TRANSMISSION, OR CHANGE SPEED GEARS
Owing to the peculiar character of Internal Combustion Engines, there
is always a certain speed at which it will work more satisfactorily,
and with greater economy.
In this respect it is unlike the steam engine, which has a much wider
range of effectiveness. Since all cars now use internal combustion
motors, and throttling is unsatisfactory, as a means of controlling the
engine, or changing the speed and power, so as to use it economically,
a mechanical speed change system is essential.
This contains certain gears, which are designed to change the speed of
the transmission shaft relative to the engine shaft.
TRANSMISSION LEVERAGE.--It is simply using leverage in order to produce
a more effective pull, or to attain greater speed, from a shaft which
runs at a certain number of revolutions.
If we have a motor with a shaft speed of, say, 800 revolutions per
minute, and an axle with a speed of 400 revolutions, the ratio would
be 2 to 1. Now, to speed up the machine, so that the axle will turn
800 revolutions, would require an engine speed of 1600, which might be
impossible.
[Illustration: Fig. 42. Progressive Transmission. Low.]
ECONOMY OF TRANSMISSION GEARING.--From an economical standpoint, also,
it would be undesirable, even though the engine should be able to make
the speed.
Owing to the explosion impulses of the gasoline motor, a heavy fly
wheel is necessary on the engine shaft, in order to store up power by
momentum, and also to give a uniform speed.
In hill climbing, or in carrying heavy loads, the transmission shaft
must have its speed cut down, while permitting the engine to run at
full or normal speed.
[Illustration: Fig. 43. Neutral Position.]
The transmission gearing is, therefore, the most satisfactory solution
of the problem, because changing the engine speed destroys its
effectiveness, and we shall, therefore, consider some of the types for
that purpose.
There are two distinct systems of transmission, namely: The Positive,
and the Frictional. Of the positive system we have the planetary and
the sliding gear types. The sliding gear type has two methods of
control, one known as the _progressive_, and the other the _selective_.
CHARACTERISTICS OF TRANSMISSION.--The progressive, selective and
planetary types, are entirely different from the frictional system, for
the reason that they effect the changes by step movements, the speeds
being produced at certain ratios, whereas the frictional method has
indefinite and infinite ratios.
[Illustration: Fig. 44. Intermediate.]
The following diagrams will clearly bring out the distinctive features
of each. Fig. 42 shows a shaft A, which derives power from the engine,
having in line with it a shaft B, which connects with the driven shaft.
The shaft B is squared, but it has a round end C, which is socketed
axially within the head of the shaft A.
THE PROGRESSIVE.--The head D has a small pinion E, and on its side is
provided with projecting teeth F. The loosely-revolving squared shaft
B has thereon a pair of spur gear G H, separated from each other a
trifle more than the width of each gear, and they are united by an
intermediate hub so they turn in unison.
[Illustration: Fig. 45. High.]
Below the shaft B, and parallel therewith, is a shaft J, which carries
a spur gear K, that is constantly in mesh with the pinion E. To the
right is a smaller gear L, which is the same diameter as the gear G,
with which it is adapted to mesh; and a small gear M, about one-third
the diameter of the gear H, is also mounted on the right-hand end of
the shaft, which meshes with the gear H, when the latter is moved to
the right on its shaft B.
Behind the two gears H, M, is a shaft N, parallel with shaft J, which
is so mounted that it has a longitudinal movement, and this carries a
broad-faced pinion O, so that it is wide enough to engage with both of
the gears H, M, when they are not in line, or in engagement with each
other, as shown in Fig. 44.
This latter shaft N, is moved longitudinally by means of the reversing
lever P. This lever, together with the gear-shifting lever, hereafter
explained, are merely indicated in their present manner, in order to
show, diagrammatically, how the gears are shifted.
LOW GEAR.--The gear-shifting lever Q, in Fig. 42, in this instance,
shows the large gear H, moved into mesh with the gear M, so that power
is transmitted from the engine shaft A, through gears E, K, shaft J,
and gears M, H, to the driven shaft B.
