TRANSMISSION—(_Continued_)
While the =planetary type= of change-speed mechanism, which is in
extensive use for runabouts and light commercial wagons, also employs
gears, their arrangement is along different lines. The first three
diagrams in Fig. 35 serve to illustrate the principle.
[Illustration: _REVERSE_ _SLOW SPEED_ _HIGH SPEED_ _PRINCIPLES OF
PLANETARY GEAR TRANSMISSION_ _SECTION PLANETARY GEAR TRANSMISSION_
FIG. 35.—PLANETARY TYPE.]
The gear _A_ in these diagrams is attached directly to the crank
shaft, and in mesh with it are four other gears (_B_) of the same
size. Surrounding them is an =internal gear= (_C_), this being a
ring with teeth cut on its inner face, the four gears meshing with
it. The shafts, or =studs=, on which the four gears revolve are
supported by a metal ring (_D_), which maintains the gears at equal
distances from each other. The first diagram shows the mechanism in
the reverse position, for driving the car backward, the car being
driven by the internal gear. To have the internal gear revolve in the
direction opposite to that of the crank shaft, as is necessary, the
ring supporting the four gears is held stationary, with the result
that as the crank-shaft gear revolves the four gears are revolved on
their studs. As these gears are in mesh with the internal gear, that
is revolved, and moves in the same direction as the four gears and in
the opposite direction to the crank shaft.
For the low-speed forward, the ring is released and the internal gear
held stationary, the car now being driven by the ring instead of by
the internal gear. If the four gears were free from the internal
gear, they and their ring would revolve with the crank-shaft gear
without rotating on their studs, but being in mesh with the internal
gear, they roll around it as a wheel rolls along the ground, rotating
on their studs. A simple experiment that will illustrate this motion
is to crook the forefinger around a napkin ring or similar object,
placing a pencil between it and the finger, and revolving the ring
with the other hand. The finger being stationary, the pencil, which
is revolved in the opposite direction to the ring, will roll along
it. In this the napkin ring represents the crank-shaft gear, the
pencil one of the four gears, and the finger the internal gear. As
the four gears roll around, the ring moves also, for it is carried
by the studs on which the four gears revolve. If each of the four
gears has fifty teeth, and the internal gear two hundred teeth,
each gear must make four revolutions in order to roll around the
internal gear to the point where it started. The crank-shaft gear
also having fifty teeth, it revolves at the same speed, and as four
revolutions of the four gears are necessary in order that they may
roll completely around the internal gear, the crank-shaft gear will
make four revolutions in the same time. The ring moves with the four
gears, and revolves once around the crank shaft in the same time. As
the car moves according to the rotation of this ring, it will go at
one quarter the speed that it would make if the wheels were directly
connected with the crank shaft instead of with the ring.
For the high speed, the internal gear and the ring are locked to the
crank shaft so that all revolve together, the wheels being driven by
either the ring or the internal gear.
In these diagrams the drive of the wheels is supposed to be shifted
from the internal gear to the ring, which is not a practical
arrangement, and the planetary change-speed mechanism as applied to
an automobile is shown in the lower diagram in Fig. 35.
In this there are two sets of crank-shaft gears, gears and rings,
and internal gears, one set being for the reverse and the other for
low and high speeds. Between the two crank-shaft gears is a loose
sleeve, one end of which forms the internal gear for the reverse,
and the other end the ring supporting the studs on which revolve the
four gears for the low speed. The sprocket for the chain drive to
the rear axle is carried on this sleeve. Two more loose sleeves are
on the shaft, one forming the ring on which revolve the four gears
for the reverse, and being extended to form a brake drum outside of
the internal gear, and the other carrying the internal gear for the
low-speed combination, its outside face serving as a brake drum.
To obtain the reverse, a brake band is tightened on the drum of the
reverse combination, which holds stationary the ring supporting the
four gears, giving the result shown on the first diagram of the four
gears revolving on their studs, and rotating the internal gear in the
direction opposite to that of the crank shaft. The sleeve bearing the
sprocket is thus revolved, and the car backs.
