Brake pad
Under a brake pad (also called brake stone) is meant for vehicles, a component to reduce the speed of the vehicle, wherein the kinetic energy by friction against a drum or brake disc is transformed into heat. Brake pads are also used on the brakes of moving parts of stationary machines.
The widespread term brake pad or brake shoe describes the carrier components of the brake linings, also on bicycles or trains. In the case of disc brakes , the material is glued to the carrier; in the case of drum brakes , it is glued or riveted to the brake shoe.
composition
The commercially available brake pads consist of the following elements and materials:
- Support plate made of steel or cast iron. However, there are repeated efforts to use plastic carrier plates to save weight. Rail coverings have z. T. no carrier plate, but pressed-in metal sheets with which the block is inserted into a holder.
- Rubbing agents, including antimony trisulfide and copper , previously also lead and asbestos ; next to it heavy spar ( barium sulphate ), the binding resin , which cokes under the influence of heat in the course of production and bonds all particles together; in addition, substances that give the brake lining the desired properties, such as metals (copper, brass, iron, gray cast iron as powder or fibers), mineral fibers, the sulfides of iron, copper, antimony, zinc, tin, molybdenum
- sometimes a noise insulation plate made of metal or plastic for brake pads
Phenol-formaldehyde resin or polyacrylonitrile are predominantly used as binders . Graphite can serve as both a friction and a binding agent. A manufacturer tries to develop a cheaper and more environmentally friendly binder based on cement .
Friction agent categories
Four different categories of friction material are mentioned internationally. The differences can be fluid. The term “ceramic” is particularly broad. Most manufacturers have several frictional mixtures, the composition of which is usually a closely guarded secret. Exotic products such as cashew nut resin, but also aramids such as Kevlar® from DuPont or Twaron® from Teijin Aramid are used.
Semi-metallic
Rubbing agents in the “Semi-Metallic” category contain 30 to 65% metal (steel, iron, copper, brass, etc.) mixed with graphite, fillers and binders. These linings have a reputation for low cost with a long service life and good hot braking behavior, but also high wear on the brake disc and susceptibility to squeaking. Until now, the main market for these brake pads has been North America, where they are increasingly being replaced by ceramic pads.
Organic
Rubbing agents in the “Organic” category (sometimes also called “Non Asbestos Organic” or NAO) consist of fibers that can be obtained from glass, rubber, carbon or Kevlar® and Twaron®. They also consist of fillers and temperature-resistant synthetic or natural resins. These linings are considered to be softer, quieter, but they have a higher level of wear when exposed to thermal stress, but they wear the brake disc less and thus develop less visible brake dust, which mainly consists of the worn cast iron of the brake disc. In addition, they develop a poorer coefficient of friction behavior, especially after exposure to increased temperature. The main markets for these brake pads are Japan and the USA.
Low metallic
Friction agents in this category consist of the aforementioned organic substances mixed with a proportion of 10 to 30% of metal, usually copper or steel. These pads have a reputation for better hot braking behavior, especially at high speeds. On the negative side, brake dust and a tendency to squeak are mentioned. The main market for these rubbers is Europe.
Ceramic
Friction media in the ceramic category are relatively new. Ceramic coverings were first used by vehicle manufacturers in 1985. These friction agents consist of ceramic fibers, fillers, binders and possibly small amounts of metal. These rubbers are usually lighter, quieter, but also more expensive than other rubbers. When using ceramic linings, the wear on the brake discs is considered higher. The main markets for these rubbers are North America and Japan. Although the term “ceramic” has been widely used, experts do not consider it a generic term for a friction agent. Any surface that contains ceramic ingredients can be called "ceramic". It may well be that a flooring that is sold as “low metallic” in Europe is marketed as “ceramic” in the USA.
Sports and luxury cars sometimes have brake pads made from carbon fiber reinforced silicon carbide .
asbestos
Before 1990 a considerable amount of asbestos (up to 42% share) was added, which has since been banned in the EU . However, the increased asbestos concentration can still be detected today in areas in which a lot is slowed down (intersections, motorway exits, runways, train stations, parking garages). The manufacture, sale and installation of asbestos-containing brake linings has been prohibited in the EU since the introduction of the Asbestos Ordinance (1989 version).
Brake pad requirements
The brake pad should:
- have as constant a coefficient of friction as possible (brake fading due to overheating),
- temperature resistant (up to 800 ° C) to prevent "glazing" and
- be mechanically resilient.
New brake pads only develop their full braking effect after a certain break-in period. During this phase, the lining surface adapts to the surface of the disc or drum until a uniform contact pattern develops. During this phase, the material on the contact surface with the disc / drum also changes. Friction charcoal is formed. Only then is the optimal connection of the friction pairing (brake lining or disc, brake lining or drum) and thus the maximum deceleration achieved when braking .
wear
During the braking process, the brake lining rubs off on the brake disc, so brake pads are wearing parts. Brake pads are changed several times during the service life of a vehicle, even more frequently than the brake discs . Brake pads (and also brake disks) may only be changed on the entire axle and not individually. Since the brake pads adapt to the brake discs through abrasion, the brake pads must also be replaced when the brake disc is changed.
