CONSTRUCTION FIXINGS


Tightening torques explained By Mark Salmon, Independent Fixings Consultants


Mark Salmon, of Independent Fixing Consultants, is always banging on about the benefits of tightening a fixing to the manufacturer’s recommended torque because this will “induce a clamping force greater than the recommended tensile load so the fixture won’t move”. But does this mean it can also be used to test fixings? Mark explains in detail how torque relates to the holding power of a fixing in order to understand when we can and cannot use this relationship.


force through the fixture. As long as that clamping force is greater than the applied load the fixture will not move. With fixings like the one shown below – a throughbolt, but it could be any fixing not just one that is set by tightening - as we tighten it up the two forces shown below develop together. The tensile force in the bolt itself, shown by the red arrow, and an equal and opposite force going from the nut through the washer and fixture and into the base material - the clamping force.


B


efore we start talking about torque we need to remember what we are trying to achieve. With most fixings, where we are fixing something to a base material, what we need to do is to generate a clamping


backed up with practical tests, to determine the installation torque he wants to recommend. But before he determines the clamping force on which to


base this torque he will (or should, if he knows what he’s doing) take into account a phenomenon that affects all fixings. This phenomenon is called “Load Relaxation”.


As these forces increase so does the friction between the nut


and the washer and that between the threads of the nut and the bolt. This friction – this resistance to turning - is what the torque wrench is actually indicating.


So we have established that there is a direct relationship


between the torque and the clamping force we are looking for. There is in fact a formula which can be used to work out the bolt tension and hence clamping force induced by a particular torque for a bolt of a particular diameter and finish (oiled, dry zinc plated, galvanised etc - stainless is a bit tricky) but this relationship is relatively approximate and depends very much on the actual condition of the mating surfaces being clean and undamaged and, in the case of anchors, free from any oil or grease. The fixings manufacturer will use such a formula, maybe


High local stress induced at the time of installation will dissipate with time, relaxing the bolt load and clamping force in the process.


When any fixing is tightened stresses are induced in the base


material. In the case of an expansion anchor, like the one shown above, these stresses emanate from the point of expansion. Even


66 Fastener + Fixing Magazine • Issue 67 January 2011


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