LEEVENSPARK

Braking System The function of the braking system is to retard the speed of the moving vehicle or bring it to rest in a shortest possible distance whenever required. The vehicle can be held on an inclined surface against the pull of gravity by the application of brake. Brakes are mechanical devices for increasing the frictional resistance that retards the turning motion of the vehicle wheels. It absorbs either kinetic energy or potential energy or both while remaining in action and this absorbed energy appears in the form of heat. While moving down a steep gradient the vehicle is controlled by the application of brakes. In this case brakes remain in action for a longer period making it imperative to dissipate the braking heat to atmosphere as rapidly as possible. Automobiles are fitted with two brakes; the service or foot brake and the emer­gency or hand brake. The foot brake is used to control the speed of the vehicle and to stop it, when and where desired, by the application of force

Types of Brake The two main types of friction brake are drum brake and disc brake (Fig. 28.4). In both types a fixed (non-rotating) shoe or pad rubs against a moving drum or disc. To increase the friction between the rubbing surfaces, a special friction material is attached to the fixed part. Earlier this friction material had a high content of asbestos, but its dust is injurious to health so that a safe asbestos free friction material is nowadays used. Fig. 28.4. Types of brake. Drum Brakes. This internal expanding type of brake contains two shoes that are attached to a back-plate and are fixed to a stub axle or axle tube. The section of each shoe is of "T" shape. A friction lining is riveted or bonded to the outer face of the shoe. A drive is fitted at one end of the shoe so that the shoe expands when the brake pedal is applied. In a simple brake (Fig. 28.5) a cam is used as a shoe expander, but modern systems for cars use hydraulically operated pistons for shoe expansion.

Brake Shoe and Pad Fundamentals 28.4.1. Brake Shoe Self Energization The drum type brake consists of two internal semicircular shoes, which are lined with friction material matching to the internal rubbing face of the drum. The shoes are fitted on a back plate, also known as a torque plate, between a pivot anchor or wedge type abutment at the lower shoe end and at the upper shoe top end by either a cam or hydraulic piston type expander. In Fig. 28.9 the expander is simply represented by two opposing arrows and the shoe linings by two small segmental blocks in the mid region of the shoes. During clockwise rotation of the drum, the upper tips of the shoes are pushed apart by the expander force, Fe, and a normal inward reaction force, N, provided by the drum, resists any shoe expansion. Consequently the drum slid over the shoe linings and a tangential frictional force Ft = \iN is produced between each pair of rubbing surfaces. The friction force or drag on the right hand shoe (Fig. 28.9)

Drum Brakes 28.5.1. Brake Layouts There are two types of drum brakes, such as externally contracting and internally expand­ing. External contracting brakes are preferred for automatic gearboxes. The internal expanding type, along with a drum, is commonly used in braking systems of vehicles. The drum brakes, used with light vehicles, are hydraulically operated. These brakes are commonly used for the rear wheels to complement a disc system at the front. This disc/drum layout permits the front wheels to undertake more braking effort. In addition, the compatibility of a mechanical hand-brake with a drum brake makes this type an obvious option for rear brakes. Various shoe arrangements in use include : • Leading and trailing shoe (L&T) • Two leading shoe (2LS) • Duo-servo. 28.5.2. Leading and Trailing Shoe (L&T) The layout of a leading and trailing shoe brake is presented in Fig. 28.14. This arrangement uses a pair of shoes pivoted at a common anchor point. The free ends of both sh