Timing Roller Chain
Roller chain or bush roller chain is the type of chain drive most commonly used for transmission of mechanical power on many kinds of domestic, industrial and agricultural machinery, including conveyors, wire- and tube-drawing machines, printing presses, cars, motorcycles, and bicycles. It consists of a series of short cylindrical rollers held together by side links. It is driven by a toothed wheel called a sprocket. It is a simple, reliable, and efficient means of power transmission.
Though Hans Renold is credited with inventing the roller chain in 1880, sketches by Leonardo da Vinci in the 16th century show a chain with a roller bearing.
Construction of the chain
There are actually two types of links alternating in the bush roller chain. The first type is inner links, having two inner plates held together by two sleeves or bushings upon which rotate two rollers. Inner links alternate with the second type, the 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. This has the advantage of removing 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. This distributed 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 provided rolling contact with the teeth of the sprockets resulting in excellent resistance to wear of both sprockets and chain as well. There is even very low friction, as long as the chain is sufficiently lubricated. Continuous, clean, lubrication of roller chains is of primary importance for efficient operation as well as correct tensioning.
Many driving chains (for example, in factory equipment, or driving a camshaft inside an internal combustion engine) operate in clean environments, and thus the wearing surfaces (that is, the pins and bushings) are safe from precipitation and airborne grit, many even in a sealed environment such as an oil bath.
But many other industrial-use chains run unprotected, and in those cases, internally sealed roller chain manufacturers such as Tsubaki, Renold, Diamond, Morse, and Rexnord produce low-maintenance versions wherein o-rings or x-rings seal in the lubricant for life.
There are also many chains that have to operate in dirty conditions, and for size or operational reasons cannot be sealed. Examples include chains on farm equipment, bicycles, and chain saws. These chains will necessarily have relatively high rates of wear, particularly when the operators are prepared to accept more friction, less efficiency, more noise and more frequent replacement as they neglect lubrication and adjustment.
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 but the smooth drive of a smaller pitch is required, the chain may be "siamesed"; instead of just two rows of plates on the outer sides of the chain, there may be three ("duplex"), four ("triplex"), 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 called strands.
Roller chain is made in several sizes, the most common American National Standards Institute (ANSI) standards being 40, 50, 60, and 80. 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 chain with half inch pitch would be a #40 while 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 (08B-1) would be equivalent to an ANSI #40. Most roller chain is made from plain carbon or alloy 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 hooked up using a master link (also known as a connecting link), which typically has one pin held by a horseshoe clip rather than friction fit, allowing it to be inserted or removed with simple tools. Chain with a removable link or pin is also known as cottered chain, which allows the length of the chain to be adjusted. Half links (also known as offsets) are available and are used to increase the length of the chain by a single roller. Riveted roller chain has the master link (also known as a connecting link) "riveted" or mashed on the ends. These pins are made to be durable and are not removable.
The effect of wear on a roller chain is to increase the pitch (spacing of the links), causing the chain to grow longer. Note that this is due to wear at the pivoting pins and bushes, not from actual stretching of the metal (as does happen to some flexible steel components such as the hand-brake cable of a motor vehicle).
With modern chains it is unusual for a chain (other than that of a bicycle) to wear until it breaks, since a worn chain leads to the rapid onset of wear on the teeth of the sprockets, with ultimate failure being the loss of all the teeth on the sprocket. The sprockets (in particular the larger of the two) suffer a grinding motion that puts a characteristic hook shape into the driven face of the teeth. (This effect is made worse by a chain improperly tensioned, but is unavoidable no matter what care is taken). The worn teeth (and chain) no longer provides smooth transmission of power and this may become evident from the noise, the vibration or (in car engines using a timing chain) the variation in ignition timing seen with a timing light. Both sprockets and chain should be replaced in these cases, since a new chain on worn sprockets will not last long. However, in less severe cases it may be possible to save the smaller of the two sprockets, since it is always the larger one that suffers the most wear. Only in very light-weight applications such as a bicycle, or in extreme cases of improper tension, will the chain normally jump off the sprockets.
