Question

Discuss the validity of EACH of the following statements with respect to large slow speed diesel engines:

(a) Bearing clearances obtained by taking leads (or use of plastic inserts) are fundamentally more accurate than those obtained with the use of feelers.

(b) Bearing wear down can be measured by taking deflections.

(c) A timing chain should be renewed when its slackness causes late fuel injection and exhaust valve operation.

(d) Timing chain slackness is solely due to stretch of the link plates.

Answer 1

Part (a)

Bearing clearances obtained by taking leads (or using plastic inserts) are fundamentally more accurate than those obtained using feelers.

Validity: True.

Measurement of bearing clearances by lead wire is a traditional method in which lead wires are inserted circumferentially at different places and tightened to rated torque. Measurement of the squeezed lead wire gives circumferential clearance of the bearing
This method is most accurate if good quality lead wire is used
Use of a feeler gauge is an easier method, but the result obtained is approximate data, which generally varies depending upon the user’s accuracy
Moreover, the feeler gauge can take only edge clearances and not inside clearances
Feeler gauge tends to wear out over some time due to continuous usage and may affect the results
Therefore, bearing clearances obtained by taking leads (or use of plastic inserts) is fundamentally more accurate than those obtained with the use of a feeler gauge

Part (b)

Bearing wear down can be measured by taking deflections:

Validity: Partially True

Bearing teardown is not the only reason for crankshaft deflection
Deflection may vary due to other reasons like loose foundation bolts and tie rods, deformation of bedplate, slippage of crankshaft, which may be due to exceptional loading, damaged bearing pockets, worn out stern tube or intermediate shaft bearing, etc
When all other reasons for deflection are checked to be in good order, only then can it be concluded that the bearing has worn out.
So, wear out of bearing cannot be measured by deflection to an accurate value but only suggested by deflection

Part (c)

A timing chain should be renewed when its slackness causes late fuel injection and exhaust valve operation:

Validity: False

A chain drive transmits the motion from the crankshaft to the camshaft, which governs the fuel pump and exhaust valve timing.
Slack chain may lead to late injection of fuel, late closing of exhaust valve, power loss, etc
Slight elongation can be compensated by VIT (1 to 2 degrees)
Slack chain can be re-tightened and not necessarily renewed
Renewal is only required if the elongation increases more than 1% of the original length
So, the timing chain is not necessarily renewed when fuel injection timing and exhaust valve operation are affected. It totally depends upon the maximum allowable chain elongation

Part (d)

Timing chain slackness is solely due to the stretch of the link plates:

Validity: False

Slackness of the chain drive is due to wear down between the pins and the bushes, which causes the chain to elongate by effectively increasing the pitch
Change in the pitch of the chain will not match with the pitch of the chain wheel and will further increase the wear rate
Also, elongation of the chain will cause excessive vibration, which further increases the wear and, thus, slackness of the chain drive
However, the link plates, which are made of nickel chrome molybdenum, have a very high factor of safety and may not stretch
So, the slackness of the timing chain may not be solely due to the stretch of link plates.

✓ Verified Model Answer (Text Solution)

Bearing clearances obtained by taking leads (or using plastic inserts) are fundamentally more accurate than those obtained using feelers.

Bearing wear down can be measured by taking deflections:

A timing chain should be renewed when its slackness causes late fuel injection and exhaust valve operation:

Timing chain slackness is solely due to the stretch of the link plates: