[Sketch notes: In large built-up (semi-built) slow-speed crankshafts, each crank is composed of a crank journal (pin) and adjacent webs built into one piece (or webs shrunk onto pins). In the classic construction the Crankpins/webs are heated so they expand, then forced onto, or all components are machined and the webs are shrunk onto the journal pins by heating the web hole or cooling the pin. In semi-built cranks, the individual crankthrows (each consists of a pin and two webs in one forging) are shrunk onto the main journals/neck. The web-pin is machined to an interference size: the journal is finished with a slightly larger diameter than the bore of the web, the web is heated (expanded) and dropped over the cooled journal, on cooling the web grips the pin with an interference fit. A keyway may assist alignment during manufacture, but the torque is carried by the interference friction.]

The sketch shows: the web with its bore, the journal pin, the interference fit, and the position/face where the five degree twist is measured.

(ii) How a five-degree twist might be caused to a shaft assembled by shrinkage (4 marks)

A twist between adjacent crank throws occurs if the shrink-fit slips. Under heavy torsional load, or if the interference fit was insufficient (heat applied unevenly, size wrong, or the surfaces oily), the frictional grip is overcome and one crank throw rotates relative to the next about the shaft axis through up to several degrees. Also, if during assembly one web was not angularly clocked correctly (indexed) relative to the next journal, or if a locking key was left out and the shrink surfaces slipped, a permanent twist results. It can also come from a serious torsional vibration/overtorquing event or from a bearing seizure torqueing the crank. The five degree twist is thus a relative rotation between the two end webs of a throw, i.e. the crank pin is angularly displaced relative to the main journals.

Effects:

• Each cylinder experiences altered (retarded or advanced) fuel injection timing and valve timing relative to the rest, so that cylinder will fire out of phase with the others, causing misfiring, rough running and vibration, and an unbalanced torque.
• The twist creates high torsional stress concentration at the shrink-fit and in the webs, which with continued running can crack the web, initiate a fatigue failure of the crankshaft, or cause the joint to slip further, with potentially catastrophic failure of the running gear.
• It changes the crank-throw angular orientation, so the main bearing loads and the piston travel are altered; a gross twist would put the connecting rod/position of the crankpin out of true, causing abnormal side thrust and possible hitting.

Conclusion/action at a port with no repair facilities: The engine should NOT be operated at high load or in that damaged condition if the twist is confirmed and large. If it must be moved (e.g. to complete the voyage/safe mooring), it should be limited to a very low speed/load, the twist and its effect on injection/valve timing checked, and the engine monitored closely for excessive vibration, noise and for any further slippage. Immediate steps: stop on arrival, secure the engine, and arrange survey/repair - the shrink-fitted joint must be re-fitted (heating, re-interference) or the crankshaft replaced at a repair yard with proper facilities. Continuing to operate at full power risks catastrophic crankshaft failure. A five-degree twist is far outside the permissible manufacturing/alignment tolerance (normally a fraction of a degree) and demands repair before normal service.