GEARS AND GEARING
For the purpose of showing how motion may be converted from a straight
line or from a circular movement into any other form or direction, and
how such change may be varied in speed, or made regular or irregular,
the following examples are given, which may be an aid in determining
other mechanical devices which can be specially arranged to do
particular work.
While cams and eccentrics may be relied on to a certain extent, there
are numerous places where the motion must be made positive and
continued. This can be done only by using gearing in some form, or such
devices as require teeth to transmit the motion from one element to the
other.
The following illustrations do not by any means show all the forms which
have been constructed and used in different machines, but they have been
selected as types merely, in order to give the suggestions for other
forms.
Racks and Pinions.--The rack and pinion is the most universal piece of
mechanism for changing motion. Fig. 101 illustrates it in its most
simple form. When constructed in the manner shown in this figure it is
necessary that the shaft which carries the pinion shall have a rocking
motion, or the rack itself must reciprocate in order to impart a rocking
motion to the shaft.
[Illustration: Fig. 101. Rack and Pinion.]
[Illustration: Fig. 102. Rack Motion.]
This is the case also in the device shown in Fig. 102, where two rack
bars are employed. A study of the cams and eccentrics will show that the
transference of motion is limited, the distances being generally very
small; so that the rack and pinions add considerably to the scope of the
movement.
The Mangle Rack.--The device called the _mangle rack_ is resorted to
where a back and forth, or a reciprocating movement is to be imparted
to an element by a continuous rotary motion.
[Illustration: Fig. 103. Plain Mangle Rack.]
[Illustration: Fig. 104. Mangle Rack Motion.]
[Illustration: Fig. 105. Alternate Circular Motion.]
The plain mangle racks are shown in Figs. 103 and 104, the former of
which has teeth on the inside of the opposite parallel limbs, and the
latter, Fig. 104, having teeth not only on the parallel sides, but also
around the circular parts at the ends.
This form of rack may be modified so that an alternate circular motion
will be produced during the movement of the rack in either direction.
Fig. 105 is such an instance. A pinion within such a rack will turn
first in one direction, and then in the next in the other direction, and
so on.
If the rack is drawn back and forth the motion imparted to the pinion
will be such as to give a continuous rocking motion to the pinion.
Controlling the Pinion.--Many devices have been resorted to for the
purpose of keeping the pinion in engagement with the teeth of the mangle
rack. One such method is shown in Fig. 106.
[Illustration: Fig. 106. Controlling Pinion for Mangle Rack.]
The rack A has at one side a plate B, within which is a groove C, to
receive the end of the shaft D, which carries the pinion E. As the
mangle rack moves to such a position that it reaches the end of the
teeth F on one limb, the groove C diverts the pinion over to the other
set of teeth G.
All these mangle forms are substitutes for cranks, with the advantage
that the mangle gives a uniform motion to a bar, whereas the to and fro
motion of the crank is not the same at all points of its travel.
Examine the diagram, Fig. 107, and note the movement of the pin A which
moves along the path B. The crank C in its turning movement around the
circle D, moves the pin A into the different positions 1, 2, 3, etc.,
which correspond with the positions on the circle D.
[Illustration: Fig. 107. Illustrating Crank-pin Movement.]
The Dead Centers.--There is also another advantage which the rack
possesses. Where reciprocating motion is converted into circular motion,
as in the case of the ordinary steam engine, there are two points in the
travel of a crank where the thrust of the piston is not effective, and
that is at what is called the _dead centers_.
In the diagram, Fig. 108, the ineffectiveness of the thrust is shown at
those points.
Let A represent the piston pushing in the direction of the arrow B
against the crank C. When in this position the thrust is the most
effective, and through the arc running from D to E, and from H to G,
the cylinder does fully four-fifths of the work of the engine.
[Illustration: _Fig. 108. The Dead Center._]
While the crank is turning from G to D, or from I to J, and from K to L,
no work is done which is of any value as power.
If, therefore, a mangle bar should be used instead of the crank it would
add greatly to the effectiveness of the steam used in the cylinder.
