The disc brake is a device for slowing or stopping the rotation of a wheel. A brake disc (or rotor in U.S. English), usually made of cast iron or ceramic, is connected to the wheel or the axle. To stop the wheel, friction material in the form of brake pads (mounted in a device called a brake caliper) is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop.
Disc Designs
The design of the disc varies somewhat. Some are simply solid cast iron, but others are hollowed out with fins joining together the disc's two contact surfaces (usually included as part of a casting process). This "ventilated" disc design helps to dissipate the generated heat and is commonly used on the more-heavily-loaded front discs.
Many higher performance brakes have holes drilled or cast through them. This is known as cross drilling and was originally done in the 1960's on racing cars. Brake pads will outgas and under use may create boundary layer of gas between the pad and the disc hurting braking performance. Cross drilling was created to provide the gas someplace to escape. Although modern brake pads seldom suffer from outgassing problems, water residue may build up after a vehicle passes through a puddle and impede braking performance. For this reason, and for heat dissipation purposes, Cross Drilling is still used on some braking components. Discs may also be slotted, where shallow channels are machined into the disc to aid in removing dust and gas. Some discs are both drilled and slotted.
Slotted discs are generally not used on standard vehicles because they quickly wear down brake pads, however, this removal of material is beneficial to race vehicles since it keeps the pads soft and avoids vitrification of their surfaces
On the road, drilled or slotted discs still have a positive effect in wet conditions because the holes or slots prevent a film of water building up between the disc and the pads. Poorly-made cross drilled discs (such as those made by simply drilling through a plain faced disc) may crack at the holes under use due to metal fatigue.
Calipers
The brake caliper is the assembly which houses the brake pads and pistons. The pistons are usually made of aluminum or chrome-plated iron. There are two types of calipers: floating or fixed. A fixed caliper does not move relative to the disc. It uses one or more pairs of pistons to clamp from each side of the disc, and is more complex and expensive than a floating caliper. A floating caliper (also called a "sliding caliper") moves with respect to the disc; a piston on one side of the disc pushes the inner brake pad until it makes contact with the braking surface, then pulls the caliper body with the outer brake pad so pressure is applied to both sides of the disc.
Floating caliper (single piston) designs are subject to failure due to sticking which can occur due to dirt or corrosion if the vehicle is not operated regularly. This can cause the pad attached to the caliper to rub on the disc when the brake is released. This can reduce fuel efficiency and cause excessive wear on the affected pad.
Pistons and cylinders
The most common caliper design uses a single hydraulically actuated piston within a cylinder, although high performance brakes use as many as twelve. Modern cars use different hydraulic circuits to actuate the brakes on each set of wheels as a safety measure. The hydraulic design also helps multiply braking force. The number of pistons in a caliper is often referred to as the number of 'pots', so if someone has six pot calipers they mean each caliper has six pistons in them.
Failure can occur due to failure of the piston to retract - this is usually a consequence of not operating the vehicle during a time that it is stored outdoors in adverse conditions. For high distance vehicles the piston seals may leak, which must be promptly corrected.
Brake pads
The brake pads are designed for high friction with brake pad material embedded in the disc in the process of bedding while wearing evenly. Although it is commonly thought that the pad material contacts the metal of the disc to stop the car, the pads work with a very thin layer of their own material and generate a semi-liquid friction boundary that creates the actual braking force. Of course, depending on the properties of the material, disc wear could be faster or slower than with other pads. The properties that determine material wear involve trade-offs between performance and longevity. The brake pads must be replaced regularly, and most are equipped with a method of alerting the driver when this needs to take place. Some have a thin piece of soft metal that causes the brakes to squeal when the pads are too thin, while others have a soft metal tab embedded in the pad material that closes an electric circuit and lights a warning light when the brake pad gets thin. Although almost all road-going vehicles have only two brake pads per caliper, racing calipers utilize up to six pads, with varying frictional properties in a staggered pattern for optimum performance.
type of disc damage
Discs are usually damaged in one of three ways: warping, scarring, and cracking. Machining the discs to correct these problems also leads to reduced life. It is usually cheaper just to replace the disc instead of repairing the parts.
Warping
Warping is primarily caused by excessive heat, which softens the metal and allows it to be reshaped. The main causes of overheating are: undersized/over machined brake discs, excessive braking (racing, descending hills/mountains), "riding" the brakes, or a "stuck" brake pad (pad touches disc at all times).
Another cause of warping is when the disc is overheated and the vehicle is stopped. When keeping the brakes applied, the area where the pads contact the disc will cause uneven cooling and lead to warping.
Several methods can be used to avoid overheating brake discs. Use of a lower gear when descending steep grades to obtain engine braking will reduce the brake loading. Also, operating the brakes intermittently - braking to slower speed for a brief time then coasting will allow the brake material to cool between applications. Riding the brakes lightly will generate a great amount of heat with little braking effect and should be avoided. High temperature conditions as found in automobile racing can be dealt with by proper pad selection, but at the trade off of everyday drive ability. Pads that can take high heat usually do best when hot and will have reduced braking force when cold. Also, high heat pads typically have more aggressive compounds and will wear discs down more quickly.
Warping can also be caused by improperly torquing the lug nuts when putting on a wheel. The manual will indicate the proper pattern for tightening as well as a torque rating for the bolts. The tightening pattern varies little between manufacturers and most mechanics are familiar with them. Lug nuts should never be tightened in a circle. Some vehicles are sensitive to the force the bolts apply and tightening should be done with a torque wrench.
Warping will often lead to a thickness variation of the disc. If it has runout, a thin spot will develop by the repetitive contact of the pad against the high spot as the disc turns. When the thin section of the disc passes under the pads, the pads move together and the brake pedal will drop slightly. When the thicker section of the disc passes between the pads, the pads will move apart and the brake pedal will raise slightly, this is pedal pulsation. The thickness variation can be felt by the driver when it is approximately 0.007 inch (0.017 cm) or greater.
Not all pedal pulsation is due to warped discs. Brake pad material operating outside of its designed temperature range can leave a thicker than normal deposit in one area of the disc surface, creating a "sticky" spot that will grab with every revolution of the disc. Grease or other foreign materials can create a slippery spot on the disc, also creating pulsation.
Cracking
Cracking is limited mostly to drilled discs, which get small cracks around outside edges of the drilled holes near the edge of the disc due to the disc's uneven rate of expansion in severe duty environments.In the main small hairline cracks will appear in all cross drilled discs, this is normal. Manufacturers that use drilled discs as OEM are doing so for two reasons: looks, if they determine that the average owner of the vehicle model will not overly stress them; or as a function of reducing the unsprung weight of the brake assembly, with the engineering assumed that enough brake disc mass remains to absorb racing temperatures and stresses. A brake disc is a heat sink, so removing mass increases the heat stress it will have to contend with. Generally an OEM application that is not drilled will crack and could fail catastrophically if used over and above the original equipment design. Once cracked, these discs cannot be repaired.
Article Courtesy:
Wikipedia
