Abstract: The loom shedding mechanism is designed to achieve the heald frame motion that includes three periods of opening, pause and closing. To ensure the easiest passage of weft carrier, the pause period is an important condition for most looms' shed-forming device. However, due to the lack of pause in heald frame motion, the traditional linkage shedding mechanisms have become difficult to adapt to the technological requirements for modern high-speed and wide-width looms, and cam shedding mechanisms are becoming more widely used.
This study introduces the concept of special coupler curve guidance and applies the principle that coupler curves with Ball's point or circling-point can make the follower pause to improve the previous design approaches of shedding mechanism. Specifically, because the coupler curves with Ball's point or circling-point have long approximate straight-lines or circular arcs, they can enable the heald frame as a follower to perform linear reciprocating movement with dwells at the limit positions by reasonable design.
In this paper, we proposed two novel linkage shedding mechanisms guided by the symmetric coupler curves with two Ball's points and two circling-points (hereafter called shedding mechanism I and shedding mechanism II). The symmetric coupler curve with two Ball's points was obtained by a graphical method based on the inverted inflexion circles, while the symmetric coupler curve with double circling points and equal curvature ratios was obtained by the analytic method based on Euler-Savary's theory of curvature of point trajectory. In shedding mechanisms I and II, the length ratios of the crank, coupler, rocker and frame were 1:2:2:2.76 and 1:2:2:2.3, with the position-angles of the coupler-points of 62.49°and 49.15°, respectively, and the lengths of the connecting rods adjacent to the rockers were equal to that of the couplers. Shedding mechanism I was connected to the frame's pushing rod to directly drive the heald frame, and shedding mechanism II drives the heald frame through a composite component composed of the connecting rod, swing-rod and frame's pushing rod.
The results of the heald frame motion show that: (1) both shedding mechanism I and II can make the heald frames pause at two limit positions, with dwell times of around 150° as well as 40°and 200° (loom's main shaft angle), respectively, indicating that there is a long shed dwell time; (2) the velocities and accelerations of the heald frames at two limit positions are both 0, indicating that the motions of the heald frames are stable, without flexible impact.
In conclusion, the proposed new linkage-type shedding mechanisms meet the requirements of loom shedding motion. Our study fully demonstrates application prospects of coupler curve guidance mechanisms in textile machinery, and the replacement of high pair with low pair will create conditions for the development of low-cost looms.

Key words: loom, shedding mechanism, four-bar linkage mechanism, coupler curve, shed

摘要： 传统连杆开口机构难以使梭口保持停歇，无法适应高速、宽幅的现代新型织机的工艺要求。引入特殊连杆曲线导引的概念，利用带鲍尔点和带圆点连杆曲线能使从动件产生停歇的原理，设计了两种新型连杆开口机构。该机构由四连杆机构和综框导引组件组成；四连杆机构的曲柄、连杆、摇杆和机架杆杆长比分别为1∶2∶2∶2.76和1∶2∶2∶2.3，连杆点的位置角分别为62.49°和49.15°，其与摇杆相连的连接杆与连杆等长。综框运动规律显示：梭口满开时主轴的相对静止角分别为约150°和40°、200°；综框处于最高和最低位时速度和加速度均为0，表明综框运动平稳，无冲击。该结构突破了传统连杆开口机构的局限，为低成本新型织机的开发创造了条件。

关键词: 织机, 开口机构, 四连杆机构, 连杆曲线, 梭口