[Illustration: _Fig. 109. Crank Motion Substitute._]
Crank Motion Substitute.--In Fig. 109 the pinion A is mounted so that
its shaft is in a vertical slot B in a frame C. The mangle rack D, in
this case, has teeth on its outer edge, and is made in an elongated
form. The pinion shaft moves up and down the slot and thus guides the
pinion around the ends of the rack.
[Illustration: Fig. 110. Mangle Wheel.]
Mangle Wheels.--The form which is the most universal in its application
is what is called the _mangle wheel_. In Fig. 110 is shown a type
wherein the motion in both directions is uniform.
Mangle wheels take their names from the ironing machines called
_mangles_. In apparatus of this kind the movement back and forth is a
slow one, and the particular form of wheels was made in order to
facilitate the operation of such machines. In some mangles the work
between the rollers is uniform back and forth. In others the work is
done in one direction only, requiring a quick return.
In still other machines arrangements are made to provide for short
strokes, and for different speeds in the opposite directions, under
certain conditions, so that this requirement has called forth the
production of many forms of wheels, some of them very ingenious.
[Illustration: _Fig. 111. Quick Return Motion._]
The figure referred to has a wheel A, on one side of which is a
peculiarly-formed continuous slot B, somewhat heart-shaped in general
outline, one portion of the slot being concentric with the shaft C.
Within the convolutions of the groove is a set of teeth D, concentric
with the shaft C. The pinion E, which meshes with the teeth D, has the
end of its shaft F resting in the groove B, and it is also guided within
a vertical slotted bar G.
The pinion E, therefore, travels over the same teeth in both directions,
and gives a regular to and fro motion.
Quick Return Motion.--In contradistinction to this is a wheel A, Fig.
111, which has a pair of curved parallel slots, with teeth surrounding
the slots. When the wheel turns nearly the entire revolution, with the
pinion in contact with the outer set of teeth, the movement transmitted
to the mangle wheel is a slow one.
[Illustration: _Fig. 112. Accelerated Circular Motion._]
When the pinion arrives at the turn in the groove and is carried around
so the inner teeth are in engagement with the pinion, a quick return is
imparted to the wheel.
Accelerated Motion.--Aside from the rack and mangle type of movement,
are those which are strictly gears, one of them being a volute form,
shown in Fig. 112. This gear is a face plate A, which has teeth B on one
face, which are spirally-formed around the plate. These mesh with a
pinion C, carried on a horizontal shaft D. This shaft is feathered, as
shown at E, so that it will carry the gear along from end to end.
[Illustration: _Fig. 113. Quick Return Gearing._]
The gear has cheek-pieces F to guide it along the track of teeth. As the
teeth approach the center of the wheel A, the latter impart a motion to
the gear which is more than twice the speed that it receives at the
starting point, the speed being a gradually increasing one.
Quick Return Gearing.--Another much more simple type of gearing, which
gives a slow forward speed and a quick return action, is illustrated in
Fig. 113. A is a gear with internal teeth through one half of its
circumference, and its hub B has teeth on its half which is opposite the
teeth of the rim.
A pinion C on a shaft D is so journaled that during one half of the
rotation of the wheel A, it engages with the rim teeth, and during the
other half with the hub teeth. As the hub B and gear C are the same
diameter, one half turn of the pinion C will give a half turn to the
wheel A.
[Illustration: _Fig. 114. Scroll Gearing._]
As the rim teeth of the wheel A are three times the diameter of the
pinion C, the latter must turn once and a half around to make a half
revolution of the wheel A.
Scroll Gearing.--This is a type of gearing whereby at the close of each
revolution the speed may be greater or less than at the beginning. It
comprises two similarly-constructed gears A, B, each with its perimeter
scroll-shaped, as shown.
The diagram shows their positions at the beginning of the rotation, the
short radial limb of one gear being in line with the long limb of the
other gear, hence, when the gears rotate, their speeds relative to each
other change, being constantly accelerated in one or decreased in the
other.