Reclining device for vehicle seat

A reclining device for a vehicle seat, comprising:

• a first arm member for attachment to a frame structure of a seat cushion or a back rest of the vehicle seat, said first arm member being formed with an annular protruded portion concentric with the hinge center axis line;
• a second arm member for attachment to a frame structure of the other of the back rest or the seat cushion and connected with said first arm member for relative rotation about a hinge center axis line, said second arm member being formed with an annular recessed portion coupled with the annular protruded portion of said first arm member for relative rotation about the hinge center axis line and being formed with an arc shaped internal tooth portion that extends less than 180.degree. and is concentric with said hinge center axis line and located radially inside said annular recessed portion;
• a slide pawl slidably supported within said first arm member to be moved in a radial direction with respect to said hinge center axis line and having an arcuate external tooth portion engageable with the internal tooth portion of said second arm member;
• a release shaft rotatably supported on said first arm member coaxially with said hinge center axis line and being loaded by resilient means in a rotational direction;
• a release arm integral with one end of said release shaft to rotate said release shaft against the load of said resilient means;
• and
• interlocking means provided between said release shaft and said slide pawl to maintain engagement of said external tooth portion of said slide pawl with said internal tooth portion of said second arm member under the load of said resilient means, wherein
• said slide pawl and said interlocking means are disposed between said first and second arm members radially inside said internal tooth portion of said second arm member,
• said first arm member is formed with an arcuate internal tooth portion concentric with said hinge center axis line, the pitch diameter and profile of which are the same as those of the internal tooth portion of said second arm member, the internal tooth portion of said first arm member being arranged to be engageable with the external tooth portion of said slide pawl, and
• the profile and number of teeth of said external tooth portion are determined based on tangents of outer-most teeth surfaces of first and second teeth in said external tooth portion, said first and second teeth being opposed to one another relative to a longitudinal centerline of said first arm member and crossing a pitch circle defined by a pitch radius of said external tooth portion at a maximum angle whereby the tangents of said opposing first and second teeth are defined parallel to one another or converging toward one another relative to the crest side of said external tooth portion.