THE DIFFERENTIAL
THE MEANING OF DIFFERENTIAL.--This is a term used to designate the
difference in the turning movement of two wheels on opposite ends of an
axle. For various reasons they do not turn at the same rate of speed,
particularly in turning corners, where the outer wheel must travel a
greater distance than the inner wheel.
If both wheels are fixed to the shaft the latter would be submitted to
a torque, or one of the wheels would slip, and thus be destructive of
tires.
On the other hand, if one wheel should be loose, then, as power is
applied to the shaft, the tractive action would be on one wheel only,
and this would be bad practice, and frequently cause the wheel to slip,
and thus unduly increase the wear of the tire.
The differential is made up of a system of gears, which are so arranged
that one wheel may turn independently of the other, and at the same
time the effective driving power is utilized by each.
Various forms of this mechanism have been developed. While the
differential is an exceedingly simple piece of mechanism, it is not
such an easy matter to describe its operation, so that the principle
will be explained by a series of illustrations.
EQUALIZER BAR.--Examine Fig. 28. Let A be an equalizer bar, mounted
on the end of a thrust bar B, by a pivot C, so the ends will swing
back and forth freely. A horizontal bar D is hinged at each end of the
equalizer, which bars project forwardly parallel with each other and
these are provided with right-angled bends E E, simply for convenience
in describing the operation.
[Illustration: Fig. 28. Equalizing Mechanism.]
[Illustration: Fig. 29. Resistance in Equalization.]
While differential gears are very simple structurally, it is not
an easy matter to explain the principle on which a faster motion is
transmitted to one wheel than another, and under conditions where the
speed is constantly changing.
[Illustration: Fig. 30. Equalizer and Differential Movements.]
For instance, in Fig. 30, a cord A, over a pulley B, has weights C, D,
at its ends. If the pivot or fulcrum E, of the wheel, is stationary, as
in sketch 1, and the wheel is turned, say a quarter of the way around,
one weight will move down below the line X the same distance that the
other weight moves above it, as shown in 2.
Thus far we have an equalizer, pure and simple. But a differential
requires something more. It is necessary, under certain conditions, for
the weight D to move a greater distance in the same time than C, or the
reverse. Or, as sometimes happens, one of the weights, as for instance,
in 3, remains fixed while the other moves.
In this case, with the pivot pin E fixed, such a thing would be
impossible, hence, in order to make such a relative movement between
the two weights, the pin must move, and this motion is shown in 3,
where it moves down from the line F. That movement, or change of
position of the pivot E, is what takes place in the small intermediate
gears in a train of differential gearing.
TRANSMISSION WHEEL.--In Fig. 32 is shown a section of the differential
housing, 1, in which, for convenience, all refinements of construction
are eliminated. This shows the divided axle shafts 5, 6. In Fig. 33 is
shown a side view of the same housing. This may be connected with the
motor shaft by means of bevel gears, or driven by a sprocket chain. In
either case the housing 1 is the substitute for the thrust bar B, in
Fig. 28, and the bevel pinions 2, which are mounted within the wheel 1,
represent the equalizer bar of that figure.
[Illustration: Fig. 31. Differential in Housing.]
The gears which make up the train are usually put into a suitable
casing, as illustrated in Fig. 31, which gives a good example of the
construction. The housing A is fixed to the side of a large bevel gear
B, this gear being designed to receive power from the motor through a
bevel pinion C. One part of the axle D passes through the gear B, and
is fixed to a bevel gear E within the housing, and the other part of
the axle F passes through the housing and is fixed to a bevel gear G,
the same size as gear E.
Intermediate the two gears is a pair of bevel pinions H, H, and these
latter are mounted on pivots I, I, projecting inwardly from the
housing.
The fact that the pinions are attached to housing has the effect of
complicating the matter, so that it may be well to show the relative
arrangement of the gears without the housing.
[Illustration: Fig. 32. Section of Differential.]
[Illustration: Fig. 33. Side View of Differential Wheel.]
In Fig. 34 we have added to Fig. 33, two bevel gears 3, 4, which are
mounted on the axles 5, 6, these representing the rear drive axles of
the car.
ACTION OF TRANSMISSION GEARING.--From the foregoing it will be seen
that the axles abut each other, within the hub of the large gear 1,
within which they are journaled. We might, therefore, call these
pinions the counterparts of the bars E E.
[Illustration: Fig. 34. Top View of Differential Wheel.]
As long as the resistance to the turning movements of the pinions 3, 4
is the same, the housing through pinions 2, 2, will simply carry the
bevel gears 3, 4 around with it, without turning them, just the same
as the equalizer bar B was moved forward without either end swinging
back or forth; but the moment the wheel of the shaft 5, for instance,
is compelled to travel at a higher rate of speed, or the wheel on shaft
6 meets with a greater resistance, the small equalizing gears 2 will
turn, and the revoluble motion of the housing 1, while transmitting the
power, and also carrying the gears, will act, in effect, the same as
the push bar shown in the previous illustration.
Like the equalizing bar, the effect is to turn one wheel, say 3, with
less, and the other wheel 4 with more than the normal power or speed.
Fig. 28 shows the principle on which all differential automobile
gearing is based, that is, that both wheels receive half of the driving
power even if one wheel should turn faster, as shown at Fig. 29, which
is the case when turning a corner. This is what causes the power to
drive both wheels at all times, whether going straight or on a turn.
[Illustration: Fig. 34a. Differential Gears.]
If, however, one wheel gets on slippery ground, then A, Fig. 29, will
move forward, without pulling on the lower end. As the lever A has the
same action as the pinion in a differential, shown in Fig. 34a, it will
be seen that if the pinion center is moved in the direction of the
arrow, and if the wheel W^1 slips, the pinion will simply roll on the
bevel gear G^2 without driving it on the wheel W^2.
This is the disagreeable characteristic of a differential, that makes
one wheel spin when it touches a slippery spot on the road, and stalls
the car, because the other wheels cannot get any driving power.