Constructional feature of CROSS HEAD, CONNECTING ROD AND CRANKSHAFT

Crosshead:

• In marine two stroke engine the side thrust bu piston is high due to angularity of connecting rod.
• The side thrust is also responsible for high wear of cylinder liner.
• Crosshead are used in large two stroke marine desiel engine as to reduce the impact of side thrust and reduce cylinder liner wear.

• The side side thrust is transferred from crosshead to crosshead guides.
• Crosshead guides are attached to engine frame and due to transverse component of connecting rod it is subjected to fluctuating load.
• The end of Crosshead are provided with guide shoes.
• The cross head bearing is accomodated in crosshead journal which is at the centre of of crosshead design.
• The bearing surface is white metal lined bearing shell.
• The piston rod foot rest on on crosshead bearing.
• To match different engine layout shims are inserted between the piston rod and cross head.
• The piston rod is mounted on the Crosshead by hydraulic stud and nuts which are tightened by hydraulic tools.

• Telescopic pipe mounted on one of the guide shoes, supplies the lubricating oil to crosshead, crankpin and piston.
• To counter the weight of telescopic pipe counter weights are added.
• The outlet pipe for piston cooling oil is mounted on the other side of guide shoes.
• The oil is led to the control device and then to lube oil sump tank. The control device checks the temperature and flow.

Connecting rod:

• The connecting rod connects the cross head to the crankshaft.
• The connecting rod top end is connected to the crosshead bearing and bottom end is connected to crankpin bearing.
• The crankpin bearing have bearing shell lined with white metal.

Crankshaft:

Marine diesel engine Crankshaft can be divided as

• Fully built.
• Semi built.
• Welded crankshaft.
• Solid single piece ( used in very small engine).

Fully built

• Fully built crankshaft are those in which all the components are separately fabricated and then shrink fitted to each other.
• Seperate webs, crankpin , journal are forged to consist the crankshaft.
• The crank pin, journal are machined and web are bored with holes which are slightly smaller in diameter.
• The webs are then heated up and crankpin and journal are fitted into the webs.
• The webs are then allowed to cool down which will enable the crank pin and journal to grip tightly and doe not allow slippage during the operation.

Semi built:

• This consist of crankthrow ( two webs and crank pin which are made from single forging) and journal which are shrink fitted.
• When two webs and crankpin are made from single forging the grain flow in the steel follows the web rounds into the crank pin and then back down to other web.
• This helps in reduction of size of webs and result in reduction of weight without compromising on strength.

Welded crankshaft:

• In this type each of the forging is made up of web, crankpin and half of the main journal on each side.
• These forging are then welded using submerged arc welding and stress relieved and then machined to form the crankshaft.
• The continuous grain flow helps in reduction of size of webs and which inturn result in lighter and shorter crankshaft without compromising on strength.

Solid single piece:

• These are forged and casted in single piece.
• These can only be used in small size engine.