Thread Helix and how it effects torque values.

In my first post of Flange head nuts verses regular hex nuts, I mentioned that I will continue this informative series on fasteners to help to understand the mechanics of the fastener world.

Thread Helix, and its corresponding Helix angle, directly affect torque values in several ways.  Fine pitch threads have less helix angle, so it takes more turns to tighten a fastener.  The finer pitch threads also allow the minor diameter of a bolt to be bigger to allow a higher torque value, (tensile stress area).  Fine pitch fasteners have a greater holding power, then their course pitch counter parts.  So the main reason fine pitch fasteners have a higher torque values over course pitch fasteners, is strictly because their minor diameter is larger, (tensile stress area).  When you increase the diameter of the bolt, you are increasing the tensile stress area.

The larger diameter requires a higher torque value to properly ‘stretch’ the fastener for proper loading that is still in the ‘plastic’ region of the fastener material.  Over tightening a fastener stretches it into the ‘plastic’ region of the fastener material, which makes the fastener permanently longer, and failure prone after the torque value is reduced.  Proper loading of a fastener is dependent upon the material it is being used in, and the forces it is subjected to in its application.

Thread lubrication, or coatings, directly change the torque values to properly stretch the fastener.  Plating type, oils, waxes, or greases, directly change the torque required to properly stretch the fastener.  Bolt stretch gauges are one of the best ways to properly stress the fastener.

Also of interest is why a nut must be torqued to a slightly higher torque value then if tightening by a bolt head, in a bolt and nut combination.  A nut generally has a smaller thrust surface then the bolt. This smaller thrust surface resists in turning, as the pounds per square inch comes into play.  The thrust surface under the head of a bolt is generally larger, as to provide for an under the head ‘root radius’ for strength.  The nut has no ‘under head radius’.

Threads have three major applications.  They fasten things together, they transmit force, and or torque, and they are used to adjust.

Bob ‘Automan’ Ottow