The Principles of Friction Involved in Brake Action
A. Friction is the distance to movement that results from two objects moving or rubbing against each other. There are two types of friction
1. Kinetic friction occurs between two objects, one of which is moving. Kinetic friction always produces heat. Automotive brakes use kinetic friction
2. Static friction occurs between two objects, both of which are stationary. Automotive brakes use static friction to hold a vehicle while parked.
B. Factors that affect the amount of friction produced between two objects:
1.The rougher the surface of two objects, the more friction
they will produce.
a. Extremely rough surfaces create the most friction, but rough surfaces also wear down quickly. Automotive brakes therefore, use relatively smooth surfaces to avoid rapid wear.
b. In order to compensate for their smoother surfaces, automotive brakes are applied with a greater amount of pressure over a relatively large contact area.
2. The greater the pressure bringing the objects together, the more friction they will produce. Therefore, the greater the pressure applied to the brakes, the greater their stopping power.
3. The greater the amount of shared contact area between two objects, the greater the amount of friction the objects will produce.
a. Automotive braking systems use the largest contact area possible.
b. The greater the contact area of a brake shoe or pad, the less heat the shoe or pad will generate. Less heat allows for more friction, which makes the brakes more efficient.
4. The hotter the friction surface of two objects, the less friction they will produce.
a. All heat created by the brake system must be dissipated as rapidly as it is created. The brake system can store little or no heat.
b. Brake friction surfaces are made of material that can conduct heat easily.
c. Braking system components that produce friction are positioned so that air cools them. In some braking systems, forced air is used to cool components.
C. The amount of friction produced by two objects rubbing against each other is called the coefficient of friction.
D. An important brake friction surface is the brake lining, which is mounted on either a brake shoe or pad. The brake lining produces friction by directly contacting another friction surface (either a brake drum or disc). The brake lining and the material with which it comes into contact must have special characteristics.
1.The brake drum or disc must do the following:
a. Conduct heat easily
b. Hold its shape under extremely high heat
c. Withstand rapid temperature changes
d. Resist warping and distortion
e. Wear well in general. Therefore, brake drums and discs are typically constructed of iron or steel combined with aluminum.
2. The brake lining must be somewhat softer than the brake drum or disc. At present, most brake linings are made of organic material, metallic particles, and other minerals held together by a bonding agent.
3. When the brake lining is applied to a drum or disc, the proper coefficient of friction must be produced if the brakes are to be effective.
a. If the friction coefficient is too great, the brakes may be 'grabby' or overly sensitive. Overly sensitive brakes may cause the vehicle to skid too easily.
b. If the friction coefficient is too low, brake application will require excessive pressure. Applying brakes with excessive pressure creates excessive heat, which could result in brake failure.
c. If brakes create more heat then can be dissipated, the friction coefficient will be reduced, causing the brakes to fade. Excessive heat also causes bonding agents in the lining to melt and flow to the surface, thus producing a glaze on the shoe lining. This glaze will reduce the brake's friction coefficient and cause more brake fading. Brake application will then require more pressure, thus creating more heat and more glazing.
