TRANSMISSION
The =transmission= of an automobile consists of those parts that
transmit to the driving wheels the power developed by the engine.
THE CLUTCH
Because a gasoline engine must be in operation before it can deliver
power, a =clutch= is provided by means of which it may run free or
be so connected that it drives the car, one of its two chief parts
being attached to the engine and the other to the transmission.
When the two parts are in contact, the transmission is driven, and
when separated, the engine and transmission are independent of each
other, and may be stationary or in motion. A clutch must be of such
a nature that it does not apply the power of the engine instantly,
but gradually, so that the car starts slowly and without jerking.
If the power were to be applied suddenly, the effort of starting the
stationary car would either overcome the momentum of the engine and
stop it, or would jerk the car into such sudden motion that it might
be badly wrenched. By making the clutch so that it is permitted to
slip when first applied, the part that is driven is gradually brought
to the speed of the part that drives, when slipping ceases and the
two make firm contact.
The most usual form of clutch is the =friction cone=, in which the
fly wheel of the engine is utilized as the driving part, the rim
being broad and thick, with its inner side funnel-shaped, or beveled.
A metal =cone= that fits the bevel is carried on the end of a shaft
of the transmission, the shaft at this point being square, to fit
a square hole in the hub of the cone. This arrangement permits
the cone to slide along the shaft while always revolving with it.
When the cone is pressed to a seat in the fly wheel, which is
accomplished by means of a heavy coil spring, the friction between
its leather-covered surface and the surface of the fly wheel causes
the two parts to revolve together, and by its fit on the square
shaft the transmission is set in motion, and through that the car.
The clutch is thrown out of contact by means of a foot pedal that
acts on a ring fitting in a groove around the hub of the cone; when
the pedal is released, the spring forces the cone to its seat (Fig.
31). In order to support it, the end of the shaft carrying the cone
projects into the hub of the fly wheel, where it rests in a bearing,
this arrangement in no manner preventing the two parts from acting
independently of each other.
In the =reversed type= of friction-cone clutch a funnel-shaped ring
is bolted to the rim of the fly wheel and forms the seat, the cone
fitting inside of it (Fig. 31). Depressing the pedal moves the cone
toward the fly wheel instead of away from it, as in the regular type,
and while there is no difference in the effect of one as against the
other, the reversed type is more compact.
[Illustration: _FRICTION CONE CLUTCH_ _REVERSED FRICTION CONE CLUTCH_
FIG. 31.—FRICTION CONE CLUTCHES.]
The =multiple-disk clutch=, which is rapidly coming into use, depends
on the friction between the flat surfaces of metal when pressed
together. An experiment illustrating this is to place a silver
dollar between two half dollars, and to press them together between
the thumb and finger. It will be found that a light pressure is
sufficient to produce friction that will make it difficult to revolve
the large coin between the smaller ones.
The parts of a simple form of multiple-disk clutch, as shown in
Fig. 32, are a =flange= on the engine shaft, a smaller flange with
a square-hole, square-shaft arrangement on the transmission shaft,
and large and small =rings= placed alternately. The large rings are
driven by the large flange, fitting loosely on pins, or =studs=,
projecting from it, and the small rings are similarly attached to
the transmission shaft. The openings in the large or driving rings
are large enough to contain the studs carrying the small rings, so
that when the parts are assembled the outer surfaces of the small or
driven rings are in contact with the inner surfaces of the driving
rings. A heavy coil spring is arranged to press the small flange
toward the flange on the engine shaft, binding the rings that it
carries between the driving rings, and the latter are often faced
with leather to increase the friction between them. When the small
flange is released from the pressure of the spring by depressing
the pedal, the driving and driven flanges with their rings are
independent of each other, and the engine may run free while the
transmission shaft is stationary or revolving. A clutch of this type
is incased and runs in oil, which prevents the rings from gripping
suddenly; when the pressure of the spring is applied, the oil is
gradually squeezed out from between them, and the slipping of the
driving and driven rings is reduced as they are forced into contact.
