How can the wear in a roller chain be accurately measured and when is the chain considered worn out?
Roller chain can be measured relatively accurately by measuring the tight span of the drive, when the drive is OFF, by measuring from center of one pin to the center 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. The 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 center distances, chains running in parallel or where smooth operation is required, the amount of wear should be limited to approximately 1.5%.
Working out if a chain has excessive wear is involves measuring and math’s. Great care needs to be taken when measuring as even small errors locating pin centres and reading the tape measure can result in errors. It is generally accepted that chain should be replaced when it is 2% worn on roller chain and 3% on leaf chain. An example of the calculation for a ¾” (19.05 mm) pitch chain. Measuring 21 pins or 20 pitches the original length is 381 mm, therefore 2% wear will be 7.62 mm and 3% wear is 11.43 mm.
The quickest and simplest way to measure chain wear is to use a chain wear gauge. Most gauges only tell you a chain is worn or not worn. The FB chain wear gauge is a “how much worn” gauge and it shows chain wear in ¼ percent increments from 0% worn to 4 % worn with a red warning box appearing at 2%. . This type of chain wear gauge makes it easy to decide if a chain could be dangerously worn out before the next service or inspection. Keeping a record of wear enables you to track chain wear at each service and take any necessary action like increasing or changing lubrication regime.
