塑性締めボルトの疲労強度 疲労強度と締付け回転角度の関係

Abstract

Tightening bolts in plastic regions, which can generate stable and high tightening force, has come to be used for joints of various machine parts. In this study, the fatigue strength of bolts tightened in plastic regions was investigated in its relation to the turning angle of the bolt or nut. The bolts used in fatigue tests were (A) shank-elongation bolts with a shank diameter (SD) of 7.2mm, (B) shank-elongation bolts (SD: 7.5mm) and (C) thread-elongation bolts (SD: 8.3mm). The fatigue tests were carried out for bolted joints and bolts alone. The former was tightened in elastic or plastic regions by varying the turning angle of the bolt or nut. The latter was chucked not only under no-turning but also under turning while loading axially till the initial mean load for simulating the actual tightening.The results obtained are as follows:(1) The endurance limit of the external load of the bolted joints in plastic regions was higher for shank-elongation bolts than for thread-elongation bolts, and that of the shank-elongation bolts with smaller shank diameter was higher. The reason is considered that the force generated in lower-rigidity bolts is smaller than that in high-rigidity bolts even if the external load is equal.(2) The endurance limit load of the bolts was higher for the bolts tightened to the turning angle (TA) of 90° in elastic regions than for bolts alone (no turning), and also higher for the bolts tightened (TA<180°) in plastic regions than in elastic regions. It is considered that these are caused by the increase in the uniformity rate of the load on the circumference of the thread ridge because of more uniform contact with scrape of the thread surface during tightening. These phenomena were similar to the results of the simulation tests for bolts alone.(3) However, the endurance limit loads of the bolted joint tightened to the turning angles of 270° and 360° in plastic regions were lower than that in elastic regions. In the simulation tests of the bolts alone, the endurance limit load reduced rapidly when the turning angle was over 180°, and it was almost constant between 180° and 1080°. The results of the bolted joints corresponded well with the above relation. These are thought to be because the load distribution of the first thread ridge increases due to the screw micro-pitch-error induced by high load.