1. Field of the Invention
The present invention relates to a bolt set made of steel used for fastening structural bodies, members, and parts in civil engineering structures, building structures, and machinery (machine tools, automobiles, rolling stock, rolling mills, etc.) upon which repeated force (load) acts.
2. Description of the Related Art
In a screw joint, even when the external thread and the internal thread geometrically correctly fit with each other, the problem arises that the load distribution of the tensile stress acting upon the screw thread is largely biased to the tensile stress side and causes plastic deformation from the crests on the tensile stress load side and breaks the crests.
In order to solve this problem, as a screw joint improved in fatigue characteristic, Japanese Patent Publication No. 52-79163, and Japanese Patent Publication No. 52-131060, which represents Conventional Examples 1 and 2, respectively, disclose to shape an external thread to be engaged with an internal thread to become thinner toward the tip at the bolt head side in the direction of tensile force so as to reduce a contact area between the external thread and the internal thread toward the tensile direction and equalize the load applied on the crests and thereby achieve an improvement of the fatigue life. Japanese Patent Publication No. 53-29780, which represents Conventional Example 3, discloses to improve the shapes of the crests so as to relieve the stress concentration at the roots and thereby achieve an improvement of the fatigue life. Japanese Patent Publication No. 58-160613, which represents Conventional Example 4, discloses to make one or both of the crest flank angles of the internal thread to be engaged with the external thread smaller than the flank angle of the external thread to equalize the load borne by the crests and thereby achieve an improvement of the fatigue characteristic. Japanese Patent Publication No. 2003-4016, which represents Conventional Example 5, discloses a set of a high strength bolt, nut, and washer reduced in stress concentration occurring at the engaged end roots and improved in delayed breakage resistance characteristic and fatigue resistance characteristic by forming the shapes of the roots by a high order curve.
Further, Report on Survey and Research for Standardization Concerning Fastening Performance of High Strength Bolts (First Report), Japan Research Institute for Screw Threads and Fasteners, 1978, which represents Conventional Example 6, discloses that the fatigue strength can be improved if the pitch of the external thread is made smaller than the pitch of the internal thread and the pitch difference thereof is about 0.3%.
However, forming an external thread to be engaged with an internal thread to become thinner toward the tip in the direction of tensile force on the bolt head side as in Conventional Examples 1 and 2 has the problem that the production cost is high and the problem that the optimum engagement position of the nut is fixed, so there is almost no leeway left for a change of the fastening length. Further, improving the thread shape so as to relieve stress concentration of the roots as in Conventional Example 3 has the problem that the flank angle differs between a metric thread and unify thread, so the production cost becomes high. Making one or both of the thread flank angles of an internal thread to be engaged with an external thread smaller than the flank angle of the external thread to equalize the load borne by the crests as in Conventional Example 4 has the problem that sophisticated production technology is necessary and production control and quality control take enormous time, so the production cost swells. Forming the shape of the roots by a high order curve so as to reduce stress concentration occurring at the engaged end roots as in Conventional Example 5 has the problem that sophisticated production technology is necessary and production control and quality control take enormous time, so the production cost swells.
Further, making the pitch of the external thread smaller than the pitch of the internal thread and making the pitch difference thereof about 0.3% as in Conventional Example 6 has the problem that while there is an effect of improvement, but this extent of pitch difference is frequently absorbed by the tolerance between the external thread and the internal thread in an actual bolt set, so the obtained effect of improvement of fatigue strength is remarkably small in practical use.
The present invention has as its object to provide a bolt set made of steel solving the above problems of the prior art, free from technical difficulties in production, inexpensive in production cost, and improved in fatigue strength by a simple configuration.
According to a first aspect of the present invention, there is provided a bolt set made of steel wherein a tensile fluctuation stress acts upon an external thread side, characterized in that a pitch difference ΔP(=Pf−Pm) between a pitch Pm of an external thread and a pitch Pf of an internal thread is set within a range of 0.5-0.8% of a pitch P prescribed in JIS B 0205 of the Japan Industrial Standard (hereinafter referred to as the “JIS pitch P”).
A second aspect of the present invention provides a bolt set made of steel as set forth in the first aspect of the present invention, wherein either of the pitch Pm of said external thread or the pitch Pf of said internal thread coincides with the JIS pitch P.
A third aspect of the present invention provides a bolt set made of steel as set forth in the first or second aspect of the present invention, wherein the strength of said bolt set is higher than a strength class 12.9 prescribed in JIS B 1051.
A fourth aspect of the present invention provides a bolt set made of steel as set forth in any one of the first to third aspects of the present invention, wherein the external thread of said bolt set is shaped by rolling after heat treatment of the bolt.
According to the present invention, by the constitution of making the pitch difference ΔP(=Pf−Pm) between the pitch Pm of the external thread and the pitch Pf of the internal thread a range of 0.5-0.8% of the pitch P prescribed in JIS B0205 (hereinafter referred to as the “JIS pitch”), the load borne by the external thread when tensile force acts upon the bolt set made of steel is schematically equalized, whereby the occurrence of local stress concentration is prevented and the fatigue strength is much improved. This is also effective for improvement of the delayed breakage characteristic. Further, an adjustment of the fastening length is possible. If the pitch difference ΔP is larger than 0.8%, the engagement of the external thread and the internal thread becomes difficult, while if the pitch difference ΔP is smaller than 0.5%, a sufficient effect cannot be exhibited.
