Disc Brake Cooling
Disc Brake Cooling
The cooling of the brake disc and its pads takes place mostly by air convection, however wheel hubs also conducts away some of the heat. The rubbing surface between the rotating disc and the stationary pads is exposed to the frontal air stream of the vehicle and hence to the directed air circulation. Therefore, under continued brake application the disc brake is considerably more stable than the drum brake. Also, the high conformity of the pad and disc, and the uniform pressure allows the disc to withstand higher temperatures compared to the drum brake. Since far less distortion takes place in discs compared to drums, the disc can operate at higher temperatures. Also, the disc expands towards the pads, whereas the drum expands away from the shoe linings. Consequently, in the hot condition the disc brake reduces its pedal movement, but the drum brake increases its pedal movement.
The discs are made of cast iron and are ventilated to considerably improve the cooling capacity during rotation (Fig. 28.12B). These discs are consisted of two annular plates ribbed together by radial vanes, which also act as heat sinks. Due to centrifugal force, air is pushed through the radial passages formed by the vanes from the inner entrance to the outer exit so that cooling is improved. The ventilated disc provides more exposed surface area, so that an increase of about 70% in convection heat dissipation occurs compared to a solid disc of similar weight. Consequently, a ventilated disc reduces the friction pad temperature to about two-thirds that of a solid disc under normal operating conditions. Also, pad life is considerably increased with the lower operating temperatures, but a very little effect on the frictional properties of the pad material takes place. However, at low speeds ventilated discs have very little influence on the cooling rate. Very high speeds produce a pressure difference between the inside and outside of the disc, which forces air to flow through the vents towards the disc and pads. This additional air flow contributes about 10% increase in the disc's cooling rate. The exposure of the disc and pads to water and dirt considerably increases pad wear, and causes reduction in the frictional properties of the rubbing pairs. Therefore, protecting the rubbing surfaces from atmospheric dust and the road surface spray is most important.
The discs are made of cast iron and are ventilated to considerably improve the cooling capacity during rotation (Fig. 28.12B). These discs are consisted of two annular plates ribbed together by radial vanes, which also act as heat sinks. Due to centrifugal force, air is pushed through the radial passages formed by the vanes from the inner entrance to the outer exit so that cooling is improved. The ventilated disc provides more exposed surface area, so that an increase of about 70% in convection heat dissipation occurs compared to a solid disc of similar weight. Consequently, a ventilated disc reduces the friction pad temperature to about two-thirds that of a solid disc under normal operating conditions. Also, pad life is considerably increased with the lower operating temperatures, but a very little effect on the frictional properties of the pad material takes place. However, at low speeds ventilated discs have very little influence on the cooling rate. Very high speeds produce a pressure difference between the inside and outside of the disc, which forces air to flow through the vents towards the disc and pads. This additional air flow contributes about 10% increase in the disc's cooling rate. The exposure of the disc and pads to water and dirt considerably increases pad wear, and causes reduction in the frictional properties of the rubbing pairs. Therefore, protecting the rubbing surfaces from atmospheric dust and the road surface spray is most important.
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