机械运动和动力学中英文对照外文翻译文献

(文档含英文原文和中文翻译)

中英文对照翻译

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 earthbound 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 creating 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 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 troublesome 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 move 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 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 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 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 , 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 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 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 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 joint 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 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 parts’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 can 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 often 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 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 mass (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 stiffness 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 why 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 element 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 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 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 the 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 enough 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 and 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 strength,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 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 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 property 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 said to be ductile.

The elongation of a material is usuallu measured over 50mm gauge length.siece this did 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 expressed as the percentage reduction in cross sectional area.

The characteristic of a ductile material which permits it to aborb largeoverloads is an additional safety factot in design. Ductility is also important because it is a measure of that property of a material which permits it to be cold-worked .such operations as bending and drawing are metal-processing operations which require ductile materials.

When a materals is to be selected to resist wear , erosion ,or plastic deformaton, hardness is generally the most important property. Several methods of hardness testing are available, depending upon which particular property is most desired. The four hardness numbers in greatest usse are the Brinell, Rockwell,Vickers, and Knoop.

Most hardness-testing systems employ a standard load which is applied to a ball or pyramid in contact with the material to be tested. The hardness is an easy property to measure , because the test is nondestructive and test specimens are not required . usually the test can be conducted directly on actual machine element .

Virtually all machines contain shafts. The most common shape for shafts is circular and the cross section can be either solid or hollow (hollow shafts can result in weight savings). Rectangular shafts are sometimes used ,as in screw driver bladers ,socket wrenches and control knob stem.

A shaft must have adequate torsional strength to transmit torque and not be over stressed. If must also be torsionally stiff enough so that one mounted component does not deviate excessively from its original angular position relative to a second component mounted on the same shaft. Generally speaking,the angle of twist should not exceed one degree in a shaft length equal to 20 diameters.

Shafts are mounted in bearing and transmit power through such device as gears, pulleys,cams and clutches. These devices introduce forces which attempt to bend the shaft;hence, tha shaft must be rigid enough to prevent overloading of the supporting bearings ,in general, the bending deflection of a shaft should not exceed 0.01 in per ft of length between bearing supports.

In addition .the shaft must be able to sustain a combination of bending and torsional loads. Thus an equivalent load must be considered which takes into account both torsion and bending . also ,the allowable stress must contain a factor of safety which includes fatigue, since torsional and bending stress reversals occur.

For fiameters less than 3 in ,the usual shaft material is cold-rolled steel containing about 0.4 percent carbon. Shafts ate either cold-rolled or forged in sizes from 3in. to 5 in. for sizes above 5

in. shafts are forged and machined to size . plastic shafts are widely used for light load applications . one advantage of using plastic is safty in electrical applications, since plastic is a poor confuctor of electricity.

Components such as gears and pulleys are mounted on shafts by means of key. The design of the key and the corresponding keyway in the shaft must be properly evaluated. For example, stress concentrations occur in shafts due to keyways , and the material removed to form the keyway further weakens the shaft.

If shafts are run at critical speeds , severe vibrations can occur which can seriously damage a machine .it is important to know the magnitude of these critical speeds so that they can be avoided. As a general rule of thumb , the difference betweem the operating speed and the critical speed should be at least 20 percent.

Many shafts are supported by three or more bearings, which means that the problem is statically indeterminate .text on strenth of materials give methods of soving such problems. The design effort should be in keeping with the economics of a given situation , for example , if one line shaft supported by three or more bearings id needed , it probably would be cheaper to make conservative assumptions as to moments and design it as though it were determinate . the extra cost of an oversize shaft may be less than the extra cost of an elaborate design analysis.

Another important aspect of shaft design is the method of directly connecting one shaft to another , this is accomplished by devices such as rigid and flexiable couplings.

A coupling is a device for connecting the ends of adjacent shafts. In machine construction , couplings are used to effect a semipermanent connection between adjacent rotating shafts , the connection is permanent in the sense that it is not meant to be broken during the useful life of the machinem , but it can be broken and restored in an emergency or when worn parts are replaced.

There are several types of shaft couplings, their characteristics depend on the purpose for which they are used , if an exceptionally long shaft is required in a manufacturing plant or a propeller shaft on a ship , it is made in sections that are coupled together with rigid couplings. A common type of rigid coupling consists of two mating radial flanges that are attached by key driven hubs to the ends of adjacent shaft sections and bolted together through the flanges to form a rigid connection. Alignment of the connected shafts in usually effected by means of a rabbet joint on the face of the flanges.

In connecting shafts belonging to separate device ( such as an electric motor and a gearbox),precise aligning of the shafts is difficult and a fkexible coupling is used . this coupling connects the shafts in such a way as to minimize the harmful effects of shafts misalignment of loads and to move freely(float) in the axial diection without interfering with one another .

flexiable couplings can also serve to reduce the intensity of shock loads and vibrations transmitted from one shaft to another .

