Bolt Torque Effect

Since there be STX and other plastic components that are bolted to other components, it is important to consider the effects of the bolt torque, namely the compressive and shear stresses under the head of the bolt. To retain the maximum torque, the following parameters must be considered (1) STX material, when loaded will tend to relax (long term effects) and (2) based on the fastener requirements, there is a minimum torque suggested to stretch the bolt (bolt preload). 

When the torque drops below this minimum value, the fastener tends to loosen. If the STX part is a vessel for containment of fluids, leakage could result at the flanges even with a gasket. The load due to the torque is computed by 

This load (F) is then used to calculate the compressive and shear stress as shown by the following equations respectively  

These equations show that the use of a flat washer to distribute the load and reduce the respective stresses is recommended (permitted design = 7 000 psi, and permitted Sj design = 6 000 psi). To illustrate the torque and relaxation relationship, see Fig. 4.64. The ordinate represents the ratio of original stress to the actual or time dependent stress and the abscissa is the time cycle. The relaxation curve shows that there is no difference in relaxation between stress levels of 5 000 psi and 1 5 000 psi and that most of the relaxation takes place during the first twenty hours. 

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Croccolo, D., De Agostinis, M., Vincenzi, N., 2011. Failure analysis of bolted joints effect of fiiction coefficients in torque-preloading relationship. Eng. Fail. Anal. 18, 364-373. 

Cooper C and Turvey G J (1995), Effects of joint geometry and bolt torque on the structural performance of single bolt tension joints in pultruded GRP sheet material. Composite Structures, Vol. 32, Nos. 1, pp. 217-226. 

Several publications have described the use of molybdenum disulphide pastes for the lubrication of screw threads. This is an application in which the paste assists assembly and subsequently acts as an anti-seize. The critical factors in screw or bolt assembly are the friction between male and female thread and between bolt-head and workpiece, and their effect on the relationship between assembly torque and bolt tension (clamping force). The stiffness and integrity of a bolted assembly are determined by the bolt tensions, but since it is impracticable to measure bolt tension directly, the required bolt tension is controlled by the torque applied to the bolt or nut. However, the relationship between assembly torque and bolt tension is determined by head friction and thread friction. If the friction is reduced, the required bolt tension will be obtained at a lower assembly torque. 

To illustrate their effectiveness as anaerobic adhesives several combinations of the adduct, with or without added filler, reactive diluents, or peroxide, were screened using nut/bolt combinations. The results in terms of breakaway torque are listed in Table 2. The strengths obtained, although none of the systems were optimized, demonstrated the use of the methacrylated polyester compounds as anaerobic curing or thermal curing adhesives. 

Figure 2.23 The bolt tension generates friction which is manifest in the torque required to remove the nut (Figure 2.22). This is additional to the combined effects of adhesion and crushing load induced by anaerobic adhesives (Figure 2.21). Here, with the two sets of curves combined, it may be seen that very high torques can be required to initiate movement.

Problems can arise if thread friction is unknown - for example when the bolt has been cleaned or coated with an aerobic adhesive. In an extreme situation the use of an adhesive with a sufficiently different lubricating effect to machine oil could result in a given torque generating insufficient clamping force. The clamped parts might move under severe vibration even though the fastener itself would not loosen. 

Specially developed lubricated grades of anaerobic adhesive counter this and the effect of one such is shown in Figure 2.28. Coating both oiled and degreased bolts with this particular composition gives the same sort of torque/tension relationship as an oiled as received fastener. 

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