This study considered the effects of needle penetrating force, needle thread tightening tension, and pressing force of the presser foot on torque produced in the arm shaft of an industrial sewing machine. The results obtained are as follows.
(1) Penetrating force, especially the initial triangular wave fluctuation, induces noticeable transient vibration in torque.
(2) The torsional natural frequency of the arm shaft of the feed mechanism motion system is more than twice that of the thread take-up lever and needle bar mechanism motion system. Penetrating force, according to the relations with the natural period of the system, is considerably more transient than pressing force.
(3) Consequently, when estimating torque, it is necessary that the arm shaft of the thread take-up lever and needle bar mechanism motion system be considered as elastic. On the other hand, the arm shaft of the feed mechanism motion system can be treated as a rigid body.