Bottom thread take-up compensation device of crank arm type sewing machine

A bottom thread take-up compensation device of a crank arm type sewing machine comprises a main body, a spring positioning piece, a torsion spring and a stud, a pair of spaced threading holes and a stud penetrating hole are formed in the main body, and a bottom thread can penetrate through the threading holes; a stud through hole and a torsional spring positioning hole are formed in the spring positioning sheet; the torsional spring comprises an elastic arm, a torsional spring ferrule and a positioning arm inserted into the torsional spring positioning hole of the spring positioning piece; the stud sleeves the torsion spring ferrule, the stud through hole of the spring positioning sheet and the stud through hole of the main body and then fixes the torsion spring ferrule, the spring positioning sheet and the main body on a shell of the sewing machine; and the elastic arm is positioned between the pair of threading holes of the main body. The bottom line sewing device is more reasonable in structure, stability