For the low speed, the reverse brake band is loosened, and the
internal gear of the low-speed combination held stationary by the
tightening of the brake band surrounding its drum. The revolution
of the crank-shaft gear causes the four gears to revolve on their
studs and to roll around the internal gear, revolving the ring and
the sleeve bearing the sprocket, which now turns in the direction
opposite to that resulting to the application of the reverse, or in
the same direction as the crank shaft.
For the high speed, a clutch is engaged that locks the internal
gear to the crank shaft, and the four gears then being held between
these two are carried around with them, and the sprocket rotates
accordingly. When this combination is used, none of the gears are in
motion, all revolving with the crank shaft but not on their studs.
The planetary change-speed mechanism gives excellent results for
light work, but having only two speeds forward is not adapted to
high-powered cars. As the speeds result from the tightening of brake
bands on the drums, there is no danger of damaging the gears by
mishandling, for the brakes will slip before the teeth will give
way. The brakes, which are leather-lined strips of steel, require
attention from the wearing of the leather, and the slipping that
results from oil working in between them and their drums. No foot
clutch is necessary, for the tightening and loosening of the brake
bands is controlled by a lever; in some designs, the reverse is
applied by means of a foot pedal, and this may be used in braking the
car.
[Illustration: _NEUTRAL—INDIVIDUAL CLUTCH DRIVE—LOW SPEED_ _HIGH
SPEED_ _LOW SPEED_ _REVERSE_ SINGLE FRICTION DRIVE _HIGH SPEED_
_LOW SPEED_ _DOUBLE FRICTION DRIVE_ FIG. 36.—INDIVIDUAL CLUTCH AND
FRICTION DRIVE.]
The =individual-clutch= type of change-speed mechanism consists
of two shafts, one being an extension of the crank shaft, and the
other parallel to it (Fig. 36). On the crank shaft is a sleeve
bearing the sprocket and a gear, this sleeve being so arranged
that it may revolve loosely, or be locked to the crank shaft and
made to revolve with it by a clutch. The crank shaft in addition
bears two fixed gears, one being for the low speed and the other
for the reverse. On the countershaft is a fixed gear in mesh with
the gear carried on the sleeve on the crank shaft, and two loose
sleeves bearing gears that are in mesh with the fixed low-speed
and reverse gears on the crank shaft. These sleeves are provided
with clutches by which they may be locked to the countershaft to
revolve with it, or disconnected from it. When the three clutches are
disengaged, the crank shaft in revolving carries with it the fixed
low-speed and reverse gears, the sleeve bearing the sprocket and
gear remaining stationary. The sleeves on the countershaft revolve
because their gears are in mesh with the fixed crank-shaft gears, but
the countershaft remains stationary. In engaging the low speed, the
clutch is thrown in, forcing the countershaft to revolve with the
fixed gear, the fixed gear on the countershaft then revolving the
gear and driving sprocket carried on the sleeve on the crank shaft.
Because of the difference in the size of the gears, the countershaft
will revolve at a slower speed than the crank shaft, and the driving
sprocket will make but one revolution while the crank shaft makes
several. This it is free to do, for the sleeve carrying the sprocket
is in no way connected with the crank shaft. For the high speed,
the low-speed clutch is withdrawn, and the driving-sprocket clutch
engaged, causing the sprocket to revolve with the crank shaft.
The reverse is caused by the introduction of an idler gear between
the gears of the crank shaft and countershaft, by which the movement
of the latter is reversed.
The application of the =friction type= of change-speed mechanism to
automobiles is recent, and is giving good results for light work. It
consists in its simplest form of a heavy disk carried on the engine
shaft, on the face of which runs a wheel sliding on a square shaft,
so that the two may be in contact at any point from the edge to the
center of the disk (Fig. 36). When the wheel is at the center of the
disk it is not moved, and the number of speeds at which the square
shaft may be driven in relation to that of the disk varies from
nothing to the limit, which is obtained when the wheel is in contact
with the outer edge of the disk. For the reverse, the wheel is moved
across the center of the disk, where it is revolved in the opposite
direction.
