_GEARING AS A MEANS OF TRANSMITTING POWER._
The term gearing, which was once applied to wheels, shafts, and the
general mechanism of mills and factories, has now in common use
become restricted to tooth wheels, and is in this sense employed
here. Gearing as a means of transmitting motion is employed when
the movement of machines, or the parts of machines, must remain
relatively the same, as in the case of the traversing screw of an
engine lathe--when a heavy force is transmitted between shafts
that are near to each other, or when shafts to be connected are
arranged at angles with each other. This rule is of course not
constant, except as to cases where positive relative motion has
to be maintained. Noise, and the liability to sudden obstruction,
may be reasons for not employing tooth wheels in many cases when
the distance between and the position of shafts would render
such a connection the most durable and cheap. Gearing under
ordinary strain, within limited speed, and when other conditions
admit of its use, is the cheapest and most durable mechanism
for transmitting power; but the amount of gearing employed in
machinery, especially in Europe, is no doubt far greater than it
will be in future, when belts are better understood.
No subject connected with mechanics has been more thoroughly
investigated than that of gearing. Text-books are replete with
every kind of information pertaining to wheels, at least so far
as the subject can be made a mathematical one; and to judge from
the amount of matter, formulæ, and diagrams, relating to the teeth
of wheels that an apprentice will meet with, he will no doubt be
led to believe that the main object of modern engineering is to
generate wheels. It must be admitted that the teeth of wheels
and the proportions of wheels is a very important matter to
understand, and should be studied with the greatest care; but it is
equally important to know how to produce the teeth in metal after
their configuration has been defined on paper; to understand the
endurance of teeth under abrasive wear when made of wrought or cast
iron, brass or steel; how patterns can be constructed from which
correct wheels may be cast, and how the teeth of wheels can be cut
by machinery, and so on.
A learner should, in fact, consider the application and operative
conditions of gearing as one of the main parts of the subject, and
the geometry or even the construction of wheels as subsidiary; in
this way attention will be directed to that which is most difficult
to learn, and a part for which facilities are frequently wanting.
Gearing may be classed into five modifications--spur wheels, bevel
wheels, tangent wheels, spiral wheels, and chain wheels; the last
I include among gearing because the nature of their operation is
analogous to tooth wheels, although at first thought chains seem
to correspond more to belts than gearing. The motion imparted
by chains meshing over the teeth of wheels is positive, and not
frictional as with belts; the speed at which such chains may run,
with other conditions, correspond to gearing.
Different kinds of gearing can be seen in almost every engineering
establishment, and in view of the amount of scientific information
available, it will only be necessary to point out some of the
conditions that govern the use and operation of the different kinds
of wheels. The durability of gearing, aside from breaking, is
dependent upon pressure and the amount of rubbing action that takes
place between the teeth when in contact. Spur wheels, or bevel
wheels, when the pitch is accurate and the teeth of the proper
form, if kept clean and lubricated, wear but little, because the
contact between the teeth is that of rolling instead of sliding.
In many cases, one wheel of a pair is filled with wooden cogs;
in this arrangement there are four objects, to avoid noise, to
attain a degree of elasticity in the teeth, to retain lubricants
by absorption in the wood, and to secure by wear a better
configuration of the teeth than is usually attained in casting, or
even in cutting teeth.
Tangent wheels and spiral gearing have only what is termed line
contact between the bearing surfaces, and as the action between
these surfaces is a sliding one, such wheels are subject to
rapid wear, and are incapable of sustaining much pressure, or
transmitting a great amount of power, except the surfaces be hard
and lubrication constant. In machinery the use of tangent wheels
is mainly to secure a rapid change of speed, usually to diminish
motion and increase force.
By placing the axes of tangent gearing so that the threads or
teeth of the pinions are parallel to the face of the driven teeth,
as in the planing machines of Messrs Wm. Sellers & Co., the
conditions of operation are changed, and an interesting problem
arises. The progressive or forward movement of the pinion teeth
may be equal to the sliding movement between the surfaces; and an
equally novel result is, that the sliding action is distributed
over the whole breadth of the driven teeth.
In spiral gearing the line of force is at an angle of forty-five
degrees with the bearing faces of the teeth, and the sliding
movement equal to the speed of the wheels at their periphery;
the bearing on the teeth, as before said, is one of line contact
only. Such wheels cannot be employed except in cases where an
inconsiderable force is to be transmitted. Spiral wheels are
employed to connect shafts that cross each other at right angles
but in different planes, and when the wheels can be of the same
size.
It may be mentioned in regard to rack gearing for communicating
movement to the carriages of planing machines or other purposes of
a similar nature: the rack can be drawn to the wheel, and a lifting
action avoided, by shortening the pitch of the rack, so that it
will vary a little from the driving wheel. The rising or entering
teeth in this case do not come in contact with those on the rack
until they have attained a position normal to the line of the
carriage movement.
(1.) Into what classes can gearing be divided?--(2.) What
determines the wearing capacity of gearing?--(3.) What is the
advantage gained by employing wooden cogs for gear wheels?--(4.)
Why are tangent or worm wheels not durable?