
of machinery for transmitting power, but to examine into the
conditions that should determine which of the several plans of
transmitting is best in certain cases--whether belts, gearing, or
shafts should be employed, and to note the principles upon which
they operate. Existing examples do not furnish data as to the
advantages of the different plans for transmitting power, because
a given duty may be successfully performed by belts, gearing,
or shafts--even by water, air, or steam--and the comparative
advantages of different means of transmission is not always an easy
matter to determine.
Machinery of transmission being generally a part of the fixed
plant of an establishment, experiments cannot be made to institute
comparisons, as in the case of machines; besides, there are special
or local considerations--such as noise, danger, freezing, and
distance--to be taken into account, which prevent any rules of
general application. Yet in every case it may be assumed that some
particular plan of transmitting power is better than any other, and
that plan can best be determined by studying, first, the principles
of different kinds of mechanism and its adaptation to the special
conditions that exist; and secondly, precedents or examples.
A leading principle in machinery of transmission that more than
any other furnishes data for strength and proper proportions
is, that the stress upon the machinery, whatever it may be, is
inverse as the speed at which it moves. For example, a belt two
inches wide, moving one thousand feet a minute, will theoretically
perform the same work that one ten inches wide will do, moving at a
speed of two hundred feet a minute; or a shaft making two hundred
revolutions a minute will transmit four times as much power as a
shaft making but fifty revolutions in the same time, the torsional
strain being the same in both cases.
This proposition argues the expediency of reducing the proportions
of mill gearing and increasing its speed, a change which has
gradually been going on for fifty years past; but there are
opposing conditions which make a limit in this direction, such as
the speed at which bearing surfaces may run, centrifugal strain,
jar, and vibration. The object is to fix upon a point between what
high speed, light weight, cheapness of cost suggest, and what the
conditions of practical use and endurance demand.
(1.) What does the term "machinery of transmission" include,
as applied in common use?--(2.) Why cannot direct comparisons
be made between shafts, belts, and gearing?--(3.) Define
the relation between speed and strain in machinery of
transmission.--(4.) What are the principal conditions which limit
the speed of shafts?