The present invention relates to a double end stud which is used in connecting two structural members to each other.
Drawbacks of conventional double end studs will be explained with specific reference to FIGS. 16 through 19. As will be seen from these Figures, a conventional double end stud has a nut 1 and externally threaded portions 2 integrally formed on both axial ends of the nut 1. Two portions 2 are threaded in opposite directions: namely, one has a clockwise screw thread, while the other has counter-clockwise screw thread. In use, the double end stud is placed between the opposing surfaces of two structural members A and B which are to be connected together, as shown in FIG. 17. As the nut 1 is rotated by a suitable tool, the threaded portions 2 are screwed into female screw members 3, 3 provided on the structural members A and B, so that the structural members A and B are pulled and connected together in a manner shown in FIG. 16.
For the purpose of firmly connecting two structural members A and B together, it is essential that the tightening margins of both threaded portions 2, 2 be determined to meet certain requirements with respect to the structural members. More specifically, referring to FIG. 17, the tightening margins, which are the axial distances to be travelled by respective structural members as the threaded portions 2 are driven, are represented by LA and LB, respectively. The tightening margins LA and LB have to match with the thicknesses tA and tB of both structural members and the effective lengths L.sub.1 and L.sub.2 of both threaded portion 2. For instance, when both threaded portions have an equal effective length, i.e., when the condition of L.sub.1 =L.sub.2 is met, it is necessary that the condition of LA +tA =LB +tB is met. If there is any difference between the thicknesses tA and tB, the connecting force is reduced by an amount corresponding to the difference in the thickness.
For strongly connecting both structural members by the double end stud, it is also necessary that the rotational phases of starting ends of the threads on both threaded portions 2 coincide with the starting ends of corresponding female screw threads on the structural members A and B.
FIG. 18 shows an example in which female screw members 3A and 3B on both structural members A and B have opposing surfaces which are to be contacted by adjacent axial end surfaces of the nut 1. In this case, it is necessary that the effective lengths L.sub.1 and L.sub.z of both threaded portions match with each other.
FIG. 19 shows another example in which one A of the two structural members is thin and backed up by a flat washer 4. In this case, one of the threaded shanks has to extend over a large thickness tA'. Since this thickness tA' is determined by the thicknesses of a plurality of members, error in the thickness size tends to occur with regard to the structural member A. Moreover, the effective lengths L.sub.1 and L.sub.2 of the threaded shanks 2 have to strictly match with the tightening margins L.sub.A ', L.sub.B in consideration of the difference between the thicknesses tA' and tB. Actually, however, it is almost impossible to meet such requirement.
For these reasons, conventional double end studs often fail to provide the desired strength of connection between two structural members.