Asbestos was added as an 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 Asbestos brake pads have largely been replaced by non-asbestos organic (NAO) materials in first world countries.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, 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.However, because the ceramic materials causes the braking sound to be elevated beyond that of human hearing, they are exceptionally quiet.
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Xiangxi Yangtze River Highway Bridge was initially approved
The Xiangxi Yangtze River Highway Bridge in Yichang, Hubei Province, was initially approved by the Ministry of Transportation and Communications in recent days, marking a major breakthrough in the preliminary work of the project.
The project was approved by the National Development and Reform Commission in June 2013. It is a project group consisting of a large number of oversized bridges, tunnels and large landslides. The project boasts a high technological content, a large investment scale, and a construction scale and difficulty to establish the history of independent construction of the Yichang Highway.
According to the preliminary design approval, the Xiangxi Yangtze River Highway Bridge starts from the east side of Guojiaba in Zigui County, connects to Provincial Highway 255, terminates at the Xiangjia Store on the west bank of Xiangxi River in Guizhou Town, and is multiplexed with Provincial Highway 255. The total length is 5.617. Kilometers. Among them, the cross-Yangtze River Bridge is 0.805 kilometers long, the Xiangxi River Bridge is 1.058 kilometers long, the length of the two banks is 3.754 kilometers, and the project budget is 2.099 billion yuan. The total project duration is 4 years (calculated from the date of commencement). The entire highway adopts the first-class highway standard construction, with a design speed of 60 kilometers per hour and a subgrade and bridge width of 23 meters. The trans-Yangtze Bridge uses the main span 498 m long-span steel box truss arch bridge scheme. The Xiangxi River Bridge adopts a 470-m double-tower double-tower composite composite beam cable-stayed bridge scheme.
At present, the Xiangxi Bridge Construction Command is in accordance with the preliminary design approval requirements and organizes domestic authoritative bridge experts to conduct technical demonstration of the bridge's key technologies and report the results of the demonstration to the Ministry of Transport. At the same time, the bidding work was quickly started and the project was started as soon as possible.