Another form of friction drive provides two driving disks, the wheel
bearing against both, so that its movement is more positive, and
there is less chance for slipping.
FINAL DRIVE
From the change-speed mechanism the power is passed to the driving
wheels by the =final drive=.
[Illustration: FIG. 37.—_A_, Propeller or driving-shaft drive; _B_,
single-chain drive.]
In the most usual construction the engine is so placed that the
crank shaft is at right angles to the axle, and it is therefore
necessary to change the direction in which the power acts, which is
done by means of =bevel gears=. In ordinary spur gears the teeth
are parallel to the shaft, and the two shafts that carry them are
parallel, while in bevel gears the teeth are at an angle, and the
shafts may be at right angles to each other. In Fig. 37 the diagram
of the =single-chain drive= illustrates a car in which the engine is
in the center of the frame, and as the crank shaft is parallel to
the axle, the power may be directly applied. In the illustration of
the =propeller= or =driving-shaft drive= the crank shaft is at right
angles to the axle, and the power is turned by means of the bevel
gears at the rear axle.
The single-chain drive can only be used for light cars, and is
usually applied in connection with a change-speed mechanism of the
planetary type.
[Illustration: FIG. 38.—TYPICAL UNIVERSAL JOINT.]
The propeller-shaft drive requires the use of =universal joints=,
which are devices that permit one shaft to drive another, even though
they are at an angle with each other. A typical universal joint is
illustrated in Fig. 38. The ends of the shafts bear yokes, the ends
of which are pivoted to a block of metal of + shape. When the two
shafts are in line, the joint will force one to rotate with the
other, and this will not be prevented if the two are out of line, for
then the pivots will act, the + swinging on its pivots in the yokes.
[Illustration: FIG. 38A.—TYPES OF SHAFT DRIVES. DRIVE THROUGH
UNIVERSAL JOINTS “STRAIGHT LINE” DRIVE GEAR BOX ON REAR AXLE]
The change-speed mechanism is carried on the frame of the car, and
is therefore supported by the springs, but the axle end of the
driving shaft follows the axle as that follows the inequalities of
the road. One end of the propeller shaft is therefore comparatively
stationary, while the other is in constant motion, and if the shaft
were inflexible it would be jammed in its bearings and twisted out of
line. This is prevented by the universal joints with which the shaft
is provided, there being one and often two in the shaft, and usually
one between the clutch and change-speed mechanism.
[Illustration: FIG. 39.—LIVE AXLE—NON-FLOATING TYPE.]
[Illustration: FIG. 40.—LIVE AXLE—FULL FLOATING TYPE.]
The single-chain and driving-shaft drives require the use of a =live
axle=, which is an axle that revolves with the wheels. The simple
type of live axle consists of the shaft to which the wheels are
attached, and the =housing= that contains and supports it (Fig. 39).
This axle is continuous, and usually has square ends that fit into
the square hubs of the wheels so there may be no slipping. The
second diagram in Fig. 40 shows a live axle of the =floating= type,
in which the revolving part serves only to turn the wheels. The
housing is extended, and the wheels run on its ends, the driving part
projecting beyond the housing and having square ends that are secured
to the outside of the hubs by square caps. The wheels thus run on the
housing, which takes the weight of the car from the driving part. A
live axle must be divided into two parts in order that a differential
gear may be fitted, and the housing must therefore be strong enough
to support the weight and prevent sagging. The efficiency of a bevel
gear is greatly reduced if the teeth are not in their exact mesh, and
sagging of the axle will throw them out to such an extent that they
will be noisy, and wear rapidly. The floating type of live axle, in
relieving the driving part of the weight, has a great advantage over
the simple type, and is in general use.
