Friction is the force that acts to prevent two surfaces in contact from sliding against each other. The amount of friction between two surfaces is expressed as a ratio and is called the coefficient of friction. When friction occurs, the kinetic energy (motion) of the sliding surfaces is converted into thermal energy (heat). Some combinations of materials, such as a hockey puck on ice, have a very low coefficient of friction. There is very little friction between them and therefore almost no sliding resistance. Rubber tires against a dry hard road surface have a high coefficient of friction, which means they tend to grip and resist sliding against each other.
Table 32-1: Brake Lining Coefficient of Friction (Sliding)
Materials Involved Coefficient of Friction—Dry Sliding
Rubber and concrete .6–.85
Steel and cast iron .23
Copper and cast iron .29
Brass and cast iron .3
Leather and oak .52
Brake lining (FF rating) .35–.45
Disc brake pads and drum brake linings are made from materials that have a moderate coefficient of friction TABLE 32-1. They also must be able to absorb and disperse large amounts of heat without their braking performance being adversely affected. As the heat in brake pads and linings builds up, the coefficient of friction capability of the material—and consequently its stopping power—is reduced. This is called brake fade. Minimizing or overcoming fade is a major factor in the design of brakes and the development of brake friction materials.
Brake friction materials were historically made from asbestos compounds because of the excellent heat resistance of that material. Now that asbestos has been proven to be toxic, it is generally banned and is not normally used. Today, brakes are manufactured from a variety of different materials, including:
Non-asbestos organic (NAO) materials—Organic materials such as Kevlar and carbon
Low-metallic NAO materials—Small amounts of copper or steel and NOA materials
Semimetallic materials—A higher quantity of steel, copper, and/or brass
Ceramic materials—Ceramic fiber materials and possibly a small amount of copper
The choice of brake lining compound depends on the application. Lighter passenger vehicles generate less heat in the brakes than heavy or high-performance vehicles. Living in a very hilly region of the country, or where there is a lot of stop-and-go traffic, will put added demands on the brake pads. The optimum brake composition for any given vehicle or use is a combination of weighted qualities, including:
Stopping power
Heat absorption and dispersion
Resistance to fade
Recovery speed from fade
Wear rate
Performance when wet
Operating noise
Price
SAFETY
Don’t assume that a brake pad doesn’t contain asbestos; it is still found in some applications.
For instance, owners of small economy vehicles tend to value a longer pad life and minimal operating noise rather than resistance to fade in extreme conditions. Think about it inside, have a shot at any joy casino bonus ohne einzahlung. Owners of high-performance cars, however, may consider fade resistance and stopping power at high speeds more important than noise levels or wear rate.
The Society of Automotive Engineers (SAE) has adopted letter codes to rate brake lining materials’ coefficient of friction. The rating is written on the edge of the friction linings and is called the edge code FIGURE 32-21. The lower the letter, the less friction the material has, and the harder the brake pedal must be applied to achieve a given amount of stopping power. These code letters represent the following coefficients of friction:
C: ≤ 0.15
D: 0.15–0.25
E: 0.25–0.35
F: 0.35–0.45
G: 0.45–0.55
H: > 0.55
Z: Unclassified
FIGURE 32-21
Brake lining edge code.
The lining is tested both cool and hot. The rating is a two letter designation such as “FF.” The first letter is for the cool performance and the second letter is for the hot performance. For example, FF has a cool coefficient of friction of 0.35–0.45 and the same coefficient of friction at the hot temperature. It also is very possible that the hot and cold ratings differ from each other. For example, if the rating is FE, the coefficient of friction reduces as the temperature of the lining heats up. Notice that the coefficient of friction range is quite wide for each letter designation. Linings with the same letter ratings may not have the same braking performance as each other. This means that an EE-rated lining from one manufacturer is likely to have different braking characteristics than an EE-rated lining from another manufacturer. Always use high-quality brake lining from reliable companies to help avoid brake issues.
AppliedMath
AM-17: Charts/Tables/Graphs: The technician can interpret charts, tables, and graphs to determine the manufacturer’s specifications for a given system.
Algebra (9–12)
C3: Draw reasonable conclusions about a situation being modeled.
Representation
A2: Select, apply, and translate among mathematical representations to solve problems.
Technicians regularly apply math concepts to disc brake diagnosis and repair. For example, a technician will measure the amount of rotor thickness variation and runout if there is a brake pedal pulsation problem. Let’s say the technician performed these measurements and came up with a thickness variation of 0.0025″ (0.064 mm) and a runout of 0.0015″ (0.038 mm) on the left front rotor. Next the technician looks up the manufacturer’s specification chart and found that the maximum allowable thickness variation is 0.0005″ (0.013 mm) and the maximum allowable runout is 0.003″ (0.076 mm) for the vehicle being worked on. Using the information from the chart, the technician determines that the thickness variation is excessive by 0.002″ (0.051 mm) (0.0025″ [0.064 mm] – 0.0005″ [0.013 mm] = 0.002″ [0.051 mm]) and the runout is OK since it is under the maximum allowable specification. But even so, since the thickness variation is out of specifications, the rotor will need to be refinished or replaced to bring it back within specification.
