外文翻译---机械运动和动力学

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1、中文 4615字 附录 附录一 英文资料 Kinematics and dynamics of machinery One princple aim of kinemarics is to creat the designed motions of the subject mechanical parts and then mathematically compute the positions, velocities ,and accelerations ,which those motions will creat on the parts. Since ,for most earthbo

2、und mechanical systems ,the mass remains essentially constant with time,defining the accelerations as a function of time then also defines the dynamic forces as a function of time. Stress,in turn, will be a function of both applied and inerials forces . since engineering design is charged with creat

3、ing systems which will not fail during their expected service life,the goal is to keep stresses within acceptable limits for the materials chosen and the environmental conditions encountered. This obvisely requies that all system forces be defined and kept within desired limits. In mechinery , the l

4、argest forces encountered are often those due to the dynamics of the machine itself. These dynamic forces are proportional to acceletation, which brings us back to kinematics ,the foundation of mechanical design. Very basic and early decisions in the design process invovling kinematics wii prove tro

5、ublesome and perform badly. Any mechanical system can be classified according to the number of degree of freedom which it possesses.the systems DOF is equal to the number of independent parameters which are needed to uniquely define its posion in space at any instant of time. A rigid body free to mo

6、ve within a reference frame will ,in the general case, have complex motoin, which is simultaneous combination of rotation and translation. In three-dimensional space , there may be rotation about any axis and also simultaneous translation which can be resoled into componention along three axes, in a

7、 plane ,or two-dimentional space ,complex motion becomes a combination of simultaneous along two axes in the plane. For simplicity ,we will limit our present discusstions to the case of planar motion: Pure rotation the body pessesses one point (center of rotation)which has no motion with respect to

8、the stationary frame of reference. All other points on the body describe arcs about that center. A reference line drawn on the body through the center changes only its angulai orientation. Pure translation all points on the body describe parallel paths. A reference line drawn on the body changes its

9、 linear posion but does not change its angular oriention. Complex motion a simulaneous combination of rotion and translationm . any reference line drawn on the body will change both its linear pisition and its angular orientation. Points on the body will travel non-parallel paths ,and there will be

10、, at every instant , a center of rotation , which will continuously change location. Linkages are the bacis building blocks of all mechanisms. All common forms of mechanisms (cams , gears ,belts , chains ) are in fact variations of linkages. Linkages are made up of links and kinematic pairs. A link

11、is an (assumed)rigid body which possesses at least two or more links (at their nodes), which connection allows some motion, or potential motion,between the connected links. The term lower pair is used to describe jionts with surface contact , as with a pin surrounded by a hole. The term higher pair

12、is used to describe jionts with point or line contact ,but if there is any clerance between pin and hole (as there must be for motion ),so-called surface contact in the pin jiont actually becomes line contact , as the pin contacts actually has contact only at discrete points , which are the tops of

13、the surfaces asperities. The main practical advantage of lower pairs over higher pairs is their better ability to trap lubricant between their envloping surface. This ie especially true for the rotating pin joint. The lubricant is more easily squeezed out of a higher pair .as s result , the pin join

14、t is preferred for low wear and long life . When designing machinery, we must first do a complete kinematic analysis of our design , in order to obtain information about the acceleration of the moving parts .we next want te use newton s second law to caculate the dynamic forces, but to do so we need

15、 to know the masses of all the moving parts which have these known acceletations. These parts do not exit yet ! as with any design in order to make a first pass at the caculation . we will then have to itnerate to better an better solutions as we generate more information. A first estimate of your p

16、arts masses can be obtained by assuming some reasonable shapes and size for all the parts and choosing approriate materials. Then caculate the volume of each part and multipy its volume by material s mass density (not weight density ) to obtain a first approximation of its mass . these mass values c

17、an then be used in Newton s equation. How will we know whether our chosen sizes and shapes of links are even acceptable, let alone optimal ? unfortunately , we will not know untill we have carried the computations all the way through a complete stress and deflection analysis of the parts. It it ofte

18、n the case ,especially with long , thin elements such as shafts or slender links , that the deflections of the parts, redesign them ,and repeat the force ,stress ,and deflection analysis . design is , unavoidably ,an iterative process . It is also worth nothing that ,unlike a static force situation

19、in which a failed design might be fixed by adding more mass to the part to strenthen it ,to do so in a dynamic force situation can have a deleterious effect . more mass with the same acceleration will generate even higher forces and thus higher stresses ! the machine desiger often need to remove mas

20、s (in the right places) form parts in order to reduce the stesses and deflections due to F=ma, thus the designer needs to have a good understanding of both material properties and stess and deflection analysis to properlyshape and size parts for minimum mass while maximzing the strength and stiffnes

21、s needed to withstand the dynamic forces. One of the primary considerations in designing any machine or strucre is that the strength must be sufficiently greater than the stress to assure both safety and reliability. To assure that mechanical parts do not fail in service ,it is necessary to learn wh

22、y they sometimes do fail. Then we shall be able to relate the stresses with the strenths to achieve safety . Ideally, in designing any machine element,the engineer should have at his disposal should have been made on speciments having the same heat treatment ,surface roughness ,and size as the eleme

23、nt he prosses to design ；and the tests should be made under exactly the same loading conditions as the part will experience in service . this means that ,if the part is to experience a bending and torsion,it should be tested under combined bending and torsion. Such tests will provide very useful and

24、 precise information . they tell the engineer what factor of safety to use and what the reliability is for a given service life .whenever such data are available for design purposes,the engineer can be assure that he is doing the best justified if failure of the part may endanger human life ,or if the part is manufactured