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

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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.

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• 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.

3

• 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.

4

• 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.

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• 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).

6

• 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.

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• On MAN-BW 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.

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• When checking of the tightness of the tie bolts,
  refer to the manufacturers 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.

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