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3.Welded Crankshafts. In welded crankshafts, the usual way of construction is: The crank-throw and half journals on both sides is forged in one piece, machined and heat treated. The pieces so formed are then joined together by welding at the journal in the proper alignment and angle. Another way of construction is the forging of the journal, webs on both sides and half pins on both sides to make one piece. The pieces are then welded at the pin. The welding is full penetration, narrow gap welding. The pieces are kept together with a narrow gap, and the welding is done right from the lower edge to the top edge, the work piece being rotated slowly. The journal/pin is hollow. During welding a ‘dummy ring’ is positioned for aligning the two pieces correctly. This ring also forms the backing plate for the welding. Afterwards, the dummy ring is drilled away. Submerged arc welding technique is used.
ED 2011/REVISION 1 MDE/STUDENT HANDOUT (VOLUME-1)Page 125 of 155After welding, the shaft is machines and heat treated. Welded crankshafts are strong and light weight. The only disadvantage is the cost of manufacture. For this reason, very few engines are fitted with welded crank shafts.
ED 2011/REVISION 1 MDE/STUDENT HANDOUT (VOLUME-1)Page 126 of 155Connecting rod, crosshead and the bottom end bearing: The connecting rod connects the piston with the crankshaft. It forms the part of the mechanism that converts the reciprocating motion of the piston to rotary motion of the shaft. It is a steel forging.
ED 2011/REVISION 1 MDE/STUDENT HANDOUT (VOLUME-1)Page 127 of 155In large bore crosshead type engines, the connecting rod is connected to the piston rod through the crosshead bearing, and is connected to the crankshaft through the crankpin (or the bottom end) bearing. In most marine propulsion engines, the piston rod is connected to the crosshead pin by bolting it to the top of the pin. The pin is supported on the lower shell of the crosshead bearing. The crosshead bearing is a tri-metal shell type bearing fitted on the connecting rod top. Two bearing covers are mounted on either side of the piston rod on top of the crosshead pin. The pin is fitted with crosshead guide shoes on either side which slide on the crosshead guides fitted on the A frame.
ED 2011/REVISION 1 MDE/STUDENT HANDOUT (VOLUME-1)Page 128 of 155Lubricating oil is supplied by a flexible linkage to the crosshead pin. The lubricating oil is distributed to the crosshead bearing, the bottom end bearing, the guide shoes and for piston cooling through passages in the pin, connection rod and the piston rod.
ED 2011/REVISION 1 MDE/STUDENT HANDOUT (VOLUME-1)Page 129 of 155For B&W SMC engines, crosshead bearing top clearance is maximum 0.45 mm and minimum 0.2 mm. Lubrication of the crosshead bearing poses a problem as the crosshead bearing has only a oscillating motion and cannot generate hydrodynamic lubrication. In two stroke engine the crosshead is loaded in the downwards direction at all times. To distribute the oil to the highly loaded bottom half, the lower bearing shell has both axial as well as circumferential grooves.