Main Engine Chain Drive

A chain drive is used to transmit motion from crankshaft to camshaft of the engine.This drive is known as timing chain. It is responsible for the rotation of camshaft which governs the fuel pump and exhaust valve timing.

In 2 stroke engine rotational speed of camshaft is same as crankshaft. In 4 stroke engine rotational speed of camshaft is half that of crankshaft. An intermediate wheel serves as a guide and can be connected to a separate chain to drive governor,lubricators etc.This wheel is also called as idler wheel. An adjuster wheel serves to adjust the chain tension.Chain rollers run over chain sprocket wheels, one being bolted to crankshaft and other to camshaft.Chain are run between guide bars with rubber shock absorbing pads.

Adjuster wheel is mounted on levers which can be held at steady measured tension.Chain rollers rolling on wheels are amply lubricated by lubricating nozzles.

Chain details and material:

Link plates - Chromium Molybdenum steel

Pin - Hardened steel

Rollers - Alloy Steel

Reasons for chain elongation in service:

Chains have a very high factor of safety to prevent stretching. Any loss in tension in chain is due to wear while in use.Elongation is due to

• wear between pins and bushing
• wear between roller and bushing
• wear between roller and sprocket wheel teeth

Maximum elongation allowed is 2%. At 1.5% of elongation renew the chain.Elongation changes camshaft position with respect to crankshaft. As fuel and valve timings depends on camshaft position, chain elongation has adverse effect on timing.

Chain Elongation also results for greater shock loading,vibration,cyclic stresses and possible fatigue failure.Too tight chain results in overloading of chain wheel and bearings, wear of rollers,links and bearings, cracking of links.

Chain Tightness checking [small engines]:

• Turn the engine to slacken the longest free length of chain
• At the middle of longest face length, pull the chain by hand away from the guide bar
• Limit of slackness is half to one chain pitch length.If too slack or tight , adjustment is done for 1 chain pitch link length.

Chain inspection:

• Chain tightness is checked
• Inspect and check lub oil pipes and jet nozzles
• Check rubber track on the guide bars for condition
• Place a short straight edge plate and cover two points A and B on chain sprocket wheel tooth.Measure wear at point 1

• Check for cracks on rollers or side plates.Check free running of rollers for one complete revolution

Chain tightening procedure: As per MAN B&W MCC engines

 

• Open tab washers for nut A,B,C,D.
• Loosen the nuts A,B,C,D to free the chain tightener bolt
• Turn the engine so that slack side of the chain is on the same side as tightener wheel
• Make sure balance weight of moment compensators if fitted are hanging downwards
• Now tighten nut B until there is clearance of 0.1mm between the shaft and the nut. After tighten nut B to prescribed tightening angle as per manual.12 sides of hexagon nut for 6S60MCC engine. 12*60=720degrees
• Tighten nut C hard against contact face of the shaft
• Tighten nut D and lock nuts C&D with tab washer
• Tighten nut A and then lock nuts A&B with tab washer.
• Measure distance X

• If chain is worn ie, X>165mm.Repeat the whole tightening procedure.But tighten nut B on a reduced tightening angle of 600degrees[10 sides of hexagon].
• If still the X distance exceeds maximum distance of 165mm.Find and eliminate the cause of abnormal chain elongation.

Readjustment of camshaft angular position would be necessary after repeated chain tightening procedure as this causes camshaft position to be altered with respect to crankshaft.Maximum limit is 2degrees increase in lead angle.