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Fixed centres

When adjustable centres or idlers cannot be used, the exact centredistance must be calculated and built into the drive. Drives with fixed centres should be conservatively selected and well lubricated to minimize the rate of chain wear. Adjustment for wear elongation in fixed centre distance drives is accomplished only by removing links or pitches to compensate for wear elongation.

Chain wear

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. Elongation is normal and may be minimized by proper lubrication anddrive maintenance. The rate of wear is dependent upon: the relationship between the load and the amount of bearing area between pin and bushing, the material and surface 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. Measurement of chain for wear elongation Chain does not ‘stretch’ - material is removed from pin and bushing Relatively accurate wear measurements can be made by using theabove illustration. Measure as closely as possible from the centre of one pin to the centre of another. The more pitches (pins) contained within the measurement increase the accuracy. If the measured value exceeds the nominal by more than the allowable percentage the chain should be replaced. The maximum allowable wear elongation is approximately 3% for most industrial applications, based upon sprocket design. The allowable chain wear in percent can be calculated using the relationship: 200/N, where N is the number of teeth in the large sprocket. This relationship is often useful since the normal maximum allowable chain wear elongation of 3% is valid only up to 67 teeth in the large sprocket. In drives having fixed centre distances, chains running in parallel or where smoother operation is required, wear should be limited to approximately 1.5%.For example, if 12 pitches (12 pins) of a 16B-1 chain were measuredand the result was 313.9mm or greater (using 3% as the maximum allowable wear), the chain should be replaced. Anything less than 313.9mm would still be acceptable by most industry standards.
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