机械专业毕业设计外文翻译--机械传动系统介绍

Transmission System introduced

The important position of the wheel gear and shaft can’t falter in traditional machine and modern machines. The wheel gear and shafts mainly install the direction that delivers the dint at the principal axis box. The passing to process to make them can is divided into many model numbers, used for many situations respectively. so we must be the multilayers to the understanding of the wheel gear and shaft in many ways.

In the force analysis of spur gears, the forces are assumed to act in a single plane. We shall study gears in which the forces have three dimensions. The reason for this, in the case of helical gears, is that the teeth are not parallel to the axis of rotation. And in the case of bevel gears, the rotational axes are not parallel to each other. There are also other reasons, as we shall learn.

Helical gears are used to transmit motion between parallel shafts. The helix angle is the same on each gear, but one gear must have a right-hand helix and the other a left-hand helix. The shape of the tooth is an involute helicoid. If a piece of paper cut in the shape of a parallelogram is wrapped around a cylinder, the angular edge of the paper becomes a helix. If we unwind this paper, each point on the angular edge generates an involute curve. The surface obtained when every point on the edge generates an involute is called an involute helicoids.

The initial contact of spur-gear teeth is a line extending all the way across the face of the tooth. The initial contact of helical gear teeth is a point, which changes into a line as line as the teeth come into more engagement. In spur gears the line of contact is parallel to the axis of the rotation; in helical gears, the line is diagonal across the face of the tooth. It is this gradual of the teeth and the smooth transfer of load from one tooth to another, which give helical gears the ability to transmit heavy loads at high speeds. Helical gears subject the shaft bearings to both radial and thrust loads. When the thrust loads become high or are objectionable for other reasons, it may be desirable to use double helical gears. A double helical gear (herringbone) is equivalent to two helical gears of opposite hand, mounted side by side on the same shaft. They develop opposite thrust reactions and thus cancel out the thrust load. When two or more single helical gears are mounted on the same shaft, the hand of the gears should be selected so as to produce the minimum thrust load.

Crossed-helical, or spiral, gears are those in which the shaft centerlines are neither parallel nor interesting. The teeth of crossed-helical fears have point contact with each other which changes to line contact as the gears wear in. for this reason they will carry out very small loads and are mainly for instrumental applications, and are definitely not recommended for use in the transmission of power. There is on difference between a crossed helical gear and a helical gear until they are mounted in mesh with each other. They are manufactured in the same way. A pair of meshed crossed helical gears usually have the same hand; that is, a right-hand driver goes with a right-hand driven. In the design of crossed-helical gears, the minimum sliding velocity is obtained when the helix angle are equal. However, when the helix angle are not equal, the gear with the larger helix angle should be used as the driver if both gears have the same hand.

Worm gears are similar to crossed helical gears. The pinion or worm has a small number of teeth, usually one to four, and since they completely wrap around the pitch cylinder they are called threads. Its mating gear is called a worm gear, which is not a true helical gear. A worm and worm gear are used to provide a high angular-velocity reduction between nonintersecting shafts which are usually at right angle. The worm gear is not a helical gear because its face is made concave to fit the curvature of the worm in order to provide line contact instead of point contact. However, a disadvantage of worm gearing is the high sliding velocities across the teeth, the same as with crossed helical gears.

Worm gearing are either single or double enveloping. A single-enveloping gearing is one in which the gear wraps around or partially encloses the worm. A gearing in which each element partially encloses the other is, of course, a double-enveloping worm gearing. The important difference between the two is that area contact exists between the teeth of double-enveloping gears while only line contact between those of single-enveloping gears. The worm and worm gear of a set have the same hand of helix as for crossed helical gears, but the helix angles are usually quite different. The helix angle on the worm is generally quite large, and that on the gear very small. Because of this, it is usual to specify the lead angle on the worm, which is the complement of the worm helix angle, and the helix angle on the gear; the two angles ate equal for a 90-deg. Shaft angle.

