Flange Head verses Hex Head Fasteners

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In the nut and bolt world of fasteners, there are two distinct groups.  Hex headed, and Flange headed bolts and nuts.  Each group have different and varied applications.  Think of the flange headed as hex heads with a built in washer.  Flange headed nuts and bolts were developed in Europe and have slowly crept into the American manufacturing markets.  Early use of the flange headed fasteners were met with mixed results.  The problems were mostly in regard to proper torquing in the areas of clamping the head to the block of engines.  Early head gasket failures lead to the development of torquing to a certain number, then additional turning by degrees.  This was done to provide proper ‘stretch’ of the fastener in its application.

As it turns out the ‘register’, or land areas of the standard hex head bolt and nut have less under the head surface area then the flange headed bolt and nut.  This means if you torque a flange bolt or nut to standard SAE torque values for hex head bolts and nuts you will not achieve proper fastener ‘stretch, or loading’.  This ‘extra’ register area acts like a bigger ‘frictional clutch’, and resists turning to cause the torque wrench to indicate you have reached the torque you have set the torque wrench to.  This leaves the flange headed fastener not stretched to proper loading, which leads to failure of the clamped load of the intended assembly.

Think of the flanged fastener pre load torque value to the use of a Rockwell hardness tester.  You engage the surface with the penetrator, with a known amount of preload to position the penetrator through a rough finish or other issues with the surface you are testing.  Only then, is the final penetrating load applied to push the penetrator to a final depth, depending on the hardness.

You tighten the flange headed fastener to a torque value to take all play out of the assembly.  Then you turn the flange headed fastener an additional number of degrees to ‘stretch’ the fastener for proper loading.  Only in this way will you have a better ‘degree’, of assembly success.

Also of interest, is that you must use different torque values if you are torquing by the head of the bolt, or by torqueing the nut on the bolt.

I will address this little known area about fasteners in my next blog article……….Bob ‘Automan’ Ottow

One thought on “Flange Head verses Hex Head Fasteners

  1. Howard says:

    good job BOB

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