_PNEUMATIC MACHINERY FOR TRANSMITTING POWER._
Pneumatic machinery, aside from results due to the elasticity of
air, is analogous in operation to hydraulic machinery.
Water may be considered as a rigid medium for transmitting power,
corresponding to shafts and gear wheels; air as a flexible or
yielding one, corresponding to belts. There is at this time but
a limited use of pneumatic apparatus for transmitting power, but
its application is rapidly extending, especially in transporting
material by means of air currents, and in conveying power to mining
machinery.
The successful application of the pneumatic system at the Mont
Cenis Tunnel in Italy, and at the Hoosac Tunnel in America, has
demonstrated the value of the system where the air not only served
to transmit power to operate the machinery but to ventilate the
mines at the same time. Air brakes for railway trains are another
example illustrating the advantages of pneumatic transmission;
the force being multiplied at the points where it is applied,
so that the connecting pipes are reduced to a small size, the
velocity of the air making up for a great force that formerly had
to be communicated through rods, chains, or shafts. The principal
object attained by the use of air to operate railway brakes is,
however, to maintain a connection throughout a train by means of
flexible pipes that accommodate themselves to the varying distance
between the carriages. Presuming that the flow of air in pipes is
not materially impeded by friction or angles, and that there will
be no difficulty in maintaining lubrication for pistons or other
inaccessible parts of machinery when driven by air, there seems to
be many reasons in favour of its use as a means of distributing
power in manufacturing districts. The diminished cost of motive
power when it is generated on a large scale, and the expense and
danger of maintaining an independent steam power for each separate
establishment where power is employed, especially in cities, are
strong reasons in favour of generating and distributing power by
compressed air, through pipes, as gas and water are now supplied.
Air seems to be the most natural and available medium for
transmitting and distributing power upon any general system like
water or gas, and there is every probability of such a system
existing at some future time. The power given out by the expansion
of air is not equal to the power consumed in compressing it, but
the loss is but insignificant compared with the advantages that
may be gained in other ways. There is no subject more interesting,
and perhaps few more important for an engineering student to study
at this time, than the transmission of power and the transport of
material by pneumatic apparatus.
In considering pneumatic machinery there are the following points
to which attention is directed:--
1. The value of pneumatic apparatus in reaching places where steam
furnaces cannot be employed.
2. The use that may be made of air after it has been applied as a
motive agent.
3. The saving from condensation, to which steam is exposed,
avoidance of heat, and the consequent contraction and expansion of
long conducting pipes.
4. The loss of power by friction and angles in conducting air
through pipes.
5. The lubrication of surfaces working under air pressure, such as
the pistons and valves of engines.
6. The diminished cost of generating power on a large scale,
compared with a number of separate steam engines distributed over
manufacturing districts.
7. The effect of pneumatic machinery in reducing insurance rates
and danger of fire.
8. The expense of the appliances of distribution and their
maintenance.
In passing thus rapidly over so important a subject, and one that
admits of so extended a consideration as machinery of transmission,
the reader can see that the purpose has been to touch only upon
such points as will lead to thought and investigation, and
especially to meet such queries as are most likely to arise in
the mind of a learner. In arranging and erecting machinery of
transmission, obviously the first problem must be, what kind of
machinery should be employed, and what are the conditions which
should determine the selection and arrangement? What has been
written has, so far as possible, been directed to suggesting proper
means of solving these questions.
(1.) In what respect are air and water like belts and gearing,
as means to transmit power?--(2.) What are some of the
principal advantages gained by employing air to operate railway
brakes?--(3.) Name some of the advantages of centralising motive
power.--(4.) Are the conditions of working an engine the same
whether air or steam is employed?