When gears are to be used to transmit motion between intersecting shaft, some of

bevel gear is required. Although bevel gear are usually made for a shaft angle of 90 deg. They may be produced for almost any shaft angle. The teeth may be cast, milled, or generated. Only the generated teeth may be classed as accurate. In a typical bevel gear mounting, one of the gear is often mounted outboard of the bearing this means that shaft deflection can be more pronounced and have a greater effect in the contact of teeth. Another difficulty, which occurs in predicting the stress in bevel-gear teeth, is the fact the teeth are tapered.

Straight bevel gears are easy to design and simple to manufacture and give very good results in service if they are mounted accurately and positively. As in the case of squrgears, however, they become noisy at higher values of the pitch-line velocity. In these cases it is often good design practice to go to the spiral bevel gear, which is the bevel counterpart of the helical gear. As in the case of helical gears, spiral bevel gears give a much smoother tooth action than straight bevel gears, and hence are useful where high speed are encountered.

It is frequently desirable, as in the case of automotive differential applications, to have gearing similar to bevel gears but with the shaft offset Such gears are called hypoid gears because their pitch surfaces are hyperboloids of revolution The tooth action between such gears is a combination of rolling and has much in common with that of worm gears.

A shaft is a rotating or stationary member usually of circular cross section, having mounted upon it such elementsas gears pulleys flywheels, cranks sprockets and other power-transmission elements Shaft may be subjected to bending tension compression or torsional loads acting singly or in combination with one another .When they are combined one may expect to find both static and fatigue strength to be important design considerations since a single shaft may be subjected to static stresses completely reversed, and repeated stresses, all acting at the same time

The word “shaft” covers numerous wariations, such as axles and spindles. Anaxle is a shaft, wither stationary or rotating nor subjected to torsion load. Ashirt rotating shaft is often called a spindle.

When either the lateral or the tosional deflection of shaft must be held to close limits, the shaft must be sized on the basis of deflection before analyzing the stresses The reason

for this is that if the shift is made stiff enough so that the deflection is not too large, it is probable that the resulting stresses will be safe. But by no means should the designer assume that they are within acceptable limits. Whenever possible the power-transmission elements such as gears or pullets, should be located close to the supporting bearings. This reduces the bending moment, and hence the deflection and bending stress.

Although the von Mises-Hencky-Goodman method is difficult to use in design of shaft, it probably come closest to predicting actual failure. Thus it is a good way of checking a shaft that has already been designed or of discovering why a particular shaft that has already been designed or of discovering why a particular shaft has failed in service. Furthermore, there are a considerable number of shaft-design problems in which the dimension are pretty well limited by other considerations, such as rigidity, and it is only necessary for the designer to discover something about the fillet sizes, heat-treatment, and surface finish and whether or not shot peening is necessary in order to achieve the required life and reliability.

Because of the similarity of their functions, clutches and brakes are treated together. In a simplified dynamic representation of a friction clutch, or brake, two inertias I1and I2 traveling at the respective angular velocities W1 and W2, one of which may be zero in the case of brake, are to be brought to the same speed by engaging the clutch or brake. Slippage occurs because the two elements are running at different speeds and energy is dissipated during actuation, resulting in a temperature rise. In analyzing the performance of these devices we shall be interested in the actuating force, the torque transmitted, the energy loss and the temperature rise. The torque transmitted is related to the actuating force, the coefficient of friction, and the geometry of the clutch or brake. This is problem in static, which will have to be studied separately for each geometric configuration. However, temperature rise is related to energy loss and can be studied without regard to the type of brake or clutch because the geometry of interest is the hear-dissipating surfaces. The various types of clutches and brakes may be classified as fallows:

Rim type with internally expanding shoes

Rim type with internally contracting shoes

Band type

Disk or axial type

Cone type

Miscellaneous type

The analysis of all type of friction clutches and brakes use the same general procedure. The following step are necessary:

1. Assume or determine the distribution of pressure on the frictionalsurfaces.

2. Find a relation between the maximum pressure and the pressure at any point

3. apply the condition of statical equilibrium to find (a) the actuating force, (b) the torque, and (c) the support reactions.