In examining Fig. 43, it will be seen that the shifting lever I, has
moved the gears G, H, so they are intermediate to the gears L, M. The
mechanism is now at what is called the neutral position, which means
that the engine drives only the shaft A, and the shaft J, through the
gears E, K.
INTERMEDIATE GEAR.--Now, when the lever is moved over another step, as
in Fig. 44, the gears G L mesh together, and motion is transmitted from
the gear E, to gear K, through shaft J, and gears L G, to the shaft B.
[Illustration: Fig. 46. Reverse.]
This is called the _intermediate_, which in this size gears, drives the
shaft at half the engine speed, or half of the speed of shaft A, for
the reason that gears G L, are of the same diameter, and gears E and K
are in the ratio of 1 to 2.
HIGH GEAR.--When the lever is shifted another notch, as shown in Fig.
45, the crown teeth F G, of the respective gears E G, engage, and the
two shafts A B, are locked together, thus turning the two shafts in
unison. This is called direct drive, in which case the shaft B, turns
with the engine.
REVERSING.--When the car is not running the gears G H are always in a
neutral position, as shown in Fig. 41, and in order to reverse shaft B,
the lever P, is drawn back, as shown in Fig. 44, so that the small gear
O, will engage with the large and the small gears H M, respectively.
The result is, gear H, is reversed, and this reversal can take place
only when the two gears G H are in a neutral position.
The term _progressive_ takes its name from the motion of the control
lever involved in changing the gears. It proceeds regularly from the
lowest to the highest.
SELECTIVE TYPE.--The second method, the _selective_, enables the
operator to select any speed at will, and in doing so, it is not
necessary to go through the other speeds to reach the high or the low,
as is the case with the progressive.
Where there are only three speeds forward, and one in reversing,
this is not so material, but as the better class cars have four
speeds forward, it means that in order to reach _high_ the gear in a
progressive system must go through two intermediate speeds.
The shaft B, Fig. 47, which connects with the engine through a clutch,
has its end journaled in a driven shaft A, and a gear C is fixed to the
shaft B, and provided with a recessed side. This has internal teeth to
receive the teeth of a sliding gear D. Another, smaller, sliding gear E
is also on the shaft.
[Illustration: Fig. 47. Selective Transmission. Low Gear.]
Below the shafts A B is a shaft F, which carries a gear G, about half
the diameter of the gear C, with which it is constantly in engagement.
This shaft, further, has a gear I, the same diameter as the gear D,
with which it meshes, and the shaft also carries a gear K, smaller than
gear J.
Behind the gear K is an idler pinion L, in such position that it may be
slid into contact with K, and the gear E, on shaft B, is also adapted
to be meshed with the pinion L by sliding contact.
All the gears G I J K are keyed to the shaft F, and only the gears D E
and L are capable of being shifted.
LOW GEAR.--Fig. 47 shows the gears E J in engagement, and the motion
is, therefore, transmitted from the shaft B, through gears E J and gear
G to C, thereby giving a slow speed to the driven shaft A. This is
called _low_ gear.
[Illustration: Fig. 48. Intermediate.]
INTERMEDIATE GEAR.--To change into the intermediate, the gear D,
engages with I, Fig. 48, so that both shafts B F run at the same speed,
but in opposite directions, since these two gears are of the same
diameter. The selective mechanism, as hereinafter explained, shows
how this may be done so that the gear E, will also be thrown out of
engagement with J at the same time.
It will, of course, be understood that while the gears E J turn the
shaft F in a direction opposite the shaft B, the shaft A is again
reversed by the gears G C, so that both shafts A B, turn in the same
direction, but the shaft A, now turns at just half the speed of shaft
B, because the gear G is only half the diameter of C.
[Illustration: Fig. 49. High.]
