MRO Magazine

Feature

The myth of chain stretch

Did you know that drive chain does not actually ‘stretch’? Its elongation is caused when material is removed from the pins and bushings. The individual joints in a roller chain articulate as they enter and leave the sprockets. This...


Did you know that drive chain does not actually ‘stretch’? Its elongation is caused when material is removed from the pins and bushings. The individual joints in a roller chain articulate as they enter and leave the sprockets. This articulation results in wear on the pins and bushings. As material is worn away from these surfaces, the chain will gradually elongate.

So what can you do about it? Elongation is normal and may be minimized by proper lubrication and drive maintenance. The rate of wear is dependent upon the relationship between the load and the amount of bearing area between the pin and bushing, the material and condition of the bearing surfaces, the adequacy of lubrication, and the frequency and degree of articulation between pins and bushings. The latter is determined by the quantity of sprockets in the drive, their speeds, the number of teeth, and the length of the chain in pitches.

Check chain wear

Roller chains should be replaced when worn (elongated beyond 3%) or when the chain rollers begin to ‘ride high’ near the tips of the teeth on relatively large sprockets.

Do not connect or splice a new section to a worn chain. Do not continue to run a chain worn in excess of 3% (or less in some applications), as the chain will not engage the sprockets properly and increased damage to the sprockets may occur.

Roller chain wear gauge instructions

1 | As a safety precaution, shut off the power to the drive and lock out gears and sprockets before attempting to measure chain wear.

2 | Determine the pitch of the chain. This is typically stamped on the outer link plates of the chain. It can also be determined by measuring the distance from the centre of one pin to the centre of the next pin. Refer to a product catalogue for a list of ANSI standard chain models and correlating pitch measurements. 

3 | For reliable linear measurement, a taut span of chain must be used. Using slack chain will result in inaccurate measurements.

4 | Choose either a 1.5% or 3% wear elongation limit to check your span of chain. Each percentage correlates to a different side of the scale. The maximum allowable wear elongation is typically 3% for most industrial applications, depending upon sprocket design. In drives having fixed centre distances, chains running in parallel, or where smoother operation is required, chain wear should be limited to approximately 1.5%.

5 | Refer to the table on the wear gauge for the number of pitches to inspect. The more pitches (pins) that are included in the measurement, the more true the representation will be of the average amount of wear distributed throughout the chain.

6 | Place the inside corner of the wear scale around one pin, using that pin as ‘0’, your starting point. This corner is the zero point on the scale.

7 | Starting at ‘0’, count the number of pins/pitches to be measured for your chain’s length.

8 | If the centre of the indicated pin does not reach the wear line for the corresponding chain size, the chain has not reached the wear limit.

9 | If the centre of the indicated pin is at or beyond the indicated line, the chain is worn to the wear limit (1.5% or 3%, depending on the scale used) and may need to be replaced. 

This article was prepared by the Diamond Chain Company Inc., Indianapolis, IN. For more information, visit the website at             www.diamondchain.com. For a free roller chain wear gauge, e-mail the company at marketing@diamondchain.com.

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