The crankshafts for slow speed engines are normally of the builtup type because of the convenience in manufacturing.
The crank shaft comprising of crank pins , webs , journals and the aft end coupling are individually forged from good quality steel whose chemical composition is Carbon- 0.6 %max,silicon—0.4% max,sulphur—0.035% max, and phosphorous—-0.035% max.
The individual components are then machined to the required dimensions after which the crank shaft is built up in a setting and press jig or device , where the webs are arranged at the correct angular sequence and the pins and journals provided with the correct shrinkage allowance. The shrinkage allowance varies between 1/570 to 1/650 Of the respective pin and journal diameter. The webs are locally heated to the right temperature to allow the pins and journals to be pressed easily , thereby preventing overheating of the webs.
After the crank shaft is built up it is mounted on a crankshaft grinding machine and the pins and journals are ground to mirror finish standard at their correct dimensions. The crank shaft is subjected to a static balance test carried out on the grinding machine finally and suitably compensated for any small imbalance .
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