Miscellaneous clutches include several type, such as the positive-contact clutches, overload-release clutches, overrunning clutches, magnetic fluid clutches, and others.

A positive-contact clutch consists of a shift lever and two jaws. The greatest differences between the various types of positive clutches are concerned with the design of the jaws. To provide a longer period of time for shift action during engagement, the jaws may be ratchet-shaped, or gear-tooth-shaped. Sometimes a great many teeth or jaws re used, and they may be cut either circumferentially, so that they engage by cylindrical mating, or on the faces of the mating elements.

Although positive clutches are not used to the extent the frictional-contact type, they do have important applications where synchronous operation is required.

Devices such as linear driver or motor-operated screw drivers must run to definite limit and then come to a stop. An over load-release rype of clutch is required for these applications. These clutches are usually spring-loaded so as to release at a predetermined toque. The clicking sound which is heard when the overload point is reached is considered to be a desirable signal.

An overrunning clutch or coupling permits the driven member of a machine to “freewheel” or “overrun” because the driver is stopped or because another source of power increase the speed of the driven. This type of clutch usually uses rollers or balls mounted between an outer sleeve and an inner member having flats machined around the periphery. Driving action is obtained by wedding the rollers between the sleeve and the flats. The clutch is therefore equivalent to a pawl and ratchet with an infinite number of teeth.

Magnetic fluid clutch or brake is a relatively new development which has two parallel magnetic plates. Between these plates is a lubricated magnetic powder mixture. An electromagnetic coil is inserted somewhere in the magnetic circuit. Bu varying the excitation to this coil, the shearing strength of the magnetic fluid mixture may be accurately controlled. Thus any condition from a full slip to a frozen lockup may be obtained.

机械传动系统介绍

在传统机械和现代机械中齿轮和轴的重要地位是不可动摇的。齿轮和轴主要安装在主轴箱来传递力的方向。通过加工制造它们可以分为许多的型号，分别用于许多的场合。所以我们对齿轮和轴的了解与认识必须是多层次多方位的。

在直齿圆柱齿轮的受力分析中，是假设各力作用在单一平面的。我们将研究作用力具有三维坐标的齿轮。因此，在斜齿轮的情况下，其齿向是不平行于回转轴线的。而在锥齿轮的情况中各回转轴线互相不平行。像我们要讨论的那样，尚有其它道理需要学习，掌握。

斜齿轮用于传递平行轴之间的运动。倾斜角度每个齿轮都一样，但一个必须右旋斜齿，而另一个必须是左旋斜齿。齿的形状是一渐开线螺旋面。如果一张被剪成平行四边形（矩形）的纸张包围在齿轮圆柱体上，之上印出齿的角刃边就变成斜线。如果我展开这张纸，在斜角刃边上的每一个点就发生一渐开线曲线。

直齿圆柱齿轮轮齿的初始接触处是跨过整个齿面而伸展开来的线。斜齿轮轮齿的初始接触是一点，当齿进入更多的啮合时，它就变成线。在直齿圆柱齿轮中，接触是平行于回转轴线的。在斜齿轮中，该线是跨过齿面的对角线。它是齿轮逐渐进行啮合并平稳的从一个齿到另一个齿传递运动，那样就使斜齿轮具有高速重载下平稳传递运动的能力。斜齿轮使轴的轴承承受径向和轴向力。当轴向力变的大了或由于别的原因而产生某些影响时，那就可以使人字齿轮。双斜齿轮（人字齿轮）是与反向的并排地装在同一轴上的两个斜齿轮等效。他们产生相反的轴向推力作用，这样就消除了轴向推力。当两个或者跟多个单向斜齿轮在同一轴上时，齿轮的齿向应作选择，以便产生最小的轴向推力。

