04 Tie Rods or Tie Bolts

8/21/2019 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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04 Tie Rods or Tie Bolts