OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 Superposition of shear vibrations on the assembled threaded parts The experimental studies used an M8 bolt-nut pair of normal accuracy with a strength class of 5.8 as specimens. The standard torque for this joint size was 24.5 Nm. The experimental stand is shown in Fig. 1. In order to create shear polarization vibrations, an ultrasonic rod three-half-wave oscillatory system PMS 2.0/22 was used, consisting of a magnetostrictive transducer, a waveguide, and an emitter 1. The oscillatory system was powered by an ultrasonic generator UZG5-1,6/22. A device for fixing the nut was screwed to the end of the radiator through a stud, which consists of two brackets 2 and 3 with slots for a hexagon and has the ability to adjust the size. After installing and fixing the nut, the bolt 4 was screwed into it with a wrench 5. A dynamometer 6 was used to measure the torque, and a helical gear 7 was used to smoothly change it. When the ultrasonic generator is turned on, vibrations are transmitted to the nut, the direction of which is perpendicular to the axis of the threaded joint. According to this scheme, the main factor determining the nature of the thread assembly is the vibration amplitude ξm of the ultrasonic instrument surface, which is the end surface of the bracket. During the experiment, the amplitude values were set according to the readings of a millivoltmeter VZ-28B connected to an electrodynamic sensor 8, previously correlated with the readings of an hour-type indicator with a division price of 0.001 mm. The amplitude varied in the range of 1 to 9 µm. This rangewas chosen based on preliminary experiments because after exceeding 9 µm, the joint heats up above 50 °C, which in turn significantly affects the assembly conditions and polymerization of the adhesive composition. The resonant vibration frequency was f = 22,000 Hz. The study was carried out as follows: after tightening with a torque wrench to the standard level and checking the torque Tclose, the vibration source was switched on. Ultrasonic vibrations decreased the tightening torque ∆Tclose. Further, under vibration conditions, the joint was re-tightened to the standard level of Tclose, after which the vibrations were turned off. Then the joint was disassembled and the loosening torque Topen was measured without imposing ultrasonic vibrations. This torque was compared with the loosening torque of the test joint obtained without ultrasonic treatment – Tlwut. As a result, the effect of vibrations on the threaded joint was estimated by the relative loosening torque Tl, which was calculated using the equation: 100 % open lwut T T l T = × . (1) Five joints were assembled for each vibration mode. The axial force change during tightening was estimated by measuring the roughness parameters on the bearing surface of the nut, which affects the friction force on the bearing surface and the thread turns. The measurement was carried out using a model 130 profilometer, which is based on probing the surface with a Fig. 1. Experimental stand: 1 – ultrasonic emitter; 2, 3 – brackets of the fastening device; 4 – bolt; 5 – wrench; 6 – dynamometer; 7 – screw drive; 8 – electrodynamic sensor; 9 – millivoltmeter; 10 – ultrasonic generator
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