| The friction phenomenon widely exists in all kinds of kinematic pair. The negative efficacy of friction has a great influence on the dynamic performance of mechanism, especially in a plane linkage mechanism consisting of a lower pair. There is a large deviation between the result of velocity fluctuation curve and the measured, when ignoring the friction in the kinematic pairs of this mechanism or simply calculating a smaller driving moment from a general transmission efficiency value instead of all frictions in every transmission links in the analysis of the offset crank slider mechanism dynamic performance. Therefore, it is necessary to analyze all frictions on the mechanism in every transmission links to distinguish their different negative effects of power on the mechanism.First, this paper discusses about the dynamics and structural analysis of the speed test bed system. For the unidirectional load offset crank slider mechanism drove by DC motor, we take the crank as equivalent components, list the equivalent kinetic equations according to the energy theorem of kinetic, solve these equations through numerical iterative method, and lastly obtain the crank velocity fluctuation curve at a variety of conditions without considering the driving efficiency and kinematic pair under friction.After that, this paper introduces the problem caused by numerical analysis and the experimental test results which cannot be compared directly because of that numerical analysis uses the crank angle to partition the calculation point while the experimental test uses timeline as its sampling period. Hence, we reasonably transform the axis of the angular velocity in experimental test system data from timeline to angle-line which represents the same results as numerical calculation. Also, the "zero" position in one period should be intercepted from the sampled data because the experimental test bed cannot use the crank limit position before the trip back as the starting point like numerical analysis does. The data processing and improvement above make it more conveniently and intuitively to compare theoretical curves with experimental curves.And then, the dynamic model of the ideal state was modified by adding the influence of friction factors in the numerical solution. The revolute pair frictions of link sides have a presented strong nonlinear feature because of velocity fluctuations on link sides and component location changes, et al. However, it greatly increases complexity of systems dynamical problem solving. Through the slider stress analysis and solution, we get the pressure of link spherical bearings sides and slide-rail, calculate all kinematic pair frictions(moment) according to the " Coulomb and Viscous " friction model and Coulomb friction model, and make equivalent to the crank, multi-rounds of numerical iterative solution crank speed fluctuation curve. Finally, by changing the model parameters, repeating numerical calculations and comparing with the measured curve, we identify the factor(vf andcf) of "Coulomb and Viscous" friction model and the factor(3f) of linear guide friction model.This paper improved the data acquisition and processing system of the speed test bed system. Based on the bed, we establish a dynamics model considering kinematic pair friction, explore a method for identifying friction parameter, and further improve the accuracy of the numerical solution of the speed test bed. |
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