Rotary hook for sewing machine

A bobbin assembly for a single needle lockstitch sewing machine having a rotary driving shaft;

• said assembly being engagable with a rotary hook (23);
• said rotary hook (23) being mountable one said shaft;
• said assembly comprising a bobbin holder (11) mounted on said hook (23) for rotational movement of said hook relative thereto and having a bottom wall (11a) and a bobbin (12) having a convexly curved outer end plate (12a) and a flat inner end plate (12b) seated within said holder (11) with said flat end plate (12b) being against said bottom wall 11a;
• (a) said holder (11) having a needle opening (19), an opening (18) for positioning said holder (11) on the sewing machine positioner (41), a relatively wide undergroove (16) spaced adjacent said opening (18), a beveled surface (20), a bobbin post (14) on which said bobbin (12) is mounted, and means (13) for applying force axially on a said bobbin so as to drive and maintain said flat bobbin end plate (12b) in frictional contact with said bottom wall (11a);
• a bobbin thread (30) passing without obstruction from said bobbin (12) to contact said undergroove (16) and pass therefrom outwardly of said holder (11).
• (b) said bobbin (12) being freely rotatable on said bobbin post (14);
• said convexly curved outer end plate (12a) directly contacting and supporting needle thread (40) when said needle thread (40) is looped around said holder (11);
• said bobbin being characterized as being self-threading within said assembly (11) when mounted on said post;
• whereby said needle thread (40) is wrapped aroung said bobbin holder (11), said beveled surface (20), and said curved bobbin plate (12a) to form a uniform stitch with the self-adjusting tension pull of the bobbin thread (30);
• the tension of said bobbin thread (30) being maintained constant by the force exerted by said force means (13) on said bobbin in conjunction with said relatively wide undergroove (16).