Brake pad

Brake pads are a component of disc brakes used in automotive and other applications. Brake pads are steel backing plates with friction material bound to the surface that faces the disc brake rotor.

Function

Brake pads convert the kinetic energy of the vehicle to thermal energy through friction. Two brake pads are contained in the brake caliper, with their friction surfaces facing the rotor.[1] When the brakes are hydraulically applied, the caliper clamps or squeezes the two pads together onto the spinning rotor to slow/stop the vehicle. When a brake pad heats up due to contact with the rotor, it transfers small amounts of its friction material onto the disc, leaving a dull grey coating on it. The brake pad and disc (both now 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. These are held in place and actuated by a caliper affixed to the wheel hub or suspension upright. Most vehicles have two brake pads per caliper. However racing calipers can utilize up to six pads, with varying frictional properties in a staggered pattern for optimum performance. Depending on the properties of the material, disc wear rates may vary. The brake pads must usually be replaced regularly (depending on pad material) to prevent brake fade. 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 and embedding a soft metal tab in the pad material that closes an electric circuit and lights a dashboard warning light when the brake pad wears thin.

Technology

Disc brake advantages

Disc brakes offer better stopping performance as compared to drum brakes. They provide better resistance to “brake fade” 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, the 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 lessen the driver’s pedal effort.

Types

A set of pads for high-performance disk brakes

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 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, the problem can be minimized by installing better quality and more aggressive brake pads.

Materials

The five most important characteristics that are considered when selecting a brake pad material are as follows:

  • The material’s ability to resist brake fade
  • The effects of water on brake fade (all brakes are designed to withstand at least temporary exposure to water)
  • The ability to recover quickly from either increased temperature or moisture
  • Service life as traded off with wear to the rotor
  • 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 or dents).[2]

For many years straightforward asbestos was viewed as having an optimal performance in all five categories. However, as the serious health-related hazards of asbestos became apparent, other materials had to be found. 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, aramid, PAN, and sintered glass. 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.[2] However, because the ceramic materials causes the braking sound to be elevated beyond that of human hearing, they are exceptionally quiet.

Phenol formaldehyde resin is frequently used as a binding agent. Graphite can serve as a friction material as well as binding agent.[3] An Italian producer conducts research to use cement as a cheap and less energy-intensive binding agent.[4]

There are environmental factors that govern the selection of brake pad materials. For example, the bill SSB 6557 [5] adopted in Washington State in 2010 will limit 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.[6] 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 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.

Cataloguing

There are different systems for the cataloguing of brake pads. The most frequently used system in Europe is the WVA numbering system.[7]

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.”[8]

Cartridge brake pad

A type of brake pad used on rim brakes.

References

  1.  Henderson, Bob; Haynes, John H. (1994). “Disc Brakes”. The Haynes Automotive Brake Manual. Haynes North America. pp. 1–20.
  2.  b Cliff Owen (21 June 2010). Today’s Technician: Automotive Brake Systems Classroom and Shop Manual. Cengage Learning. pp. 27–28. ISBN 978-1-4354-8655-3.
  3.  Entry on brake pads (Bremsbelag) in Kfz-Tech.de
  4.  Essay Forschungsprojekt Cobra – Die Bremse der Zukunft besteht aus Zement, February 2015 in: Ingenieur.de
  5.  Limiting the use of certain substances in brake friction material
  6.  Rampin, Ilaria; Zanon, Matteo; Echeberria, Jon; Loreto, Antonio Di; Martinez, Anemaite (2014-05-19). “Development of copper-free low steel brake pads for passenger cars”.
  7.  WVA numbering system
  8.  http://fmsi.org/home/
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