To understand the importance of the role played by the tie bolts or tie rods, it is necessary to appreciate what is happening inside the cylinder of the engine.
When the piston is just after top dead center the pressure inside the cylinder can rise as high as 140 bar (14000kN/m2). This acts downwards through the piston rod and con-rod, pushing the crankshaft down into the bearing pockets. At the same time, the pressure acts upwards, trying to lift the cylinder cover. The cylinder head studs screwed into the entablature prevent this happening and so this upward acting force tries to lift the entablature from the frames and the frames from the bed-plate, putting the fitted location bolts into tension.
As the piston moves down the cylinder 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 are cyclic stress. Because they are not designed to withstand such stresses they would soon fail with disastrous consequences.
To hold the bedplate , frames 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 then tightened hydraulically. To prevent excessive bending moments in the transverse girders, the tie bolts are positioned as close to the centre of the crankshaft as possible. Because the tie bolts are so close to the crankshaft, some engines employ jack bolts to hold the crankshaft main bearing cap in position instead of conventional studs and nuts.
Operating the engine with loose tiebolts will cause the fitted bolts holding the bedplate, frame and entablature in alignment to stretch and break. The machined mating surfaces will rub together, corrode and wear away (this is known as fretting). Once this has happened the alignment of the engine running gear will be destroyed. Loose tie bolts will also cause the transverse girders to bend which could lead to cracking, and main bearing misalignment.
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 subject to forces which may lead to them breaking. If fretting has occurred, then the only solution is to remove the entablature or/and 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; and the engine not overloaded. If a breakage does occur, this is not disastrous, as the engine can be operated with care for a limited period (the load on the engine may have to be reduced). The position of the fracture will dictate how the broken pieces are removed. However in the worst possible scenario where the bolt is broken at mid length, then one solution is to lift out the top half,remove the bottom nut, and then feed a loop of braided wire cable (about 7mm diameter) down the tie bolt tube, down the side of the broken tie bolt and once it emerges at the bottom a supporting piece can be fitted to the wire enabling the broken tie bolt to be withdrawn.
On the MAN B&W MC-C engine the tie bolts do not pass through the bedplate transverse girder in the traditional way. Instead there are two pairs of tie bolts fitted either side of the single plate A frame and screwed into the bedplate transverse girder. This, it is claimed, reduces the distortion of the bedplate during engine operation.
When checking the tightness of tie bolts, refer to manufacturers instructions for tightening pressures for the jacks and the order in which to carry out the check. The normal order is to start at the centre and work outwards checking the bolts in pairs. The MC -C engine with its twin tie bolts is an exception, starting at the fwd end and working aft. If the engine is fitted with bearing jacking bolts, then these must be slackened before tightening the tie bolts. Any pinch bolts fitted must also be slackened off.