中文翻译

机械运动和动力学

运动学的基本目的是去设计一个机械零件的理想运动，然后再用数学的方法去描绘该零件的位置，速度和加速度，再运用这些参数来设计零件。因为，对于大部分固着在地球上的机械系统来说，与之联系最密切的是时间，将加速度和动态力定义成时间作用的结果。相应地，应力是作用在物体上的外力和惯性力的作用结果。所以机械设计的内容是要建立一种在该机器的使用寿命内保证其安全的系统，目的是要在一定的工况要求下，对材料进行选择，使材料的应力在许用极限应力之内。这一点很明显要求所有的系统要在理想的限制内工作。在机械设计中，零件受到的最大力是取决于材料本身的动态性能。这些动态力引起了零件的加速度，加速度又要回到运动学中去计算，这是机械设计的基础。运动分析

是最基本的也是最早出现在设计的过程中的，它对与任何一个零件的成功设计够起着至关重要的作用。在设计过程中很差的运动学分析会带来麻烦和错误。

根据机构所具有的自由度，任何机械系统都可以被分类。系统的自由度是在任何时候限制它的位置独立的参数数目。

在通常情况下，刚体在相关的平面内能实现复杂的自由运动，这个运动同时包括转动和平移。在三纬空间内，在可以饶任何轴转动的同时可以沿着三个坐标平移。在一个平面或是一个二维的空间内，复杂运动变成了饶一个(垂直与这个平面的)轴线的转动和同时发生的可以被分解为沿在这个平面内的两个坐标轴的平移分量。为了简化，我们将当前的讨论限制在二维的运动系统中。接下来将要介绍相关的术语:

纯转动物体围绕着在相对于一个静止的坐标系静止的一点(回转中心)转动。其他所有物体上的点都可以用相对中心的弧来描述。在物体上的参考线通过中心，只有在角度方向上有变化。

纯平动所有在物体上的点在平行的路径上平移。物体上的参考线在线性位置上有变化，而在角度方向上没有发生变化。

复杂运动同时包含转动和平动的运动。在物体上的参考线在沿线性点平动的同时又在角度方向上有变化。物体上的点不会在沿着平行的路径移动，他们在饶中心转动的同时也不停着改变着位置。

铰链是联接所有机构的基本的构件。所有一般形式的机械，(齿轮，带，链)实际上都是不同类型的铰链，铰链组成了联接和运动部件。

联接是一个刚体和另外的连接件至少有两个结点。

运动部件(也称接头)是在两个连接件的结合部分，这个结合允许相对的运动，允许连接件之间潜在的运动。

术语低副是用来描绘接头间的面接触。，如针和孔的结合面。高副是用来描绘接头间的点和线接触。但是如果在针和孔之间有间隙存在(当它们之间用于有相对运动时)当针和孔只有一面接触时，在针间的面联接实际上已经变成了线接触。类似的，微观上看，在平面滑动的杆件实际上只存在一些相关点的接触，那是表面凹凸不平的突点，低副相对于高副的优点是有利于接触表面之间的润滑，这一点对于旋转接头来说是确实存在的。在高副中润滑易被挤出来。结果铰接接头能够减少摩擦，延长寿命。

当我们设计机械时，为了取得运动部件的加速度信息必须首先对我们的设计进行全面的运动分析。接下来再运用牛顿第二运动定律去计算动态力。但是这样做，我们需要知道所有运动部件的质量，和加速度，这些零件还没有存在，正如碰到的所有设计问题，我们在设计决定零件最佳尺寸和形状时缺少足够的信息。为了通过最初的计算我们必须估计零件的质量和设计的其它部分。当我们得到更多的信息时，再得到更好的解决方案。

在估计你设计的零件质量的初期通过合理的假设零件的形状和尺寸及其合理选择材料来获得。然后计算每个零件的体积，再去乘以所选材料的质量密度，去取得零件最初的

合理质量。这些质量值在牛顿方程中可以运用。

我们如何来判断我们所选择的尺寸和形状是否合理呢?很不巧，我们要到分析完所受应力和失效分析后才能知道，特别是细长零件，如轴，细长的连杆，甚至在很小的应力条件下，零件在动载的的失效形式将限制我们的设计。这种情况我们经常碰到。

我们可能将会发现零件在动载荷的情况下会失效。然后我们将反过来检查我们初选时假设的形状，尺寸和材料，重新来选择设计。然后重复力，应力和失效分析。设计不可避免地成为一个迭代过程。

值得注意的是，在静力作用下，可以通过增加零件的质量来提高其强度，将不合格的设计变为合格，而在动态力作用的情况下，这样做可能产生有害的后果。在相同的加速度条件下，更大的质量将会产生更大的力，随即也会有更大的应力。为了降低应力和失效，设计者要从零件上去除一些质量。同时设计者需要对材料的特性和应力实效分析都要有很好的了解才能通过用合理的形状和尺寸来达到最小的质量，与此同时，抵御动态力的强度和刚度最大。

在设计任何机器或者机构时，所考虑的主要事件之一是其强度应该比它所承受的应力要大的多，以确保安全可靠性，要保证机械零件在使用过程中不发生失效，就必须知道它们在某些时候会发生失效的原因，然后，才能将应力和强度联系起来，以确保其安全。

