TRANSMISSION
The parts of a tractor by which the power of the engine is applied
to the driving wheels are called the _transmission_, and include the
_clutch_, _the change speed gear_, the _differential_ and the _drive_.
It has been shown that a gas engine delivers power only when it is
running at speed; it cannot run until some outside power drives it
through the inlet and compression strokes.
The tractor cannot move until the engine is running and delivering
power, and it follows, therefore, that it must be possible to
disconnect the engine from the driving mechanism in order that it may
run independently. This is done by means of a _clutch_, which is a
device that connects two shafts, or disconnects them.
[Illustration: FIG. 60.—INTERNAL CLUTCH]
A clutch must be so made that when it is engaged it takes hold, not
suddenly, but gradually. If it took hold suddenly, the tractor would be
required to jump at once into full motion; this would cause a severe
straining of the parts and probable breakage. The alternative would be
the abrupt stopping of the engine, and this would also strain things.
By making the clutch in such a way that it slips, and takes hold little
by little, the tractor starts slowly, and gradually comes up to speed;
the slipping of the clutch then ceases, and it takes hold firmly.
All clutches operate by the friction of one surface against another; in
some, the surfaces are curved and in others flat, while in still others
the clutch is a band around a wheel, or _drum_. A clutch is operated by
a hand lever or by a foot pedal.
Figure 60 shows a type of clutch that operates inside a drum, which is
often the overhanging rim of the flywheel. The shaft in the center is
independent of the flywheel, and it is the purpose of the clutch, which
is attached to the shaft, to lock the shaft and flywheel together when
the tractor is to be started.
The brake shoes, which bear against the drum, form the ends of pivoted
levers, and are lined with an asbestos material that resists the heat
caused by the friction against the drum.
A cone-shaped block of steel slides lengthways on the shaft; when it
is pushed into position, it forces out the yokes, and thus presses the
brake shoes against the drum.
A _plate clutch_, or _disk clutch_, is shown in Figure 61. The
principle of a plate clutch may be illustrated by placing a half-dollar
between two quarters and pinching them with the thumb and forefinger.
If they are held loosely, the half-dollar may be turned between the
quarters, but if they are pinched tightly, the friction between the
coins will be so great that one cannot be turned without turning the
others.
[Illustration: FIG. 61.—PLATE CLUTCH]
Attached to the flywheel are studs, which support a disk, or plate;
this plate revolves with the flywheel, and is practically a part of it.
On either side of this plate are other plates that are supported on the
drive shaft; they revolve with it, but can slide along it. The end of
the shaft is square and fits a square hole in a collar, so that while
the collar may slide along the shaft, the two must turn together. Cams
are mounted on the hub of one of the plates in such a position that
they can press the outside plates together and pinch the flywheel plate
between. The cams are operated by pressing the collar against them.
The first drawing shows the clutch out, or released; the flywheel may
then turn without turning the shaft, for the plates are not in contact.
The second drawing shows the clutch in, or engaged. The collar is
pressed against the cams, and the plates in turn are drawn together,
pinching the flywheel plate between them. The flywheel and the drive
shaft then revolve together.
Plate clutches are often made with more than three plates; some makes
run in a bath of oil, and some are intended to work dry.
In a cone clutch, the overhanging rim of the flywheel is funnel-shaped,
and into it fits a cone-shaped disk carried on the end of the drive
shaft. To engage the clutch, the disk is slid along the shaft against
the flywheel, the friction between the two being sufficient to drive
the shaft.
When a clutch is thrown in it should take hold gradually, slipping at
first, but finally having a firm grip. When it is thrown out, it should
release instantly and completely.
The power delivered by an engine depends on the _bore_ and _stroke_
of the cylinder, and on the speed. The greater the bore, or diameter
of the cylinder, and the greater the stroke, or distance the piston
moves in a half-revolution of the crank shaft, the larger will be the
combustion space, and the larger will be the charge of mixture that
it can take in; the larger the charge, the greater will be the power
produced when the charge burns.
Each cylinder produces power once during every two revolutions of the
crank shaft; if the engine runs at 1,000 revolutions per minute there
will be twice as many power strokes as there would be if it ran at 500
revolutions per minute, and during that minute it will produce twice as
much power.
A traction engine is intended to run at a certain speed, at which it
will produce its greatest power without overstraining its parts. This
_normal speed_ for any particular engine depends on the number of
cylinders, their size and design, and other details established by the
manufacturer. To get the best from the engine, this is the speed at
which it should always be run.
