Frishauf, Holtz, Goodman & Woodward
Coupling pin for sucker rod made of fiber-reinforced plastic material
 A sucker rod coupling pin comprises a hole into which the sucker rod is to be inserted, and has a plurality of annular depressions which surround the inner wall of the hole. As viewed crosswise, the annular depressions respectively have tapered surface portions arranged in series from the open end to the closed end of the coupling pin hole. The tapered surface portions whose cross sections progressively increase toward the closed end of the coupling pin hole have their respective lengths progressively shortened in relation to the preceding tapered portion, toward the closed end of the coupling pin hole, and the angle of inclination of the respective tapered surface portions progressively increases toward the closed end of the coupling pin hole. 
Nippon Kokan Kabushiki Kaisha
F F F E Y Y
F16B F16B F16B E21B Y10T Y10T
inventional inventional additional inventional additional additional
Toshio Masaru Atsuhiko Akihiko Takashi Masakazu
Iwasaki Nasu Murao Katoh Nakano Tsukada