CrankshaftThe crankshafts of modern two stroke

crosshead main engines can weigh over 300 tons. They are too big to be made as a single

unit and so are constructed by joining together individual forgings. On older engines,

so called fully built up units was used. This consisted of forging separate webs, crankpin

and main journals. The crankpins and journals were machined and matching holes bored into the web, which were of a slightly smaller diameter. The webs were heated up and the crankpins and main journals were fitted in the expanded holes. As the webs cooled down, so the diameter of the web holes would try and shrink back to their

original size.

• In doing so the crankpins and main journals would be gripped tightly enough to stop them from slipping, when the engine is operated. This method of construction had its origin from the early days of steam reciprocating engine crankshaft manufacture, where apart from shrink fitting dowel pins were also used (mainly because tightness of shrink fitting could not be guaranteed). This fitting of dowel pin is never used in construction of crankshaft of diesel engine. This would act as a stress raiser point from where cracks may emanate.

• Today, crankshafts of large crosshead type diesel engines are semi-built up type. In this method of construction, the crankshaft “throws” consisting of two webs and crankpin are made of a single forging from 0.4% carbon steel. The webs are bored to take separately forged and machined main journal, which are fitted to the webs by shrink fitting, described earlier. The shrink fitting allowance is 1/570 to 1/600 of the diameter.

• The advantages of construction of two webs and crankpin in a single forging is that the grain flow in the steel follows the web into the crankpin and into the other web. Because the crankpin and webs are in a single forging, the web thickness can be reduced and a hole can sometimes be drilled through the crankpin, reducing weight, without compromising in strength. Note: There is a need for a good amount of material around the holes bored to take the main journals. This is because of the large hoop stress present in the material after shrink fitting. This could lead to a crack in the web if the thickness here is not adequate.

Welded crankshaftWelded crankshaft was developed in 1980s. It was made up of series of forgings, each comprising of half a main journal, web, crankpin, second web and half a main journal. These forgings were then welded by submerged arc welding process to form a crankshaft. After welding, the crankshaft was stress relieved and machined. The advantages of this method was continuous grains and thinner web because there was no shrink fitting stresses; leading to a shorter lighter crankshaft. Only 20 crankshafts were produced by this method and they have performed very well. But production costs are very high. Hence this method is not used.