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• Problems Tightening HSFG Assemblies

Problems Tightening HSFG Assemblies

Q: Why are the problems occasionally encountered in trying to obtain the specified direct tension indicator gaps when tightening HSFG assemblies?

The causes of the occasional problems encountered in trying to obtain the specified direct tension indicator gaps when tightening HSFG assemblies can be generally categorised under three headings:-

1) Fit – Ensure that the bolts fit the holes freely and that poor alignment of the holes is not causing the bolts to trap.

2) Tooling – Hand wrenches may be satisfactory for tightening smaller diameters of bolts, but power tools or torque multipliers must be used for sizes M22 diameter and larger. It is most important that impact wrenches have adequate capacity to tighten bolts within about 15 seconds.
Prolonged impacting can damage the bolt assembly to such an extent that the bolt may be fractured. In order to obtain optimum tool performance the tool manufacturer’s specification regarding air pressure, air flow volume and hose size must be followed. The only way to confirm that a particular tool is suitable is by judging its performance in bolt tightening under actual site conditions.

3) Additional Lubrication - During the tightening of HSFG bolts and nuts high frictional stress can develop in the assemblies, the likelihood of this occurring is greater when the bolts and nuts have one of the thicker sacrificial coatings i.e. galvanizing or sherardizing, but it can occur with zinc electroplating and occasionally with self colour products. In certain circumstances the frictional stress can lead to torsional failure of the bolts before the proof load is achieved. This high frictional stress can be reduced by the application of a suitable high pressure lubricant which should be applied both to the nut threads and nut washer face. In Cooper & Turner’s experience the most effective and economical lubricant for this purpose is tallow*.

The use of a high pressure lubricant in the way described has no detrimental effect whatsoever on the functioning of the installed HSFG assemblies. The lubricant simply allows the torque applied to the nut to be more efficiently transferred into bolt shank tension.

* In the UK, tallow is generally available from plumbers’ merchants. In parts of the world where tallow may not be available a product called ‘stick wax’, manufactured by a number of companies including Johnson and Castrol, is used for on site lubrication of fasteners, alternatively a grease with a high molybdenum content may be effective. The high pressure lubricant selected should be of medium to high viscosity and should be applied both to the nut threads and nut washer face in order to prevent the possibility of contaminating the inner faces of the HSFG joint.

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