设计任何机械零件的理想情况为，工程师可以利用大量的他所选的这种材料的强度试验数据。这些试验应该采用与所设计是零件有着相同是热处理，表面粗糙度和尺寸大小的试件进行，而且试验应该在与零件使用过程中承受的载荷完全相同的情况下进行。这表明，如果零件将要承受弯曲载荷，那么就应该进行弯曲载荷的试验。如果零件将要受弯曲和扭转的复合载荷，那么就应该进行弯曲和扭转复合载荷的试验，这些种类的试验可以提供非常有效和精准的数据。它们可以告诉工程师应该使用的安全系数和对于给定的寿命时的可靠性。在设计过程中，只要能获得这种数据，工程师就可以尽可能好地进行工程设计工作。

如果零件的失效可能危害人的生命安全，或者零件有足够大的产量，则在设计前收集这样广泛的数据所花费的费用是很值得的，例如，汽车和冰箱的零件的产量非常的大，以在生产之前对它们进行大量的试验，使其具有较高的可靠性。如果把进行这些试验的费用分摊到所生产的零件上的话，则摊到所生产每个零件的费用是非常低的。

你可以对下列四种类型的设计作出评价。

(1)零件的失效可能危害人的生命安全，或者零件的产量非常大，因此在设计时安排一个完善的试验程序会被认为是合理的。

(2)零件的产量足够大，可以进行适当的系列试验。

(3)零件的产量非常小，以至于进行试验根本不合算;或者要求很快地完成设计，以至于没有足够的时间进行试验。

(4)零件已经完成设计，制造和试验，但其结果不能令人满意。这时候需要采用分析的方法来弄清楚不能令人满意的原因和应该如何进行改进。

我们将主要对后三种类型进行讨论。这就说，设计人员通常只能利用那些公开发表的屈服强度，极限强度和延伸率等数据资料。人们期望工程师在利用那些公开发表的资料的基础上，对静载荷和动载荷，二维应力状态与三维应力状态，高温与低温以及大零件和小零件进行设计! 而设计中所能利用的数据通常是从简单的拉伸试验中得到，其载荷是渐渐加上去的，有充分的时间产生应变。到目前为止，还必须利用这些数据来设计每分钟承受几千次复杂的动载的作用的零件，因此机械零件有时会失效是不足为奇的。

概括地说，设计人员所遇到的基本问题是，不论对于哪一种应力状态或者载荷情况，都能利用通过简单拉伸试验所获得的数据并将其与零件的强度联系起来。

可能会有两种具有完全相同的强度和硬度值的金属，其中一种由于其本身的延搌性而具有很好的承受超载荷的能力，延搌性是利用材料断裂时的延伸率来衡量的。通常将5%的延伸率定义为延展性的分界线。断裂时延伸率小于5%的材料称为脆性材料，大于5%的称为延性材料。

材料的伸长量通常是在50mm 的计量长度上测量的。因为这并不是对实际应变量的测量，所以有时也采用另一种测量延展性的方法，这个方法在试件断裂后，测量其断裂处的很截面的面积。因此，延展性可以表示为横截面的收缩率。

延展性材料能够承受较大的超载荷这个特性，是设计中的一个附加的安全因素。延展性材料的重要性在于它是材料泠变形能的衡量尺度。诸如弯曲和拉伸这类金属加工过程需要采用延性材料。

在选用抗磨损，抗侵蚀或者抗塑性变形的材料时，硬度通常是最主要的性能。有几种可选用的硬度试验方法，采用哪一种方法取决于最希望测量的材料特性。最常用的四种硬度是布氏硬度，洛氏硬度，维氏硬度，努氏硬度。

大多数硬度试验系统是将一个标准的载荷加在与被试验材料相接触的小球或者棱锥上。因此，硬度可以表示为所产生的压痕尺寸的函数。这表明由于硬度是非破坏性试验，而且不需要专门的试件，因而，硬度是一个容易测量的性能。通常可以直接在实际的机械零件上进行硬度试验。

实际上，几乎所有的机器中都装有轴。轴最常见的形状是圆形，其截面可以是实心的，也可以是空心的（空心轴可以减轻重量）。有时也采用矩形轴，例如，螺丝起子的头部，套筒扳手和控制旋转的杆。

为了在传递扭矩时不发生过载，轴应该具有适当的抗扭强度。轴还应该具有足够的抗扭刚度，以使在同一个轴上的两个传动零件之间的相对转角不会过大。一般来说，在长度等于轴的直径的20倍时，轴的扭转角不应该超过1度。

轴安装在轴承中，通过齿轮，皮带轮，凸轮和离合器等零件传递动力。通过这些零件传来的力可能会使轴产生弯曲变形。因此，轴应该有足够的刚度以防止支撑轴承受离过大。总而言之，在两个轴承之间，轴在每英尺长度上的弯曲变形不应该超过0。01英寸。

此外，轴还必须能够承受弯矩和扭矩的组合作用。因此，要考虑考虑扭矩与弯矩的