TechArchive

Original article date: October 1999

There’s much more to torque control than turning up with an impact wrench, as Alan Quinn found out from MHH Engineering

When a bolt is tensioned correctly, it is working at optimum efficiency and will resist coming undone. However, if the tension is too low, the nut could vibrate or work loose. If the tension is too high (overstretched), it is possible for the bolt to break.

Every bolt has a correct optimum torque/tension figure for each fastening application. It is important to have these figures available, so that the end product will be safe, efficient and economical.

The correct tightening torque can be determined from formulae which relate the fastener details, the tensile stress in the threaded section, the preload and the tightening torque. This procedure is covered, together with worked examples in a free brochure available from MHH Engineering (Torqueleader).

The formulas used are applicable to metric and unified thread forms which have a thread flank angle of 60». The calculation procedure distinguishes thread and underhead friction, as well as differences which can be caused by bearing face diameter variations.

To determine the tensile stress in the fastener, it is first necessary to establish what proportion of the yield strength you wish the tightening process to utilise. Normally, a figure of 90% is acceptable, but this may be varied to suit the application. Because of the torque being applied to the threads, torsion reduces the tensile stress available to generate preload.

Upper and lower limits to friction values are normally stated. Traditionally, a mean value of friction is used when calculating the tightening torque and preload value. But if other conditions remain constant, the higher the value of friction, the higher is the required tightening torque and lower is the resulting preload.

To measure torque, ensure that a calibrated torque wrench is used and that a torque value is specified on the tightening specification. Certain automatic tightening tools, such as impact wrenches, can result in significant variations occurring in the torque value and a bolt’s preload. A calibrated torque wrench should therefore be used for the final tightening operation.

Whenever feasible, specify the tightening torque based upon actual test results, rather than a theoretically calculated value. Experimental determination of the tightening torque can be established by measurement of bolt extension, strain gauges or by the use of a load cell embedded in the joint.

The majority of joints consist of more than one bolt and bring together surfaces which are not completely flat. The sequence of tightening bolts influences on the resulting preloads. With such joints, specify the sequence in which bolts are to be tightened: tightening one bolt in the vicinity of another will affect the preload generated by the first bolt tightened.

Use caution when specifying plain washers: clearance between the bolt shank and the washer hole can result in relative lateral motion occurring. Conventional gaskets creep, too, resulting in a reduction of preload over time.

On relatively soft materials, or where high tensile bolts are used, consideration should be given to the use of flange-headed bolts and nuts to reduce the surface pressure under the nut surface and a form of plastic deformation called embedding.

Specifying torque values for fasteners

Generally, in the majority of applications, the reliability of a bolted joint is dependent upon the ability of the bolt to clamp the parts together. Adequate clamping prevents relative motion between parts of the joint and leakage from joints containing gaskets.

Measuring the clamp force of a bolt is difficult, especially under production assembly conditions. The clamp force generated by a bolt can be indirectly controlled by regulating the applied torque. This method, known as torque control, is by far the most popular. The initial clamp force generated by the bolt is frequently called preload.

There is a link between the torque applied to a bolt and the resulting preload. A problem exists, in that friction has a large influence on how much torque is converted into preload. Besides the torque required to stretch the bolt, torque is also required to overcome friction in the threads and under the nut face. Typically, only 10-15% of the torque is used to stretch the bolt. Of the remaining torque, typically 30% is dissipated in the threads and 50-55% under the nut face.

Because friction is such an important factor in the relationship between torque and preload, variations in friction have a significant influence on the bolt’s preload. Different bolt surface finishes have different friction values.

The torque required for a socket-headed screw will not be the same as that required for the same size standard hexagon bolt. The larger bearing face of the standard bolt will result in an increased torque being required, compared to a socket-headed screw. This is because more torque is being dissipated between the nut face and the joint surface.

•  MHH Engineering

October 1999