Tie rod of 2 stroke marine diesel engine

Marine diesel engines are constructed with different components, which are held together with the help of tie rod to form a complete structure of an engine.

Tie rods are long metal rods that are found at the periphery of the engine.

Functioning of tie rods

Tie rods hold the engine components (like entablature cylinder block, "A" frame, and crankcase) under compression and transmits the firing load to the bedplate.

Tie rods are placed as close to the centreline of the crankshaft of the engine to minimise the bending movement in the transverse girder.

To understand the purpose of tie rods, it is important to know what is happening inside the cylinder of the engine. When the piston is just after the top dead centre (TDC), the pressure inside the cylinder can raise high upto 140 bar. This acts downwards through piston rod and connecting rod, pushing the crankshaft down into the bearing pockets. At the same time, the pressure acts upwards trying to lift the cylinder head.

The cylinder head studs screwed into the entablature prevent this from happening and so this upward acting force tries to lift the entablature from A-frame and the A-frame from the bedplate, putting the fitted bolts in tension. As the piston moves downwards, the pressure in the cylinder falls and then rises again as the piston changes direction and moves upwards on the compression stroke. This means that the fitted bolts are under cyclic stresses, which they are not designed to withstand. This would lead to disastrous consequences.

To hold the bedplate, A-frame and entablature firmly together in compression and to transmit the firing forces back to the bedplate, long tie bolts are fitted through these three components and tightened hydraulically. To prevent excessive bending moment in the transverse girders, the tie bolts are positioned as close as possible to the centre of the crankshaft. On some engines to achieve this, jack bolts are used to hold the crankshaft main bearing cap in position instead of the conventional studs and nuts. Operating the engine with loose tie bolts will cause the fitted bolts, used to hold the bedplate, A-frame and entablature together in alignment, to stretch and break. The machined mating surfaces will rub together, corrode and wear away (fretting). Once this has happened, the alignment of the engine running gear will be destroyed. Loose tie bolts will also cause bending of transverse girders of bedplate to bend, which could lead to cracking and misalignment of the main bearing.

Once fretting between the mating surfaces has occurred, then tightening of the tie bolts will pull the engine out of alignment. The crosshead guides, the cylinder liner and the stuffing box will no longer be in line and excessive wear will occur. Because the tie bolts will no longer be pulled down squarely, they will be subjected to forces which may lead them to breaking. If fretting has occurred, then the only solution is to remove the entablature and/or A-frame and machine the fretted mating surfaces (a very costly exercise).

Tie bolts can break in service. To reduce the risk of this happening, they must be checked for tightness (not overtightened). If a breakage does occur, the engine may be operated for a limited period on reduced load, without replacing the broken tie bolt.

Effect of loose or broken tie rod

Marine engine will show the following abnormalities when tie rod bolts are loosen or broken:

• It can lead to main engine vibration.
• It can lead to crankshaft misalignment.
• Bearing damage due to prolonged conditions.
• Foundation bolts and chocks may get loose.
• Due to vibrations in the engine, turbocharger can damage or misalign the rotor or damage the bearing.

What are the causes of the Tie Rod breaking?

• Tie rods are not properly tightened.
• Tie rod material and threading are of poor quality.
• Prolong use of tie rod can lead to breakage.
• Tie rods crossing its elasticity limits when tightened by hydraulic pressure.
• Overloaded engine or peak pressure of the cylinders are very high.
• Previous fretting of engine mating surface.
• If the ship faces very heavy weather, the fluctuation in the marine engine load may cause loosening or breakage of the rods.
• Foundation bolts have become loose or chocks are damaged leading to transmission of vibration, in to tie rods.
• Tie rods consist of quenching screws, if they are loose it will lead to heavy vibration leading to loose or crack in the rod.
• Scavenge fire loosen the rods as they pass from the scavenge space and the heat leads to the expansion of the rods.