Types of Brake
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.
The shoe anchor is rigidly attached to the back-plate and takes the form of a large pin that passes through the shoes, or housing. The shoes butt against the anchor. Springs pull the brake shoes on the back-plate and also return the shoes to the 'off position after the brake has been applied. In some layouts separate springs execute the retention and return functions.
The inner cylindrical surface of the cast iron drum is made smooth on to which the brake linings rub. The drum is generally fixed to the hub flange using counter-sunk screws and secured by the wheel nuts. It is necessary to adjust excessive clearance due to wear of the friction facing, so that they are always positioned very close to the drum. This is carried out either manually adjusting the brakes periodically, or having an automatic adjuster that continually sets the shoes.
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.
The shoe anchor is rigidly attached to the back-plate and takes the form of a large pin that passes through the shoes, or housing. The shoes butt against the anchor. Springs pull the brake shoes on the back-plate and also return the shoes to the 'off position after the brake has been applied. In some layouts separate springs execute the retention and return functions.
The inner cylindrical surface of the cast iron drum is made smooth on to which the brake linings rub. The drum is generally fixed to the hub flange using counter-sunk screws and secured by the wheel nuts. It is necessary to adjust excessive clearance due to wear of the friction facing, so that they are always positioned very close to the drum. This is carried out either manually adjusting the brakes periodically, or having an automatic adjuster that continually sets the shoes.
Disc Brakes.
One problem with drum brakes is fade and to minimize this problem the disc brake was developed. It uses an exposed disc that is attached to the hub flange. The two friction pads are pressed on to this disc to provide braking action (Fig. 28.6). The pads are actuated by hydraulic pistons placed in cylinders formed in a caliper, which is secured to a fixed part of the axle.
Fig. 28.5. Cam-operated drum brake.
Fig. 28.6. Disc brake.
The hydraulic pressure forces the friction pads against the rotating cast iron disc. Consequently, the disc motion is retarded and heat generated from the energy of motion is conducted to the disc. Since a large part of the disc is exposed to the air, heat is easily dissipated, so that the brake can be used continuously for long periods before serious fade occurs. In this layout the friction pads move at a right angle to the disc, so that any drop in the friction value does not affect the force applied to the pad. ,
For the application of a disc brake, a greater pedal pressure is necessary to produce a given retardation than that required for a drum brake. Adjustment of pad wear is automatic on a disc brake. In this brake system the pads can also be inspected easily and in the absence of corrosion, the pads can be renewed easily.
The hydraulic pressure forces the friction pads against the rotating cast iron disc. Consequently, the disc motion is retarded and heat generated from the energy of motion is conducted to the disc. Since a large part of the disc is exposed to the air, heat is easily dissipated, so that the brake can be used continuously for long periods before serious fade occurs. In this layout the friction pads move at a right angle to the disc, so that any drop in the friction value does not affect the force applied to the pad. ,
For the application of a disc brake, a greater pedal pressure is necessary to produce a given retardation than that required for a drum brake. Adjustment of pad wear is automatic on a disc brake. In this brake system the pads can also be inspected easily and in the absence of corrosion, the pads can be renewed easily.
Brake Operating Systems
Brake shoes and pads, nowadays, are generally operated hydraulically. The mechanically operated system is discussed in the chapter for the purpose of appreciating the features offered by modern systems.
28.3.1.
28.3.1.
Mechanically Operated System
The layout of a simple mechanical system is illustrated in Fig. 28.7A. Four adjustable rods or cables connect the brake shoe operating levers to a transversely mounted cross-shaft. The footbrake and handbrake controls are connected to the cross-shaft using links with elongated holes that allow independent operation of each control.
The mechanical system provides the same brake pedal force to each brake only when the mechanism is balanced so that all the shoes with the drums are operated simultaneously. If one brake has a much smaller shoe-drum clearance than the others, the total force on the brake pedal is directed to that brake and the unbalanced braking action causes the vehicle of 'pull' violently to the side with that brake. Compensation devices are installed in the system to overcome this problem. Figure 28.7B shows a simple arrangement for balancing two brakes. A fully compensated mechanical brake system requires three compensators that is front (to balance the front brakes), rear (for the rear brakes) and centre (to equalize front and rear).
The mechanical system provides the same brake pedal force to each brake only when the mechanism is balanced so that all the shoes with the drums are operated simultaneously. If one brake has a much smaller shoe-drum clearance than the others, the total force on the brake pedal is directed to that brake and the unbalanced braking action causes the vehicle of 'pull' violently to the side with that brake. Compensation devices are installed in the system to overcome this problem. Figure 28.7B shows a simple arrangement for balancing two brakes. A fully compensated mechanical brake system requires three compensators that is front (to balance the front brakes), rear (for the rear brakes) and centre (to equalize front and rear).
Fig. 28.7. Mechanical brake layout. A. Brake layout. B. Brake compensator to balance two brakes.
