|
5. The differential in your car
|
The differential from an early
motor-car.
|
The automobile differential is very similar to the
elementary rotary differential. The main difference is
that the blue gear has been moved outside the green.
But the same equation holds between the three rotations:
purple gear (a), green gear (b) and
blue gear (c) : c = (a+b)/2 , or a+b=2c.
A housing
is built on the blue gear to hold the pinion or pinions. The
diagram shows how power is transmitted by bevel gearing from
the driveshaft to the blue gear (called crown gear in
this context).
Another differential from an early motor-car,
with a view of the inside.
This car was chain-driven, and sprockets on the
housing (of which two are visible) take tke place
of the crown gear. The images of early differentials are taken from
"The Automobile Instructor" by Clyde H. Pratt,
Shrewsbury Publishing Co., Chicago 1917.
|
In an automobile, the differential connects the driveshaft to the axles. Its
role is to balance the flow of power from
the engine to the wheels. The equation a+b=2c
means that the wheels share between them the rotation coming from
the engine. When the car is traveling in a straight line, the
wheels roll the same amount: a=b, and both are equal to
c. When the car goes around a curve, the differential
allows the outside wheel to travel farther, and the inside
wheel to travel less far. Only the sum of their rotations is
steady at 2c. If the engine is locked and the wheels
are free to turn, turning one wheel will cause the other to
rotate in the opposite directon (a+b=0). On the other
hand if one wheel is free to turn (e.g. in snow or mud)
and the engine is running,
the other wheel will not turn at all: a=2c, b=0.
This is frequently experienced.
|