[工学]Lecture4-4.ppt

4.4 Dimensional Synthesis (综合) Dimensional synthesis(综合) of a mechanism is the determination of the kinematic dimensions necessary to achieve(获得) the required motion. 4.4.1 Body Guidance(刚体导引) A linkage mechanism is to be design to guide a line segment on the coupler passing through some specified positions. Such a synthesis problem is called body guidance. 铸造车间大型翻砂机的翻箱机构 热处理炉门机构 (1) The fixed pivots(铰链) have not been determined A revolute four-bar mechanism ABCD is to be designed to guide a line segment(段) EF on the coupler passing through three specified positions E1F1, E2F2 and E3F3. Points B1 and C1 can be chosen arbitrarily(任意地) on the first position of the coupler. The shape of the quadrilateral(四边形) BCFE should remain the same in all positions. Constructing quadrilaterals B2C2F2E2 ?B1C1F1E1, we get points B2, and C2. Constructing quadrilaterals B3C3F3E3 ?B1C1F1E1, we get points B3 and C3. Since the locus(轨迹) of the point B relative to the frame is a circle the centre of which is the fixed pivot A, a circle is constructed passing through the three points B1, B2 and B3. Since the locus(轨迹) of the point B relative to the frame is a circle the centre of which is the fixed pivot A, a circle is constructed passing through the three points B1, B2 and B3. The centre of the circle is the fixed pivot A. Similarly, bisect(平分) C1C2 and C2C3. The intersection(交点) of the two bisectors(平分线) is the fixed pivot D. If the three points C1, C2, C3 locate on a straight line, the the rocker become a sliding block. The accuracy(精度) of the graphical methods by hands is insufficient(不够的). However, the accuracy of the graphical methods is good enough if AutoCAD is used. ABCD is to be designed to guide EF on the coupler BC through three specified positions. The points E and F are selected as the moving revolute centres B and C, respectively. The centre of a circle passing through the three points B1, B2 and B3 is the