Tie rods or Tie bolts To understand the role played by tie rods, it is necessary to appreciate 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 rise as high as 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 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 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 over tightened). If a breakage does occur, the engine may be operated for a limited period on reduced load, without replacing the broken tie bolt.
On MAN-B&W MC-C engines, the tie bolts do not pass through the transverse girder in the traditional way. Instead there are two pairs of tie bolts fitted on either side of the single plate A-frame and screwed into the transverse girder of the bedplate. They claim that this reduces the distortion of the bedplate during running of the engine.
When checking of the tightness of the tie bolts, refer to the manufacturer’s instructions for tightening pressures of the hydraulic jacks and the order in which to carry out the check. The normal order is start at the centre and work outwards, checking the bolts in pair. The MC-C engines with twin bolts is an exception; starting forward and working towards after side. If the main bearing is fitted with jack bolts, then it must be slackened before tightening the tie bolts. Any pinch bolts fitted must also be slackened off.