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Brake pads are a component of
disc brake A disc brake is a type of brake that uses the #Calipers, calipers to squeeze pairs of #Brake pads, pads against a disc (sometimes called a
rake Rake may refer to: Common meanings * Rake (tool), a horticultural implement, a long-handled tool with tines * Rake (stock character), a man habituated to immoral conduct * Rake (poker), the commission taken by the house when hosting a poker game ...
rotor) to create friction. There are two basic types of brake pad friction mechanisms: abrasive f ...
s used in automotive and other applications. Brake pads are composed of steel backing plates with friction material bound to the surface that faces the disc brake rotors.


Function

Brake pads convert the
kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...
of a vehicle to
thermal energy The term "thermal energy" is often used ambiguously in physics and engineering. It can denote several different physical concepts, including: * Internal energy: The energy contained within a body of matter or radiation, excluding the potential en ...
through
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...
. Two brake pads are contained in the
brake A brake is a machine, mechanical device that inhibits motion by absorbing energy from a moving system. It is used for Acceleration, slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of ...
with their friction surfaces facing the rotor. When the brakes are hydraulically applied, the caliper clamps or squeezes the two pads together onto the spinning rotor to slow and stop the vehicle. When a brake pad heats up due to contact with the
rotor ROTOR was an elaborate air defence radar system built by the British Government in the early 1950s to counter possible attack by Soviet bombers. To get it operational as quickly as possible, it was initially made up primarily of WWII-era syst ...
, it transfers small amounts of its friction material onto the disc, leaving a dull grey coating on it. The brake pad and disc (now both having the friction material), then "stick" to each other, providing the friction that stops the vehicle. In disc brakes, there are usually two brake pads per disc rotor, they both function together. These are held in place and actuated by a caliper affixed to the wheel hub or suspension upright. Racing calipers, however, can utilize up to six pads, with varying frictional properties in a staggered pattern for optimum performance. Depending on the properties of the material, the weight of the vehicle and the speeds it is driven at, disc wear rates may vary. The brake pads must usually be replaced regularly (depending on pad material). Most brake pads are equipped with a method of alerting the driver when this needs to be done. A common technique is manufacturing a small central groove whose eventual disappearance by wear indicates the end of a pad's service life. Other methods include placing a thin strip of soft metal in a groove, such that when exposed (due to wear) the brakes squeal audibly. A soft metal wear tab can also be embedded in the pad material that closes an
electric circuit An electrical network is an interconnection of electrical components (e.g., battery (electricity), batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e. ...
when the brake pad wears thin, lighting a dashboard warning light.


History

The concept of brake pads or disc brakes as an alternative to
drum brake A drum brake is a brake that uses friction caused by a set of Brake shoe, shoes or Brake pad, pads that press outward against a rotating bowl-shaped part called a brake drum. The term ''drum brake'' usually means a brake in which shoes press o ...
s had been around at least as early as a patent by F. W. Lanchester in 1902. However, due to high cost and inefficiencies compared to drum brakes they were not commonly implemented until after
World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...
. Once disc brake technology improved, brake performance quickly surpassed that of drum brakes. The performance difference was most noticeably exhibited in 1953 when a Jaguar outfitted with brake pads won the
24 Hours of Le Mans The 24 Hours of Le Mans () is an endurance-focused Sports car racing, sports car race held annually near the city of Le Mans, France. It is widely considered to be one of the world's most prestigious races, and is one of the races—along with ...
Grand Prix of Endurance race. The success of the Jaguar is commonly attributed to the car’s disc brakes, which allowed the drivers to approach turns faster and brake later than their opponents, which ultimately led to its victory. As late as 1963 the majority of automobiles using disc brakes were European made, with American cars adopting the technology in the late 1960s after the invention of fixed calipers that made installation cheaper and more compact.


