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.