04 Tie Rods or Tie Bolts
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Transcript of 04 Tie Rods or Tie Bolts
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To understand the role played by tierods, it is necessary to appreciate whatis happening inside the cylinder of the
engine. When the piston is just afterthe top dead centre (TDC), thepressure inside the cylinder can rise ashigh as 14 bar. This acts downwardsthrough piston rod and connecting rod,
pushing the cran!shaft down into thebearing poc!ets. "t the sa#e ti#e, thepressure acts upwards trying to lift the
cylinder head.
Tie rods or Tie bolts
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The cylinder head studs screwed into the
entablature pre$ent this happening and so thisupward acting force tries to lift the entablature
fro# "%fra#e and the "%fra#e fro# the bedplate,
putting the fitted bolts in tension. "s the piston
#o$es downwards, the pressure in the cylinderfalls, and then rises again as the piston changes
direction and #o$es upwards on the
co#pression stro!e. This #eans that the fitted
bolts are under cyclic stresses, which they arenot designed to withstand. This would lead to
disastrous conse&uences.
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' To hold the bedplate, "%fra#e and entablature
fir#ly together in co#pression and to trans#it
the firing forces bac! to the bedplate, long tiebolts are fitted through these three co#ponents
and tightened hydraulically. To pre$ent ecessi$e
bending #o#ent in the trans$erse girders, the
tie bolts are positioned as close as possible tothe centre of the cran!shaft. n so#e engines to
achie$e this, jac! bolts are used to hold the
cran!shaft #ain bearing cap in position instead
of the con$entional studs and nuts.
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'perating the engine with loose tie bolts willcause the fitted bolts, used to hold the bedplate ,
"%fra#e and entablature together in align#ent,
to stretch and brea!. The #achined #ating
surfaces will rub together, corrode and wearaway (fretting). nce this has happened, the
align#ent of the engine running gear will be
destroyed. *oose tie bolts will also cause
bending of trans$erse girders of bedplate tobend, which could lead to crac!ing and
#isalign#ent of the #ain bearing.
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nce fretting between the #ating surfaces has
occurred, then tightening of the tie bolts will pull
the engine out of align#ent. The crosshead
guides, the cylinder liner and the stuffing bo will
no longer be in line and ecessi$e wear will
occur. +ecause the tie bolts will no longer bepulled down s&uarely, they will be subjected to
forces which #ay lead the# to brea!ing. f
fretting has occurred, then only solution is to
re#o$e the entablature and-or "%fra#e and#achine the fretted #ating surfaces (a $ery
costly eercise).
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' Tie bolts can brea! in ser$ice. To reducethe ris! of this happening, they #ust be
chec!ed for tightness (not o$er tightened).
f a brea!age does occur, the engine #aybe operated for a li#ited period on
reduced load, without replacing the bro!en
tie bolt.
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' n "/%+0W C%C engines, the tie boltsdo not pass through the trans$erse girder
in the traditional way. nstead there are
two pairs of tie bolts fitted on either side
of the single plate "%fra#e and screwed
into the trans$erse girder of the bedplate.
They clai# that this reduces the distortion
of the bedplate during running of theengine.
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' When chec!ing of the tightness of the tie bolts,
refer to the #anufacturers instructions fortightening pressures of the hydraulic jac!s and
the order in which to carry out the chec!. The
nor#al order is start at the centre and wor!
outwards, chec!ing the bolts in pair. The C%Cengines with twin bolts is an eception2 starting
forward and wor!ing towards after side. f the
#ain bearing is fitted with jac! bolts, then it #ust
be slac!ened before tightening the tie bolts. "nypinch bolts fitted #ust also be slac!ened off.
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