To ensure a fastener performs in its application as the engineer intended, it must be adequately tensioned. As a general rule, the joint will have been designed with sufficient numbers and sizes of fasteners to apply the required clamp load at 65% of the fastener proof load; i.e. well below the fasteners yield point. (Note: gasket or soft joint components significantly alter this). To achieve a minimum pre-load in the fastener of 65% of proof load, the fastener needs to be stretched by tightening. This can be done by various methods, each with varying degrees of accuracy, certainty and cost. eg: The commonly accepted relationships are shown in the following chart. Operator Judgement, tightening by feel, is the most common tensioning method for non-engineered and DIY type applications. It is generally satisfactory in these non- critical joints where loads are static and not subject to vibration; however, it is prone to significant under and over tightening by inexperienced operators. Torque wrenches are by far the most common tensioning method for engineered joints because of low cost and simplicity, but at + or - 25%, they lack accuracy. Approximately 85-90% of the torquing effort is used to combat the frictional forces in threads and mating surfaces of the bearing and rotating units; (stainless components can be even higher). Any reduction in friction will have a marked affect on the induced tension; ie. a 10% reduction could increase tension 80-90%. Lubrication, thread fit, tightening speed, surface finish or plating, all have some effect on the friction generated. Close attention to these factors and to torque wrench calibration can improve accuracy. The minimum lubrication required would be light oiling. The residue on plain finish mild steel and high tensile bolts is usually sufficient, but all plated products should be oiled and stainless steel products can benefit from a high quality solid type lubricant such as molybdenum disulphade. Tightening torque figures to achieve 65% or proof load are shown in the James Glen Technical Catalogue for mild steel, high tensile and stainless grades. Turn of nut is commonly used in structural bolting, but requires marking of the various components to verify the degree of turn achieved from 'snug tight'.   Time consuming, but does provide some evidence for subsequent inspection, as do load indicator washers - slightly more expensive for slightly more accuracy and permanent evidence. Fastener elongation involving direct measurement of the degree of stretch along with strain gauges attached to the bolt shank, give excellent accuracy, but would only be justified economically in the most critical of circumstances. The following chart pictorially demonstrates the typical tension/elongation relationship, the various zones of elongation and points of tension. Elastic Elongation: elongation from which the fastener will recover when load is removed. Plastic Elongation: elongation which is permanent and renders the fastener non-reusable. Necking Elongation: elongation past the tensile strength of the fastener from where the diameter is reducing, the tension is decreasing and fracture results. Minimum Tension: the minimum tension used for design purposes = 65-70% of proof load and is the theoretical minimum tension the recommended tightening torque should achieve. Proof Load: the minimum point prior to permanent elongation and the test point for actual proof load testing. Yield Point: the point at which elasticity is lost and permanent elongation commences. Tensile Strength: the maximum load-carrying point prior to fracture.     Click here to download the complete Training Manual. (588kb)   You will need Adobe Acrobat to view this file. Click the logo for a free download