[Illustration: FIG. 40A.—DEAD AXLE WITH DRIVING SHAFT. Axle supports
Bevel Gear and Differential. The Driving Shaft is supported in
Bearings at the Differential end, and drives the wheels through
clutches in the hubs.]
With the driving-shaft drive it is necessary to use a =torsion rod=,
which extends from the gear case, or a crosspiece of the frame,
to the rear axle. The necessity for this is the tendency of the
driving bevel gear to roll around on the driven bevel gear rather
than to revolve it. If it were not for the torsion rod, there would
be a continual strain on the parts because of the tendency of the
axle housing to revolve around the axle, instead of the axle being
revolved inside of the housing. The torsion rod has a flexible joint
at one end, that permits it to give as the axle follows an uneven
road surface, but it retains the housing in the correct position,
preventing the bevel gears from getting out of line (Fig. 41).
[Illustration: FIG. 41.—TORSION ROD.]
[Illustration: FIG. 42.—DOUBLE SIDE-CHAIN DRIVE.]
In Fig. 42 is shown a car with =double-chain drive=, in which the
bevel gears that change the direction in which the power is applied
are contained within the =gear case= that incloses the change-speed
mechanism. As will be seen from Fig. 34, the bevel gears connect the
square shaft with the =jack shaft=, which is a shaft passing across
the car, and bearing on its ends the sprockets by which the wheels
are driven. This type of drive requires the use of a =dead axle=,
which is stationary with the wheels running loose on its ends, like
the axle and wheels of a coach. An axle of this type may have great
strength with light weight, and is usually a manganese-bronze or
steel forging. The sprockets on the rear wheels are bolted to the
spokes, and should be, of course, exactly in line with the sprockets
on the jack shaft in order that the chains may run true.
DIFFERENTIAL
When a car takes a corner, the outside wheels make a larger curve
than the inside, and cover a longer distance. As the front wheels
are loose on the axle, they accommodate themselves to this; but as
both rear wheels are driven by the engine, it is necessary to apply a
device that will permit them to rotate at different speeds without
interfering with their driving the car. This is accomplished by means
of a =compensating and differential gear=.
To understand the necessity for a differential, stand behind a wagon,
with one hand on each tire; push, and if the vehicle is steered
straight ahead, the hands will move ahead equally; but if the vehicle
turns, the hand on the outside wheel will move ahead faster than the
other. Now take a stick, and run it through the rear wheels so that
it bears against the spokes; press it forward from its center, and if
the vehicle moves straight ahead, the stick will go forward equally;
but if the vehicle turns, the outside end of the stick will go ahead
faster and farther than the other, although the pressure is being
applied to its center.
[Illustration: _BEVEL GEAR TYPE_ _SPUR GEAR TYPE_ FIG.
43.—DIFFERENTIALS.]
In applying a simple form of differential the axle is divided into
two parts, to the inner ends of which are fitted bevel gears, these
being held at a fixed distance apart by the construction of the
housing (Fig. 43). Between the bevel gears and in mesh with both of
them are small bevel gears, or =pinions=, which may revolve on
short studs carried on a ring so that they are a fixed distance
apart. When the ring is revolved it carries with it the studs and
pinions. The ring forms a housing that incloses the differential, and
is driven by the single chain or driving shaft. To understand the
action of the differential, imagine the rear wheels of a car to be
jacked up clear of the ground so that they are free to revolve, and
the housing to be revolved by hand. As it turns, the driving wheels
will turn also, for the resistance to each is the same, and the
pinions, being in mesh with both bevel gears, cannot revolve on their
studs. If one of the wheels is now held stationary and the housing
revolved, the bevel pinions will revolve on their studs, and roll
around on the stationary gear; this will drive the gear of the free
wheel at twice the speed of the housing. The revolving of the housing
in the first instance caused the wheels to turn equally at the speed
of the ring, and in the second permitted one to remain stationary
while the other turned at twice the speed of the housing, the speed
of the latter being unchanged. The first is the effect when the car
moves straight ahead, and the second the result if the car could
make so short a turn that it would pivot on one wheel.
