Selection of right fasteners

In many cases fastener failure has caused severe accidents on ships; causing fire, flooding and even loss of life. On board, general fasteners shouldn’t be used in machineries, when overhauling a machinery, sometimes it is necessary to replace fasteners like, bolts, nuts and screws; however in urgency or due to ignorance it is not checked whether new fastener is of correct specification or not. Do we ever consider checking strength requirement of a fastener for particular use?

In single machinery there may be various grades of fasteners used depending upon the force and temperature it should withstand. Ordinary bolts without grade must not be used for pressure parts, high temperature parts or rotating or running parts, it would certainly fail due to poor strength. Grades are marked on bolts, nuts, and hexagon screws as shown in these pictures, if there is no marking then it should be assumed that the fastener grade is of ordinary mild steel and should not be used on machinery. Bolts received in general store without specifications are usually ordinary bolts and are only suitable for general purpose fastening. For machinery, minimum 4.6 metric grade should be used, for higher strength grade 8.8 to be used, grade 12.6 to be used on exhaust pipes or high temperature equipment. Grade 12.6 is usually the highest grade of fastener used on board.

Many accidents, breakdowns and machinery failure occur when improper fasteners are used. Therefore it is prudent to check proper grade of fasteners before replacing it. Below information is helpful when managing fasteners on board, specially for pressure vessel, pressure pipes, assembling 2 stroke or 4 stroke engines, fuel pumps, exhaust manifold parts, boiler parts, couplings, pumps and many more;

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What should be the torque when tightening fasteners?
Once fasteners are chosen correctly then the next precaution is to use correct tightening torque, below formula is basic for applying correct torque;

Torque = K x d x F
(Example – For M16, grade 8.8 bolt, in lubricated condition, K=0.15, torque recommended is 120 ft-lbs or 162 N-M)
K = Nut Factor, (Non-plated, black finish 0.20 – 0.30, Lubricated 0.12 – 0.16)
d = Nominal Bolt Diameter (in., mm, etc.)
F = Bolt Tension (lbs., N); F = 75% of bolt material proof load for standard bolts.

(Source: Fastenal, technical reference guide.)

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