Level of Friction
The coefficient of friction should be high enough to limit the amount of effort required to depress the brake pedal. It should not be so high that it causes grab, lock, or sprag in extreme cases. The rotation of the drum becomes impossible in this situation. The friction material must be compatible with the self-energization degree. Modern friction materials have an average coefficient of friction between 0.3 and 0.5. Steelwool Fiber as a composite material helps to achieve this level of Friction Coefficient in a very cost effective way.
Following is a list of properties of Desired Friction Lining having Steelwool fiber as ingredient of the composite Mix
Heat Resistance Fades
This property enables a lining or pad material to maintain its coefficient of friction as the rubbing temperature of the drum and shoes or disc and pads increases. A decrease in the coefficient of friction necessitates more effort on the brake pedal and results in poor braking response. The changes in coefficient of friction caused by rising working temperatures are also influenced by additional curing of the pad as a result of chemical changes in the binder resin. In the higher temperature range, a gradual reduction in frictional level may be acceptable. A rapid decrease in the coefficient of friction severely reduces braking power capability, especially when the vehicle is subjected to long descents or constant stop-start travel.
Recovery from Fade
Once brake lining or pad temperature fade has occurred, this is the ability of a friction material to return to its original friction level after cooling. Even after repeated exposure to high temperatures, a high-quality material’s frictional properties are restored upon cooling. Poor recovery in the case of a poor material is primarily due to chemical breakdown in the ingredients. The friction level may be permanently altered, resulting in the linings or pads hardening, cracking, flaking, charring, or even burning. If thermoplastic binder resins are used with the lining or pads, a deposit may form on the rubbing surfaces, altering the material’s friction properties.
Because rubbing speed and pressure are responsible for material wear, the life of a friction material, both lining and pad, is greatly influenced by them. The working temperature has a significant impact on wear. The material structure is weakened at the upper limits of the lining or pad temperature range, resulting in a higher wear rate (Fig. 28.32).
Rubbing Speed Resistance
In practise, the coefficient of friction between two rubbing surfaces decreases slightly with increasing speed, particularly at higher operating temperatures, though it should be independent of speed. However, a low-quality friction material may exhibit high friction at low rubbing speeds, resulting in judder and grab.
Resistance to Pressure Intensity
The coefficient of friction, according to the laws of friction, should be independent of the pressure holding the rubbing surface together. However, current friction materials are generally compounds held together with resin binders; as a result, pressure between the rubbing surfaces reduces friction to some extent. The deceleration produced by a pressure-stable lining is proportional to the pedal effort. However, a pressure-sensitive material necessitates a significantly greater pedal force to produce a given braking effort. Disc brakes perform better at high rubbing pressures, whereas shoe linings perform worse at similar pressures.
Water Contamination Resistance
Water contamination affects almost all friction materials to some extent. As a result, a safe margin of friction should be available for use in wet conditions. During the drying out process, good quality friction materials should recover quickly and gradually to their original friction level. A low-quality material may recover slowly or have an over-recovery characteristic. Under certain driving conditions, an over-recovery tendency causes brake-grab and even wheel-lock.
Moisture Resistance Sensitivity
For the first few applications, atmospheric dampness, humidity, or dew may increase the friction level. They may cause brake noise and brake-grab for a short period of time. Moisture-sensitive friction materials should not be used in conjunction with self-energizing brakes.