13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 3. Effect of pitch difference on anti-loosening performance Based on the torque-axial force relationship obtained above, the loosening experiment is performed to investigate pitch difference effect on the anti-loosening performance. The same specimens are used in the torque-axial force and loosening experiments for α=0, α=αsmall, α=αmiddle and α=αlarge. Two specimens are investigated together for the loosening experiment. The experimental device is impact-vibration testing machine based on NAS3350 (National Aerospace Standard) whose vibration frequency is 1,800 cycles per minute, and vibration acceleration is 20G. If the vibration cycles are over 30000, we may judge the anti-loosening performance is enough. The specimens are fixed on the experimental device under the same torque of 70 N·m before the vibration starts. The experiment device is shown in Figure 4. When the device starts vibrating, a counter connected with the experiment device shows the cycles of vibrations. Table 2 shows the cycles of vibrations when dropping happens and when loosening happens. Figure 4. Loosening experimental device based on NAS3350 For α=0 and α=αsmall, the nuts drops when the vibration number is nearly 1000 cycles. For α=αmiddle, the nuts does not drop before 30,000 cycles of vibration, but the loosening is observed for one specimen. Moreover, the axial force decreases to 20 kN in this case. For α=αlarge, neither dropping nor loosening happens, however, the clamping ability of the bolt and nut decreases significantly to F=8 kN, which is not acceptable for actual application. It is found that, on the one hand, when α is Table 2. Experimental result for anti-loosening performance Pitch difference α Sample Nut drop Cycles for dropping Cycles for start loosening Prevailing torque (N·m) Axial force (kN) No.1 751 - 0 No.2 876 - 0 24 No.3 813 - αsmall No.4 Yes 1528 - 0 24 No.5 30000 21000 αmiddle No.6 30000 30000 30 20 No.7 30000 30000 67 αlarge No.8 No 30000 30000 57 8 Vibration stroke 11mm Nut Bolt Impact stroke 19mm Mark line
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