The primary cause of chain elongation is wear-down between the pins and bushes in the roller chain. Over time, this wear effectively increases the pitch of the chain (the distance between the centres of adjacent pins), causing the chain to elongate.

As the chain's pitch increases, it no longer matches the pitch of the sprocket teeth, leading to:

• Excessive wear on the teeth of the sprockets.
• Increased vibration, which further accelerates wear on the chain and sprockets.
• Damage to the guide bars and oil spray nozzles due to misalignment and excessive vibration.

Elongation of chain causes angular slippage between the crankshaft & camshaft leading to:

• Change in fuel pump timing
• Change in fuel injection timing
• Improper combustion
• Loss of power
• Change in exhaust valve timing
• Change in starting air distributor timing
• Trouble with engine starting

(ii) Method of Assessing Percentage Increase in Length:

To measure the percentage increase in chain length:

• Measure the total length of 10 consecutive chain links in the current chain.
• Compare the measured length with the standard value provided in the manufacturer’s manual.
• Calculate the percentage elongation using the formula:

$$Percentage\:elongation=\frac{Measured\:length-Standard\:length}{Standard\:length}\times100$$

(c) Correcting the Effects of Chain Elongation:

Tightening the Chain:

For minor elongation, the chain can be tightened following the procedure:

• Turn the engine so that slack is on the side of the chain tightener unit.
• Adjust nuts (A, B, C, and D) according to the manufacturer's guidebook to achieve the required tension.
• Ensure no excessive compression of the spring during the adjustment.

Compensation with Variable Injection Timing (VIT):

• A small amount of elongation (up to 1-2°) can be compensated by adjusting the VIT.

Camshaft Readjustment:

• Turn the engine by bringing the No.1 unit to TDC.
• Check the camshaft's angular position using a pin gauge and markings.
• If the lead angle exceeds 2°, the camshaft timing must be restored:
  • Use a hydraulic pump to float the coupling.
  • Adjust the coupling alignment with a special spanner.
  • Verify alignment with the pin gauge, then allow the coupling flange to settle before sealing.

  Chain Renewal:

  • If the elongation exceeds the allowable limit (generally 1% to 1.5%, but not more than 2%), the chain must be replaced, and timing should be rechecked and restored.

  Reason for Limiting Percentage Chain Elongation

  A limit is placed on percentage chain elongation because elongation directly alters the angular position of the camshaft relative to the crankshaft. This results in:

  • Significant changes in fuel injection and valve timing.
  • Inefficient combustion and engine performance issues.
  • Increased wear and mechanical failure risks.

  Example:

  Manufacturers typically recommend renewing the chain when the elongation reaches 1.5%. In some cases, the allowable elongation may vary, but it should not exceed 2%.