曲柄(导杆)滑块机构设计分析外文翻译.doc

Mechanism Introduction to Mechanism Mechanisms may be categorized in several different ways to emphasize their similarities and differences. One such grouping divides mechanisms into planar, sphe-rical, and spatial categories. All three groups have many things in common; the criterion, which distinguishes the groups, however, is to be found in the characteristics of the motions of the links. A planar mechanism is one in which all particles describe plane curves in space and all these curves lie in parallel planes; i. e., the loci of all points are plane curves parallel to a single common plane. This characteristic makes it possible to represent the locus of any chosen point of a planar mechanism in its true size and shape on a single drawing or figure. The motion transformation of any such mechanism is called coplanar. The plane four-bar linkage, the plate cam and follower, and the slider-crank mechanism are familiar examples of planar mechanisms. The vast majority of mechanisms in use today are planar. A spherical mechanism is one in which each link has some point which remains stationary as the linkage moves and in which the stationary points of all links lie at a common location; i.e., the locus of each point is a curve contained in a spherical surface, and the spherical surfaces defined by several arbitrarily chosen points are all concentric. The motions of all particles can therefore be completely described by their radial projections, or shadows, on the surface of a sphere with properly chosen center. Hookes universal joint is perhaps the most familiar example of a spherical mechanism.Spherical linkages are constituted entirely of revolute pairs. A spheric pair would produce no additional constraints and would thus be equivalent to an opening in the chain, while all other lower pairs have nonspheric motion. In spheric linkages, the axes of all revolute pairs must intersect at a point.Spatial mechanisms, include no restrictions on the relative motions of the particles. The motion transformation is not necessarily coplanar, nor must it be concentric. A spatial mechanism may have particles with loci of double curvature. Any linkage which contains a screw pair, for example, is a spatial mechanism, since the relative motion within a screw pair is helical. Thus, the overwhelming large category of planar mechanisms and the category ofspherical mechanisms are only special cases, or subsets, of the all-inclusive category spatial mechanisms. They occur as a consequence of special geometry in the particular orientations of their pair axes: If planar and spherical mechanisms are only special cases of spatial mechanisms, why is it desirable to identify them separately?Because of the particular geometric conditions, which identify these types, many simplifications are possible in their design and analysis. As pointed out earlier, it is possible to observe the motions of all particles of a planar mechanism in true size and shape from a single direction. In other words, all motions can be represented graphically in a single view. Thus, graphical techniques are well suited to their solution. Since spatial mechanisms do not all have this fortunate geometry, visualization becomes more difficult and more powerful techniques must be developed for their analysis. Since the vast majority of mechanisms in use today are planar, one might question the need of the more complicated mathematical techniques used for spatial mechanisms. There are a number of reasons why more powerful methods are of value even though the simpler graphical techniques have been mastered. 1. They provide new, alternative methods, which will solve the problems in a different way. Thus they provide a means of checking results. Certain problems by their nature may also be more amenable to one method than another. 2. Methods which are analytical in nature are better suited to solution by calculator or digital computer than graphical techniques.3. Even though the majority of useful mechanisms are planar and well suited to graphical solution, the few remaining must also be analyzed, and techniques should be known for analyzing them. 4. One reason that planar linkages are so common is that good methods of analysis for the more general spatial linkages have not been available until quite recently. Without methods for their analysis, their design and use has not been common, even though they may be inherently better suited in certain applications.5. We will discover that spatial linkages are much more common in practice than their formal description indicates. Consider a four-bar linkage. It has four links connected by four pins whose axes are parallel. This parallelism is a mathematical hypothesis; it is not a reality. The axes as produced in a shop in any shop, no matter how good will only-be approximately parallel. If they are far out of parallel, there will be binding in no uncertain terms, and the mechanism will only move because the rigid links flex and twist, producing loads in the bearings. If the axes are nearly parallel, the mechanism operates because of the looseness of the running