 Towing Vehicle 1
In order to gain access to the actual design of the compressed air brake, we have recorded the absolutely necessary components from the beginnings of the two-circuit brake and connected them
hydraulically. The only electrical system refers to a few control lights later.
We start historically relatively early and at the same time didactically simply with one of the first two-circuit braking systems. This is not artificially further simplified, but it has been given in this form in
practice. It begins with a compressor (1) driven by the engine, in this case air-cooled. It contains a crankshaft drive with one-way valves in the cylinder head which produces 8 to 12 bar pressure.
Relatively undesirable is the heat generated together with the pressure. Therefore, the line coming from the compressor is often longer and formed to spirals. It also does not go directly to the pressure
regulator (2), because the compressed air is previously mixed with antifreeze so that all functions of the compressed air brake system are guaranteed even at minus degrees.
The pressure regulator (2) ensures that the maximum pressure is maintained. Since it has not been so important to observe the CO2-values, it simply blows the superfluous energy that has
just been acquired into the atmosphere. The compressed air then reaches the compressed air tanks (4), carefully shielded from each other by the four-circuit protection valve (3).
Here, for the first time, a hierarchy between the four circles becomes visible. If for example the pressure in the compressed air tanks dropped slightly overnight, the driving of a truck starts with the idling
and pumping up of the circuits 1 and 2, responsible for the foot brake of the front and rear axle. The assignment above is by the way not mandatory. Depending on the manufacturer, the two brake circuits
can also be interchanged.
In any case, it must be clear that driving without circles 1 and 2 with maximum pressure would be much too dangerous. The four-circuit protection valve prevents such a drive by operating the handbrake
circuit 3. The handbrake must still function even if any pressure has escaped from the system. For this reason, in contrast to the front axle, there are not only simple cylinders (8) which transmit pressure
via a linkage to the (formerly exclusive) drum brake.
We also refer to the rear cylinders as spring-loaded brakes (9) because a strong spring is also installed, which prevents a truck from rolling loose under normal conditions, too. This is only possible by
pressing the spring together by compressed air, thus releasing the rear wheels. In turn, it would also apply the brake when the pressure drops during the ride for some reason.
Return to the four-circuit protection valve (3). The releasing of the hand brake via the manually actuated valve (5) in the cockpit thus is not possible until the two brake circuits 1 and 2 are supplied with
sufficient compressed air. At the same time, it favors its initial filling, even before any other circuit. By the way, the compressed air tanks (4) had small drainage valves at their bottoms (lowest points),
which had to be opened regularly and let the water go out.
So, now the truck is finally driving with full reservoir pressure. Decisive for a possible actuation of the foot brake is the two-circle brake valve (6) usually arranged underneath the brake pedal. At that time it
was almost one of the most complicated components in the compressed-air brake system. It was still necessary to convert even the finest actuation maneuvers exactly into the pneumatic of the brake
system.
This is or has been regulated at the time, because it is particularly unpleasant if the brake operates much more than is intended, and of course also for those who are driving behind the truck. After the two-
circuit brake valve (6), the two circuits are finally separated. At the top, a load-dependent brake valve (7) is drawn into the brake circuit to the rear axle, nowadays also to the front axle. In the era of the only
steel suspension, there was always a linkage to the axis to feel the spring deflection height.
Here too, the task is to prevent over-braking. Specifically, the braking force should be somewhat proportional to the loading of the vehicle, especially with partial braking. Please note that the weight of a
truck, unlike the normal car, can be doubled by load, even triple the one of the trailer. The brake system must take this into account.
And so it goes on each axle to the brake cylinders, formerly with pistons, now equipped with diaphragms. Their respective left side is the same in front and back. The component 10 will be subsequently
explained in connection with the trailer.
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