Disc Brake Layouts (Automobile)


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.
Double-piston-caliper disc brake.
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

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.
 Two-cylinder caliper.
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.

Single-cylinder caliper.
Fig. 28.27. Single-cylinder caliper.

Disc Brake Cooling

The cooling of the brake disc and its pads takes place mostly by air convection, however wheel hubs also conducts away some of the heat. The rubbing surface between the rotating disc and the stationary pads is exposed to the frontal air stream of the vehicle and hence to the directed air circulation. Therefore, under continued brake application the disc brake is consid­erably more stable than the drum brake. Also, the high conformity of the pad and disc, and the uniform pressure allows the disc to withstand higher temperatures compared to the drum brake. Since far less distortion takes place in discs compared to drums, the disc can operate at higher temperatures. Also, the disc expands towards the pads, whereas the drum expands away from the shoe linings. Consequently, in the hot condition the disc brake reduces its pedal movement, but the drum brake increases its pedal movement.
The discs are made of cast iron and are ventilated to considerably improve the cooling capacity during rotation (Fig. 28.12B). These discs are consisted of two annular plates ribbed together by radial vanes, which also act as heat sinks. Due to centrifugal force, air is pushed through the radial passages formed by the vanes from the inner entrance to the outer exit so that cooling is improved. The ventilated disc provides more exposed surface area, so that an increase of about 70% in convection heat dissipation occurs compared to a solid disc of similar weight. Consequently, a ventilated disc reduces the friction pad temperature to about two-thirds that of a solid disc under normal operating conditions. Also, pad life is considerably increased with the lower operating temperatures, but a very little effect on the frictional properties of the pad material takes place. However, at low speeds ventilated discs have very little influence on the cooling rate. Very high speeds produce a pressure difference between the inside and outside of the disc, which forces air to flow through the vents towards the disc and pads. This additional air flow contributes about 10% increase in the disc’s cooling rate. The exposure of the disc and pads to water and dirt considerably increases pad wear, and causes reduction in the frictional properties of the rubbing pairs. Therefore, protecting the rubbing surfaces from atmospheric dust and the road surface spray is most important.

Disc Brake Pad Support Arrangements Swing Yoke Type Brake Caliper

This disc brake caliper is a single cylinder unit and is of light weight. The caliper unit uses a rigid yoke of steel pressing, a cylinder assembly, two pads and a carrier bracket bolted to the suspension hub carrier. A tongue and groove joint rigidly secure the cylinder to one side of the yoke frame while the yoke itself pivots at one end on it supporting carrier bracket. The disc is mounted on the transmission drive shaft hub which it is mounted provides the drive to the disc. The lining pads are supported on either side of the disc in the yoke frame (Fig. 28.28).
Swing yoke type brake caliper.
Fig. 28.28. Swing yoke type brake caliper.
During operation of the foot brake, hydraulic pressure pushes the piston and inboard pad against their adjacent disc face. At the same time, the hydraulic reaction moves the cylinder in the opposite direction so that the outboard pad and cylinder body are bridged. Then the yoke pivots, forcing the outboard pad against the disc face opposite to that of the inboard pad.
As the pads wear the yoke moves through an arc about its pivot. The lining pads are tapered in shape to compensate for this tilt. The operating clearance between the pads and disc is maintained roughly constant by the inherent distortional stretch and retraction of the pressure seals as the hydraulic pressure is increased and reduced respectively.

Sliding Yoke Type Brake Caliper.

In this type of caliper unit the cylinder body is rigidly fixed to the suspension hub carrier. The yoke slides between parallel grooves formed in the cylinder casting (Fig. 28.29).
Application of the foot brake causes hydraulic pressure to push the pistons apart. The direct piston forces the direct pad against the disc whereas the indirect piston forces the yoke to slide in the cylinder in the opposite direction until the indirect pad contacts the outstanding disc face. Further increase in pressure provides an equal but opposing force which sandwiches the disc > between the friction pads.
Sliding yoke type brake caliper.
Fig. 28.29. Sliding yoke type brake caliper.
The rubber seals distort due to pressure increase as the pistons move apart. As the hydraulic pressure collapses the seals retract and withdraw the pistons and pads from the disc surface. Yoke rattle between the cylinder and yoke frame is minimized by incorporating either a wire or leaf spring between the sliding joints.
Slide pin type brake caliper.
Fig. 28.30. Slide pin type brake caliper.

Sliding Pin Type Brake Caliper.

This type of disc brake caliper unit incorporates a disc, a carrier bracket, a cylinder caliper bridges, piston and seals, friction pads and a pair of support guide pins (Fig. 28.30). The carrier bracket is bolted onto the suspension hub carrier. It supports the cylinder caliper bridge and absorbs the brake torque reaction. The cylinder caliper bridge is mounted on a pair of guide pins that slide in holes in the carrier bracket. The guide pins only support and guide the bride. These are sealed against dirt and moisture by dust covers. The frictional drag of the pads is absorbed by the carrier bracket.
The application of the foot brake generates hydraulic pressure that pushes the piston and cylinder apart. The inboard pad moves up to the inner disc face. In contrast, the cylinder and bridge react in the opposite direction until the outboard pad touches the outside disc face. Further increase of hydraulic pressure imposes equal but opposing forces against the disc faces via the pads.
Similar to all other types of caliper units, in slide pin type also, the brake calipers pad to disc free clearance is provided by the pressure seals. The pressure seals are fitted inside recesses in the cylinder wall. They grip the piston when hydraulic pressure forces the piston outwards, getting distorted in the process. When the foot brake is released the pressure is withdrawn from the piston crown and the strain energy of the elastic rubber pulls back the piston so that the pressure seal is restored to its original shape.

Sliding Cylinder Body Type Brake Caliper.

This type of brake caliper unit uses a carrier bracket bolted to the suspension hub carrier and a single piston cylinder bridge caliper. The caliper straddles the disc and slides laterally on guide keys located in wedge-shaped grooves formed in the carrier bracket (Fig. 28.31).
The hydraulic pressure, generated due to the application of the foot brake, pushes the piston along with the direct acting pad onto the inside disc face. The cylinder body caliper bridge is pushed in the opposite direction, so that it reacts and slides in its guide groove at right angles to the disc. This causes the indirect pad to contact the outside disc face, equalizing the forces acting on both sides of the disc.
Slide cylinder body type brake caliper.
Fig. 28.31. Slide cylinder body type brake caliper.
A pad to disc face working clearance is provided as in the other units by the retraction of the pressure seal, after the hydraulic pressure collapses. Anti-rattle springs are incorporated alongside each of the two-edge-shaped grooves to avoid vibration and noise caused by the relative movements between the bridge caliper and carrier bracket sliding joint.

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