[Illustration: _PARTS OF MULTIPLE DISK CLUTCH_
_MULTIPLE DISK CLUTCH ASSEMBLED_
FIG. 32.—MULTIPLE DISK CLUTCH.]
An =internal-expanding= clutch consists of a broad ring, or =drum=,
against the inner surface of which bear two pieces of metal shaped to
fit. The pieces of metal, or =shoes=, are pivoted together at one end
so that they may be moved in or out, after the manner of the handles
of a pair of scissors; when open they bear against the inside surface
of the drum, and when closed they are free from it. The drum is
attached to the engine shaft and the shoes to the transmission shaft,
the friction between them being so great that the transmission shaft
is carried around as the drum revolves. The shoes are kept in contact
with the drum by a coil spring, the depression of a pedal releasing
them from its pressure.
CHANGE-SPEED MECHANISM
The =change-speed mechanism=, to which the clutch transmits the power
of the engine, is to the engine what a block and tackle is to a man
who lifts a heavy weight, and is necessary because of the varying
resistance to the movement of the car in traversing steep, rough
hills and smooth avenues. A change-speed mechanism may be defined as
an arrangement by which the relative number of revolutions of the
crank shaft and driving wheels may be altered to suit conditions.
If the driving wheels revolve but once while the crank shaft makes
twelve revolutions, the car will move at one sixth the speed that it
would have if the wheels revolved once to every two revolutions of
the crank shaft, but it will have six times the ability to overcome
the resistance presented by a hill or sandy road.
If a gasoline engine were so connected that the relative number of
revolutions of the crank shaft and wheels could not be changed,
a slowing down of the car through the resistance presented by a
rough hill would slow the engine to correspond, and as speed is an
important factor in the power that the engine delivers, it would be
prevented from doing the work of which it is capable at the time
when it was most necessary. By means of the change-speed mechanisms
in most general use, the relative number of revolutions of the crank
shaft to one of the wheels may be varied to from two to eighteen, the
former giving the car high speed over a smooth road, and the latter
slow speed, but greater ability to overcome hills and heavy roads.
To attain this result, gears are used. If two gears having the same
number of teeth are in mesh, they will make the same number of
revolutions, and the force with which the driven gear will revolve
will be the same as that of the driving gear, less the friction of
the teeth. If the driven gear has twice the number of teeth of the
driving gear, it will revolve at half the speed, but with twice the
force.
The forms of change-speed mechanism most largely used are based on
this principle, which is so applied that the gear driven by the
engine may be in mesh with a gear that has many more teeth and
revolves much slower in consequence, or a gear that has possibly
one and a half times the number of teeth, or a gear that has the
same number of teeth, and therefore revolves at the same speed. The
=sliding-gear= mechanism takes its name from the arrangement by which
the changes in the combination of gears is effected by sliding them
along a shaft, to mesh with other gears on a shaft driven by the
engine.
[Illustration: _NEUTRAL_ _SLOW SPEED_ _SECOND SPEED_ _HIGH SPEED_
_REVERSE_ FIG. 33.—SLIDING GEAR—PROGRESSIVE TYPE. _A_, Sleeve driven
by engine; _B_, gear on sleeve; _C_, gear on countershaft; _D_,
low-speed gears; _E_, second-speed gears; _F_, idler for reverse;
_G_, clutch for high speed; _H_, connected to rear wheels; _J_, gears
sliding on square shaft.]