Technology


Disc brake advantages

Disc brake A disc brake is a type of brake that uses the #Calipers, calipers to squeeze pairs of #Brake pads, pads against a disc (sometimes called a
rake Rake may refer to: Common meanings * Rake (tool), a horticultural implement, a long-handled tool with tines * Rake (stock character), a man habituated to immoral conduct * Rake (poker), the commission taken by the house when hosting a poker game ...
rotor) to create friction. There are two basic types of brake pad friction mechanisms: abrasive f ...
s offer better stopping performance as compared to
drum brake A drum brake is a brake that uses friction caused by a set of Brake shoe, shoes or Brake pad, pads that press outward against a rotating bowl-shaped part called a brake drum. The term ''drum brake'' usually means a brake in which shoes press o ...
s. They provide better resistance to "
brake fade Brake fade (or vehicle braking system fade) is the reduction in stopping power that can occur after repeated or sustained application of the brakes of a vehicle, especially in high load or high speed conditions. Brake fade can be a factor in any ...
" caused by the overheating of brake pads, and are also able to recover quickly from immersion (wet brakes are less effective). Unlike a drum brake, a disc brake has no self-servo effect—the braking force is always proportional to the pressure applied on the braking pedal lever. However many disc brake systems have servo assistance ("Brake Booster") to reduce the driver's pedal effort. Disc brake pads are easier to inspect and replace than drum brake friction lining.


Types

There are numerous types of brake pads, depending on the intended use of the vehicle, from very soft and aggressive (such as racing applications) to harder, more durable and less aggressive compounds. Most vehicle manufacturers recommend a specific kind of brake pad for their vehicle, but compounds can be changed (by either buying a different make of pad or upgrading to a performance pad in a manufacturer's range) according to personal tastes and driving styles. Care must always be taken when buying non-standard brake pads as the
operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
ranges may vary, such as performance pads not braking efficiently when cold or standard pads fading under hard driving. In cars that suffer from excessive
brake fade Brake fade (or vehicle braking system fade) is the reduction in stopping power that can occur after repeated or sustained application of the brakes of a vehicle, especially in high load or high speed conditions. Brake fade can be a factor in any ...
, the problem can be minimized by installing better quality and more aggressive brake pads.


Materials

The most important characteristics that are considered when selecting a brake pad material are as follows: * The material's ability to resist brake fade, caused by an increase in temperature the material will experience from the conversion of kinetic energy into thermal energy. * The effects of moisture on brake fade. All brakes are designed to withstand at least temporary exposure to water. * The ability to recover quickly from increased temperature or moisture, and exhibit approximately the same friction levels at any point in the drying or cooling process. * The
friction coefficient Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...
of modern brake pads should be low enough prevent locking of the wheels but high enough to provide sufficient stopping power. Friction coefficients are typically between 0.3 and 0.5 for brake pad materials. * The ability to resist wear due to friction, but not to the extent that rotor wear occurs more quickly than brake material is sacrificed. * The ability of the material to provide smooth and even contact with the rotor or drum, instead of a material that breaks off in chunks or causes pits, dents, or other damage to the surface in contact. * The ability to apply appropriate frictional force while also operating quietly. Another material requirement that is considered is how compressible the brake pads are; if they are too compressible then brake travel or brake booster fluid displacement can be excessive. Brake pad material must also be porous to minimize the effect water has on the friction coefficient.
Asbestos Asbestos ( ) is a group of naturally occurring, Toxicity, toxic, carcinogenic and fibrous silicate minerals. There are six types, all of which are composed of long and thin fibrous Crystal habit, crystals, each fibre (particulate with length su ...
was added as a common ingredient to brake pads post-WWI, as car speeds began to increase, because research showed that its properties allowed it to absorb the heat (which can reach 500 °F) while still providing the friction necessary to stop a vehicle. However, as the serious health-related hazards of asbestos eventually started to become apparent, other materials had to be found. Asbestos brake pads have largely been replaced by non-asbestos organic (NAO) materials in first world countries. Today, brake pad materials are classified into one of four principal categories, as follows: * Non-metallic materials – these are made from a combination of various synthetic substances bonded into a composite, principally in the form of
cellulose Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...
,
aramid Aramid fibers, short for aromatic polyamide, are a class of heat-resistant and strong synthetic fibers. They are used in aerospace and military applications, for ballistic-rated bulletproof vest, body armor cloth, fabric and ballistic composites ...
, PAN, and
sintered glass Fritted glass is finely porous glass through which gas or liquid may pass, made by sintering together glass particles into a solid but porous body. This porous glass body can also be called a frit. Applications in laboratory glassware includ ...
. They are gentle on rotors, but produce a fair amount of dust, thus having a short service life. * Semi-metallic materials – synthetics mixed with varying proportions of flaked metals. These are harder than non-metallic pads, more fade-resistant and longer lasting, but at the cost of increased wear to the rotor/drum which then must be replaced sooner. They also require more actuating force than non-metallic pads in order to generate braking torque. * Fully metallic materials – these pads are used only in racing vehicles, and are composed of sintered steel without any synthetic additives. They are very long-lasting, but require more force to slow a vehicle while wearing off the rotors faster. They also tend to be very loud. * Ceramic materials – Composed of clay and porcelain bonded to copper flakes and filaments, these are a good compromise between the durability of the metal pads, grip and fade resistance of the synthetic variety. Their principal drawback, however, is that unlike the previous three types, despite the presence of the copper (which has a high thermal conductivity), ceramic pads generally do not dissipate heat well, which can eventually cause the pads or other components of the braking system to warp. However, because the ceramic materials cause the braking sound to be elevated beyond that of human hearing, they seem exceptionally quiet.
Phenol formaldehyde resin Phenol formaldehyde resins (PF), also called phenolic resins or phenoplasts, are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Used as the basis for Bakelite, PFs were the first commercial syntheti ...
is frequently used as a binding agent.
Graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
can serve as a friction material as well as binding agent. Another friction material commonly used is zirconium silicate. An Italian producer is conducting research to use
cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...
as a cheap and less energy-intensive binding agent. The table below outlines the make up of a common brake pad. There are environmental factors that govern the selection of brake pad materials. For example, the bill SSB 6557 adopted in Washington State in 2010 limits the amount of copper that is allowed to be used in friction materials, to be eventually phased out to trace amounts, due to the negative impact of high copper levels on aquatic life. For its substitution, different material combinations have been developed, though no direct replacement is available yet. Other materials, such as compounds made with antimony, are being studied. Vehicles have different braking requirements. Friction materials offer application-specific formulas and designs. Brake pads with a higher coefficient of friction provide good braking with less brake pedal pressure requirement, but tend to lose
efficiency Efficiency is the often measurable ability to avoid making mistakes or wasting materials, energy, efforts, money, and time while performing a task. In a more general sense, it is the ability to do things well, successfully, and without waste. ...
at higher temperatures. Brake pads with a smaller and constant coefficient of friction do not lose efficiency at higher temperatures and are stable, but require higher brake pedal pressure.


