A Theroretical Study On Mechanism Dynamics With Multi-clearances

With the maturity of modern machine manufacturing industry, the kinematicaccuracy of machines is highly required. However, the kinematic pair clearance whichis inevitable in real mechanical system is now heavily affecting the dynamicperformance of mechanical system. As the Kinematic pair has clearance, which leadsto the deviation of the position of the system, the instability of the dynamic state, andunnecessary collision, a deeper research of the mutual relationship between theclearance and the dynamic traits of mechanic system is extremely necessary.For now, in the field of the research on clearance of mechanic dynamic system,the researchers are searching for an effective and accurate way to do modelinganalysis on the clearance mechanic system. In the modeling analysis, there are stillsome problems to be solved. The estimate on the separated state of the kinematic pairclearance contact is still inaccurate; the research on clearance on mechanical systemwith the consideration of flexibility needs more work; the searches on the dynamictraits of multiple clearance of mechanical system are rarely discussed. Therefore, thecontent of the thesis, focusing on the above problems, are as follows.First and foremost, the thesis is based on the theory of the collision of twoobjects, and includes the collision issue in the studying theory of clearance kinematicpair. Taking the revolute pair as an example, seven aspects, like the description ofclearance kinematic pair, the kinematic modeling, the mechanic modeling, the contactand collision modeling, the friction modeling, the judging approaches of contact andthe clearance agency many-body dynamic equation, are focused to do comprehensiveanalysis on clearance of rotation.Secondly, the thesis combines the clearance of rotation model and the dynamicof mechanical model, constructs the model of clearance mechanism dynamics, andexplores on the target of offset slider-crank mechanism. At first, the thesis doesresearches on the dynamic traits of objects which only considers crank and connectingrod position of rotation clearance. Then, the thesis studies the relationship between thegap size and the dynamic characteristics. Last, the thesis does researches on thedynamic characteristics which considers the clearance of the elastic connecting rodoffset slider-crank. The results of these studies show the following conclusions: theexistence of clearance leads to hysteresis of the output displacement, the output speedand the accelerated speed vibrate dramatically, and the pairing elements produce inconsecutive high frequency oscillation contact force; with the increase of the size ofclearance, the indirect contact and collision rate between the kinematic pair clearancedecreases, but the impact force is growing; in the meantime, the research showsconsidering the connecting rod as flexibility has more stability compared with that inrigidity.Thirdly, the thesis works on the clearance from two aspects. On one hand, itcompares the dynamic characteristics of two different positions in the offsetslider-crank mechanism. On the other hand, it studies the dynamic characteristics ofthe objects considering the existence of multiple kinematic pair clearance in themeantime. The conclusions that can be drawn from the analyses are as follows. In theoffset slider-crank mechanism, the gap between the connecting rod and the slide blockposition of rotation has more influence on the output state compared to gap betweenthe crank and connecting rod position of rotation clearance. When multiple clearancesare considered, the contact and collision is tenser in the moving process of thekinematic pair, and has greater influence.