交错轴斜齿轮或螺旋齿轮，他们的轴中心线既不相交也不平行。交错轴斜齿轮的齿彼此之间发生点接触，它随着齿轮的磨合而变成先接触。因此它们只能传递小的载

荷和主要用于仪器设备中，而且肯定不能推荐在动力传动中使用。交错轴斜齿轮与斜齿轮之间在倍安装后互相啮合之前是没有任何区别的。它们是以同样的方法进行制造。一对相啮合的交错轴斜齿轮通常具有同样的齿向，即左旋主动齿轮跟右旋从动齿轮相啮合。在交错轴斜齿设计中，当该齿的斜角相等时所产生滑移速度最小。然而当该齿的斜角不相等时，如果两个齿轮具有相同齿向的话，大斜角齿轮应用作主动齿轮。

涡轮与交错轴斜齿轮相似。小齿轮即蜗杆具有较小的齿数，通常是一到四齿，由于他们完全缠绕在节圆柱上，因此它们被称为螺纹齿。与其相配的齿轮叫做涡轮，涡轮不是真正的齿轮。蜗杆和涡轮通常是用于向垂直相交轴之间的传动提供大的角速度减速比。涡轮不是斜齿轮，因此其齿顶面做成中凹形状以适配蜗杆曲率，目的是要形成先接触而不是点接触。然而蜗杆涡轮传动机构中存在齿间有较大滑移速度的缺点，正像交错轴斜齿轮那样。

蜗杆涡轮机构有单包围和双包围机构。单包围积垢就是涡轮包裹着蜗杆的一种机构。当然，如果每个构件各自局部地包围着对方的涡轮机构就是双包围涡轮蜗杆机构。这两者之间的重要区别是，在双包围涡轮组成的轮齿间有面接触，而在单包围的涡轮组成的轮齿间有线接触。一个装置中的蜗杆和涡轮正像交错轴斜齿轮那样具有相同的齿向，但是其斜齿齿角的角度是极不相同的。蜗杆上的齿斜角度通常很大，而涡轮上的则极小，因此习惯常规定蜗杆的导角，那就是蜗杆齿斜角的余角：也规定了涡轮上的齿斜角，该两角之和就等于90度的轴线交角。

当齿轮要用来传递相交轴之间的运动时，就需要某种形式的锥齿轮。虽然锥齿轮通常制造成能构成90度轴交角，但它们也可产生任何角度的轴交角。轮齿可以铸出，铣制或滚切加工。仅就滚齿而言就可达一级精度。在典型的锥齿轮安装中，其中以一个锥齿轮常常装于轴承的外侧。这意味着轴的挠曲情况更加明显而使在轮齿接触上具有更大的影响。

另外一个难题，发生在难于预示锥齿轮轮齿上的应力，实际上是由于齿轮被加工成锥状造成的。

直齿锥齿轮易于设计且制造简单，如果他们安装的精密而确定，在运转中会产生良好效果。然而在直齿圆柱齿轮情况下，在线速度较高时，他们会发生噪音。在这些情况下，螺旋锥齿轮比直齿轮能产生平稳的多的啮合作用，因此碰到高速运转的场合是很有用的。当在汽车的各种不同用途中，有一个带偏心轴的类似锥齿轮的机构，那是常常所希望的。这样的齿轮机构叫做准双曲面齿轮机构，因为它们的界面是双曲回

转面。这种齿轮之间的轮齿作用是沿着一根直线上产生滚动与滑动相结合的运动并和蜗轮蜗杆的轮齿作用有着更多的共同之处。

轴是一种转动或静止的杆件。通常有圆形横截面。在轴上安装像齿轮，皮带轮，飞轮，曲柄，链轮和其他动力传递零件。轴能够承受弯曲，拉伸，压缩或扭转载荷，这些里相结合时，人们期望找到静强度疲劳强度作为设计的重要依据。因为单根轴可以承受静压力，变应力和交变应力，所有的应力作用都是同时发生的。

