机械工程专业英语lesson12 翻译

Spur Gears
Gears , defined as toothed members transmitting rotary motion from one shaft to another , are among the oldest devices and inventions of man . In about 2600 B.C. , the Chinese are known to have used a chariot incorporating a complex series of gears . Aristotle , in the fourth century B.C. , wrote of gears as if they were commonplace . In the fifteenth century A.D. , Leonardo da Vinci designed a multitude of devices incorporating many kinds of gears .
齿轮，在最古老的设备和发明人中，被定义为通过轮齿将旋转运动从一根轴传递到另一根轴，大约在公元前2600年，中国人就以能够用一系列复杂的齿轮机构做成战车而出名。西元前四世纪，亚里士多德记述了齿轮就好像是他们司空见惯的一样。在十五世纪，达芬奇设计了大量的包含各种各样齿轮的设备。
Among the various means of mechanical power transmission (including primarily gears , belts , and chains ) , gears are generally the most rugged and durable . Their power transmission efficiency is as high as 98 percent . On the other hand , gears are usually more costly than chains and belts . As would be expected , gear manufacturing costs increase sharply with increased precision -- as required for the combination of high speeds and heavy loads , and for low noise levels . ( Standard tolerances for various degrees of manufacturing precision have been established by the AGMA , American Gear Manufacturers Association. )
在众多的机械传动方式中（包括齿轮传动，带传动，链传动），一般来说，齿轮是最经久耐用的，它的能量传递效率高达98%。另一方面，齿轮一般比带传动和链传动成本更高。可以想象，齿轮制造的成本会随着齿轮的精密度急剧增加--基于对高速和重载的组合以及低噪音的要求。（美国齿轮制造盟已经确立了齿轮制造精度的公差标准）
Spur gears are the simplest and most common type of gears . They are used to transfer motion between parallel shafts , and have teeth that are parallel to the shaft axes .
直齿圆柱齿轮是最简单也是最常见的齿轮。它们通过齿的啮合在两中轴线平行的轴间传递运动。
The basic requirement of gear-tooth geometry is the provision of angular velocity ratios that are exactly constant . For example , the angular velocity ratio between a 20-tooth and a 40-tooth gear must be precisely 2 in every position . ( It must not be , for example , 1.99 as a given pair of teeth comes into mesh , and then 2.01 as they go out of mesh. ) Of course , manufacturing inaccuracies and tooth deflections will cause slight deviations in velocity ratio ; but acceptable tooth profiles are based on theoretical curves that meet this criterion .
轮齿几何学的基本要求是提供确定不变的角速度比例。例如，齿数为20和齿数为40的齿轮在任何位置的角传动比必须为精确的两倍（这样一定不能行

，例如1.99为给定的进入啮合的传动比，而脱离啮合时传动比变为2.01）,当然，齿轮制造的误差和轮齿变形也将引起轻微的传动比偏差，但是可接受的齿侧面是基于符合标准的理论曲线。
As gears mesh , both rolling and sliding occur , causing pitting and wear ; pitting is primarily a consequence of rolling contact and wear is the result of sliding contact . Pitting is initiated when contact stresses are high ; it is hastened by sliding action . Although sliding causes wear , it also creates hydrodynamic action that counteracts wear . This relative motion , minute as it may be , is sufficient to provide conditions necessary for hydrodynamic action : a narrow , wedge-sharped opening between surfaces that have relative motion . Each time a gear goes through mesh with its accompanying rolling and sliding , surface and subsurface material is subjected to compressive , shear , and tensile stresses . The result is slow surface destruction in the form of pockmarks and metal erosion . These phenomena , for clarity , are often described as occurring separately , but this is not the case in practice . Two or more may occur simultaneously . In fact , one type may promote the other . The result is roughening of tooth surface , alteration of tooth profile , and loss of conjugate motion . Failure has definitely occurred when performance is unsatisfactory because of noise , vibration , or overheating .
由于齿轮啮合，滚动和滑动同时发生，就会产生点蚀和磨损；点蚀主要是由于滚动接触造成，磨损是由滑动接触导致的结果。点蚀发生是由于接触应力过高；它的加剧是由于滑动摩擦的影响。虽然滑动接触引起磨损，但它产生的水动力作用也减缓一部分磨损。这种相对运动，短时间内就足够提供产生水动力作用的必须条件：一个狭小的开放的尖锐的接触面之间的相对运动，每一次经过一个齿轮啮合其伴随滚动和滑动，表层和次表层的物质经历压缩，剪切和张力。结果导致以麻点或金属冲蚀形式的缓慢的表面破坏。为了清楚解释这种现象，这个过程经常描述为分别发生的，但实际中的情况并非如此。两个或多个是同时发生的。实际上一种作用会促进另一种作用，这些因素的结果是齿面变得粗糙，齿侧面变形，失去共轭运动。当表现出噪音、振动、过热等不尽人意的现象时，故障必然会发生。
Pitting first occurs at the pitch line , where the absence of sliding favors early breakdown of protective oil films . Excessive contact stress causing surface fatigue is the real cause of pitting . This may be due to (1) a narrow gear width , (2) radii of the involute surfaces being too small ,and (3) frequent overloads . The smaller the radius of either surface , the narrower is the contact band and the greater the unit stress . After what is usually a very large nu

