OBRABOTKAMETALLOV Vol. 26 No. 3 2024 technology In this case, ultrasonic vibrations lead to two effects that contribute to the tightening of the threaded joint: 1. Occurrence of an additional mechanical force Fus arising from the vibration of the assembled element, which increases with increasing amplitude of vibrations; 2. Reduction of the friction force under conditions of ultrasonic vibrations due to its transformation into a quasi-viscous one (this effect is described in detail in [37–39]). The same factors can explain the absence of significant changes in the dependence of Tl(ξm) after reaching 5 µm. This amplitude value seems to be optimal for reducing friction for these experimental conditions. A further increase in amplitude leads to an increase in Fus and a simultaneous increase in Ffr. In addition, the factors affecting Tl will be the joint temperature, which increases due to heating with increasing amplitude of vibrations, and the friction force on the contact surface of the nut. Temperature measurements after the assembly of the joint showed that an increase in amplitude above 9 µm leads to heating to ≈ 50 °C, which will affect the conditions of polymerization of the adhesive and at the same time will not provide an increase in Tl. Therefore, studies at high amplitudes have not been conducted. The change in the friction force of the contact surface of the nut can be indirectly found by the amount of crushing the profile of the micro-dimensions when tightening the joint. For this purpose, roughness was measured on the support surface of a nut tightened without ultrasound and a nut tightened with imposing shear-type ultrasonic vibrations, as well as a nut before tightening. The main height and step roughness parameters are presented in Table 1; examples of profilograms obtained as a result of measurements are shown in Fig. 5. The results showed that when assembled with ultrasound, the surface underwent deformations and changes in microrelief more heavily than when assembled without ultrasound. So, if during conventional assembly, the altitude parameters decreased by 30 %, then during assembly with ultrasound, there was Ta b l e 1 Values of the roughness parameters of the nut bearing surface Parameter roughness Nut before tightening Nut tightened without vibra- tion Nut tightened with vibrations Ra, μm 0.47 0.32 0.28 Rz, μm 3.05 1.94 1.56 Rmax, μm 5.65 3.19 2.98 Sm, μm 47.30 63.91 72.83 tp, % 2.77 4.13 12.22 Fig. 5. Bolt thread profile after assembly with different Tclose
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