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220 readings | Published on 19 November 2009
Infomanejo Number 64
All the versions of this article: [es] [pt]

There are differences between the brakes of heavy vehicles and light. The foremost of these is the fluid used to transmit each driver’s action on the pedal to the wheels.

In InfoManejo 52 we saw characteristics of liquids and gases, so the operating principle and therefore each system mechanisms are clearly distinct. Put aside the hydraulic brakes, which were analyzed in the delivery 24 and lets focus on pneumatic systems.

(Click so the animation starts)

Before entering the various constituent parts we should know that the brakes of a truck can be classified into three categories:

Service brakes, are those that the driver is using while driving.

Parking brakes; are applied to leave the vehicle stationary on a permanent basis.

Emergency brakes, those that are automatically activated when air pressure is not sufficient to ensure safe working order.

While the brake elements may be the same, differences in the forms of assets. The first act when they get air from the brake pedal command, while those of emergency are put into operation by releasing a spring that is content as it has sufficient pressure. In between, the parking can run through a manually operated valve that releases the spring pressure above achieving the same end effect. (Note that these are simplifications and there is a variety of mechanisms and different methods of operation so you should always use the vehicle manual).

Here are the main components of these systems.

Compressor: It is powered by the vehicle’s engine can be connected through belts or gears and having lubrication and cooling systems themselves or make use of the engine. In the case of being belt-driven, it must be inspected regularly as any other vehicle belt, with the exception of the importance this has for security. Note: With the engine running normally and without stepping on the pedal brake, the pressure should not take longer than specified in the manual to reach their work value, otherwise, you should do a review.

Controller: It is responsible for commanding the compressor operation, making it operate when the pressure is less than needed and stopping when superior. Note: That the compressor does not stop when the pressure is excessive or does not start when it is low is a sign of malfunction.

Air Tanks: These tanks have adequate capacity to store sufficient quantity of air which would cover peak demand or power to stop the vehicle when the compressor has stopped working. Note: When compressed air, water vapour in it condenses inside the circuit. This water, along with oil from the compressor, can cause various types of faults. Also be alert to cold temperatures weather that can turn water into ice.

Blowdown Valves: We recommend using drain valves at least at the end of the daily activity in order to avoid them. There are manual and automatic valves, many vehicles have both.

Safety valve: Responsible for releasing air into the atmosphere when pressure in the tanks exceeds safe limits. Note: When we hear that they are constantly working, something is wrong in the system and should be verified.

Pedal Brake: Command responsible for the valve controlling the amount of air entering the wheel mechanisms. When it is pressed, the valve allows air passage that slows the wheels, when released, the pressure decreases and is then released. Tip: The more pressed, the greater the amount of air will pass and therefore will be stronger braking.

Brake mechanism: Is in charge of preventing wheels rotation, it can have different operation methods. The modern mechanisms have automatic adjustment devices, they compensate the inevitable wear of components. Warning: if the brakes are poorly regulated, these devices fail to be sufficient to ensure the proper functioning and therefore should never be relied upon to compensate for possible errors of regulation.

Instruments: Because the system works with air pressure constantly. Tip: You must have instruments that can inform this parameter. Besides knowing what happens while driving, are useful to perform various field tests to verify the operation of some parts of the circuit. Along with the classic needle gauges you can find lights or sound alerts.

How to test the components?

There are different tests to check system components of air brakes. Some of them were mentioned here, others we will see in a forthcoming paper on which also discuss good management practices for air brakes. A smart driver checks all the points that are within reach and immediately goes to a specialist when it detects that something is not working properly.


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