fits of the bearings or flexibility of the links. A common way of compensating for small no parallelism is to connect the links with self-aligning bearings, actually spherical joints allowing three-dimensional rotation. Such a planar linkage is thus a low-grade spatial linkage. Degrees of Freedom A three-bar linkage (containing three bars linked together) is obviously a rigid frame; no relative motion between the links is possible. To describe the relative positions of the links in a four-bar linkage it is necessary only to know the angle between any two of the links. This linkage is said to have one degree of freedom. Two angles are required to specify the relative positions of the links in a five-bar linkage; it has two degrees of freedom. Linkages with one degree of freedom have constrained motion; i. e., all points on all of the links have paths on the other links that are fixed and determinate. The paths are most easily obtained or visualized by assuming that, the link on which the paths are required is fixed, and then moving the other links in a manner compatible with the constraints. Four-Bar Mechanisms When one of the members of a constrained linkage is fixed, the linkage becomes a mechanism capable of performing a useful mechanical function in a machine. On pin-connected linkages the input (driver) and output (follower) links are usually pivotally connected to the fixed link; the connecting links (couplers) are usually neither inputs nor outputs. Since any of the links can be fixed, if the links are of different lengths, four mechanisms, each with a different input-output relationship, can be obtained with a four-bar linkage. These four mechanisms are said to be inversions of the basic linkage. Slider-Crank Inversions When one of the pin connections in a four-bar linkage is replaced by a sliding joint, a number of useful mechanisms can be obtained from the resulting in Fig. 1 (top) the connection between links 1 and 4 is a sliding joint that permits block 4 to slide in the slot in link 1. It would make no difference, kinematically, if link 1 were sliding in a hole or slot in link 4. If link 1 in Fig. 1 (top) is fixed, the resulting slider-crank mechanism is shown in Fig. 1 (center). This is the mechanism of a reciprocating engine. The block4 represents the piston; link 1, shown shaded, is the block that contains the crankshaft bearing at A and the cylinder; link 2 is the crankshaft and link 3 the connecting rod. The crankpin bearing is at B, the wrist pin bearing at C. The stroke of the piston in twice AB, the throw of the crank. The slider-crank mechanism provides means for converting the translator motion of the pistons in a reciprocating engine into rotary motion of the crankshaft, or the rotary motion of the crankshaft in a pump into a translator motion of the pistons. In Fig. 1 (center), when B is in position B, the connecting rod would interfere with the crank if both were in the same plane. This problem is solved in engines and pumps by offsetting the crankpin bearing from the crankshaft bearing. By using an eccentric-and-rod mechanism in place of a crank, no offsetting is necessary and very small throws can be obtained. In Fig.1 (bottom) the crankpin bearing at B has become a large circular disk pivoted at A with an eccentricity or throw AB. The connecting rod has become the eccentric rod with a strap that encircles and slides on the eccentric. The mechanisms in the center and bottom drawings of Fig. 1 are kinematically equivalent. By fixing links 2, 3, and 4 instead of link 1, three other inversions of the linkage in Fig. 1 (top) are obtained. Fig.1译文：机构机构机构可用几种不同的方式进行分类。以强调其相近与差异之处。其中一种分类法将机构分为平面、球面与空间三类。所有这三类有许多共同之处；然而从连杆运动的特性可以看出区分这几类机构的标准。平面机构是这样一种机构，其所有质点在空间描出的是平面曲线，并且所有这些曲线都在平行平面上，也就是说，所有点的轨迹都与一个单一公共平面相平行的平面曲线。这一特点使得有它可能代表的轨迹所选择的任何质点的平面机构的位置，这个平面机构在一个单一的图形或模型中有其真实的大小和形状。任何这类机构的转变，就是所谓的共面。平面四连杆机构、凸轮、导杆机构，以及曲柄滑块机构是我们所熟悉的平面机构。在今天所使用的绝大多数的机构是平面机构。一种球形机构是平面机构之一，在各个杆件有一些质点，这仍然是平稳传动，就像是联结的移动而且在其中的所有杆件的固定质点的各个连接，都处于一个共同的位置，即每一点的运动轨迹是一个曲线并处于一个球面内，几个任意选择的质点所确定的球面都是同心的。因此，所有质点的运动都可以完全由它们的径向向外的方向来分析，或者称为“影子” ，位于正确选择的中心的球的表面。虎克的普遍联结，就是一个球形的机构最熟悉的例子。球形的联结，构成了完整的运动副。一个球形联结一个运动副不会产生任何额外的约束，并会因此等于链中的开环，而所有其他低副，则不是球形运动。在球形的联结中，所有运动副的轴必须相交于一点。在相对运动的质点中，空间机构不包括约束。机构运动的传动，并不一定是共面，也不一定是同心。一个空间的机构可能有质点的运动轨迹发生双面弯曲。任何带有螺旋副的联结，举例来说，它是一个空间的机构，因为螺旋副的相对运动是螺旋状的。因此，绝大多数的大的平面机构和类似球形的机构，只有在特殊情况下，或者在亚特殊情况下，包含各方的类似空间的机构。在两个轴上的特殊方向上，它们作为特殊几何关系作用的结果： 如果平面和球形机构仅仅是空间机构的特殊情况，为什么要分别分析来它们呢？由于用来区分这些类型的特殊几何条件，在他们的设计和分析中，许多是可能得到简单化的。 若能更快的指出来，从一个单一的方向去观察一个在真实的大小和形状的平面机构的所有质点的运动是可能的。换句话说，所有的运动在一个方向上可以由图形来表示。 因此，图解技法非常适合去解决它们的问题。 因为空间机制不可能全部都有这种几何关系，将其视觉化变得更加困难，并且必须开发出更强的技术来对它们进行分析。 因为现在所使用的绝大多数机构是平面机构，也许有的人对于空间机制更复杂的数学技术的需要会表示怀疑。虽然更简单的图解法已经为我们所掌握，但是有很多的原因可以告诉我们为什么更加强有力的方法是有价值。 1它们提供新的、可交替的方法，这些方法可以用不同的方式解决问题。 因而他们能提供检验结果的方法。具有它们的属性的某些问题也可能比其他方法更有效的到解决。2通过计算器或数字计算机，分析它们性质的方法比图解法更合适来分析问题。3虽然大多数有用的机构是平面机构和非常适合对图形分析，剩下的少数机构也必须得到分析，并且也应该有针对它们进行分析的方法。4平面联结如此普遍的一个原因是在这之前能为更广泛的空间联结进行分析的好方法还未得到应用。虽然空间机构也许在本质上能更适合于某些应用，但是没有对他们进行分析的方法，它们的设计和用途就不一样。5我们发现空间连接比他们的外部结构的描述在实践应用上是更加普遍的。考虑到四杆连接， 它由四个杆件相连，这些链接由四个轴为平行的销连接。 这里的“平行性”只是一个数学假设; 并不是真实的。 轴如果是由同一家生产的，无论有多好，都将只是近似平行。如果他们离平行差的太远，组合起来都是不确定的，并且这