28.3.
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.
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).
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.
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,
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.
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.
