LECTURE4-3
57页1、4.3 Characteristics(特性) Analysis of Four-bar Linkages,through belt mechanism or gears. Therefore a designer must ensure(确保) that one side link is a crank.,4.3.1 Grashof Criterion格拉霍夫准则 (Condition for having a crank),In a revolute four-bar mechanism, the input motion is usually obtained through a side link driven by an electric motor directly or indirectly,Suppose we wish to design a crank-rocker mechanism ABCD, in which the side link AB is an input crank.,in BCD must obey (服从) thetriangle inequa
2、lity(不等式),If the RRR Assur group can be assembled(装配) onto the basic mechanism by the two outer revolutes B and D, the lengths of the three sides,-(4-1),The distance f is a variable(可变的) value during the motion of the mechanism.,(4-2),(4-3),(4-2),Again, from the inequalities (4-2), we can conclude that the sum of the shortest and the longest links must be less than the sum of the remaining(剩余) two links. This is called Grashof criterion(准则) or the Condition for having a crank.,Thus from the ineq
3、ualities (4-3), we can see that the crank in a crank-rocker mechanism must be the shortest link.,(4-3),The Grashof criterion can be expressed as: LMAX +LMIN Lb +Lc, the linkage mechanism is an non-Grashof linkage mechanism, in which no link can rotate through 360o relative to any other link and all inversions(倒置) are double-rocker mechanisms.,In a non-Grashof linkage mechanism, no link can rotate through 360o relative to any other link.,However, in a Grashof double-rocker mechanism, the coupler
4、can rotate 360o with respect to other links.,If LMAX +LMIN =Lb +Lc, the centre lines of the four links can become collinear(共线).,At these positions, the output behavior may become indeterminate(不确定的). These positions are called change-points. Such linkage mechanisms are called change-point mechanisms.,The configuration AB2C2D is called a parallel-crank mechanism, while the configuration AB2C2D is called an antiparallel-crank mechanism.,In Fig.4-22, the change-points are handled by providing the
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