1.  Field of the Invention
The present invention relates to a bolt and a nut preventing seizure and biting in screwing the bolt into the nut. 
2.  Description of the Related Art
A bolt provided with a guide portion at the tip portion of a bolt shank has been known.  Such a bolt is appropriately adjusted an orientation of the bolt with respect to the nut and is able to screw the bolt into the nut even in a case where the bolt is inserted into the nut with their axes not being aligned with each other. 
As an example of the above-described bolt, a bolt disclosed in U.S. application Ser.  No.  6,120,227 has been known.  FIG.  9 illustrates the bolt.  In this conventional bolt, a constant diameter cylindrical part 4 with a diameter smaller than a bolt shank 2 is provided at the tip portion of the bolt shank 2 having a screw thread 1 formed thereon via a short tapered inclined part 3.  A complete screw thread 5 is formed on the constant diameter cylindrical part 4.  In the example illustrated in FIG.  9, an edge tapered part 6 is further provided at the other end of the constant diameter cylindrical part 4. 
In general, it is difficult to form a complete screw thread on the tapered inclined part 3 between the bolt shank 2 and the constant diameter cylindrical part 4.  Therefore, an incomplete screw thread 9 having a height lower than that of the complete screw thread is likely to form on the inclined part. 
As described in FIG.  9, an incomplete screw thread 10 is also formed in an opening of a female screw 7 of the nut.  The height of the incomplete screw thread 10 is lower than that of a screw thread 8 of the female screw 7 and the crest thereof is sharper than that of the screw thread 8. 
Therefore, when a bolt is inserted into a nut with their axes not being aligned with each other, the incomplete screw thread 9 of the bolt easily bites in the incomplete screw thread 10 of the nut as shown in FIG.  10, and then resulting in the occurrence of the seizure. 
An object of the present invention is to provide a bolt and a nut capable of appropriately adjusting the orientation of the bolt and then fastening the bolt and the nut without fastening defects such as slipping and seizure and the like even when the bolt is obliquely inserted into the nut. 
The bolt according to the present invention is a bolt comprising a head part; and a shank part extending from the head part,
wherein the shank part comprises a first constant diameter cylindrical part having a first diameter which is constant along the axial direction and having a screw thread with a predetermined pitch formed on the side surface thereof; a transition part having a tapered shape with a diameter gradually decreasing with distance from the head part in the axial direction, concentrically extending from the tip portion of the first constant diameter cylindrical part, and having a screw thread with a predetermined pitch on the side surface thereof; a cylindrical connection part having a diameter smaller than the first diameter in the axial direction and concentrically extending from the tip portion of the transition part; and a second constant diameter cylindrical part having a second diameter which is constant and smaller than the first diameter in the axial direction, concentrically extending from the tip portion of the cylindrical connection part, and having a screw thread with a predetermined pitch on the side surface thereof, and a crest of the screw thread formed on the side surface of the transition part is truncated. 
Since the crest of the thread screw formed on the side surface of the transition part is truncated, the biting between the screw thread of the nut and the screw thread formed on the transition part can be prevented even when the bolt is inserted into the nut with their axes not being aligned with each other.  Accordingly, it is possible to appropriately adjust an orientation of the bolt with respect to the nut.  In addition, since the orientation of the bolt can be appropriately adjusted, it is possible to easily screw the bolt.