【正文】 ons. The decrease of the output speed is at the price of consuming the servomotor input power. In order to save energy, the output angular speed should avoid or reduce running in the decreasing region. The two drive schemes of the hybrid mechanism of the mechanical press According to eq. (3), the output angular velocity of the hybrid mechanism is equal to the reduced value of their sum of the angular velocities of both the conventional motor and the servomotor. As shown in Figure 1, the output angular velocity of the differential gear train is connected with the crank axis of the mechanical press, so the two axes have the same angular velocity. When either of the conventional motor or the servomotor is driven, or when both of them are driven, the different angular velocity of the crankshaft of the mechanical press can be obtained. Therefore, two kinds of the schemes of the hybrid mechanism of the mechanical press are proposed: (i) the conventional motor and the servomotor are simultaneously driven。 and the servomotor provides its adjustable speed. Therefore, in this way, it not only makes the output motion of the crankshaft of the mechanical press flexible, but also avoids using high power servomotor. Hence it could save both the machine manufacturing costs and the machine operating costs. Figure 1 Working principle of hybrid mechanism of differential gear train. 3 The velocity characteristics of the mechanical press slide The work of the mechanical press presents the regularity of the periodic change[12]. The displacement and velocity changes of the mechanical press slide in an ideal work circulation are illustrated in the Figure 2. The slide starts to move from top dead center to the working start point at high velocity (called quick feeding stage). When the slide of mechanical press approaches the working point, its high velocity is shifted to slow velocity and then it begins to stamp work piece at low velocity (called low working stage). The low velocity of the slide is to avoid great impact on the die, and benefit the plastic shaping of the work piece. After the slide finishes the stamping work and reaches the bottom dead center, the slide es back at high velocity and stops at the top dead center (called quick back stage). Hence, the motion velocity of the mechanical press slide can be divided three kinds: high down velocity V1 , slow working velocity V2 , and high back velocity V3 . The velocity V1 and velocity V3 should be as possible as high and the velocity V2 should be slow and flexible in order to ensure high working times of the mechanical press per minute and satisfy the requirements of different technologies. Actually the mechanical press only works in a very short course before the bottom dead center, and in the other courses it does not do work to the work piece. 4 The nomenclatures and equations of the hybrid mechanism The relationship of the angular velocity in all axes There are three external axes in the differential gear train as illustrated in Figure 1. In order to conveniently express the relationship of the three axes, the axis connected with the conventional motor is called axis 1, the axis connected with servomotor is called axis 2, and the axis connected with crankshaft is called axis 0. The angular velocities of the three axes are respectively expressed as n1 , n2 and n0 . The torques of the three axes are respectively expressed as M1 , M2 and M0 . Because there are two freedoms in the differential gear train, only the third axis is fixed, such that the drive ratio of the other two axes can be determined. So the relationships of drive ratio and angular velocity of the three axes need to be expressed as the character with superscript and subscript. Figure 2 Ideal displacement cure of the slide. The conventional motor’s influence on the output motion. When the servomotor stops, n2 = 0 , only the angular velocity of the conventional motor affects the output angular velocity n0 . The servomotor’s influence on the output motion. When the conventional motor stops, n1 = 0 , only the angular velocity of the servomotor affects the output angular velocity n0 , where 1 n0 represents axis 0 angular velocity, and 1 i20 represents total drive ratio from axis 2 to axis 0 including differential gear train and reducing unit II when axis 1 (servomotor axis) is fixed. The conventional motor’s and servomotor’s influence on the output motion. When the conventional motor and the servomotor run at the same time through the position of the differential gear train, the output velocity can be expressed as . Because the servomotor can run at arbitrary angular velocity between the zero and specific velocity in both positive and negative direct