| Planar linkage mechanisms are used very widely in engineering and can also be regarded as the theoretical structure model of all other basic mechanisms, thus they have been gained attention by domestic and overseas researchers and engineers all the time. Nevertheless, the widely existed overconstraints in the mechanisms will make the quality of the mechanisms very sensitive to the inevitable manufacturing, assembly errors (especially, the errors resulted from the shape and position between kinematic pair elements and their axes). At the same time, because the internal and external uncertain factors such as non-plane characteristic of dynamic parameters of links (such as mass distribution, inertia forces, etc.), bending and twist of links, and the changes of environment and operating condition are inevitable, they will form the disturbance to planar constraints and make them possess the characteristic of "uncertain". The uncertainty of planar constraints causes overconstraints, which have no influences in theory on smooth work of a machine with planar linkages, to form obstruction in fact and bring about a series of pernicious influences on the performance of the machines, especially on their dynamic behavior and the quality.In order to make the mechanisms possess high efficiency, high stability, and low cost, a thorough study for constraints and overconstraints in the linkage mechanisms, self-adjusting mechanism and self-adjusting structures of the mechanisms, the degree of the influences of errors in different kinematic pairs on performance of the mechanisms, and the theory and method of designing for the mechanisms without overconsstaint was carried out in this dissertation.On the basis of the generalized constraints and on principle of designing and controlling the constraints advantageously, the conceptions such as basic constraint, statically constraint, and over-statically constraint were put forward for the first time. With this understanding, the common constraints in mechanisms and mechanical systems were classified. The uniform definition of the overconstraints was presented. According to the differences of mechanism of forming overconstraints, the overconstraints in closed chain mechanisms were divided into theoretical-plane overconstraint and special-plane overconstraint. A method, which is used to analyze for overconstraints of mechanisms with theoretical-plane overconstraint and to eliminate the overconstraints in the mechanisms, was proposed for the first time. Thecharacteristics and number of overconstraints in these mechanisms, and the self-adjusting mechanisms without overconstraint were gained.The mechanisms and conditions for the mechanisms to adjust itself were analyzed in detail. An accurate method of analyzing for the properties of the mechanisms without overconstraints was given for the first time, we obtained the equations of self-adjusting displacement in the cylindrical pair, the allowable kinematic-pair-axis errors which enable the mechanisms realize function of self-adjustment, and the most rational arrangements of kinematic pairs for the three common four-bar mechanisms. The results of analysis show that the influences of the errors of crank-pin on the self-adjusting properties of the mechanisms are bigger than that of the other pairs.A method of describing and analyzing for the degree of influences (DOI) of the kinematic-pair-axis errors on the performance of a mechanism was presented for the first time, and results of DOI of the different errors from the different kinematic pairs were obtained. The results of analysis show that: 1) DOI of the pin-axis errors, especially the crank-pin-axis errors, is bigger than other axis errors; 2) From the calculating results of DOI of the errors of the different kinematic pairs, DOI of rotate-completely pairs is bigger than that of oscillate pairs; 3) The forced coupler-deformations resulted from the constraints out of motion plane consist of twisting and bending deformation, and the deformations are variat...
|
PDF Full Text Request