The power required to move the tractor depends on various things;
the hardness and smoothness of the ground, the grade, the load it is
pulling, and so on. The tractor might be running on level ground,
pulling so great a load that the engine is called on for all of the
power that it can deliver.
On coming to a hill, still more power will be required, for now the
tractor and its load must be lifted as well as moved forward. The
engine, already working at its limit, cannot deliver the extra power
needed, and will slow down and stop unless something is done to aid
it. In such a case, the change speed gear is used to give the engine
a greater leverage on its work, just as a block and tackle gives a
greater leverage or purchase to a man who must lift a heavy weight.
Let us say that the normal speed of the engine is 1,000 revolutions per
minute, and that it is so connected that it makes 40 revolutions while
the driving wheels make 1, the speed of the tractor being 3 miles per
hour. If it is a 4-cylinder engine there will thus be 80 power strokes
to every revolution of the driving wheels. The engine is delivering its
full power and cannot do more should the tractor be called on for an
extra exertion, such as climbing a hill or crossing rough ground.
By changing the connections between the engine and the driving wheels,
the engine can be made to run twice as many revolutions to one turn of
the driving wheels, which will give double the number of power strokes;
the wheels will thus be turned with twice the force. As no change is
made in the speed of the engine, the wheels will now turn at half their
former speed, and the tractor will run at 1½ miles per hour. It will,
however, have twice the ability to overcome obstacles.
This change in the connections between the engine and the drive is
performed by the _change speed gear_, which is driven by the engine and
which in turn drives the wheels.
There are many varieties of change speed gears, but the main principle
in them all is the same, for they depend on the action of cog-wheels,
or _gears_.
When two gears running together, or _in mesh_, have the same number of
teeth, they will revolve at the same speed. If one has half as many
teeth as the other—10 teeth and 20, let us say—the 10-tooth gear will
make two revolutions while the 20-tooth gear is making one.
There are two shafts in a change speed gear, one driven by the engine
and the other driving the wheels; each carries gears that mesh with
gears on the other shaft. These pairs of gears are of different sizes,
and any pair may be used; the shaft driven by the engine runs as the
engine runs, while the speed of the other shaft depends on the pair of
gears that is being used.
By changing from one pair of gears to another, the driven shaft, and,
consequently, the wheels, may be run at a greater or less number of
revolutions, while the speed of the engine and the driving shaft do not
change. The number of power strokes that occur during one revolution of
the wheels is thus changed, and they turn with more force or with less.
_High speed_, or _high gear_, means the combination of gears that
gives the greatest speed to the wheels, but the fewest power strokes
to each revolution. The combination that gives the slowest speed to
the wheels, but the greatest number of power strokes, is called _low
speed_, or _low gear_.
Many tractors have but two speeds, a low and a high; but others have an
intermediate combination for conditions too severe for running on high
gear but too easy for low.
The change speed gear mechanism also provides for reversing or backing
the tractor. Two gears running together turn in opposite directions,
while in a train of three gears the outside gears turn in the same
direction. The usual combination in a change speed gear uses two gears
for going ahead; to run the driven shaft the other way, which will make
the tractor back, a third gear is meshed between the two.
The differences between various makes of change speed gears are in the
methods used to put into action the desired pair of gears.
[Illustration: FIG. 62.—PRINCIPLE OF SLIDING GEAR]
Two general plans are used. In one of them, a gear of each pair can
slide endways on its shaft, but must revolve with it; thus it can be
slid into mesh or out. In the other, the gears of a pair are always in
mesh, but one of them is loose on its shaft, so that shaft and gear can
revolve independently. To make the pair of gears operate, the loose
gear is locked to its shaft.
Figure 62 shows the principle of the _sliding gear_ type. One part of
the shaft driven by the engine is square, and fits into square holes
in its gears, which may thus slide along it, but must revolve with it.
Each sliding gear is moved by a shifter block, which is operated by
a shift lever. There is a shifter block for each gear, and the shift
lever may be moved sideways to operate either one of them.
Figure 63 shows the _jaw clutch_ type of change speed gear, in which
the gears are in mesh all of the time, but run loose on their shaft
when they are not working. The drawing shows _bevel gears_, which are
used when the driving and driven shafts are at a right angle. The same
principle is used for _spur gears_ on shafts that are parallel, as in
Figure 62.
[Illustration: FIG. 63.—PRINCIPLE OF JAW CLUTCH CHANGE SPEED GEAR]
The center of the shaft is square, and fits a block that can slide
endways, but that must revolve with it. The ends of the block have
heavy teeth that can mesh with teeth on the hubs of the loose gears;
meshing the block with one of the gears forces that gear to revolve
with the shaft.
