外文翻译机械运动和动力学精品.docx

1、外文翻译机械运动和动力学精品附录附录1外文原文Kinematics and dynamics of machineryOne 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 earthb

2、ound 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 crea

3、ting 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

4、largest 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 tr

5、oublesome 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 m

6、ove 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

7、a 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 i

11、s 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 ti describe jionts with surface contact , as with a pin surrounded by a hole. The term higher pair i

12、s 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 t

13、he 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 joint

14、 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 newtons second law to caculate the dynamic forces, but to do so we need t

15、o 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 par

16、ts 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 materials mass density (not weight density ) to obtain a first approximation of its mass . these mass values can

17、then be used in Newtons 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 often the

18、 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 in whi

19、ch 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 mass (in

20、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 stiffness need

21、ed 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 why they

22、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 element he pr

23、osses 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 precise

24、 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

25、is manufactured in sufficiently large quantities. Automobiles and refrigrerators, for example, have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufacture , the cost of making these is very low when it is divided by th

26、e total number of parts manufactrued. You can now appreciate the following four design categories :(1)failure of the part would endanger human life ,or the part ismade in extremely large quantities ;consequently, an elaborate testingprogram is justified during design .(2)the part is made in large en

27、ough quantities so that a moderate serues of tests is feasible.(3)The part is made in such small quantities that testing is not justified at all ; or the design must be completed so rapidlly that there is not enough time for testing.(4) The part has already been designed, manufactured, and tested an

28、d found to be unsatisfactory. Analysis is required to understand why the part is unsatisfactory and what to do to improve it . It is with the last three categories that we shall be mostly concerned.this means that the designer will usually have only published values of yield strenth , ultimate stren

29、gth,and percentage elongation . with this meager information the engieer is expected to design against static and dynamic loads, biaxial and triaxial stress states , high and low temperatures,and large and small parts! The data usually available for design have been obtained from the simple tension

30、test ,where the load was applied gradually and the strain given time to develop. Yet these same data must be used in designing parts with complicated dynamic loads applied thousands of times per minute . no wonder machine parts sometimes fail. To sum up, the fundamental problem of the designer is to

31、 use the simple tension test data and relate them to the strength of the part ,regardless of the stress or the loading situation. It is possible for two metal to have exactly the same strength and hardness, yet one of these metals may have a supeior ability to aborb overloads, because of the propert

32、y called ductility. Dutility is measured by the percentage elongation which occurs in the material at frature. The usual divding line between ductility and brittleness is 5 percent elongation. Amaterial having less than 5 percent elongation at fracture is said to bebrittle, while one having more is

33、said to be ductile. The elongation of a material is usuallu measured over 50mm gauge length.siece this id not a measure of the actual strain, another method of determining ductility is sometimes used . after the speciman has been fractured, measurements are made of the area of the cross section at the fracture. Ductility can then be expr