FISCHER Profi Cartech Manual Del Usuario página 18

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Model: Vehicle with Differential
(see the assembly instructions on page 32)
Of course, only driving one wheel of the rear axle is not ideal, because then
the vehicle has less drive power. An ingenious invention, which allows drive
of both wheels without them having to be connected rigidly to one another,
is represented by differential gears. First assemble the model, and then we
will take a closer look at the technology.
Experiment:
First have the car go straight, then around a curve and observe
whether all wheels always rotate and whether the model takes the
curve cleanly without pushing forward.
Observation:
If the model has been assembled correctly, both wheels on the rear
axle rotate when the car goes straight and when it goes around a
curve too. The model takes curves cleanly.
Result:
Without knowing how the gearbox functions exactly, we can see that
it is obviously able to drive both rear wheels simultaneously.
Model: Functional Model of Differential Gears
(see the assembly instructions on page 34)
2 3
To make it easier for you to see how differential gears are constructed on
the inside, assemble it as a large function model from fischertechnik con-
struction components. It is important that you do not tighten the hub nut
(4) on the hub (1), so that it can rotate freely on the axle.
16
1 4
Experiment 1:
Rotate each wheel individually. What happens to the other wheel?
Observation:
It turns in the opposite direction.
Experiment 2:
Switch on the engine. Both wheels must rotate at the same speed.
What happens if you hold one wheel?
Observation:
The other wheel rotates faster.
Task:
Can you explain from the observation in Experiment 2 why you can-
not go forward with a car in winter if only one of the driven wheels
spins on ice?
Solution:
The rotating wheel revolves quickly, and the wheel on solid ground
remains stationary (it is "held").
Mode of Operation of Differential Gears:
The differential is driven via a belt pulley (1) (see the diagram above). The
power is transmitted to the two wheels via the differential bevel gears (2).
They are capable of compensating for the rotational differences between the
wheel on the inner and outer positions of a curve by rotating around their
own axle and hobbing on the axle shaft wheels (3) at the same time.
When the car drives straight forward, the differential wheels do not rotate.
Then they act like a fixed connection between the two driven axles on which
the wheels are mounted. The inner wheel is slightly braked in a curve, and
the differential wheels start to rotate and make the outer wheel faster. The
outer wheel is always faster at the exactly the same rate as the inner wheel
is slower.
Too complicated? No problem. The main thing is that you have seen how
differential gears function and now know an elegant way of driving both
wheels on the rear axle of a vehicle.
Model: Rear Engine and Differential Gears
(see the assembly instructions on page 36)