The drawing shows only one speed forward; the reverse is obtained by a
second gear on the same shaft, which is placed on the opposite side of
the center of the driven gear, and turns it in the opposite direction.
When a tractor turns, the outside wheel makes a larger circle than the
inside wheel, and has a longer path to travel. Both wheels travel their
paths in the same time, so it follows that the outside wheel must move
faster than the inside wheel, although both are being driven by the
engine. This is allowed for by the _differential_, which is driven by
the change speed gear, and which in turn drives the wheels; it operates
automatically by the difference in the resistance to the rolling of the
wheels.
The action of the differential is illustrated by an experiment that
requires a pair of wheels on an axle, like buggy wheels, and a stick
long enough to reach from one to the other. With the wheels on smooth
ground, put the ends of the stick through the wheels at the top, each
end pressing against a spoke. Hold the stick at its center and push it
forward; the stick will transmit the pressure to the spokes, and the
wheels will turn. The wheels being on smooth ground, there is equal
resistance to their movement, and they will run straight forward.
Now repeat the experiment with the wheels so placed that one is on
a smooth roadway and the other on sand; as the wheel on the smooth
surface meets with less resistance than the other does, it moves
faster, and the pair of wheels circles, although the stick applies
equal pressure to both.
The power developed by the engine is transmitted by the differential
to both rear wheels; when the wheels meet with equal resistance, they
turn equally, but when one wheel meets with greater resistance than the
other, it slows down, while the other speeds up to correspond.
A tractor with two driving wheels must use a differential in order to
make turns easily. Without a differential, the wheels would run always
at equal speed, and in making a turn one would be obliged to slip.
The use of a differential has a disadvantage, however. If one wheel
is in a mudhole and the other is on hard ground, the wheel in the mud
meets with little resistance, and all of the power of the engine goes
to it; it spins without moving the tractor, while the other wheel
remains stationary. In such a case all of the power should be applied
to the wheel that has traction in order to move the tractor, but this
the differential fails to allow.
In some tractors the differential is so made that the parts may be
locked together. This lock is used when one wheel is in a mud hole, and
as by its use power is transmitted equally to both wheels, the tractor
moves.
Great care must be taken to unlock the differential as soon as the need
for the lock has passed, for otherwise the wheels would slip on a turn,
and the parts of the transmission might be strained or broken.
A differential is usually made with two bevel gears placed face to
face; between them is a frame holding three or more small bevel gears
that are in mesh with them both. The engine revolves the frame with its
small gears; each of the large bevel gears revolves a driving wheel.
When the tractor moves straight ahead the differential turns as if it
were one solid piece. When there is less resistance to one driving
wheel than to the other, the small bevel gears, in addition to
revolving with the frame that carries them, turn on their shafts. This
transmits the power of the engine to one wheel more than the other,
according to the resistance of the wheels.
[Illustration: FIG. 64.—“I. H. C.” CHAIN DRIVE, SHOWING THE
DIFFERENTIAL]
Figure 64 shows one of the large bevel gears of a differential, with
the three small gears, the other large bevel gear being removed. A
differential in section is shown in Figure 65.
A tractor with only one driving wheel has no differential. Such
tractors usually have two wheels, but one of them runs loose on
the axle, and serves only to support the tractor. The rear axle
construction of a tractor with a 1-wheel drive is shown in Figure 66,
which should be compared with the 2-wheel rear construction shown in
Figure 65.
[Illustration: FIG. 65.—“CASE” REAR AXLE]
There are a number of methods used for transmitting power to the
driving wheels. In Figure 64 a chain is used; there are tractors with
but one chain, and others with a chain for each driving wheel.
[Illustration: FIG. 66.—“OIL-PULL” REAR AXLE]
The most usual method is by a _master gear_, or _bull gear_, which is a
large and heavy gear attached to the driving wheel, as shown in Figures
65 and 66. In some tractors this gear is nearly the size of the wheel,
and is fully exposed; in others it is smaller, and enclosed in an
oil-tight housing.
[Illustration: FIG. 67.—DRIVING WORM]
The small gears that drive the bull gears are on the ends of the cross
shaft, called the _jack shaft_, that carries the differential.
In the Fordson tractor the differential is built into the axle, as it
is in an automobile, and power is applied by a _worm_. The worm is
driven by the change speed gear, and is a screw meshing with a gear on
the differential, whose teeth are cut at the proper angle to make them
fit the threads of the worm. A worm, which is shown in Figure 67, is
always enclosed, and runs in oil.