The driver changes the gears by moving a lever that in the
=progressive type= moves forward by degrees to move the car on
the slow speed, the intermediate speeds, and the high. A typical
arrangement of the progressive type of sliding change-speed mechanism
is shown in Fig. 33. The power of the engine is transmitted to a
short, hollow shaft, called a =sleeve= (_A_), which carries a gear
(_B_) that is in permanent mesh with a gear (_C_) on the end of the
=countershaft=. Parallel to the countershaft is another shaft, one
end of which is held in a bearing in the hollow sleeve; while the
sleeve supports this shaft, the two may revolve independently of each
other. The second shaft is square, or of such a construction that
while the two gears that it carries may slide along, they revolve
with it. The gears on the square shaft are of different sizes, and
in sliding on it come successively into mesh with gears carried on
the countershaft. Because of the gears between them, the countershaft
revolves when the engine revolves the sleeve; but the speed of the
square shaft depends on the combination of gears in mesh between it
and the countershaft. When the sliding gears are in such a position
that they are not in mesh with the countershaft gears, the square
shaft is independent of the countershaft, and may revolve or be
stationary, the gears then being in the =neutral position=. When
the sliding part is moved so that its largest gear is in mesh with
the smallest of the countershaft gears (_D_), the square shaft will
revolve at a slower speed than the countershaft, because its gear
is larger than the one driving it. Again sliding the moving part
will separate these gears, and bring the next pair (_E_) into mesh,
the square shaft then moving at a higher speed, but still slower
than the countershaft because of the difference in the size of the
gears. Sliding the moving part still farther along the shaft will
disengage the =second-speed= gears and engage the =high speed=, in
which the square shaft revolves at the speed of the sleeve and crank
shaft, this being effected by locking the moving part to the sleeve
by means of a clutch (_G_). This clutch consists of several fingers
projecting from the moving part, corresponding to the spaces between
similar fingers on the end of the sleeve. The locking together of the
square shaft and sleeve gives what is known as the =direct drive=,
which is of comparatively recent development; many designs of sliding
gears still use a third pair of gears which, being of the same size,
give the square shaft the speed of the crank shaft. By the use of
the direct drive, the power of the engine is directly applied to the
square shaft, avoiding the loss that occurs through the friction of
the teeth of the gears.
The revolution of the square shaft is transmitted to the driving
wheels, the speed of the car therefore corresponding to the speed at
which the square shaft is driven by the gear combinations between it
and the countershaft. To obtain the =reverse=, which enables the car
to be backed without reversing the engine, a third gear is introduced
between the low-speed gears of the square shaft and countershaft.
When two gears are in mesh, they revolve in opposite directions,
but when one of them is in addition meshed with a third gear, the
first and third will revolve in the same direction, and opposite to
the direction in which the middle gear revolves. When the car is
going forward, the square shaft and countershaft revolve in opposite
directions, but when the reverse gear is introduced between them, the
square shaft is revolved in the same direction as the countershaft,
reversing the rotation of the driving wheels.
The ends of the teeth of the gears are chisel-shaped, instead of
being flat, as in ordinary gears, so that they will go into mesh
easily.
The greatest economy in the operation of a gasoline engine results
from its running at as nearly constant a speed as possible, and the
gear is therefore changed when the resistance of the road to the
movement of the car decreases the speed of the engine, or permits it
to increase.
The =selective type= of sliding change-speed mechanism as shown on
Fig. 34 is in use on many of the high-grade cars, and its control
differs from that of the progressive type described in that the lever
has only a short movement forward and back, and in addition may slide
sideways. To the lower end of the lever is attached a shaft that
rocks in its bearings as the lever is moved forward or back, and
slides lengthways when the lever is moved toward or away from the car.
[Illustration: FIG. 34.—SELECTIVE TYPE. _A_, Sleeve driven by engine;
_B_, fixed gear on sleeve; _C_, fixed gear on countershaft; _D_,
low-speed gears; _E_, second-speed gears; _F_, third-speed gears;
_G_, clutch for direct drive; _H_, rod and arm for third-speed and
direct drive; _J_, rod and arm for low and second speeds; _K_, rod
and arm for reverse, _L_, rocking-shaft finger in groove; _M_, guide
plate and control lever; _N_, bevel gears on square shaft and jack
shaft; _O_, idler gear for reverse.]