HIGH GEAR.--The direct drive, Fig. 49, is arranged by connecting the
two shafts A B together, and this is done by means of the teeth of the
wheel D, engaging with the internal teeth of the gear C, so that shaft
A turns with the engine.
REVERSE GEAR.--The reversing engagement is brought about by putting the
gears K L E into mesh with each other, as in Fig. 50, thus making the
transmission from shaft B, through gears E L and K, shaft F, and back
to A, through gears G C.
[Illustration: Fig. 50. Reverse.]
A four-speed selection transmission uses four, instead of three, gears
on the driving shaft, without in any way changing the principles above
outlined.
CONTROL LEVER FOR PROGRESSIVE TRANSMISSION.--A careful study of the
following mechanism, taken in connection with the accompanying sketch
of the change speed gear, and the relations of the several elements,
will explain the method now generally employed in the use of the
_progressive_ type.
The diagram, Fig. 51, shows the engine 1, with its shaft 2, connected
directly with the shaft A of the transmission gearing. Intermediate the
gear box and the engine 1, is a clutch 4, with which the foot pedal 5
is connected.
[Illustration: Fig. 51. Progressive Control Mechanism.]
The gear box has thereon a fore and aft sliding bar 6, the forward end
of which projects through the case and is pivotally connected with the
change speed lever 7. The lever has a quadrant 8, alongside, with four
notches therein, for the low, intermediate, and high and also for the
neutral positions of the lever.
The sliding bar 6, has an arm, the fork of which spans the hub I of the
gears G H, so they may be carried in either direction when the speed
lever swings to and fro.
The reversing lever 10 may be connected up with a bar, similar to 6,
but for convenience herein, we employ a vertical lever R, pivoted to a
cross rock-shaft S. The lower end of this lever has a fork T to engage
the collar of the shaft N of the idler pinion. The upper end of the
lever is connected with the reversing lever 10 by a link U.
The quadrant, alongside the reversing lever, has two notches, as shown,
one being designed to hold the lever in a cut-out position, whereas the
other notch is to hold the lever 10 when the running gear is in action.
OPERATION OF THE PROGRESSIVE GEAR.--The relative arrangement of the
parts gives a comprehensive idea of the mechanical ideas involved, and
by referring to the description and illustrations of the gears, it will
be seen how the change lever 7, in moving back one notch, from its
neutral position, will throw the gear G into mesh with L, and another
movement of the lever to the next notch, will cause the crown teeth on
G, to engage with the teeth on gear E, and thus effect a high gear
connection.
THE SELECTOR MECHANISM.--This is more or less confusing to the novice,
and the accompanying illustration, Fig. 52, shows a perspective view,
in which some of the parts are drawn out of proportion, merely for the
sake of clearness. The aim is to show principles and not details of
exact mechanical construction.
[Illustration: Fig. 52. Selective Control Mechanism.]
SELECTOR BARS.--The two selector bars A B, are mounted in guide ways so
they move longitudinally alongside each other a limited distance. Each
bar has an arm, as at C D, the end of each having a curved finger E to
engage the annular grooves on the hubs of the shifting gears.
Above these bars, and at right angles thereto, is a rock-shaft F,
mounted in bearings G G, so that it is longitudinally-movable a limited
distance, to shift the selector lever H from one bar A to the other bar
B.
SHIFTING LEVER.--The selector I has two fore and aft slots J K,
these slots being of such width that the gear shifting lever L can
travel therein back and forth. Midway between the ends of the bar the
intermediate wall of the selector plate has a cut-out portion as at M,
so the lever may pass through.
This opening, or gate-way, is in such a position, relative to the cross
lots N O, of the bars A B, that when the lever is in line with the
gate-way, the slots N O are also in line, and in a neutral position,
so that when a lateral motion is imparted to the lever L, and the
rock-shaft F is moved longitudinally, the selector lever H will then
engage with the other bar.
