Brake linings are the consumable surfaces in brake systems, such as drum brake linings and disc brakes pad used in transport vehicles.
History
Brake linings were invented by Bertha Benz (the wife of Karl Benz who invented the first patented automobile) during her historic first long-distance car trip in the world in August 1888. The first asbestos brake linings were developed in 1908 by Herbert Frood.
Structure and function
Brake linings are composed of a relatively soft but tough and heat-resistant material with a high coefficient of dynamic friction (and ideally an identical coefficient of static friction) typically mounted to a solid metal backing using high-temperature adhesives or rivets. The complete assembly (including lining and backing) is then often called a brake block or brake shoe. The dynamic friction coefficient “µ” for most standard friction pad is usually in the range of 0.35 to 0.42. This means that a force of 1000 Newtons on the block will give a resulting brake force close to 400 Newtons. There are some racing friction pads that have a very high µ of 0.55 to 0.62 with excellent high-temperature behaviour. These pads have high iron content and will usually outperform any other friction pads used with iron discs. Unfortunately nothing comes for free, and these high µ friction pads wear fast and also wear down the discs at a rather fast rate. However they are a very cost effective alternative to more expensive materials.
Since the lining is the portion of the braking system which converts the vehicle’s kinetic energy into heat, the lining must be capable of surviving high temperatures without excessive wear (leading to frequent replacement) or out gassing (which causes brake fade, a decrease in the stopping power of the brake).
Due to its efficacy, chrysotile asbestos was often a component in brake linings. However, studies such as a 1989 National Institutes of Health item showed an uncommonly high proportion of brake mechanics were afflicted with pleural and peritoneal mesothelioma, both of which are linked to chrysotile and asbestos exposure. Public health authorities generally recommend against inhaling brake dust, chrysotile has been banned in many developed and developing countries, such as Australia,China etc. in late 2003, and chrysotile has been progressively replaced in most brake linings and blocks by other fibers such as the synthetic aramids.
Maintenance
When the lining is worn out, the backing or rivets will contact the rotors or drums during braking, often causing damage requiring re-machining or replacement of the drums or rotors. An annoying squeal caused by the warning tang is the typical alert that the pads need to be replaced; if the squeal is ignored for too long, drum or rotor damage (usually accompanied by an unpleasant grinding sound or sensation) will be the typical result.
