Functions and Problems of the Automotive Brake Pads.
The disc brake pads are a main works of the Automotive Brake System. Pads are consist of a steel backing plate and a molded, bonded or riveted friction material puck. A brake set usually contains 4 pads 2 inner and 2 outer for one front or rear axle. There are many types of friction materials depending on the use of the vehicle.
Function
When you push on the your brake pedal, hydraulic fluid is forced from the master cylinder through the brake lines and hoses, down to the brake calipers, forcing the pistons to expand, putting pressure on the pads. The piston squeezes the brake pads against the rotors causing the wheel to slowdown and stop.
The Disc Brake Pads convert kinetic energy to heat energy. During braking there is a little amount of friction material transferred from the pads to the rotors, this helps the rotor and pads to stick providing the friction to bring the vehicle to a stop. The material transfer is also why you need to break in the pads and rotors. If the pads are not properly broken in, they may leave an uneven deposit of pads friction material on the rotor surface resulting in a high runout, Disc Thickness Variation and other problems.
Here are some typical Brake Pads problems:
- Brake fade: A reduction or loss in braking force due to loss of friction between the brake pad and the brake rotor. Fade is caused by heat build-up through repetitive or extended brake application
- Outgas: At high temperature the resins in the bake pad vaporize forming a barrier layer in between the pad and rotor. This barrier cusses the brakes to fade. When outgassing occurs there is still a firm pedal but the amount of friction being generated is reduced.
- Creep Groan: A noise that occurs at very low vehicle speed (under 2 mph) when applying or holding the brake ON. Creep groan is caused by brake pad “stick-slip”. It usually experienced on vehicles with automatic transmissions.
- High speed Brake judder: A Vibration during high speed braking. Its caused by Brake Torque Variation, hot spots and on even brake material transfer on to the rotor. High Speed Brake Judder vibrations are felt trough the chassis or body.
- Brake Judder: A low speed Vibration during braking which is caused by Disc Thickness Variation. Brake judder vibrations are transferred trough the brake pedal and the chassis.
- Brake Howl: Brake Noise generated in the range of 500 to 1,000 Hz and is caused by stick slip.
- Rotor scoring: Grooves on the friction surface of the brake rotor, usually caused by the brake pads.
- Squeal (low frequency): Noise generated in the range of 800 to 5,000 Hz, caused by the positive mechanical feedback excitations from the brake linings.
- Squeal (high frequency): Noise in the range of 5,000 to 18,000 Hz and caused by the positive mechanical feedback excitations from the brake linings.
- Stick-slip: Stick-Slip is caused by a non-linear change in the coefficient of friction. When the coefficient of friction increases, the two surfaces will stick. As the rotational force increases or the coefficient of friction decreases, a sudden release or slip will occur between the two surfaces.
- Transfer layer: The transfer of friction material to the brake drum or rotor. The thickness increases with temperature and the number of braking cycles; thickness is also directly related to the amount of stick-slip.
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