SPEED SELECTORS.--The selector I, in Fig. 52, while made substantially
the same in all cars, has a different order of lever movement. Each
manufacturer has his own preferential type. In some cases the lever
must be thrown forward in order to reverse, and in others it is drawn
back.
In certain cars the lever is moved forwardly to throw the gears into
first, or low, while a number of makers insist that the first movement
should be to the rear.
[Illustration: Types of Speed Selectors.
Fig. 53. 3-Speed.
Fig. 54. 3-Speed.
Fig. 55. 3-Speed.
Fig. 56. 4-Speed.
Fig. 57. 4-Speed.
Fig. 58. 4-Speed.
Fig. 59. 4-Speed.]
This is, really, an immaterial matter, so long as there is no standard,
and each claims some distinctive feature of value for his particular
choice.
3-SPEED SELECTORS.--Figs. 53, 54 and 55 show the 3-speed selectors,
in each of which the reverse is brought about by moving the lever to
the forward end of the selector. In Figs. 53 and 54 the lever slot, for
reversing, is in the inside, whereas in 55 it is in the outside slot.
The form 53 also has, in certain makes, the reverse at the rear end of
the selector.
4-SPEED SELECTORS.--The greatest variety is found in the 4-speed types,
represented by Figs. 56, 57, 58 and 59, the almost universal plan being
to place the reverse in the single side slot, as shown in Fig. 56.
[Illustration: Fig. 60. Control-Lever Bracket.]
One of the most practicable and easily operated selectors is shown in
Fig. 60, which is used on the Jeffery car.
CONTROLLING THE SELECTOR.--It will be seen, on examination of the
selector, that if, in starting, the lever is at its neutral position,
as it should be, and it is moved inwardly the distance of about an
inch, it will be in position where it can be moved forward to the
_first speed_ position.
The clutch of the car may then be disengaged gently, by pressing the
foot down slowly, and at the same time pressing the accelerator with
the right foot, so as to increase the speed of the motor sufficiently
to take care of the load.
After the clutch engages and the car has traveled about ten feet,
pressure on the accelerator is released, and the clutch pedal pulled
down quickly, and the lever is then pulled straight back to the _second
speed_.
USING THE CLUTCH AND SELECTOR.--For the third and fourth speeds the
same course is followed. If, in hill climbing, or in going through a
heavy stretch of mud or sand, lower speed is required, the clutch is
thrown out, and, if traveling on fourth speed, the control lever is
quickly pulled to the rear end of the slot, and then the clutch thrown
in.
If it is on third speed, the clutch is disengaged, the control lever
pushed forward, at the same time pressing it inwardly so it will pass
through the gate, and then pulling it back to the second speed.
[Illustration: Fig. 61. Planetary Transmission.]
PLANETARY TRANSMISSION.--Fig. 61 shows the general arrangement of the
planetary transmission. The disk A, carries four small planet gears
B, B, B, B, the hub C´ of which is attached to the transmission shaft.
These four planet wheels mesh with and travel around a central gear C,
of the same diameter this gear being attached to the engine shaft D.
E is a loosely-revolving drum, with internal teeth, to mesh with the
planet wheels B. The drum E, and the disk or planet wheel carrier A,
are provided with braking mechanism so that either may be slowed down
or entirely stopped.
For slow speed E is stationary; for high speed A and E revolve with the
gear C; and for reversing A is locked by means of the brake.
[Illustration: Fig. 62. Frictional Transmission.]
FRICTIONAL TRANSMISSION.--A single illustration will suffice to show
the principle involved in _Frictional_ transmission. Fig. 62 represents
a driven shaft A, which receives its power from the engine, and on
which is mounted a friction wheel B, that is adapted to travel along on
the shaft in front of a friction disk C, secured to the transmission
shaft D.
The shaft A has a spline E, and means are provided at the end of the
wheel B to draw it back and forth on the shaft, the slightest movement
toward the center of the friction disk C serving to increase the speed
of the driven shaft D.