何孔德设计双语版_第一章.ppt

AB2C2D Parallel-crank mechanism AB2C’2D AntiParallel-crank mechanism The centre lines of the four links can become collinear. At these positions, the output behavior may become indeterminate. These positions AB1C1D are called change-points. The change-points are handled by inertia of flywheel fixed with the driven crank or providing the duplicatelinkage 90? out of phase. Slider-Crank Mechanisms ----conditions for having a crank Right-angled Triangle △AC1E： AC1＞AE b-ae Conditions for having a crank: ba+e offset slider-crank mechanism ba centric slider-crank mechanism Right-angled Triangle △ AC2E： AC2＞AE a+be Example： (1) (2) Whichever link is the frame, the mechanism is double-rocker mechanism . If link AB is the frame. ——Double-crank mechanism If Link CD is the frame. ——Double-Rocker mechanism If link BC or AD is the frame. ——Crank-rocker mechanism (3) Satisfy the Grashof Criterion Double-crank mechanism Parallel-crank mechanism Antiparallel-crank mechanism The length of two cranks are the same 4.3.2 Pressure Angle α(压力角) and Transmission Angle γ(传动角) Neglect the friction force gravity force crank-rocker mechanism: input link: AB output link: CD The acute angle(锐角) between the directions of the force F and the velocity of the point receiving the force on the follower is defined as the pressure angle ? of the mecha- nism at that position. Transmission angle γ ：The complement（余角） of the pressure angle α is transmission angleγ. —— Only the tangential component Ft can create the output torque on the driven link CD ——The radial component Fr increases pivot friction and does not contribute to the output torque. For this reason, it is desirable that αis not too great or γis not too small. α change during motion. For smooth operation of any mechanism without jerky movements, the maximum value of should b