齿轮英语文献.doc

Helical, Worm and Bevel GearsGears are machine elements that transmit motion by means of successively engaging teeth. Gears transmit motion from one rotating shaft to another, or to a rack that translates. Numerous applications exist in which constant angular velocity ratio (or constant torque ratio) must be transmitted between shafts. Based on the variety of gear types available, there is no restriction that the input and the output shafts need be either in-line or parallel. Nonlinear angular velocity ratios are also available by using noncircular gears. In order to maintain a constant angular velocity, the individual tooth profile must obey the fundamental law of gearing: for a pair of gears to transmit a constant angular velocity ratio, the shape of their contacting profiles must be such that the common normal passes through a fixed point on the line of the centers. Any two mating tooth profiles that satisfy the fundamental law of gearing are called conjugate profiles. Although there are many tooth shapes possible in which a mating tooth could be designed to satisfy the fundamental law, only two are in general use: the cycloidal and involute profiles. The involute has important advantages: it is easy to manufacture and the center distance between a pair of involute gears can be varied without changing the velocity ratio. Thus close tolerances between shafts are not required when utilizing the involute profile.There are several standard gear types. For applications with parallel shafts, straight spur gear, parallel helical, or herringbone gears are usually used. In the case of interesting shafts, straight bevel or spiral bevel gears are employed. For nonintersecting and nonparallel shafts, crossed helical, worm, face, skew bevel or hypoid gears would be acceptable choices. For spur gears, the pitch circles of mating gears are tangent to each other. They roll on one another without sliding. The addendum is the height by which a tooth projects beyond the pitch circle (also the radial distance between the pitch circle and the addendum circle). The clearance is the amount by which the dedendum (tooth height below the pitch circle) in a given gear exceeds the addendum of its mating gear. The tooth thickness is the distance across the tooth along the arc of the pitch circle while the tooth space is the distance between adjacent teeth along the arc of the pitch circle. The backlash is the amount by which the width of the tooth space exceeds the thickness of the engaging tooth at the pitch circle.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 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 engagement 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.Worm gears are similar to crossed helical gears .The pinlon 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 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 is one in which the gear wraps around or partially encloses the worm. A gearing in 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 gear of a set have the same hand of helix as for crossed helical gears, but the helix angle are usually quiet different. The helix angle on the worm is generally quiet 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 are equal for a 90-deg.shaft angle. When gears are to be used to transmit motion between intersecting shafts, some form of bevel gear is required. Although bevel gears 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 on the contact of the teeth. Another difficulty, which occurs in predicting the stress in bevel-gear teeth, is the fact that 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 spur gears, 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 sliding along a straight line and has much in common with that of worm gears.斜齿轮，蜗杆和圆锥齿轮齿轮是通过轮齿连续啮合的方式传递运动的机器零件。齿轮把运动从一个回转轴传递到另一个回转轴，或者传递到一传动齿条上。多数应用中都以恒定的角速度（或者恒定的扭矩比）而存在，恒定角速比中必应用于轴向传动。在各种各样有用类型齿轮的基础上，输入轴和输出轴需要在同一条直线上或者相互平行都不受任何限制。由于使用非圆柱齿轮，非线性角速比也是很有用的。为了保持恒定的角速度，每个轮齿的齿廓必须服从齿轮啮合的基本规律：为了让一对齿轮能传递一个恒定的角速度比，它们接触齿廓的形状必须要是这样的：公法线通过两齿轮中心连线上的固定点。满足齿轮基本规律的任何两个啮合齿廓称为共轭齿廓。尽管在设计中有许多配合的齿廓能被设计出来，以满足基本啮合规律。通常使用的只有两种类型：摆线型和渐开线型齿廓。渐开线齿廓有若干重要的优点：它不但容易加工，而且在不改变速比的情况下可以改变一对配合的渐开线齿轮的中心距。当使用渐开线齿廓时，可不要求精密的轴间公差。有几种常用的标准齿轮。常用于平行轴的的齿轮有直齿圆柱齿轮，平行轴斜齿轮，人字形齿轮。在相交轴的情况下使用直齿圆锥齿轮和螺旋齿轮。用于非相交和非平行轴的齿轮有交错轴螺旋齿轮，蜗杆，端面齿轮，斜齿圆锥齿轮，准双曲面齿轮。对于直齿圆柱齿轮，相互啮合齿轮的节圆是彼此相切的。他们互相滚动而无滑动。齿顶高是齿轮伸出超过节圆的高度（也是节圆和齿顶圆之间的径向距离）。顶隙是一个给定齿轮的齿根高（在节圆以下的齿高）大于与它啮合的齿轮的齿顶高的那个量（差值）。齿厚是沿着节圆的弧度跨齿的距离，齿间距是沿着节圆的弧度相邻轮齿之间的距离。齿侧间隙是齿间距的宽度超过在节圆处啮合轮齿的厚度的差值。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，接触线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。它是轮齿逐渐进行啮合并平稳地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。当轴向推力变得大了或由于别的原因而产生某些影响时，那就可以使用人字齿轮。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。当两个或更多的单向齿斜齿轮被装在同一轴上时，齿轮的齿向应作选择，以便产生最小的轴向推力。 蜗轮和交错轴斜齿轮是相似的。小齿轮即蜗杆具有较小的齿数，通常是一到四齿，由于它们完全缠绕在节圆柱上，因此它们又被称为螺纹齿。与蜗杆相配合的齿轮叫做蜗轮，蜗轮不是一个真正的斜齿轮。蜗杆和蜗轮通常用于垂直相交轴之间的传动，它们能够传递很大的角速度减速比。蜗轮不是斜齿轮，因为它的齿顶面做成中凹形状以配合蜗杆的曲率，目的是形成线接触而不是点接触。然而蜗杆蜗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。蜗杆蜗轮机构有单包围和双包围机构。单包围机构就是蜗轮包裹着蜗杆或部分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蜗轮机构就是双包围蜗轮蜗杆机构。这两者之间的重要区别是，在双包围蜗轮组的轮齿间有面接触，而在单包围蜗轮组的轮齿间只有线接触。一个装置中的蜗杆和蜗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。一般情况下，蜗杆上的螺旋角相当大，而蜗轮上的螺旋角非常小。正是由于这个原因，通常规定蜗杆的导程角，那就是蜗杆齿斜角的余角：也规定了蜗轮上的齿斜角，该两角之和就等于90的轴线交角。 当齿轮要用来传递相交轴的运动时，就需要某种形式的锥齿轮。尽管锥齿轮通常被加工成为90度的轴交角，但是它们可以被加工成任意角度的轴交角。轮齿可以被铸造，铣削和滚切加工。只有滚切加工的轮齿可以达到一级精度。在典型的锥齿轮安装中，其中一个锥齿轮常常装于支承的外侧。这意味着轴偏差更明显并且在轮齿的接触上有更大的影响。另外一个难题，发生在难于预示锥齿轮轮齿上的应力，实际上是由于轮齿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，对于直齿圆柱齿轮，在节线速度的较高时，他们将发出噪音。在这些情况下，通常设计使用螺旋锥齿轮，实践证明是切实可行的，那是和配对斜齿轮很相似的配对锥齿轮。在斜齿轮的情况下，螺旋锥齿轮在啮合运动中比直齿锥齿轮更平稳，因此在遇到高速时螺旋锥齿轮是更有用处的。 在汽车差速器的应用方面，有一个带偏心轴的类似锥齿轮的机构，那是常常所希望的。这样的齿轮称为准双曲面齿轮机构，因为他们的节圆表面是双曲回转面。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。5