Unit One Mechanisms and Machine Theory

Lesson 1 Mechanisms

A mechanism is a member combination that more than two or two connections with the members realize the regulation motion made up by way of the activity. They are the component of machinery. Activity connections between two members that have the relative motion are called the motion pairs. All motion pairs contacts with planes are called lower pairs and all motion pairs contacts with points or lines are called high pairs. The motion specific property of mechanism chiefly depends on the relative size between the member, and the character of motion pairs, as well as the mutual disposition method etc[1]. The member that is used to support the member of motion in the mechanism is called the machine frame and used as the reference coordinate for the study of the motion system. The member that possesses the independence motion is called motivity member. The member except machine frame and motivity member being compelled to move in the mechanism is called driven member. The independent parameter (coordinate number) essential for description or definite mechanism motion is called the free degree of mechanism. For gaining the definite relative motion between the members of mechanism, it is necessary to make the number of motivity members of mechanism equal the number of free degrees.

Mechanisms may be categorized in several different ways to emphasize their similarities and differences. Mechanisms are generally divided into planar, spherical, 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[2].

A planar mechanism is one in which all particles describe plane curves in space and all these curves lie in parallel places ,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[3]. The motion transformation of any such mechanism is called coplanar. The plane four-bar linkage, the plate cam and driven parts, and the slider-crank mechanism are familiar examples of planar mechanism. The vast majority of mechanism in use today is planar. The following Fig.1.1 is cam mechanism.

A cam is a machine member that drives a follower through a specified motion. By the proper design of a cam, any desired motion to a machine member can be obtained. As such, cams are widely used in almost all machinery. They include internal combustion engines, a variety of machine tools, compressors and computers. In general, a cam can be designed in two ways.

(1) To design an optimal profile of a cam to give a desired motion to the follower.

(2) To choose a suitable profile to ensure a satisfactory performance by the follower.

A rotary cam is a part of a machine, which changes cylindrical motion to straight-line motion. The purpose of a cam is to transmit various kinds of motion to other parts of a machine.

Practically every cam must be designed and manufactured to fit special requirements. Though each cam appears to be quite different from the other, all of them work in similar ways. In each case, as the cam is rotated or turned, another part is connected with the cam, called a follower, is moved either right or left, up and down, or in and out. The follower is usually connected to other parts on the machine to accomplish the desired action. If the follower loses contact with the cam, it will fail to work.

According to their basic shapes, cams are classified into four different types as illustrated in Fig.1.1.

(1) Plate (Disc) Cam.

(2) Translation Cam.

(3) Cylindrical Cam.

(4) Face Cam.

Planar mechanisms utilizing only lower pairs are called planar linkages; they may include only revolute and prismatic pairs. Although a planar pair might theoretically be included, this would impose no constraint and thus be equivalent to an opening in the kinematics chain. Planar motion also requires that axes of all prismatic pairs and all revolute axes be normal to the plane motion.

Words and Expressions

Notes

[1] The motion specific property of mechanism chiefly depends on the relative size between the member, and the character of motion pairs, as well as the mutual disposition method etc.

① specific property：特性。

② as well as：不但……而且；和……一样；和；也，表示递进或并列关系。

[2] The criterion which distinguishes the groups，however，is to be found in the characteristics of the motions of the links．

全句翻译：然而，进行分类的标准在于连杆的运动特性。

① to be found为不定式被动语态。

② 1inks译为“连杆装置”。

[3] 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．

① makes it possible：使……可能。

② represent：描绘，展现。

③ planar mechanism：平面机构。

④ in size and shape：在大小和形状方面。

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 axes 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.

Spherical linkages are constituted entirely of revolute pairs. A spherical pair would produce no additional constraints and would thus be equivalent to an opening in the chain, while all other tower pairs have no spherical motion. In spherical linkages, the axes of all revolute pairs must intersect at a point.

Spatial mechanisms, on the other hand, include no restrictions on the relative motions of the particles. The motion transformation is not necessary 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. To take an example, worm gear pairs transmit motion between two shafts. The shafts are usually at right angles to each other but do not lie in the same plane. The teeth on the worm slide against the teeth of the worm wheel, producing a rolling action. The worm turns the worm wheel.