Gear and chain drive

 There are two different types of drives - Gear drive and Chain drive.

Gears are found on medium speed and high speed engines. There are various types of gears:-

Gears used for parallel shafts.

a. Spur gears with external teeth. The line of the gear teeth is axial.
b. Helical gears. The tooth line follows a helix. 

Gears used for non-parallel shafts

a. Bevel gears. For changing the direction. Used in governors.
b. Spiral bevel gears. Similar to helical gears. Used in Indicator drives.
c. Worm gears. Used in Turning gear for engine. 

Faults in gearing.

Noise is the most common indication of wear or damage. There may a variation of the tooth pitch of an individual wheel, which can cause this. Tooth damage takes place due to shock loading.
Pitting indicates a deficiency in lubrication. Usually found in the tooth at mid-height. Scuffing, galling and undercutting are due to incorrect meshing or total lubrication failure.

Gear drive

A train of Gear wheels is fitted, which transmits the drive from the crankshaft to the camshaft. Appropriate speed and direction is achieved, by suitably arranging the intermediate gears in different positions and sizes. Unlike a chain, the gear does not lose tension; however, gears are subject to damage and difficult/ expensive to replace.
Large super long stroke engines have Moment Compensators fitted to the gears. The balance weights rotate at the full engine speed or twice the engine speed, in synchronisation with the Crankshaft to take care of primary and secondary vibration.

Chain drive

Chain drives are used by many manufacturers, to give more flexibility in location of camshaft, as well as ease of repair / replacement of parts at a lower cost; however this gives a more complex arrangement, with a large number of moving parts subject to wear and slackening, which can affect the timing of fuel as well as other factors in engine operation.
The chain tension needs to be checked at intervals given in the maintenance schedule, or earlier if any doubt exists.
To carry out the checking, tum the engine to slacken the longest free length. At the middle of the longest free length, grasp the chain and try to pull it away from the guide bar, by a distance of approximately half a chain link. If it is possible to pull more, the chain is slack and needs to be re-tensioned. Loosen the screw of the Chain Tightener, and by turning the 'loose' plate of the Chain Tightener, adjust the tension of the chain.
Excessive chain tension overloads the chain and the bearings and will wear unnecessarily. Insufficient chain tension may give rise to vibration, in the case of resonance, which, in the long run, may damage the chain.

Lubricating the chain drive

Special attention is to be paid to the securing of the lubricating oil pipes and spray nozzles. The spray nozzles must be directed so that the oil jets correctly hit the side bars. During operation, the functioning of the spray nozzles must be periodically checked and the nozzles cleaned if necessary.

Maintenance of the chain drive

The chain drive is to be checked at the prescribed intervals given in the manual. At overhauls, the chain must be examined for signs of damage, such as cracks in the wheels or side bars, jamming links, pitted wheels, traces of blows on the outer parts on account of catching, or scouring on the insides of the side bars, due to poorly aligned sprocket wheels. The causes of these defects must be
put right immediately. Damaged or broken chains must be replaced as soon as possible.
During operation, the teeth of the sprocket wheels acquire a polished or slightly worn band above the root circle. A slight polish is often produced on the inner side bars, in spite of correctly aligned wheels. If there are signs of marked wear, however, its cause is to be investigated and the chain closely inspected. Burrs on the edges of the teeth point to vibration of the chain.
Sprocket wheels whose teeth sustained major deformation should be replaced as soon as possible. The bearings and bearing pins, their fastenings and their true and even running are to be checked at overhauls

Admissible wear of the chain

The chain wear is directly proportional to the lengthening of the chain. The maximum admissible chain elongation is approximately 1 % of the length of the chain. Also check relative position of camshaft w.r.t crankshaft, the limiting maximum deviation allowed being 2°.