Maintenance and troubleshooting

Brake pads should be checked at least every 5,000 miles for excessive or uneven wear. Although brake pad wear is unique to each vehicle, it is generally recommended that brake pads be replaced every 50,000 miles, while brake discs (or rotors) typically last longer, needing replacement every 70,000 miles. Malfunctions with brake pads can have many effects on the performance of a vehicle. The following chart outlines some common issues that can be caused by brake pad malfunctions:


Testing of materials

The
National Bureau of Standards The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sc ...
(NBS) started testing of brake material in the US in 1920. The testing setup was then shared with manufacturers who wanted them so that they could begin to test their own products. Over time the NBS continued to develop new instruments and procedures for testing pads and lining, and these standards eventually became the standards for the American Engineering Standards Committee's Safety Code for Brakes and Brake Testing. SAE J661 testing is used to determine the friction of different brake pad materials by testing a square liner with a brake drum. This testing yields values for both hot and cold coefficients of friction, which are then paired with letter designations. The table below outlines what letter goes with each range for the coefficient of friction. An example of the designation would be "GD", where "G" is the normal coefficient, while the "D" represents heated.


Cataloguing

There are different systems for the cataloguing of brake pads. The most frequently used system in Europe is the WVA numbering system. The cataloguing system used in North America, and recognized around the world, is the standardized part numbering system for brakes and clutch facings issued by the Friction Materials Standards Institute (FMSI). FMSI's mission is to, "Maintain and enhance this standardized part numbering system for all on highway vehicles in use in North America."


Cartridge brake pad

A type of brake pad used on rim brakes.


See also

* Brake lining *
Brake shoe A railway brake is a type of brake used on the cars of railway trains to enable deceleration, control acceleration (downhill) or to keep them immobile when parked. While the basic principle is similar to that on road vehicle usage, operational ...
* Brake wear indicator *
Engine braking Engine braking occurs when the retarding forces within an internal combustion engine are used to slow down a motor vehicle, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes. The term is ofte ...
* Electromagnetic brake *
List of auto parts This is a list of auto parts, which are manufactured components of automobiles. This list reflects both fossil-fueled cars (using internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the c ...


References

{{Reflist Brakes Vehicle braking technologies