mber of stress repetitions , surface failure may occur . Minute cracks form in and below the surface , then grow and join . Eventually , small bits of metal are separated and forced out , leaving pits .
点蚀会首先发生在油膜破坏失去滑动倾向的齿轮节线上。产生点蚀的本质原因是过高的接触应力引起表面接触疲劳。造成这个原因可能是由于（1）齿宽过窄，（2）渐开线曲面半径太小，（3）频繁过载。渐开线半径越小，齿宽越窄，接触带单位面积所受应力越大。经过大量重复的应力，表面失效可能发生。微小的裂缝产生在表面下，然后接着扩展并连接在一起。最后，金属颗粒分离并剥落，留下坑。
The least expensive gear material is usually ordinary cast iron , ASTM ( or AGMA ) grade 20. Grades 30,40,50,and 60 are progressively stronger and more expensive . Cast iron gears typically have greater surface fatigue strength than bending fatigue strength . Their internal damping tends to make them quieter than steel gears . Nodular cast iron gears have substantially greater bending strength , together with good surface durability . A good combination is often a steel pinion mated to a cast iron gear .
价格最便宜的齿轮材料是普通铸铁，材料等级为20.等级分别为30,40,50和60的材料性能逐渐增强价格也更贵。铸铁齿轮表面通常有比弯曲疲劳强度更大的接触疲劳强度。其内部阻尼使它们比钢齿轮噪音更小。球墨铸铁齿轮有较高的弯曲强度和良好的接触强度。通常一个很好的组合是一个小的钢齿轮与大的铸铁齿轮啮合。
Unheat-treated steel gears are relatively inexpensive , but have low surface endurance capacity . Heat-treated steel gears must be designed to resist warpage ; hence , alloy steels and oil quenching are usually preferred . For hardnesses over 250-350 Bhn , machining must usually be done before hardening . Greater profile accuracy is obtained if the surfaces are finished after heat-treating , as by grinding . But if grinding is done , care must be taken to avoid residual tensile stresses at the surface .
未经热处理的钢齿轮相对便宜，但是表面耐磨性较差。经过热处理的钢齿轮必须设计得能抵抗弯曲；因此，合金钢和油淬火都会优先考虑。对于布氏硬度在250-350的材料，应该在硬化前进行机械加工。更大的轮廓精度是热处理后再经过研磨加工得到的。但是完成研磨加工后，要小心避免表面残余拉应力。
Of the non-ferrous metals , bronzes are most often used for making gears . Nonmetallic gears ( nylon and other plastics ) are generally .quiet , durable , reasonably priced , and often can operate under light loads without lubrication . Their teeth deflect more easily than those of corresponding metal gears . This promotes effective load sharing among teeth in simultaneous contact . Since nonmetallic materials have l

ow thermal conductivity , special cooling provisions may be required . Furthermore , these materials have relatively high coefficients of thermal expansion , and this may require installation with greater backlash than metal gears .
对于有色金属，青铜最常用来制造齿轮。非金属齿轮（尼龙和其他塑料）一般较轻，耐用，价格合理，而且轻载时可以不经过润滑使用。相比金属齿轮，它们的轮齿更容易弯曲。这促进有效载荷同时分布在同时接触的轮齿上。由于非金属材料导热性差，所以可能需要特殊的散热方法。此外，这些非金属材料具有较高的热膨胀系数，因此这可能需要安装比金属齿轮更大的齿侧间隙。
Nonmetallic gears are usually mated with cast iron or steel pinions . For best wear resistance , the hardness of the mating metal pinion should be at least 300 Bhn . Design procedures for gears made of plastics are similar to those for gears made of metals , but are not yet as reliables . Hence , prototype testing is even more important than for metal gears .
非金属齿轮经常和铸铁齿轮或小钢齿轮啮合。为了更好的耐磨性，与之啮合的小钢齿轮的布氏硬度至少应为300.设计塑料制成的齿轮同金属齿轮的类似，但是不完全一样，因此，非金属齿轮的原型试验比金属齿轮更重要。