Roller chain

From Academic Kids

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Chain_and_sprocket.gif
Roller chain and sprocket

Roller chain or bush roller chain is the type of chain most commonly used for transmission of mechanical power on bicycles, motorcycles, and in industrial and agricultural machinery. It is simple, reliable, and efficient (as much as 98% efficient under ideal conditions (http://www.hw.ac.uk/mecWWW/research/mdk/res.htm)), but requires more attention to maintenance than may be desired by potential owners; therefore there has been of late a tendency towards the use of other modes of power transmission, such as the cog belt.

Contents

Construction of the chain

There are actually two types of links alternating in the bush roller chain; inner links, having two inner plates held together by two sleeves or bushings upon which rotate two rollers, alternating with outer links consisting of two outer plates held together by pins passing through the bushings of the inner links. The bushingless roller chain is similar in operation though not in construction; instead of separate bushings or sleeves holding the inner plates together, the plate has a tube stamped into it protruding from the hole which serves the same purpose, but removes one step in assembly of the chain.

The roller chain design reduces friction compared to simpler designs, resulting in higher efficiency and less wear. The original power transmission chain varieties lacked rollers and bushings, with both the inner and outer plates held by pins which directly contacted the sprocket teeth; however this configuration exhibited extremely rapid wear of both the sprocket teeth, and the plates where they pivoted on the pins. This problem was partially solved by the development of bushed chains, with the pins holding the outer plates passing through bushings or sleeves connecting the inner plates, thus distributing the wear over a greater area; however the teeth of the sprockets still wore more rapidly than is desirable, from the sliding friction against the bushings. The addition of rollers surrounding the bushing sleeves of the chain and providing rolling contact with the teeth of the sprockets provided excellent resistance to wear of both sprockets and chain as well as very low friction, provided that the chain is sufficiently lubricated.

Lubrication

This need for lubrication, not just to the outside of the chain but especially to the inner surfaces between the pins and bushings, and the bushings and rollers, is a source of aggravation to both bicycle owners, who have to clean and lubricate the chain by hand or with specialized gadgets, as well as to owners of complex machinery utilizing high speed chain drives, who have to utilize very expensive sophisticated lubrication systems to keep the chain lubricated. It is no surprise, then, that some owners on both ends of the scale opt to just not maintain the lubrication of the chain, and rather to just accept the more frequent replacement that requires (as much as 300 times more frequent, according to one estimate (http://www.gates.com/facts/documents/Gf000379.pdf)).

Variants in design

If the chain is not being used for a high wear application (for instance if it is just transmitting motion from a hand operated lever to a control shaft on a machine, or a sliding door on an oven), then one of the simpler types of chain may still be used. Conversely, where extra strength and/or durability are required, the chain may be "siamesed"; instead of just two rows of plates on the outer sides of the chain, there may be three, four, or more rows of plates running parallel, with bushings and rollers between each adjacent pair, and the same number of rows of teeth running in parallel on the sprockets to match. Timing chains on automotive engines, for example, typically have multiple rows of plates.

Roller chain is made in several sizes, the most common ANSI standards being 10, 20, 40, 60, and 60H. The first digit(s) indicate the pitch of the chain in eighths of an inch, with the last digit being 0 for standard chain, 1 for lightweight chain, and 5 for bushed chain with no rollers. Thus, a bicycle chain with half inch pitch would be a #40, a #160 sprocket would have teeth spaced 2 inches apart, etc. Metric pitches are expressed in sixteenths of an inch; thus a metric #8 chain would be equivalent to an ANSI #40. Most roller chain is made from steel, but stainless steel is used in food processing machinery or other places where lubrication is a problem, and nylon or brass are occasionally seen for the same reason.

Roller chain is ordinarily spliced using a master link, which typically has one pin held by a C clip rather than friction fit, allowing it to be inserted or removed with simple tools. Half links are available, and are used to increase the length of the chain by a single roller.

Use

Roller chain is used in low- to mid-speed drives, up to 2,000 or 3,000 feet per minute; at higher speeds, V-belts are quieter and wear better. A bicycle chain is a form of roller chain; however a master link would interfere with the operation of a derailleur bicycle gear system, therefore for removal or installation a chain tool must be used. A similar but stronger chain was traditionally used on most motorcycles but has been largely supplanted by either the cog belt or the shaft drive, which offer lower noise level and lesser maintenance requirements. For American automobile engines as well as others, roller chains would traditionally drive the camshaft(s) off the crankshaft, generating less noise than a gear drive as used in very high performance engines, and more durability than a timing belt as is frequently used on more modern engines.

Wear

The effect of wear on a roller chain is to increase the spacing of the links, causing the chain to grow longer. This is not from any actual stretching of the metal, but is due to the effect of wear at the pivoting parts. It is not impossible for well-used bicycle chains to have stretched ten percent of their nominal length. Although it is unusual for a chain to wear until it breaks, a worn chain leads to the rapid onset of a great increase in the rate of wear of the drive and driven sprockets with which it mates, since the sprockets can not change the spacing of their teeth to match the new spacing of the chain links. The wear a worn chain causes on the sprockets is asymmetrical, and grinds a characteristic hooked shape into the rearward face of the sprocket teeth. As this pattern progresses, the sprocket teeth naturally begin to hook the rollers of the chain links, preventing them from rolling off the sprocket teeth until later in the rotation than they should. This causes a jittery transmission of power rather than smooth; in bicycles, for instance, this translates into the chain "jumping" off the sprockets, and in automotive engine timing chains it translates into a jittery timing of both the valves and the distributor, which is evident when trying to time the ignition with a timing light. The only cure in such a case is to replace not only the chain, but also both the drive and driven sprockets; therefore it is advisable either to monitor the length of a drive chain (the generally accepted rule of thumb is to replace a chain which has stretched three percent or more), or just replace it at frequent intervals to minimize wear on the sprockets. Thus any savings in maintenance costs from skimping on lubrication result in increased costs for monitoring wear and for replacement. This need for frequent maintenance, comprising lubrication, assessing wear, and replacement of the chain and/or the sprockets, represents the major drawback of the roller chain.

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