An Angle on Torque

Friday, 1 March 2024
A bolt and nut is in effect a spring.  Obviously a bolt has to stretch to be utilized as a spring, and a very stiff spring at that.  However the difference between stretching the Bolt so that it can return to its original form (elastic stretch) and stretching it permanently (plastic stretch) is often the difference between a successful joint and a failed joint.
In a well designed joint the preload in the elastically stretched bolt, or sets of bolts, should exceed the external forces acting on the joint.   It therefore follows that a bolt must be tightened in a way that will give repeatable clamping force, and be evenly distributed around the joint if it has multiple bolts. 
So how do we achieve this consistency?
The most common method is to use a Torque Wrench.
When you use a Torque Wrench, you are in effect measuring the amount of friction generated by the mating threaded surfaces and the under head (or nut) surface area.  As the clamp load is increased so is the torque.  While there is a direct relationship between torque and the clamp load, the proportion of torque that actually goes into clamp load can vary widely.
Friction can be reduced with lubricants, and many have been tried from Bees’ wax to graphite anti-seizes and latterly with outstanding success the Lanotec range of Lanolin lubricants.  Unfortunately all fall short of the goal of absolute friction control.
Accuracy of torque control tightening is generally accepted to be plus or minus 25%.  Provided all bolts in a joint are from the same batch and similarly lubricated, torque control tightening will provide an acceptable level of consistency in clamp load, but the actual clamp load may vary up to 25% either way from that predicted. 
What can be done if we can’t control, this continuously variable thing called friction?
The simplest method is to use the fastener itself to obtain the required clamp load by rotating the bolt, or more commonly the nut, a given angle from a predictable threshold.  This threshold or starting point must be chosen at a low level of torque because the clamp load variation will be less than at a higher level where friction less than at a higher level where friction becomes more pronounced.   Tighten the fastener to a “snug” position, and then turn the nut a predetermined angle and the result is a predictable, repeatable, consistent and acceptable clamp load.
Because the fastener is being turned through an angle, friction and its effect are removed from the equation.  Remember the thread around a bolt is a continuous spiral and if one 360 degree turn is straightened out the result is an inclined plane that has the height of one pitch of the thread.
If follows that a half turn will stretch the bolt half thread pitch, regardless if friction.
Part Turn Tightening first can to prominence with the advent of High Strength Friction Grip Structural Bolts, when it was found that torque control tightening was inadequate due to friction variables at the clamp loads required.  In fact all bolting Standards specify that torque control cannot and must not be used.