The following guidelines are meant to help you identify pitfalls related to the tightening of bolted joints.
Use the right tool: Ensure that a calibrated torque tool is used and that a torque value is specified on the tightening specification. Be aware that certain automatic tightening tools, such as impact wrenches, can result in significant variations occurring in the torque value and the preload of bolts. Therefore, use a calibrated torque tool for the final tightening operation or inspection.
Specify the correct tightening torque: Whenever feasible, specify the tightening torque based upon actual test results rather than a theoretical value. Establish experimental determination of the tightening torque by measuring bolt extension and strain gauges or by using a load cell embedded in the joint.
Figure 1. Criss-cross tightening sequence.
Specify a tightening sequence: Most joints consist of more than one bolt and join surfaces that aren't completely flat. The sequence of bolt tightening can have a big influence on the resulting preloads. With such joints, specify the bolt tightening sequence. Because the joint surfaces compress, tightening one bolt in the vicinity of another will affect the preload generated by the first bolt tightened.
A good tightening sequence ensures that an even preload distribution is achieved in the joint (Figure 1). Since joints containing conventional gaskets have a comparatively low compressive stiffness, bolt preloads in such joints are particularly sensitive to the tightening sequence. Based on experience, if the bolts are in a circular pattern, a criss-cross sequence is normally specified. For non-circular bolt patterns, a spiral sequence starting at the middle is specified (Figure 2).
Figure 2. Spiral tightening sequence.
On critical joints, you may specify a pattern that tightens the bolts more than once to ensure an even preload distribution.
Caution with plain washers: Clearance between the bolt shank and washer hole can result in relative lateral motion. It can change the friction surface from nut and washer to washer and joint surface during tightening. This affects the torque-tension relationship and leads to large variations in preload. In some situations, such as to cover slots or to reduce the surface pressure under the bolt head, plain washers are specified. In such circumstances, ensure that they are of sufficient thickness and hardness and that they fit well to the bolt shank.
Flange-headed bolts: On relatively soft materials or when high-tensile bolts are used, consider using flange-headed bolts and nuts. Such fasteners reduce the surface pressure under the nut surface, reducing the amount of preload lost to embedding. Due to the larger-diameter bearing faces, a higher tightening torque is required because more torque is dissipated by friction.
Gaskets: Conventional gaskets are non-elastic; this results in a reduction in the bolt preload over time. This condition usually occurs shortly after installation and causes bolt relaxation. To reduce the effect of such problems, retighten the bolts after allowing a time to elapse after initial tightening.
Embedding: This plastic deformation occurs in the threads of the fastener and in the joint. It is caused by high stresses generated by the tightening process. This results in a loss of bolt extension and, hence, preload. Typically, preload loss due to embedding is around 10 percent. It increases with the number of joint surfaces being clamped and with the roughness of those surfaces. High surface pressures under the bolt head can also be a cause of excessive embedding. This can be due to the use of high-tensile fasteners in relatively soft materials. Hardened washers or the use of flanged fasteners can reduce such effects.
Exercise caution when using short bolts to clamp together several interfaces. In such joints, the small amount of bolt extension can be greatly reduced by the large amount of embedding which can be anticipated.
This article was provided by Mountz Inc., a manufacturer of torque tools. For more information, visit www.etorque.com.