“轴”这个词包含着多种含义，例如心轴和主轴。心轴也是轴，既可以旋转也可以静止的轴，但不承受扭转载荷。短的转轴常常被称为主轴。

当轴的弯曲或扭转变形必须被限制于很小的范围内是，气尺寸应根据变形来确定，然后进行应力分析。因此，如若轴要做的有足够的强度以致挠曲不太大，那么合应力符合安全要求那是完全可能的。但决不意味着设计者要保证：它们是安全的，轴几乎总是要进行计算的，知道它们是处在可以接受的允许的极限以内。因此，设计者无论何时，动力传递零件，如齿轮或皮带轮都应该设置在靠近支撑轴附近。这就减低了弯矩，因而减小变形和弯曲应力。

虽然来自M.H.G方法在设计轴中难于应用，但它可能用来准确预示实际失效。这样，它是一个检验已经设计好了的轴或者发现具体轴在运转中发生损坏原因的好方法。进而有着大量的关于设计的问题，其中由于别的考虑例如刚度考虑，尺寸得到较好的限制。

设计者去查找关于圆角尺寸、热处理、表面光洁度和是否要进行喷丸处理等资料，那真正的唯一需要是实现所要求的寿命和可靠性。

由于他们的功能相似，将离合器和制动器一起处理。简化摩擦离合器或制动器的

动力学表达式中，各自以角速度ω

1和ω

2

运动的两个转动惯量I

1

和I

2

,在制动器情况下其中之一可能是零，由于接上离合器或制动器最终导致同样的速度。因为两个构件开始以不同速度运转而使打滑发生了，并且在作用过程中能量散失，结果导致温升。在分析这些装置的性能时，我们应注意到作用力，传递的扭矩，散失的能量和温升。所传递的扭矩关系到作用力，摩擦系数和离合器或制动器的几何状况。这是一个静力学问题。这个问题将必须对每个几何机构形状分别进行研究。然而升温与能量损失有关，研究表明温升可能与制动器或离合器的类型无关。因为几何形状的重要性是散热表面。各种各样的离合器和制动器可作如下分类：

1.轮缘式内膨胀制动块；

2.轮缘式外接触制动块；

3.条带式；

4.盘型或轴向式；

5.圆锥型；

6.混合式。

分析摩擦离合器和制动器的各种形式都应用一般的同样的程序，下面的步骤必需的：

1.假定或确定摩擦表面上压力分布；

2.找出最大压力和任一点处压力之间的关系；

应用静平衡条件去找寻(a)作用力；(b)扭矩；(c)支反力。

混合式离合器包括几个类型，例如强制接触离合器、超载释放保护离合器、超越离合器、磁液离合器等等。

强制接触离合器由一个变位杆和两个夹爪组成。各种强制接触离合器之间最大的区别与夹爪的设计有关。为了在结合过程中给变换作用予较长时间周期，夹爪可以是棘轮式的，旋型或齿型的。有时使用许多齿或夹爪。他们可能在圆周面上加工齿，以便他们以圆柱轴向配合来结合或者在配合元件的端面上加工齿来结合。

虽然强制离合器不像摩擦接触离合器用的那么广泛，但它们确实有很重要的运用。离合器需要同步操作。

有些装置例如线性驱动装置或惦记操作驱动器必须运行到一定的限度然后停顿下来。为着这些用途就要超载释放保护离合器。这些离合器通常用弹簧加载，以使得在达到预定的力矩时释放。当到达超载点时听到的“碦嚓”声就被认定为是所希望的信号声。

超越离合器或连轴器允许机器的被动构件“空转”或“超越”，因为主驱动件停顿了或者因为另一个动力源使被动构件增加了速度。这种离合器通常使用装在外套筒和轴件之间的滚子或滚珠。该内轴件，在它的周边加工了数个平面。驱动作用是靠在套筒和平面之间契入的滚子来获得。因此该离合器与具有一定数量齿的棘轮棘爪机构等效。

磁液离合器或制动器相对来说是一个新的发展，它们具有两平行的磁极板。这些磁极板之间有磁粉混合物润滑。电磁线圈被装入磁路中的某处。借助激励该线圈，磁液混合物的剪切强度可被精确的控制。这样从充分滑移到完全锁住的任何状态都可以

获得。