Crankshaft is the most important part of the engine which trasmitting the power from Connecting rod to the propeller shaft. Crankshaft continously suffers from the heavy intertia load and due to that high stress is developed. Crankshaft is mainly made of High Carbon Steel for slow speed engine and for medium an high speed engine they are made of High Carbon Alloy (Chromium, tungsten, nickel and magnesium having 1.5% each).
What is shrink fitting and how does it carried out?
The crankpins and journals were machined and matching holes bored in the webs, which were slightly smaller in diameter. The webs were heated up and the crankpins and journals fitted into the holes (which due to the heat had expanded in size). As the webs cooled down, so the diameter of the bored holes would try and shrink back to their original size. In doing so, the crankpins and journals would be gripped tightly enough to stop them being able to slip when the engine was being operated normally. This method of construction had its origins in the days of early reciprocating steam engine crankshaft manufacture, when as well as shrink fitting, dowel pins were used (mainly because the tightness of the shrink fit could not be guaranteed). THIS FITTING OF DOWEL PINS IS NEVER USED IN THE CONSTRUCTION OF DIESEL ENGINE CRANKSHAFTS. It would act as a stress raising point from which a crack could start.
Which are the different types of Crankshaft ?
Normally there are four types Crankshaft according to its construction :-
1.Fully Build Up Crankshaft:-
They are seprately built and then joined with the help of shink fitting. Normally very less number engine are Fully build. Fully build is normally used in very slow speed engine.On older engines the so called fully built method was used.
Advantage :- Easy Construction & replacement with having fast repairing of any damage parts
Disadvantage :- Normally low strength to allow shrunk fitts at web
2.Semi Build up Crankshaft:-
They are shrunk fit assemblies of complete throw an dseprate joural pins. They are used in 2 stroke and medium 4 stroke engine.Today, crankshafts for large 2 stroke crosshead engines are of the semi built type. In this method of construction the
crankshaft “throws” consisting of two webs. The webs are bored to take the separately forged and machined main journals which are fitted into the webs using the shrink fitting method described above.
The advantages of this method of construction is that by making the two webs and crankpin from a single forging the grain flow in the steel follows the web round into the crankpin and back down the other web.Because the crankpin and webs are a single forging, the webs can be reduced in thickness and a hole is sometimes bored
through the crankpin as shown, reducing the weight without compromising strength. Note however, there is a need for a good deal of material around the holes bored to take the main journals. This is because of the large tensile 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 or if the shrink fit is too tight or if there is a flaw in the material.
3.Solid Single Piece Crankshaft :-
4.Welded type Crankshaft:-
The welded crankshaft was developed in the 1980s. It was made up of a series of forgings each comprising of half a main journal, web, crankpin, second web, and half a main journal. These forgings were then welded together using a submerged arc welding process to form the crankshaft. After welding the journals were stress relieved and machined. As well as having the advantage of continuous grain flow, the webs could be made thinner (no shrink fit to accommodate), leading to a lighter shorter crankshaft. They are normally not used due to its high cost and limitation of twenty crankshaft.
Article By :- Harshal Patil