In the formation of a double lock stitch, the thread carried by the sewing machine needle through the opening in the stitch plate is engaged below the stitch plate by a gripper which can, in some machines, be of the rotatable type. A double-lock-stitch sewing machine with rotating gripper generally comprises, with the gripper, a bobbin housing from which the bobbin feeds a looper thread or underthread for engagement with the thread carried downwardly by the needle to produce the double lock stitch.
In general, the bobbin housing is provided with a holding device adapted to prevent the bobbin housing from being frictionally entrained by the rotation of the gripper.
Such holding device can comprise a holding finger, on the one hand, and an abutment on the other hand. During each stitch formation in the conventional system, a throughgoing gap is opened between the retaining finger and the abutment to pass the upper thread loop. This gap formation can be effected by the thread itself, in which case, under the effect of thread tension, the bobbin housing is rotated slightly in the sense opposite the gripper rotation sense through a small angle. This results in the gap between the retaining finger and the abutment to clear the upper thread loop.
With rapidly operating industrial sewing machines, however, the force which is effective on the holding device is generally so great that the upper thread tension must be increased to an undesirable extent to bring about the aforementioned gap formation.
In one prior art double-lock-stitch sewing machine (see German patent application No. 14 39 214), the holding device for the bobbing housing comprises a two-part retaining finger with a rigid and a yieldable section. The yieldable portion of the retaining device is formed as a spring rod which is movable in the direction of withdrawal of the needle-thread loop and can be resiliently deformed by the thread to permit the aforementioned throughgoing gap to form.
Since the yieldable mass is small, a passage for the needle-thread loop is achieved without interfering with the formation of clean seam or undesirable increase in the thread tension.
However, the retaining device for the bobbin housing of this prior art teaching is suitable only for a gripper rotatable in a vertical plane and makes use of a two-part retaining finger which is fixed to the fabric-carrying plate and cooperates with abutments on the bobbin housing. Such a retaining device has been found to be unsatisfactory for grippers rotatable in a horizontal plane because of the different thread movements and the inability to mount the retaining finger and abutments in the aforedescribed manner.