Earlier brake pads already had wear indicators that indicated when the lining was worn. This wear indicator or so-called warning contact was invented at the end of the 1970s by the automotive electrician Yigal Bar-Yoav, who emigrated from Israel to Germany. He also developed what is known today as the warning contact function, which indicates the condition of the brake lining before it is completely worn out. Bar-Yoav made use of Ohm's law . The less coating there is, the lower the measured electrical resistance. Simpler and more common wear sensors have a wire inserted in the last millimeters of the brake lining; as soon as the brake is so worn that the wire touches the brake disc, there is ground contact. A control lamp in the vehicle lights up or flickers. However, this type of construction cannot be called "Fail Safe", since an interruption in the measuring line does not lead to any display in case of doubt and thus the wear can no longer be recognized via the system. Other systems of a similar design therefore work with a two-wire system. In this, the wear element, which is usually located in a recess in the backing plate, contains a loop of the conductor, which from a certain value before the wear limit of the lining (usually 2–4 mm) is cut by the brake disc and thus separates the ground connection. which then results in a wear warning. Since there must be a low-resistance ground connection, a possible contact via the brake disc is not sufficient to simulate an intact connection. In most systems, an exact resistance value is not evaluated or rather a limit value in the sense of a maximum resistance is implemented, which means that the systems can be bridged with low resistance. The advantage here is that failure of individual elements (e.g. measuring line or connector) will also result in a warning. Although this can possibly be interpreted as a "false alarm" (with regard to lining wear), it does indicate deficiencies in the system, which can be eliminated by a repair. Both evaluation and execution of the measuring line (two-wire) and the wear contact itself are slightly more complex with this system, but due to the "fail safe" issue, this system is state of the art in the area of cheap and reliable wear detection. The manufacture of modern, high-quality wear indicators, like the production of the lining, must use the most heat-resistant materials. Limit values for the operating temperature of the coverings must be withstood by the thin cable loop embedded in the covering material over the entire duration of the set. For this purpose, manufacturers whose sensors meet the requirements of the automotive industry use the most modern thermosetting plastics, whose heat resistance is even equivalent to that of the flooring.
Some disc brake pads have an acoustic wear warning. This is attached to the back of the pads in the form of a flat, bent metal plate and is made to vibrate by the friction of the pads as the pad thickness decreases. These vibrations then produce a high-pitched, squeaky sound.
As a contribution to fine dust pollution , brake linings have come under criticism again since asbestos was abolished and are an object of investigation.
quality
Since brake pads are often replaced as part of a vehicle service, there is a large market for this with a large number of manufacturers. In the EU, only brake pads that comply with the ECE-R 90 regulation may be used in the vehicle approved for this purpose. The ECE-R 90 regulation defines the following properties, among others:
- same coefficient of friction as original linings from the vehicle manufacturer (deviations of up to ± 15% allowed)
- Compressive strength, shear strength and material hardness
- Check for speed sensitivity
- Asbestos-free
- An approval mark must be permanently identifiable on the spare part. It consists of a circle with the letter "E" and the code of the country that issued the approval and the designation "90 R", followed by a hyphen and an approval number.
- The packaging of the coverings must be glued / sealed in order to clearly identify prior opening.
- The vehicles approved for the surface must be listed on the packaging.
- Assembly instructions in one of the official ECE languages and the language of the country in which they are sold must be enclosed with every package.
The aim of this regulation is that replica brake pads meet at least some minimum requirements. In the case of motor vehicle parts, there are large price differences between original parts and the aftermarket and consequently, without this regulation, customers could have doubts about the quality or safety. In particular, web shops and automotive parts dealers active in online auction houses emphasize that their products have this E-ID and advertise this as a special quality feature.
Suppliers of brake pads for the German aftermarket that comply with ECE-R 90 are z. B. Becorit (= Wabtec FRG Holding ), Bremskerl-Reibbelagwerke Emmerling , Bosch , Brembo , Continental AG (ATE, Barum), EBC Brakes , Eurobremsband (Beral), Federal-Mogul (Ferodo, Jurid, Bendix, Saxid), TMD Friction (Pagid, Textar, Mintex, Don), TRW / Lucas and PEX Automotive . Incidentally, the well-known vehicle manufacturers do not produce brake pads themselves, but have them produced by brake pad manufacturers according to their own specifications. Meanwhile, Eurobremsband only produces a few disc brake pads, but its main task is in other areas. As a result of the group split into Eurobremsband, MAT and Federal Mogul, the locations for (drum) brake linings have relocated to the Czech Republic and Ireland. The aftermarket of Beral is handled by Federal-Mogul.
Cataloging
Brake pads are cataloged according to classification systems. The most common classification system in Europe is the WVA number system , which is also used worldwide .
literature
- Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden 2001, ISBN 3-528-13114-4 .
Individual evidence
- ↑ Entry brake lining in Kfz-Tech.de
- ^ Article in the research project Cobra - The brake of the future consists of cement from February 2015 in: Ingenieur.de
- ↑ http://www.catalogs.com/info/automotive/types-of-brake-pads.html
- ↑ Archive link ( Memento of the original from September 25, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ - ( Memento of the original from September 15, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Andrews Inc .: Ceramic Brake Pad Update ( Memento from January 27, 2009 in the Internet Archive )
- ↑ sgaf de Meschi: SGAF :: The truth about the brake pad wear indicator. Retrieved June 5, 2019 .
- ↑ Brake pad wear indicator - T4 Wiki. Retrieved June 5, 2019 .
- ↑ http://www.bmvbs.de/cae/servlet/contentblob/44514/publicationFile/1233/r-90- Ersatzbremsbelag-einrichtungen-fuer-kfz-und-anhaenger- pdf.pdf ( page no longer available , search in Web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
