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Chain drive
From Wikipedia, the free encyclopedia
Chain drive is a way of transmitting mechanical power from one place to another. It is often used to convey power to the wheels of a vehicle, particularly bicycles and motorcycles. It is also used in a wide variety of machines besides vehicles.
Most often, the power is conveyed by a roller chain, known as the drive chain, passing over a sprocket gear, with the teeth of the gear meshing with the holes in the links of the chain. The gear is turned, and this pulls the chain putting mechanical force into the system.
Sometimes the power is output by simply rotating the chain, which can be used to lift or drag objects. In other situations, a second gear is placed and the power is recovered by attaching shafts or hubs to this gear. Though drive chains are often simple oval loops, they can also go around corners by placing more than two gears along the chain; gears that do not put power into the system or transmit it out are generally known as idler-wheels. By varying the diameter of the input and output gears with respect to each other, the gear ratio can be altered, so that, for example, the pedals of a bicycle can spin all the way around more than once for every rotation of the gear that drives the wheels.
The oldest known application and illustrated depiction of a chain drive date to the written horological treatise of the Song Dynasty Chinese engineer Su Song (1020-1101 AD), who used it to operate the armillary sphere of his astronomical clock tower. An earlier endless drive may have been invented much earlier by the Greek Philon of Byzantium (3rd century BC) in a magazine arcuballista, but it did not transmit power continuously.[1]
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Drive chains are similar to drive belts in many ways, and which device is used is subject to several design tradeoffs. Drive chains are most often made of metal, while belts are often rubber, plastic, or other substances. This makes drive chains heavier, so more of the work put into the system goes into moving a chain versus moving a belt. On the other hand, well-made chains are often stronger than belts. Also, drive belts can often slip (unless they have teeth) which means that the output side may not rotate at a precise speed, and some work gets lost to the friction of the belt against its rollers.
Teeth on toothed drive belts generally wear faster than links on chains, but wear on rubber or plastic belts and their teeth is often easier to observe; you can often tell a belt is wearing out and about to break more easily than a chain. Chains often last longer.
Chains are often narrower than belts, and this can make it easier to shift them to larger or smaller gears in order to vary the gear ratio. Multi-speed bicycles with derailleurs make use of this. Also, the more positive meshing of a chain can make it easier to build gears that can increase or shrink in diameter, again altering the gear ratio.
Both can be used to move objects by attaching pockets, buckets, or frames to them; chains are often used to move things vertically by holding them in frames, as in industrial toasters, while belts are good at moving things horizontally in the form of conveyor belts. It is not unusual for the systems to be used in combination; for example the rollers that drive conveyor belts are themselves often driven by drive chains.
Drive shafts are another common method used to move mechanical power around that is sometimes evaluated in comparison to chain drive; in particular shaft drive versus chain drive is a key design decision for most motorcycles. Drive shafts tend to be even tougher and more reliable than chain drive, but weigh even more (robbing more power), and impart rotational torque.
Chain drive was the main feature which differentiated the safety bicycle introduced in 1885, with its two equal-sized wheels, from the direct-drive penny-farthing or "high wheeler" type of bicycle. The popularity of the chain-driven safety bicycle brought about the demise of the penny-farthing, and is still a basic feature of bicycle design today.
Chain drive was a popular power transmission system from the earliest days of the automobile. It gained prominence as an alternative to the Système Panhard with its rigid Hotchkiss driveshaft and universal joints.
A chain drive system uses one or more roller chain to transmit power from a differential to the rear axle. This system allowed for a great deal of vertical axle movement (for example, over bumps), and was simpler to design and build than a rigid driveshaft in a workable suspension. Also, it had less unsprung weight at the rear wheels than the Hotchkiss drive, which would have had the weight of the driveshaft to carry as well, which in turn meant that the tires would last longer.
Frazer Nash were strong proponents of this system, even adapting licensed Hotchkiss drive BMW designs to chain drive. The last popular chain drive automobile was the Honda S800 of the 1960s.
Internal combustion engines often use chain drive to power the timing chain used to drive overhead camshaft valvetrains. This is an area in which chain drives frequently compete directly with belt drive systems, and an excellent example of some of the differences and similarities between the two approaches. For this application, chains last longer, but are often harder to replace. Being heavier, the chain robs more power, but is also less likely to fail. There are also other approaches to valve trains, notably pushrod systems.
- ^ Needham, Joseph [1971] (1986). Science and Civilisation in China: Volume 4, Physics and Physical Technology, Part 3, Civil Engineering and Nautics, Lu Gwei-Djen, Ling Wang (in English), Taipei: Caves Books, Ltd., 457. ISBN 9780521070607.