Brake Components and functions
Brake Components and Functions
A disc brake system usually consists of break disc rotor, two brake pads and a caliper (SAE International, 2007). The combination of these components allows the rotating wheel to experience severe braking in a short stopping distance. The braking surface is the area on which the braking action of the friction material takes place (Limpert, 1999).
Figure illustrated the disc brake system components on a passenger vehicle. The centre part of the brake disc has a circular aperture, which is locates on the wheel hub (SAE International, 2007). It is surrounded by a number of holes for the wheel bolts. The brake disc rotates along with the wheel. The normal load, produced when the brake is actuated result in the generation of an in-plane friction force at the disc-pad interface. This in turn produces a brake torque about the centre of rotation of the wheel as shown in Figure. The reaction to the brake torque is seen in the brake force, between the tire and ground, which slow the vehicle.
The ventilated brake disc is a one-piece casting with the groves and for passenger vehicle with cooling fins between the two braking surfaces. This enables air to circulate between the braking surfaces, making them less sensitive to heat build-up and more resistant to fade. Dirt and water do not generally affect the braking action since contaminants are thrown off by the centrifugal action of the brake disc or scraped off by the pads. In addition, the equal clamping action of the two brake pads tends to ensure uniform, straight-line stops (SAE International, 2007).
The two main functions of the brake rotor are the transmission of mechanical force and the dissipation of heat, produced when functioning at both medium and high temperature (Limpert, 1999). This means that the materials used for brake discs must be able to support high temperatures (Grieve et al., 1998). The rotor material must be cost effective, allowing for potential reductions in weight as well as for the stability of the components (Ioannidis et al., 2005).
A solid disc brake consists of a rubbing surface and a top hat section (Bae and Wicket, 2000). The section that connects these two parts is known as the neck (Yumoto and Okamura, 2006). The rubbing surface section is the area where a tangential friction force is generated when the disc interacts with a stationary pad to stop the moving vehicle. The disc rubbing surface area is sometimes known as the cheek (Grieve et al., 1998)(Koetniyom, 2000). A top hat section is connected to the disc rubbing surface and mounted to the vehicle wheel hub.
A disc brake which has separate inboard and outboard rubbing surfaces with cooling vanes or fins in between is known as a ventilated rotor. These vanes allow the air to flow through the structure and cool the rubbing surfaces during and after all braking events. There are two types of ventilated disc: front-vented and back-vented. Figure shows the front-vented type of disc brake, where the top hat section is connected by the neck to the outboard rubbing surface. On the other hand, a neck that connects the inboard rubbing surface with the top hat section as shown in Figure creates what is known as a back-vented disc (SAE International, 2007).
The friction between the pad and disc plays a decisive role in defining the amount of the brake pad is designed to rub against the disc surface leading to diminution of pedal force required to obtain a given rate of deceleration (Papinniemi et al., 2002). This factor is also important in designing brakes for the balanced operation of a vehicle as brake imbalance can lead to a yaw torque about the vertical that could compromise the vehicle stability. The additional task for the brake disc is to induce air movement, as air moving over the rubbing surface of the disc reduces the heats build up (Valvano and Lee, 2000).
The brake pad is designed to rub against the disc surface leading to diminution of vehicle speed, thereby converting mechanical work into thermal energy. The structure of the pads can be very complex (Nicholson, 1995). They can consist of different materials or numerous parallel layers as shown in Figure. There are many different types of friction material on the market which can be classified into the following categories: semi-metallic (SM), non-asbestos organic (NAO) and sintered metal (Anderson, 1992). A friction material is mounted to a rigid metal back plate using adhesive. A substrate material is sometimes located in between the friction material and back plate. The main function of the substrate material is to act as a thermal insulator that will prevent an excessive flow of heat towards the piston and brake fluid as well as damping vibration. The backplate distributes the force exerted by the piston over the pad contact surface (Lee et al., 2003a). An anti noise layer or shim located behind the backing plate minimizes the transmission of vibrations produced during braking action.
The caliper, which contains one or more pistons, holds the two brake pads on either side of the rotor. The movement of the pistons is controlled by a hydraulic system (SAE International, 2007). When hydraulic pressure is applied by pressing the brake pedal, the piston is pushed forward to press the inner pad against the rotor while the housing is pushed in the opposite direction to press the outer pad against the rotor, hence generating a hydraulic clamp around the rotor. For the fixed (non-floating) caliper type of disc brake system, each piston presses the brake pad against its respective side of the brake rubbing surface, as shown in Figure . Meanwhile the floating caliper housing, which is designed to slide on its support, reacts by shifting and pushing the pads against both sides of the disc as shown in Figure (SAE International, 2007).