Crosshead & Connecting Rod Arrangement of Marine Diesel Engine- Onboard Ship

• The piston is rigidly fixed to the piston rod. The rod passes through a gland to a cross head to which it is attached via a flange , or shoulder through bolt and nut. The crosshead consists of an rotating element which is attached to the connecting rod. It is through this bearing that the vertical load is transferred from the combustion space and converted into the rotary motion of the crankshaft via the connecting rod and big end bearing. Horizontal thrust generated at the crosshead are absorbed by white metal surfaced shoes which run up vertical athwartships mounted guides.
The advantages of the crosshead design are
guide faces take side thrust, this is easily lubricated, wears little and takes side forces off the piston and liner running surfaces.
Uniform clearance around piston allows for better lubricating oil distribution reducing wear
simplified piston construction designed for maximum strength and cooling. Extended load bearing skirts found on trunk pistons unnecessary
due to gland lubricating oil may be optimised for crankcase and cylinder. High alkalinity oils used in cylinder allow poorer quality fuels to be burnt.
Trunk piston construction
The piston is directly attached to the connecting rod by a small end rotating bearing. Side thrust is absorbed by extended skirts on piston.
The main advantage is reduced engine height
Opposed piston engines
Mainly built by doxford and consisted of two opposing piston moving in a common liner. Fuel injection occurred at the centre where the piston met. Construction is of the crosshead design with the upper piston connected to the crankshaft via two side rods and transverse beam. Timing was approximately 180oC except for a small angle of advance for exhaust timing.
Advantages are;
Perfect primary balance by balancing
upper reciprocating masses and lower velocity side cranks against
lower reciprocating mass and the higher velocity centre crank
No gas loading transvered to bed plate (normally via head and tie rods) on engine meaning that construction could be lighter
• The connecting rod also known as the con-rod of a typical main propulsion engine looks like the one shown in the picture below which shows two views of the rod. The diagram is labeled clearly for all its parts and as you can note from the sketch it consists of an upper forked section which fits on the crosshead bearings while the lower part fits on the crankpin bearing.
As you can imagine with this sort of arrangement there is heavy axial loading on the connecting rod which reaches its peak at the top dead center because the gas pressure and the inertial forces add to increase the overall force. Other abnormal working conditions such as piston seizure and momentary increase in peak pressure can also result in severe increase in stress on the con-rod and it could fail due to buckling due to these forces.Apart from the shape of the connecting rod shown above, there are special designs for other engines such as trunk engines. Apart from transmitting motion, the connecting rod also serves to supply oil for piston cooling and crosshead lubrication.
Normally connecting rods are forge-manufactured and the material used is typically mild and medium carbon steel. The ends where the rod is connected to the X-head or crankpin have bearings which are made of white metal working surface and shims are used to make the necessary adjustments. There are four bolts at each connection point which help for assembly and removal of the connecting rod, and are tightened to their required torque using hydraulic jack.In case of auxiliary marine diesel engines which are 4-stroke engines, the con-rod is constructed by drop forging and the material used in normally alloy steel and the alloy metals being nickel, chromium and molybdenum. The bottom end connecting bolts of 4 stroke engines often fail due to severe forces acting on them and this failure in turn can cause various structural damages in the engine in turn. Hence it is very necessary to tighten these bolts properly and keep checking for their tightness, lest one has to pay a heavy price for this minor negligence later on.