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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 smaller 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 larger of the two sprockets, since it is always the smaller 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, whilst ensuring the chain is taut. 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. Sprocket wear cancels this effect, and may mask chain wear.
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)[citation needed]. 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)2. X-ring and O-Ring chains greatly decrease wear by means of internal lubricants, increasing chain life. The internal lubrication is inserted by means of a vacuum when riveting the chain together.
Standards organizations (such as ANSI and ISO) maintain standards for design, dimensions, and interchangeability of transmission chains. For example, the following Table shows data from ANSI standard B29.1-2011 (Precision Power Transmission Roller Chains, Attachments, and Sprockets) developed by the American Society of Mechanical Engineers (ASME). See the references[8][9][10] for additional information.
ASME/ANSI B29.1-2011 Roller Chain Standard SizesSizePitchMaximum Roller DiameterMinimum Ultimate Tensile StrengthMeasuring Load25ASME/ANSI B29.1-2011 Roller Chain Standard Sizes | ||||
Size | Pitch | Maximum Roller Diameter | Minimum Ultimate Tensile Strength | Measuring Load |
---|---|---|---|---|
25 | 0.250 in (6.35 mm) | 0.130 in (3.30 mm) | 780 lb (350 kg) | 18 lb (8.2 kg) |
35 | 0.375 in (9.53 mm) | 0.200 in (5.08 mm) | 1,760 lb (800 kg) | 18 lb (8.2 kg) |
41 | 0.500 in (12.70 mm) | 0.306 in (7.77 mm) | 1,500 lb (680 kg) | 18 lb (8.2 kg) |
40 | 0.500 in (12.70 mm) | 0.312 in (7.92 mm) | 3,125 lb (1,417 kg) | 31 lb (14 kg) |
50 | 0.625 in (15.88 mm) | 0.400 in (10.16 mm) | 4,880 lb (2,210 kg) | 49 lb (22 kg) |
60 | 0.750 in (19.05 mm) | 0.469 in (11.91 mm) | 7,030 lb (3,190 kg) | 70 lb (32 kg) |
80 | 1.000 in (25.40 mm) | 0.625 in (15.88 mm) | 12,500 lb (5,700 kg) | 125 lb (57 kg) |
100 | 1.250 in (31.75 mm) | 0.750 in (19.05 mm) | 19,531 lb (8,859 kg) | 195 lb (88 kg) |
120 | 1.500 in (38.10 mm) | 0.875 in (22.23 mm) | 28,125 lb (12,757 kg) | 281 lb (127 kg) |
140 | 1.750 in (44.45 mm) | 1.000 in (25.40 mm) | 38,280 lb (17,360 kg) | 383 lb (174 kg) |
160 | 2.000 in (50.80 mm) | 1.125 in (28.58 mm) | 50,000 lb (23,000 kg) | 500 lb (230 kg) |
180 | 2.250 in (57.15 mm) | 1.460 in (37.08 mm) | 63,280 lb (28,700 kg) | 633 lb (287 kg) |
200 | 2.500 in (63.50 mm) | 1.562 in (39.67 mm) | 78,175 lb (35,460 kg) | 781 lb (354 kg) |
240 | 3.000 in (76.20 mm) | 1.875 in (47.63 mm) | 112,500 lb (51,000 kg) | 1,000 lb (450 kg |
For mnemonic purposes, below is another presentation of key dimensions from the same standard, expressed in fractions of an inch (which was part of the thinking behind the choice of preferred numbers in the ANSI standard):
Pitch (inches) | Pitch expressed in eighths |
ANSI standard chain number |
Width (inches) |
---|---|---|---|
1⁄4 | 2⁄8 | 25 | 1⁄8 |
3⁄8 | 3⁄8 | 35 | 3⁄16 |
1⁄2 | 4⁄8 | 41 | 1⁄4 |
1⁄2 | 4⁄8 | 40 | 5⁄16 |
5⁄8 | 5⁄8 | 50 | 3⁄8 |
3⁄4 | 6⁄8 | 60 | 1⁄2 |
1 | 8⁄8 | 80 | 5⁄8 |