THE DRIVE
The term used to designate the transmission of power from the engine to
the wheels, is called the _drive_.
In nearly all cars the engine shaft runs fore and aft, and consequently
is at right angles to the axles. This, of course, necessitates some
sort of gearing between the engine shaft and axle. This change is made
in the bevel gear drive hereafter explained.
As the engine is mounted on the frame of the car, which rests on
springs, and the axle is below the springs, it is obvious that the
drive must be transmitted between two parts which have a relative up
and down movement.
This necessitates several things, structurally, which should be
considered.
First. A flexible joint must be interposed in the system, where a shaft
is used to transmit the power.
Second. Torsion rods are necessary to prevent the housing or casing of
the rear axle from turning, due to reaction of the driving bevel gear.
Third. A rod, or rods, are required to prevent a fore and aft movement
of the rear axle. The rods run from the ends of the rear axle housing
to some convenient point on the frame.
ILLUSTRATING POWER TRANSMISSION.--For convenience, these mechanical
elements are illustrated on a frame.
[Illustration: Fig. 35. Radius Rods.]
Fig. 35 shows a frame which has its rear axle provided with a pair of
radius rods A A. These have their rear ends attached, in any suitable
manner, to the axle housing, near the springs and the forward ends are
brought forward and pivoted to the cross beam B.
TORSION ROD.--These rods thus take care of any undue strain which takes
place by the wheel striking obstructions.
C represents the torsion rod which has its rear end firmly secured to
the housing D, and its forward end to the cross piece E. This prevents
the housing from turning, and also serves to provide against any undue
thrust of the driving bevel.
Some cars dispense with the torsion rod, by incasing the shaft in a
torsion tube. Such a form of construction is shown in Fig. 36.
_The torque tube_ A, as it is called, is rigidly secured to the housing
B, of the rear axle, the forward end being pivoted to a cross piece C
of the frame.
[Illustration: Fig. 36. Torque Tube.]
The radius rods D D, have their forward ends attached to a sleeve E,
located near the forward end of the torque tube A, and the rear ends
are secured to the axle housing F at the spring seats.
Some manufacturers avoid the use of these radius rods by such a
construction in the springs as will prevent any forward and rearward
movement of the axle.
CHAIN DRIVE.--The chain drive machines require the radius rods, or some
other means to counteract the movement of the axle when it meets an
obstruction, particularly where the chain transmits the power to the
differential on the wheel shaft.
JACKSHAFT.--With the double chain drive no differential is used on the
axle, but, instead thereof, it is placed on the jackshaft which carries
the small driving sprocket wheels. The chain transmits the power direct
to each wheel, and a radius rod is necessary to hold the shaft of the
drive sprocket wheel the proper distance from the rear axle.
[Illustration: Fig. 37. Chain Drive.]
Such an arrangement is shown in Fig. 37, in which the drive, or
jackshaft A is mounted transversely across the vertically-movable frame
B, and the torque bar C, therefore, serves as the means for keeping
the jackshaft and the axle D the proper distance apart, and it is also
arranged to serve as a radius rod to prevent any undue tension on the
chain when a wheel strikes an obstruction.
The wheels of such a truck turn freely on the axle stubs of a dead axle.
OBJECTIONS TO CHAINS.--Few pleasure cars are now equipped in this
manner, as the shaft drive is more desirable for several reasons: The
use of chains is always objectionable, as the efficiency decreases with
wear quicker than the shaft drive, and requires the jackshaft, sprocket
chains and sprocket wheels, besides the noise and excessive wear, by
stretching of chains, which are always inherent in the use of chains.
It is impossible to prevent dirt, sand and grit from adhering to the
chains, unless they are inclosed, a thing which is difficult and
expensive. If they are not so protected the lubricant only serves to
catch the grit and retain it, so that when it is carried around by the
chain, the wheel and chain are both worn out.
Another difficulty in the use of chains is due to the inability to
keep them at a proper tension at all times. All chains will stretch in
use, consequently the tension will change, and when wear takes place,
the distance of the centers of driving and driven sprockets has to be
adjusted, calling again for another mechanical complication.
SHAFT DRIVE.--The shaft of the engine, being on the frame, has a
vertical movement, and the axle, to which power is to be transmitted,
is below. The engine must be mounted so the shaft inclines, or, be
placed low enough, so that it will be on a direct line with the rear
shaft.
In either case some flexible means must be provided between the engine
shaft and axle on account of the relative vertical motion between
engine and rear axle. The _straight line drive_ is most desirable, in
every way, as the full power of the engine is available, and this is
usually arranged for by lowering the engine bed sufficiently so that
the shaft will point straight to the axle when the car is loaded.
[Illustration: Fig. 38. Shaft Drive.]
[Illustration: Fig. 39. Straight Line Drive.]
TRAIN OF SHAFTING.--Several lengths of shafting are often interposed
between the engine shaft and axle, and some cars have two universal
joints in the shaft line, one mounted forward of the transmission case
and the other to the rear of it. Or, more frequently, one in the rear
of the transmission and one in front of the rear axle.
It seems, however, to be the most general practice to have a single
universal joint directly behind the gear case, and the shaft forward of
the case only slightly inclined.
Figs. 38 and 39, show the two types, the former being the straight line
drive, and the latter a form of construction where the two universal
joints make the drive through a line which minimizes the angles as much
as possible between the shafts.
Figs. 38 and 39 are not intended to show all the elements in the train
of shafting, such as joints and connections, but is merely designed to
illustrate the disposition of the drive shafting relative to the engine
and rear axle.