 Trailer control valve 1
So here's a quick summary: The trailer brake is controlled by the towing vehicle, and the signal is then implemented in the trailer itself. Consequently, the trailer control valve is located in the towing vehicle, and the trailer
brake valve is located in the trailer. How could it be otherwise, because the control valve needs a few connections to be able to decide whether the trailer should brake or not.
Above, you can see ports 41 and 42 of the two brake circuits. They come in with an increase in pressure if they are activated (actuated) and function. Then there's the slightly different port 43, which signals this control valve
that the handbrake or parking brake has been applied. It's important to note that the strength of a possible signal transmission is measured by the pressure drop.
In this picture here, the lines come in from the right, 41 at the top and 42 at the bottom. Just above 42, then 43, whereby the last two meet at the large plate of a pressure valve for a comparison. In the drive position, there is
no pressure drop at the handbrake because it is not applied. This means it has the same pressure as the supply line above it. This is greater than the pressure coming through port 42 from the footbrake, which is not
currently applied.
Just above 43, we have 21, and further up, 22. The former is connected to the "supply" coupling head, the latter to the "brake" coupling head. "Supply" is easily explained in that there is a direct connection to the only
connection on the left, 11. This is where the trailer control valve is supplied with stored compressed air. In this case, it is simply passed on to the trailer via the 'supply' coupling head. We'll encounter an exception later,
which is caused by the throttle valve on the left protruding from above into this channel.
Let's leave this throttle valve aside for now. It's not so closely connected to the trailer control valve in every design; it can also be designed as a separate valve and completely separate. Therefore, for the following
considerations, the two connections to the left of the throttle valve should be considered closed (image below).
In the normal driving position, as shown above, our focus should naturally be on port 22, the second one on the right from the top. It must be clear that no pressure is being sent to the trailer brake valve now, which in turn
means the brakes are not being applied there. Only when the pressure rises does the brake valve become active.
So, how do you prevent any pressure on port 22 in this driving position? The key for this lies almost exactly in the middle of the whole picture. So, if you slowly approach yourself from the second port on the right from the top
(22) to the arrows in the middle, you'll find an open valve that connects this port straight down to a small filter and then to the outside air.
So, how do you prevent any pressure on connection 22 in this driving position? The key for this lies almost exactly in the middle of the whole picture. So, if you slowly approach yourself from the second port on the right from
the top (22) to the arrows in the middle, you'll find an open valve that connects this straight down to a small filter and then to the outside air.
The opening of this valve is achieved by the previously discussed lower piston, which, in its lowest position, rests on the spring-loaded plate and pushes it downward. At the same time, the spring, also indicated by two
arrows, ensures the highest possible position of the two nested pistons at the top.
The valve in the center is the pivot point of the entire trailer control system. Just look at its position during partial braking (image below). The large lower piston remains in its position down at the stop, because during
partial braking, the full pressure from the handbrake valve still pushes the valve down more than the partial pressure during braking.
A crucial moment occurs at the top, because the same partial pressure is also present in brake circuit 1. This can push the upper piston downward, and now the pressure is redirected to the trailer control line at the
aforementioned valve (see arrows). This is now no longer connected to the outside air, but to the supply line, whereby also the trailer is partially braked.
Now, the pressure at port 22 could be increased up to the reservoir pressure, which could potentially have catastrophic consequences. An overbraking trailer can become completely uncontrollable. So, how is the pressure
in the control line to the trailer reduced to a level appropriate for the partial braking of the towing vehicle? Again, a pressure comparison is used, specifically at the largest valve plate at the top.
The pressure from brake circuit 1 acts on it from above. Its spring is positioned on the outside, counteracting this. From below, we have the pressure from port 22. The inner spring acts against this. This comparison, which
can be influenced by the screw, means that if there is too much pressure (break point) for the trailer control line 22, the large piston is pushed upward, and there is no longer any connection to the supply at the two arrows.
The pressure from brake circuit 1 acts on it from above. In contrast, its spring is far on the outside. From below, we have the pressure from port 22. The inner spring acts against this. This comparison, which can be
influenced by the screw, means that if there is too much pressure (break point) for the trailer control line 22, the large piston is pushed upward, and there is no longer any connection to the supply at the two arrows.
This system regulates itself and can ensure that the trailer has a certain amount of pressure advance, so that it brakes a little harder during partial braking to stretch the traction. As mentioned, this advance is adjustable
using the adjusting screw. There is no time advance, as the term might suggest.
Finally, this chapter discusses the effect of the parking brake. In this case, the lower piston finally moves because it is lifted by the pressure at connection 42, pushing against line 43, which has been depressurized by the
parking brake valve. Now, the lower piston ensures that the constantly critical valve in the center opens, thus applying the trailer brake.
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