The arrangement of the countershaft and square shaft is the same as
in the progressive type, but there are two sets of sliding gears
instead of one, and these are moved by means of arms that extend
from rods sliding in bearings at the side of the gear case. When
these rods are slid endways, the gears attached to their arms slide
on the square shaft to correspond, and go in or out of mesh with
the countershaft gears. Across the ends of these rods are grooves,
which when the gears are in the neutral position are in line with the
rocking shaft attached to the control lever. From the inward end of
the rocking shaft a finger (_L_) projects downward into the groove;
when the grooves are in line, the rocking shaft may be slid endways,
the finger passing from one groove to the next without affecting the
rods. When the shaft is rocked, however, the finger in engaging one
of the grooves slides the rod endways, shifting the gears controlled
by its arm. Moving the control lever into such a position that it may
enter the middle slots of the guide plate (_M_) slides the rocking
shaft so that its finger projects into the groove of the central
sliding rod (_J_), and if the control lever is then pushed forward
so that it enters the front half of the slot, the sliding rod will
be moved by the finger in the opposite direction, and the low-speed
gears (_D_) will be brought into mesh. Bringing the lever back to the
central position will separate the gears, and moving it to the back
half of the slot will slide the same gears in the opposite direction,
meshing the second-speed combination (_E_). Moving the control lever
outward so that it is in line with the outside slot brings the finger
into the groove of the sliding rod (_H_) that moves the third and
high speeds (_F_ and _G_), the latter being direct drive, and the
reverse is obtained through the movement of the sliding rod (_K_)
that is engaged by the finger when the control lever is in the inside
slot. The movement of this rod brings a third gear (_O_) into mesh
with the low-speed gears on the square shaft and countershaft, and
the rotation of the countershaft is reversed.
While this type is in general use, the gears are often so arranged
that the direct drive combination is reached when the control lever
is in the third-speed position, the gears meshed by the fourth-speed
position driving the square shaft at a still higher speed. This high
speed can only be used for running under the best road conditions.
The advantages of the selective type over the progressive are the
shorter movements of the control lever and the ability to pass from
one speed to any other without the necessity of first meshing and
unmeshing those between, or, in other words, there is a neutral
position between every combination of gears, and from neutral any
desired combination may be obtained directly without reference to the
others.
In starting up a car fitted with either of these types of
change-speed mechanism, it is necessary to withdraw the clutch before
sliding the gears, and this is also necessary in changing from one
combination to another. The square or corresponding shaft of a
change-speed mechanism is always connected with the driving wheels,
and is at rest when the car is standing. With the engine running, the
countershaft will be revolving, and it will obviously be difficult to
slide the stationary gear into mesh with a gear that is revolving.
When the clutch is withdrawn, the countershaft moves only through
momentum, and will be brought to a stop by the contact of the teeth
of the sliding gear as that is moved against it, the two then easily
going into mesh. The clutch is then thrown in slowly, and will bring
the speed of the countershaft to the speed of the crank shaft.
When the car is moving, sliding the gears without first withdrawing
the clutch will bring together two gears that are revolving at
different speeds, and as it is necessary for them to be rotating
equally in order that they may mesh, either the speed of the car
must be changed to bring the speed of the gear on the square shaft
to that of the countershaft gear, or the speed of the engine must
be changed to bring the countershaft gear to the speed of the gear
on the square shaft. If the change is from a low to a higher speed,
the countershaft will be moving much faster than the square shaft,
and their gears being brought into contact will result in the
slowing of one and speeding up of the other until the speeds are
the same, but in so doing the ends of the teeth will grind against
each other, resulting in the wear of the chisel-pointed ends, if not
in the breaking of the teeth. Withdrawing the clutch obviates this
difficulty, for it frees the countershaft, permitting its gear to
take the speed of the square-shaft gear without wear or damage, and
when the change is made, the slow engagement of the clutch brings the
speed of both to that required by the crank shaft.