With the wheels jacked up, hold one hand lightly against one of the
wheels, so that while it may turn, there is more resistance to it
than to the other. If the housing is now revolved, the bevel pinions
will revolve on their studs, and roll slowly around the gear of the
wheel that presents the resistance, the free wheel being revolved at
a higher speed than the housing. This is the condition when the car
takes a corner, for there is then more resistance to the inside than
to the outside wheel, and it slows down; this will start the pinions
revolving on their studs, and they will drive the outside wheel
correspondingly faster.
With the housing revolving at a fixed speed, the outside wheel will
revolve as much faster as the inner wheel is revolving slower; for an
illustration, if the housing makes fifty revolutions a minute, and
the inner wheel is slowed to forty, the outer will be driven at sixty
revolutions.
If one wheel of a jacked-up car is revolved by hand, the other wheel
will revolve in the opposite direction. This is caused by the
housing remaining stationary and the pinions being revolved on their
studs by the turning of the wheel, the movement being transmitted to
the free bevel gear and wheel in the reverse direction.
The bevel gear differential described was the early type, but a more
recent design employs spur gears. The axle ends carry spur instead of
bevel gears, these being in mesh with other spur gears that are long,
but of small diameter. These small gears are in pairs, as shown in
Fig. 43, being in mesh with each other at their inner ends, and each
member of a pair meshing with one of the axle gears. The small gears
revolve on studs supported by the housing that is revolved by the
drive, the studs in this case being parallel with the axle instead of
at right angles to it, as are the studs in the bevel-gear type.
If the small gears meshed only with the axle gears, and not with each
other, revolving the housing would cause them to roll around the axle
gears, all rotating on their studs in the same direction, and the
axle gears remaining stationary. Being in mesh with each other, they
cannot revolve in the same direction, for when two gears are in mesh
they must revolve in opposite directions. Thus the small gears cannot
roll around on the axle gears when the housing is revolved, and if
there is equal resistance to the turning of the wheels, the small
gears will not revolve on their studs, but will carry the axle gears
with them.
If the car is turning a corner, the greater resistance to the inner
wheel will cause the small gears to revolve on their studs, rolling
around the resisting gear and driving the other correspondingly
faster.
On cars with double-chain drive, the differential is fitted to the
jack shaft, and of course receives the drive from the change-speed
mechanism through its housing.
Both the driving-shaft and double-chain drive have points of
advantage and of weakness, and each type has its advocates. For
the double chain, great strength can be claimed with light weight,
as the axle is in one piece, and perfectly adapted to support
the car. Against it is the difficulty of keeping the chains
properly lubricated, and their consequent wear and stretching. The
driving-shaft type has the advantage of the perfect lubrication of
the parts, for all may be inclosed and running in oil or grease; the
rear axle must be divided, however, which requires it to be heavily
braced in order that the weight imposed on it may not bend or spring
it out of line. Where a bent dead axle can be straightened by a
blacksmith, a similar condition in a live axle requires the services
of an expert mechanic; on the other hand, bevel gears make less noise
than chains.
DRIVING-GEAR RATIOS
Even when on the direct drive, the crank shaft makes more revolutions
than the rear wheels, in order that the momentum of the moving
parts of the engine may be sufficient to keep the car in motion. On
shaft-driven cars, the bevel on the driving shaft has fewer teeth
than that on the axle, so that it revolves more than once to one
revolution of the axle. On chain-driven cars, the driving sprockets
are smaller than those driven. This =driving-gear ratio=, as it is
called, varies from one and a half to three and a half, or, in other
words, the wheels revolve once while the driving shaft or sprocket
makes from one and a half to three and a half revolutions. Other
conditions remaining equal, a higher driving gear gives the car lower
speed, but greater ability in hill-climbing and the traversing of
heavy roads.