Design of shedding mechanisms guided by coupler curves
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 two circling-points where the radii of curvature are equal 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 are 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 rods connected to the rockers are equal to that of the couplers.Additionally,in shedding mechanism Ⅰ,the coupler-point directly drives the heald frame,while in shedding mechanism Ⅱ,the heald frame is driven by a composite component composed of the connecting rod,swing-rod and 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.
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