The lengthening due to wear of a chain is calculated by the following formula:
M = the length of a number of links measured
S = the number of links measured
P = Pitch
In industry, it is usual to monitor the movement of the chain tensioner (whether manual or automatic) or the exact length of a drive chain (one rule of thumb is to replace a roller chain which has elongated 3% on an adjustable drive or 1.5% on a fixed-center drive). A simpler method, particularly suitable for the cycle or motorcycle user, is to attempt to pull the chain away from the larger of the two sprockets. Any significant movement (e.g. making it possible to see through a gap) probably indicates a chain worn up to and beyond the limit. Sprocket damage will result if the problem is ignored.
The most common measure of roller chain's strength is tensile strength. Tensile strength represents how much load a chain can withstand under a one-time load before breaking. Just as important as tensile strength is a chain's fatigue strength. The critical factors in a chain's fatigue strength is the quality of steel used to manufacture the chain, the heat treatment of the chain components, the quality of the pitch hole fabrication of the linkplates, and the type of shot plus the intensity of shot peen coverage on the linkplates. Other factors can include the thickness of the linkplates and the design (contour) of the linkplates. The rule of thumb for roller chain operating on a continuous drive is for the chain load to not exceed a mere 1/6 or 1/9 of the chain's tensile strength, depending on the type of master links used (press-fit vs. slip-fit) Roller chains operating on a continuous drive beyond these thresholds can and typically do fail prematurely via linkplate fatigue failure.
The standard minimum ultimate strength of the ANSI 29.1 steel chain is 12,500 x (pitch, in inches). Internal Lubrication X-Ring and O-Ring Type chains greatly decrease wear by means of keeping in internal lubrication increasing chain life. The internal lubrication is inserted by means of a vacuum when riveting the chain together
Standards organizations (such as ANSI) maintain standards for design, dimensions, and interchangeability of transmission chains. For example, the following table shows the principal data of ANSI standard B29.1 (Precision Power Transmission Roller Chains, Attachments, and Sprockets).
|ANSI B29.1 roller chain standard sizes|
|Size||Pitch||Roller diameter||Tensile strength||Working load|
|25||0.250 in (6.35 mm)||0.130 in (3.30 mm)||781 lb (354 kg)||140 lb (64 kg)|
|35||0.375 in (9.53 mm)||0.200 in (5.08 mm)||1,758 lb (797 kg)||480 lb (220 kg)|
|41||0.500 in (12.70 mm)||0.306 in (7.77 mm)||1,500 lb (680 kg)||500 lb (230 kg)|
|40||0.500 in (12.70 mm)||0.312 in (7.92 mm)||3,125 lb (1,417 kg)||810 lb (370 kg)|
|50||0.625 in (15.88 mm)||0.400 in (10.16 mm)||4,880 lb (2,210 kg)||1,430 lb (650 kg)|
|60||0.750 in (19.05 mm)||0.469 in (11.91 mm)||7,030 lb (3,190 kg)||1,980 lb (900 kg)|
|80||1.000 in (25.40 mm)||0.625 in (15.88 mm)||12,500 lb (5,700 kg)||3,300 lb (1,500 kg)|
|100||1.250 in (31.75 mm)||0.750 in (19.05 mm)||19,531 lb (8,859 kg)||5,072 lb (2,301 kg)|
|120||1.500 in (38.10 mm)||0.875 in (22.23 mm)||28,100 lb (12,700 kg)||6,800 lb (3,100 kg)|
|140||1.750 in (44.45 mm)||1.000 in (25.40 mm)||38,280 lb (17,360 kg)||9,040 lb (4,100 kg)|
|160||2.000 in (50.80 mm)||1.125 in (28.58 mm)||50,000 lb (23,000 kg)||11,900 lb (5,400 kg)|
|180||2.250 in (57.15 mm)||1.460 in (37.08 mm)||63,300 lb (28,700 kg)||13,700 lb (6,200 kg)|
|200||2.500 in (63.50 mm)||1.562 in (39.67 mm)||78,000 lb (35,000 kg)||16,000 lb (7,300 kg)|
|240||3.000 in (76.20 mm)||1.875 in (47.63 mm)||112,500 lb (51,000 kg)||22,250 lb (10,090 kg)|