Disc Brake Layouts
Disc-brake Assembly
A disc-brake has a rotating cast-ion disc, bolted to the wheel hub and a stationary caliper unit. The caliper straddles the disc and is bolted to the stub-axle or swivel-post flange. It is made of cast iron in two halves (Fig. 28.25) and each half forms a separate cylinder block with the cylinder axis perpendicular to the disc. The two cylinders are connected together by drillings at the pressure faces of the two caliper halves near to the inlet port (Fig. 28.25B). A bleed-screw drilling also intersects at this junction
Fig. 28.25. Double-piston-caliper disc brake
Each cylinder uses a rubber sealing in the form of a ring located in a groove in the body and a hollow piston protected by a dust-cover. A friction pad in the form of a segment is bonded to a steel plate and is sandwiched between each piston and the disc face. These pads fit into slots formed in each half of the caliper housing and are held in position by retaining pins, or spring plates.
The application of the brake pedal causes hydraulic pressure to be transmitted in all directions. The opposing caliper pistons and friction pads apply equal and opposite forces on the rotating disc in direct proportion to the applied effort. Once the brakes are released, the hydraulic pressure collapses and the distorted rubber seal retracts the piston and pad to clear the disc faces from frictional contact. The pistons are pushed out further to compensate for pads wear. Since, the retraction movement of the rubber seals remains constant, the pad wear adjustment is automatic.
As the pads are visible, the state of wear can be easily known. To replace the pads, the split pins and pad-retaining springs are removed. Internal drillings link the two fluid chambers, and a rubber hose supplies the fluid from the master cylinder. A bleeder screw is fitted to each caliper. Since most of the frictional contact surface is exposed to the atmosphere, dissipation of heat is improved as compared with the drum-brake arrangement. Normally disc brakes are used for front wheels and conventional L&T shoe drum brakes are fitted to the rear wheels.
The advantages of disc-brakes over drum-brakes are as follows :
(a) In the absence of any self-servo action, disc-brakes produce consistent braking. This non-assisted brake may required more effort but its action is progressive that means the brake provides a torque proportional to the applied force.
(b) A low average disc temperature is produced due to good air ventilation of the disc and the friction pads, so that pad friction fade is reduced.
(c) A uniform pad wear is developed due to flat friction contact between the disc and pads.
(d) Uniform hydraulic pressure on each side of the disc floats the pad pistons and provides equal grip on the disc, so that side-thrust from the disc to the hub is eliminated.
(c) As pads wear, the pad-to-disc clearance is automatically maintained. (/) Disc-brakes are simple in design, and use very few parts susceptible to wear or malfunction.
(g) The brake is not very sensitive to friction variation.
(h) Pedal travel does not increase with the heating up of the disc. Heating of the drum causes expansion, which increases pedal travel.
(i) The friction pads of disc-brakes can be easily removed and replaced. 28.6.2. Types of Disc Brakes
The application of the brake pedal causes hydraulic pressure to be transmitted in all directions. The opposing caliper pistons and friction pads apply equal and opposite forces on the rotating disc in direct proportion to the applied effort. Once the brakes are released, the hydraulic pressure collapses and the distorted rubber seal retracts the piston and pad to clear the disc faces from frictional contact. The pistons are pushed out further to compensate for pads wear. Since, the retraction movement of the rubber seals remains constant, the pad wear adjustment is automatic.
As the pads are visible, the state of wear can be easily known. To replace the pads, the split pins and pad-retaining springs are removed. Internal drillings link the two fluid chambers, and a rubber hose supplies the fluid from the master cylinder. A bleeder screw is fitted to each caliper. Since most of the frictional contact surface is exposed to the atmosphere, dissipation of heat is improved as compared with the drum-brake arrangement. Normally disc brakes are used for front wheels and conventional L&T shoe drum brakes are fitted to the rear wheels.
The advantages of disc-brakes over drum-brakes are as follows :
(a) In the absence of any self-servo action, disc-brakes produce consistent braking. This non-assisted brake may required more effort but its action is progressive that means the brake provides a torque proportional to the applied force.
(b) A low average disc temperature is produced due to good air ventilation of the disc and the friction pads, so that pad friction fade is reduced.
(c) A uniform pad wear is developed due to flat friction contact between the disc and pads.
(d) Uniform hydraulic pressure on each side of the disc floats the pad pistons and provides equal grip on the disc, so that side-thrust from the disc to the hub is eliminated.
(c) As pads wear, the pad-to-disc clearance is automatically maintained. (/) Disc-brakes are simple in design, and use very few parts susceptible to wear or malfunction.
(g) The brake is not very sensitive to friction variation.
(h) Pedal travel does not increase with the heating up of the disc. Heating of the drum causes expansion, which increases pedal travel.
(i) The friction pads of disc-brakes can be easily removed and replaced. 28.6.2. Types of Disc Brakes
Two-cylinder Caliper.
Figures 28.25 and 26 illustrate a brake unit. A split caliper assembly is rigidly fixed to the stub axle carrier. It houses the cylinders and two opposed pistons. These pistons act directly on friction pads, attached one on each side of the disc. A rubber seal is mounted in a groove in the cylinder and prevents fluid leakage. Also it retracts the piston and pad after application of the brake (Fig. 28.26B). This feature takes care for lining wear and keeps each pad close to the disc
Fig. 28.26. Two-cylinder caliper
Four-cylinder Caliper
A four-cylinder caliper unit is used to provide greater safety. This assembly is used in conjunction with a tandem master cylinder. In other words, one opposing pair of cylinders in the caliper is connected to the primary master cylinder piston and the other pair to the secondary. With this arrangement, if a line fails, one pair of cylinders still remains operative. Some disc pads are incorporated with an electrical contact. When the pad needs replacement, the contact rubs on the disc and activates a warning lamp.
Single-cylinder Caliper
In vehicles with a steering geometry based on negative offset (negative scrub radius) there is often limited road wheel-to-disc clearance and hence it is insufficient to accommodate a caliper
having two opposed pistons. In such vehicles a single-cylinder caliper similar to that illustrated in Fig. 28.27 is installed. The piston housing is keyed to the pad housing that is bolted to the wheel suspension member. Hydraulic pressure moves the piston in one direction and the piston housing in the opposite direction.
having two opposed pistons. In such vehicles a single-cylinder caliper similar to that illustrated in Fig. 28.27 is installed. The piston housing is keyed to the pad housing that is bolted to the wheel suspension member. Hydraulic pressure moves the piston in one direction and the piston housing in the opposite direction.
Fig. 28.27. Single-cylinder caliper.
28.6.3.
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