Four-circuit Protection Valve: The Key Assurance of the Air Braking System

What is a four-circuit protection valve?

First of all, we need to know that the braking system of a truck usually falls into two types: hydraulic braking and air braking. Hydraulic braking has a slightly slower response speed and lower intensity. Generally, hydraulic braking is mostly used in micro-vans, micro-trucks and other vehicle models. Its characteristics are small size, light weight and simple structure.

On the other hand, air braking has a rapid braking response and strong braking force, and is mostly used in medium and heavy-duty trucks. It has large components such as air storage tanks and brake cylinders. Now many light trucks have also been converted from hydraulic braking to air braking. The four-circuit protection valve exists in the air braking system and is used to divert and protect the air pressure pipelines of the entire vehicle.

The four-circuit protection valve is essentially a five-way valve, and its internal structure is of the diaphragm type. It is mainly composed of a housing, a diaphragm, a valve, a transmission ring, a push plate, a pressure regulating spring, etc. It has one air inlet and four air outlets. Among them, air outlets 21 and 22 correspond to the front axle braking system and the middle and rear axle braking systems, and air outlets 23 and 24 are connected to the parking braking system and the auxiliary braking system respectively.

● The function of the four-circuit protection valve

After dividing the braking system into four circuits that are both interconnected and independent of each other, it can ensure that when a certain circuit fails (breaks or leaks air), it will not affect the normal operation and inflation of other circuits. At the same time, it can appropriately supplement the air pressure of the failed circuit or even close the circuit, playing a protective role.
That is to say, even if a certain air circuit fails and leaks air, the four-circuit protection valve can still make the remaining circuits retain about 6.5 units of braking air pressure and provide a braking effect.

Why do trucks use four-circuit protection valves?

So why do trucks need to be equipped with four-circuit protection valves? In fact, it is still to ensure the reliability of the vehicle's braking system. Just imagine, if the braking system does not have a four-circuit protection valve, the air circuits in all directions are directly connected. In case a certain pipeline is scratched by a small stone bouncing on the road, it will cause the entire braking air circuit to completely lose air pressure, and the brakes will no longer work, which may then lead to major traffic accidents.

Therefore, according to the regulations, when a certain pipeline fails, it is necessary to ensure that the emergency braking can take effect so that the vehicle can stop. To meet the above requirements, the vehicle should use two or three independent energy storage circuits. Not only should the energy storage circuits be separated, but also their control circuits should be separated at the same time and be independent of each other to ensure that the air compressor can continue to inflate the other undamaged pipelines.

Common fault problems:

1. A certain circuit does not inflate or leaks air
   Fault phenomenon: Weak braking force, or even brake failure
   Fault cause: There is moisture in the air passage. After a long time, it will cause extensive corrosion in the inner cavity and block the air valve. Corrosive foreign matter falls off, affecting the rupture of the diaphragm and causing air leakage. At this time, the four-circuit valve needs to be disassembled and inspected, and the safety valve and check valve need to be checked. If the valve is stuck, it can be cleaned and reassembled. If the valve is damaged, it needs to be repaired or replaced.
   At the same time, also check whether the drying tank is damp or leaking, whether its service life has expired, and replace it in a timely manner.

2. Air pressure imbalance
   Fault phenomenon: Slow air inflation, fast air consumption, the air cut-off brake cannot be released, and the clutch pedal is heavy
   Fault cause: Since the opening air pressure of each branch circuit of the four-circuit protection valve has been adjusted before leaving the factory, the occurrence of an imbalance fault is likely due to damage or deformation of the diaphragm, valve seat, and piston assembly, or improper adjustment after maintenance and replacement of related parts, thus limiting the air pressure.
    

In simple terms, the four-circuit protection valve can ensure the normal operation of each air circuit in the braking system and play the role of a hub for shunting and control. Of course, if this part fails, phenomena such as the parking brake cannot be released, the clutch pedal is heavy, and the air horn does not sound will occur. All truck drivers must remember to check it regularly. 

Article source: 360che.com