A compound lever system is used to achieve large force on the brake shoe. The larger the leverage, the smaller is the force needed on the brake pedal. However is very large leverage increases wear of the brake linings so that frequent adjustment of the brake is necessary.
Due to regulation a handbrake (parking brake) must be provided to hold the car stationary when it is left unattended. A ratchet and pawl is used with this hand-lever to hold the brake in the 'on' position.
28.3.2.
A compound lever system is used to achieve large force on the brake shoe. The larger the leverage, the smaller is the force needed on the brake pedal. However is very large leverage increases wear of the brake linings so that frequent adjustment of the brake is necessary.
Due to regulation a handbrake (parking brake) must be provided to hold the car stationary when it is left unattended. A ratchet and pawl is used with this hand-lever to hold the brake in the 'on' position.
28.3.2.
Hydraulically Operated System
The hydraulic system has a high efficiency and is fully compensated. This system is suited for vehicles fitted with independent suspension. The main features of this system are shown in Fig. 28.8. The basic layout uses a master cylinder, which is connected by smallbore tubing to wheel cylinders positioned at each brake. A special brake fluid is stored in a reservoir after completely filling all pipelines and working cylinders.
As the foot brake pedal is operated, a piston in the master cylinder pumps fluid through the lines into the wheel cylinders. This causes the pistons in the wheel cylinders to move outwards so that the shoe or pad is brought into contact with the drum or disc. The pressure on the master cylinder piston is transmitted through the fluid in the system to apply a force to each brake. The greater the force applied to the pedal,
As the foot brake pedal is operated, a piston in the master cylinder pumps fluid through the lines into the wheel cylinders. This causes the pistons in the wheel cylinders to move outwards so that the shoe or pad is brought into contact with the drum or disc. The pressure on the master cylinder piston is transmitted through the fluid in the system to apply a force to each brake. The greater the force applied to the pedal,
Fig. 28.8. Hydraulic brake layout
the higher is the pressure produced in the system. Since fluid may be considered as incompressible, and the pressure is the same throughout the system, the thrust on any part of the system is proportional to the area of that part. Therefore the force applied to the pistons can be varied as per the requirement.
Presence of any air in the system causes the brake pedal to become springing, i.e. to lose its 'solid' feel. Therefore removal of air is essential if any part of the system has been disconnected. The valves are fitted at each wheel cylinder to remove air and this operation is called bleeding.
Due to legal requirements, hydraulically operated systems must be supported with a mechanical handbrake at least on two wheels. Usually a rod or cable links the handbrake with a mechanical lever-type shoe-expander fitted to the rear brakes. The single-line system shown in the figure has a drawback that the brake failure occurs if fluid leaks in any part of the system. For safety reasons modern vehicles use a dual-line system that ensures the operation of at least two brakes by the foot brake in the event of a leak.
28.3.3.
the higher is the pressure produced in the system. Since fluid may be considered as incompressible, and the pressure is the same throughout the system, the thrust on any part of the system is proportional to the area of that part. Therefore the force applied to the pistons can be varied as per the requirement.
Presence of any air in the system causes the brake pedal to become springing, i.e. to lose its 'solid' feel. Therefore removal of air is essential if any part of the system has been disconnected. The valves are fitted at each wheel cylinder to remove air and this operation is called bleeding.
Due to legal requirements, hydraulically operated systems must be supported with a mechanical handbrake at least on two wheels. Usually a rod or cable links the handbrake with a mechanical lever-type shoe-expander fitted to the rear brakes. The single-line system shown in the figure has a drawback that the brake failure occurs if fluid leaks in any part of the system. For safety reasons modern vehicles use a dual-line system that ensures the operation of at least two brakes by the foot brake in the event of a leak.
28.3.3.
Pneumatically Operated System
Nowadays both low and high pressure air brake systems are fitted in vehicles, but most light category vehicles use only air to boost the effort applied by the driver. A full compressed-air braking system, is normally used in heavy vehicles, but is too expensive, bulky and heavy for light vehicles.
Some light trucks in the 3 ton category incorporate a part-compressed-air system called air/hydraulic (air over hydraulic). This air system combined with a hydraulically operated arrangement reduces the maximum pedal pressure to a comfortable limit.
Now a days it is common to use a disc-type brake with a vacuum servo as standard equipment in many cars. This device utilizes manifold depression, or 'vacuum' produced by an engine-driven pump, to assist the driver in the application of the brake.
Some light trucks in the 3 ton category incorporate a part-compressed-air system called air/hydraulic (air over hydraulic). This air system combined with a hydraulically operated arrangement reduces the maximum pedal pressure to a comfortable limit.
Now a days it is common to use a disc-type brake with a vacuum servo as standard equipment in many cars. This device utilizes manifold depression, or 'vacuum' produced by an engine-driven pump, to assist the driver in the application of the brake.
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