Roller Chain Wear and Measurement: Understanding the Key Concepts
A roller chain consists of a series of interconnected roller bearings that hinge as they enter and leave the sprockets. This hinging action results in wear on the pins and bushings, and over time, material is worn off these surfaces. As the material wears down, the roller chain gradually elongates.
Elongation due to wear is a normal phenomenon during the operation of the drive. However, the rate of wear and elongation depends on several factors:
- Proper lubrication: Ensures minimal friction and extends the life of the chain.
- Load: The heavier the load, the greater the stress on the chain components.
- Frequency and extent of pin and bushing hinging: More frequent movements cause more wear.
The pins and bushings are the key wear components in a roller chain. Therefore, their manufacturing process requires meticulous attention to detail. From selecting the right raw materials, ensuring proper manufacturing processes, to preparing parts for assembly, every step is crucial for optimal performance. If the quality of these wear components is poor, the wear life of the roller chain will be negatively affected, regardless of other factors.
Measuring Wear and Determining When to Replace the Chain
To assess whether the chain has elongated to a point where it needs replacement, wear measurements can be taken. The key is to measure the length of the roller chain accurately when it is under tension during normal operation. If the chain is still on the sprocket, the system must be shut down, and all safety procedures must be followed.
The section of the chain that should be measured is the tight span. If the chain has been removed from the sprocket, the ANSI specified measurement load must be applied to the chain to eliminate slack before taking the measurement.
How to Measure Chain Wear
- Measure from the center of one pin to the center of the next pin.
The more pins included in the measurement, the more accurate the result. - Check if the measured value exceeds the nominal value by an allowable percentage.
If the measurement exceeds the nominal value by the permissible percentage, the chain should be replaced.- For most industrial applications, the maximum allowable wear elongation is 3%.
- For large sprockets (e.g., those with 68 teeth or more), the allowable wear percentage can be calculated using the formula:
200 / N, where N is the number of teeth on the large sprocket.
This formula is useful because the 3% maximum allowable wear elongation typically applies to sprockets with 67 teeth. For sprockets with fewer teeth, the allowable elongation percentage may differ.
Example:
Let’s say we are measuring a #80 chain with 12 pitches (12 pins). If the measurement results in 12.360 inches or more (using 3% as the maximum allowable wear), the chain should be replaced. Most industrial standards still consider any measurement below 12.360 inches acceptable.
Conclusion
Proper measurement of chain elongation is essential for ensuring the smooth and efficient operation of a chain-driven system. Regular checks and understanding the wear limits of the chain can help prevent breakdowns and reduce maintenance costs. Ensuring the pins and bushings are of high quality, lubricating the chain properly, and performing accurate elongation measurements when needed will significantly contribute to the longevity and efficiency of the roller chain.