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Air Brake Basics Part 6

Spline Shaft S-cam and Rollers

The slack adjuster is attached to one end of the splined shaft as visible in Figure 2- 13 and an S-shaped cam is on the opposite end, which is mounted inside the wheel assembly

When the slack adjuster rotates, so does the shaft and the S-cam. The S-cam then pushes against rollers which are mounted to the ends of the brake shoes, moving them outward so their linings (or blocks) contact the  rotating brake drum. This contact creates friction which, in turn slows the vehicle.

Brake Shoes, Blocks and Linings

The brake linings (or blocks) are high friction materials riveted or bolted to the brake shoes. They are four inches wide on the steer axle brakes and seven inches wide for all others. As they get worn down, they should be replaced with ones of similar functioning characteristics.

Brake blocks, shoes and linings have a two letter designation that is found on the edge of the friction material. The letters refer to the amount of traction the brake block will generate during a controlled test specified by the Society of Automotive Engineers (SAE). The first letter designates the friction coefficient tested at 200° F to 400° F, which is not unreasonably hot for brakes. The second letter designates the hot friction coefficient. The letters range from C to H with the higher letters corresponding to the higher friction coefficients. For example, a GH designated brake block would have a higher friction coefficient than an EE. This means that the GH brake block would be more aggressive in stopping.

It is very important to replace worn original equipment brake blocks with ones that have similar characteristics so as to maintain a balanced brake system. If, for example, it is apparent that the tractor brakes are supplying more stopping power than the trailer brakes, a more aggressive brake block might be needed for the trailer to balance the whole system. The brake blocks must be the same between the left and right sides, otherwise uneven braking and dangerous handling will result during braking.

Although all original equipment manufacturers (OEM) must certify that their brakes meet FMVSS121 standards, non-OEM brake blocks do not. SAE and The Maintenance Council (TMC) are now testing after market brake blocks and applying a torque value to them. By contacting either SAE or TMC, an updated list can be obtained.

Drivers should be alerted by repair facilities that after brake blocks or shoes are replaced, the braking ability of the vehicle will be reduced for a little while. This is because the full friction surface does not mate fully with the drum so only sub-optimal braking will occur until the brake linings are worn down to better engage the brake drum.

Brake Drums

Brake drums look like a large bowl and are generally cast iron with the inside portion machined smooth. Figure 2-14 portrays a brake drum. The braking process depends on friction generated between the brake linings and the brake drums when the linings are pushed with hundreds of pounds of force against the rotating brake drum. In essence, the kinetic energy of the moving truck and cargo along with the kinetic energy of the rotating tires and wheels are being converted to heat energy as a result of slowing the vehicle. As the brake drum heats up, it acts as a heat exchanger and dissipates the accumulated heat to the atmosphere.

The amount of heat generated in a brake drum depends more on vehicle speed than any other factor. Although it is not unusual for the brake drum to heat up to 300 degrees, excess heat can reduce stopping ability and is termed brake fade.

Providing the brakes are in adjustment, brake fade occurs due to one of two reasons:

I. Excessive heat, due to excessive speed or improper braking techniques, which causes the drum to heat up faster than it can dissipate the heat. This causes the brake drum to expand and the brake linings to have reduced stopping ability. The expanded drum means the brake shoes and linings have to travel farther which can reduce the amount of torque that can be applied to the brake drum by the brake shoes.

2. Contamination of the friction surfaces, namely the brake linings and the brake drums. The contamination, usually a film of oil or grease, can cause a glazing on the brake friction material which reduces the coefficient of friction between the drum and the linings. This severely reduces the stopping ability of the brakes. Finally, elevated temperatures are found to reduce the frictional properties between the surface of the shoe and drum. The higher the temperature, the less braking for the same application pressure.

Following continued use and through a wide range of hot and cold temperatures, the cast iron wears away, creating a larger inside diameter than when originally installed as well as a rougher surface. As the drum wears, some trucking companies have the brake drum interior machined or turned, which removes some or the metal in order to restore the smoothness to the interior of the brake drum.

Most manufacturers of brake drums do not recommend machining the inside surface and placing the brake drum back in service. Because some metal is removed, a machined brake drum will have a larger inside diameter which will therefore require adjustments to be made to the rest of the brake system for the brakes to work effectively. For example, a thicker brake pad might be required in order to contact the drum when the brakes are applied.

Having the brake drums machined and placed back into service is an alternative to replacement and is often a cost-cutting strategy. However, if the interior diameter is worn or machined and then exceeds the maximum allowable diameter for that brake drum, the drum must be taken out of service. The maximum ID is cast into the exterior of the brake drum.

Another type of brake drum is the centrifuge brake drum. This brake drum is also cast iron but it has a steel insert. Visually, the difference between cast iron drums and centrifuge drums is that cast iron drums have square flutes and centrifuge drums have rounded flutes on its exterior. Since the inside of centrifuge drums are made of steel, much harder linings can be used with them. These drums are generally more efficient at dissipating heat, are much lighter and have a longer life than the cast iron drums.


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