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Air Brake Basics Part 8

Caging the spring brakes requires the insertion of a special bolt into the end of the spring brake chamber as shown in Figure 2-16. The bolt grabs on to the spring retaining plate and as the nut on the bolt is turned, the bolt is drawn out of the chamber while it  compresses the spring and releases the brakes. On some spring brake chambers the bolt is stored on the exterior of the spring brake chamber. This brake release method preserves the slack adjustment for future examination. Preserving the amount of adjustment is particularly important after an accident occurs, as it is pivotal information in accident investigation.

Other similar systems exist for caging the spring brakes. In all cases, the desired effect is to compress the spring so that there is no pressure on the brake drum.

A potential disadvantage of spring brakes is if the driver is not careful, the brake components may be damaged due to brake compounding. This occurs when the driver steps on the treadle valve while the spring brakes are still applied. This results in the summation of the large force from the springs in the spring brake chamber with the force developed from the air pressure in the service brake chamber. Anti-compound valves prevent this from happening.

Engine Brakes

Engine brakes (also called brake retarders) are used to reduce the wear on the mechanical brake system. The three types of engine brakes are the Jacobs brake, Williams exhaust brake and the turbine type retarder.

The Jacobs engine brake (called Jake brake in trucker parlance) was invented by Clessie Cummins and developed by Jacobs Engineering Company. It converts the engine into an air compressor through an electrically- activated, hydraulically-operated cycle interrupter.

The Jacobs brake is placed in ready mode when the driver turns it on by the switch on the dashboard. Depending on the engine manufacturer, the driver can choose how much braking he wants by selecting 2, 4 or 6 cylinders on which to activate the Jake brake mechanism.

Then, whenever the driver removes his foot from the throttle, instead of the engine exhaust valves staying closed at the top of the piston’s compression stroke, they open, allowing all of the compressed air to escape before ignition.

This action interrupts the cycle of the engine. Since the compression is relieved as the injector is about to inject fuel, there is no power stroke. In fact, not only is there no power stroke, but since the engine is coupled to the drive wheels through the transmission, the rotating wheels act to instead turn the engine, which is resisting. If the transmission is taken out of gear, the braking effect is lost.

Another type of engine brake is the Williams exhaust brake. This device is a butterfly valve in the exhaust pipe. When the valve is turned, restricting the flow of exhaust gas, it puts back-pressure into the cylinders which reduce their efficiency.  This action generates some retarding force to the vehicle.

The third type of engine brake is the turbine retarder which uses oil flowing over turbine blades to create pressure on the engine crank shaft, and the resulting resistance helps to slow the vehicle.

All three of these systems provide braking to the truck without using the service brakes. The energy removed is dissipated by the engine and engine cooling system itself. On long down grades this is a big help by not heating up the service brakes unnecessarily.

Anyone near a heavy truck as it slows can generally hear the Jake brake come on as it sounds like the truck has just lost its muffler or has a steady backfiring.

Recent Advancements in Braking Systems

EBS Brakes

A new design for brakes is entering the marketplace . Electropneumatic braking systems (EBS) is also known as electronic brakes. This system uses electrical signals to activate the mechanical brake components. As a result, the brake chamber applies the brakes sooner, and the vehicle’s stopping distance is reduced.

Primarily used in conjunction with ABS, this system then provides shorter stopping distances and improved timing. This system uses modules that can sense wheel speed and brake wear. The system then controls the stopping power at each individual wheel and improves brake balance which improves control of the vehicle during panic stops and slippery road conditions.

Due to current United States Federal law, EBS has not been widely implemented in the U .S. trucking industry. However, experts predict that this system will be common and fully integrated with other electronic features within a few years. This system is legal in Europe.


Another braking advancement is the combination of automatic traction control (ATC) and ABS. The electronic control module (ECM) on the ABS system senses wheel speed. Upon detection of a speed difference between the drive wheels and the non-drive wheels, ATC automatically activates. It applies the brakes on an individual wheel basis, just enough to maintain traction and uniform tire rotation.

This system is an optional feature for anti-lock braking systems that can have other benefits such as reduction in wear on various components.


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