By the constitution of setting the pitch of either of the external thread or internal thread to the JIS pitch and changing the pitch of the other thread with respect to the JIS pitch by the pitch difference ΔP, the JIS pitch can be used, so the production cost can be reduced.
Production is possible just by changing the die, so production is not technically difficult and the production cost can be made inexpensive.
The invention can be applied to any type of thread, so the range of application is wide.
It is possible to apply the invention to both of anchor bolt sets and high strength bolt sets.
By rolling the bolt thread after the heat treatment, compression residual stress is imparted to the roots and the fatigue strength is improved.
FIGS. 1(a), 1(b), 1(c), and 1(d) are diagrams showing changes when tensile force acts upon a bolt set made of steel of the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
An embodiment of the present invention will be explained according to the drawings. FIGS. 1(a), 1(b), 1(c), and 1(d) show states of distribution of the load acting upon the crests when tensile force acts upon a bolt set made of steel of the present invention where the pitch Pm of the external thread is made smaller than the pitch Pf of the internal thread.
Step 1:
Note that the figure is drawn so that part of the crests of the external thread and part of the crests of the internal thread are superimposed on each other in order to visually express the magnitude of the contact pressure. The larger this superimposition, the larger the contact pressure shown. In actuality, however, the crests of the external thread and the crests of the internal thread are not superimposed on each other as in the figure. The same is true also for the following steps 2 to 4.
Step 2:
Step 3:
Step 4:
The state of the load borne by the screw threads of a bolt in the case of a general bolt set with no pitch difference and the case of a bolt set of the present invention having a pitch difference are shown below for bolt sets made of steel having a nominal size of M22. As the bolt material, a steel material having a yield point of 1300 N/mm2 and a plastic hardening rate of 0.5% was used. As the nut material, a steel material having a yield point of 900 N/mm2 and a plastic hardening rate of 0.5% was used. Further, for the shape of the external thread, the invention disclosed in Japanese Patent Publication (B) No. 6-89768 (high strength bolt, nut, and washer set) was employed.
Next, the states of stress of roots of screw threads of bolts in the case of a general bolt set with no pitch difference and the case of a bolt set of the present invention having a pitch difference are shown for bolt sets made of steel having a nominal size of M22.
Table 1 shows results of fatigue tests on bolt sets made of steel of the present invention and conventional bolt sets. The fatigue tests were executed by using servo jack test machines of 500 KN and 1000 KN, wherein the repetition load was a sine wave and the load rate was 300 times/min. In the table, the “JIS” in the column of the screw shape shows the metric thread. In the conventional bolt sets, both of the pitches of the external threads and internal threads are the JIS pitch. In the bolt sets of the present invention, the pitch difference ΔP is changed to three levels with respect to the two levels of the case where the pitch of the internal thread is fixed to the JIS pitch and the case where the pitch of the external thread is fixed to the JIS pitch P. Further, the pre-rolling was carried out by rolling the external thread before the heat treatment, and the post-rolling was carried out by rolling the external thread after heat treatment. The stress ratio is the ratio between the minimum stress degree and the maximum stress degree. The test results are values of all amplitudes. The test pieces in the shaded portions in the table show breakage in the lower portions of the bolt necks. The other pieces broke at the first screw thread of the nut.
In connection with Table 1, the following notes are pertinent: 1) the fatigue test method is executed by using servo jack test machines of 500 KN and 1000 KN; the repetition load was a sine wave, and the loading rate was 300×/min. 2) The basic set has both bolt and nut with JIS pitch P as pitch. 3) Pre rolling is rolling the external thread before heat treatment, and post rolling is rolling the external thread after heat treatment. 4) In the case of change of the external thread pitch, the internal thread pitch is made JIS pitch P. 5) In the case of change of the internal thread pitch, the external thread pitch is made JIS pitch P. 6) Stress ratio=minimum stress degree/maximum stress degree. 7) Test results are values of all amplitudes. 8) Test pieces in the shaded portions broke at the lower portion of the bolt neck, while the other pieces broke at the first screw thread in the nut.
It is learned from the fatigue test results shown in Table 1 that the bolt sets made of steel of the present invention exhibit effects of improvement of fatigue strength equivalent to that of the invention disclosed in Patent Document 1 and that the fatigue strength is much improved in comparison with the conventional bolt sets with no special threading. Note that, in the bolt sets made of steel of the present invention, since the improvements of the fatigue strength at the external threads were so great, in the fatigue tests, the fatigue strengths of the bolt sets made of steel were determined by the fatigue strengths of the lower portions of the bolt necks. Accordingly, if combining the shape of the lower portion of the neck of a bolt set made of steel of the present invention with for example a shape enabling stress concentration to be relieved as in the invention disclosed in Japanese Patent Publication (B) No. 6-89768, the fatigue strength is further improved.
The bolt set made of steel of the present invention can be applied to not only general metric thread, but also metric trapezoidal thread or pipe use parallel thread.
Further, the bolt set made of steel of the present invention can be applied to anchor bolts and stud bolts.
The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be recognized by one skilled in the art are intended to be included within the scope of the following claims.
Number | Date | Country | Kind |
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2004-082640 | Mar 2004 | JP | national |