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外文翻译--齿轮和齿轮传动-其他专业.doc

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外文翻译--齿轮和齿轮传动-其他专业.doc

兰州交通大学毕业设计(论文) Gears and gear drive Gears are the most durable and rugged of all mechanical drives. They can transmit high power at efficiencies up to 98 and with long service lives. For this reason, gears rather than belts or chains are found in automotive transmissions and most heavy-duty machine drives. On the other hand, gears are more expensive than other drives, especially if they are machined and not made from power metal or plastic. Gear cost increases sharply with demands for high precision and accuracy. So it is important to establish tolerance requirements appropriate for the application. Gears that transmit heavy loads or than operate at high speeds are not particularly expensive, but gears that must do both are costly. Silent gears also are expensive. Instrument and computer gears tend to be costly because speed or displacement ratios must be exact. At the other extreme, gears operating at low speed in exposed locations are normally termed no critical and are made to minimum quality standards. For tooth forms, size, and quality, industrial practice is to follow standards set up by the American Gear Manufactures Association AGMA. Tooth form Standards published by AGMA establish gear proportions and tooth profiles. Tooth geometry is determined primarily by pitch, depth, and pressure angle. PitchStandards pitches are usually whole numbers when measured as diametral pitch P. Coarse-pitch gearing has teeth larger than 20 diametral pitch –usually 0.5 to 19.99. Fine-pitch gearing usually has teeth of diametral pitch 20 to 200. Depth Standardized in terms of pitch. Standard full-depth have working depth of 2/p. If the teeth have equal addendaas in standard interchangeable gears the addendum is 1/p. Stub teeth have a working depth usually 20 less than full-depth teeth. Full-depth teeth have a larger contract ratio than stub teeth. Gears with small numbers of teeth may have undercut so than they do not interfere with one another during engagement. Undercutting reduce active profile and weakens the tooth. Mating gears with long and short addendum have larger load-carrying capacity than standard gears. The addendum of the smaller gear pinion is increased while that of larger gear is decreased, leaving the whole depth the same. This form is know as recess-action gearing. Pressure Angle Standard angles are and . Earlier standards include a 14-pressure angle that is still used. Pressure angle affects the force that tends to separate mating gears. High pressure angle decreases the contact ratio ratio of the number of teeth in contact but provides a tooth of higher capacity and allows gears to have fewer teeth without undercutting. Backlash Shortest distances between the non-contacting surfaces of adjacent teeth . Gears are commonly specified according to AGMA Class Number, which is a code denoting important quality characteristics. Quality number denote tooth-element tolerances. The higher the number, the closer the tolerance. Number 8 to 16 apply to fine-pitch gearing. Gears are heat-treated by case-hardening, through-hardening, nitriding, or precipitation hardening. In general, harder gears are stronger and last longer than soft ones. Thus, hardening is a device that cuts the weight and size of gears. Some processes, such as flame-hardening, improve service life but do not necessarily improve strength. Design checklist The larger in a pair is called the gear, the smaller is called the pinion. Gear Ratio The number of teeth in the gear divide by the number of teeth in the pinion. Also, ratio of the speed of the pinion to the speed of the gear. In reduction gears, the ratio of input to output speeds. Gear Efficiency Ratio of output power to input power. includes consideration of power losses in the gears, in bearings, and from windage and churning of lubricant. Speed In a given gear normally limited to some specific pitchline velocity. Speed capabilities can be increased by improving accuracy of the gear teeth and by improving balance of the rotating parts. Power Load and speed capacity is determined by gear dimensions and by type of gear. Helical and helical-type gears have the greatest capacity to approximately 30,000 hp. Spiral bevel gear are normally limited to 5,000 hp, and worm gears are usually limited to about 750 hp. Special requirements Matched-Set Gearing In applications requiring extremely high accuracy, it may be necessary to match pinion and gear profiles and leads so that mismatch does not exceed the tolerance on profile or lead for the intended application. Tooth Spacing Some gears require high accuracy in the circular of teeth. Thus, specification of pitch may be required in addition to an accuracy class specification. Backlash The AMGA standards recommend backlash ranges to provide proper running clearances for mating gears. An overly tight mesh may produce overload. However, zero backlash is required in some applications. Quiet Gears To make gears as quit as possible, specify the finest pitch allowable for load conditions. In some instances, however, pitch is coarsened to change mesh frequency to produce a more pleasant, lower-pitch sound. Use a low pressure angle. Use a modified profile to include root and tip relief. Allow enough backlash. Use high quality numbers. Specify a surface finish of 20 in. or better. Balance the gear set. Use a nonintegral ratio so that the same teeth do not repeatedly engage if both gear and pinion are hardened steel. If the gear is made of a soft material, an integral ratio allows the gear to cold-work and conform to the pinion, thereby promoting quiet operation. Make sure critical are at least 20 apart from operating speeding or speed multiples and from frequency of tooth mesh. Multiple mesh gear Multiple mesh refers to move than one pair of gear operating in a train. Can be on parallel or nonparallel axes and on intersection or nonintersecting shafts. They permit higer speed ratios than are feasible with a single pair of gears . Series trainsOverall ratio is input shaft speed divided by output speed ,also the product of individual ratios at each mesh ,except in planetary gears .Ratio is most easily found by dividing the product of numbers of teeth of driven gears by the product of numbers of teeth of driving gears. Speed increasers with step-up rather than step-down ratios may require special care in manufacturing and design. They often involve high speeds and may creste problems in gear dynamics. Also, frictional and drag forces are magnified which, in extreme cases , may lead to operational problems. Epicyclic GearingNormally, a gear axis remains fixed and only the gears rotates. But in an epicyclic gear train, various gears axes rotate about one anther to provide specialized output motions. With suitable clutchse and brakes, an epicyclic train serves as the planetary gear commonly found in automatic transmissions. Epicyclic trains may use spur or helical gears, external or internal, or bevel gears. In transmissions, the epicyclic or planetary gears usually have multiple planets to increase load capacity. In most cases, improved kinematic accuracy in a gearset decreases gear mesh excitation and results in lower drive noise. Gearset accuracy can be increased by modifying the tooth involute profile, by substituting higher quality gearing with tighter manufacturing tolerances, and by improving tooth surface finish. However, if gear mesh excitation generaters resonance somewhere in the drive system, nothing short of a “perfect” gearset will substantially reduce vibration and noise. Tooth profiles are modified to avoid interferences which can result from deflections in the gears, shafts, and housing as teeth engage and disendgage. If these tooth interferences are not compensated for by profile modifications, gears load capacity can be seriously reduced. In addition, the drive will be noisier because tooth interferences generate high dynamic loads. Interferences typically are eliminated by reliving the tooth tip, the tooth flank, or both. Such profile modifications are especially important for high-load , high-speed drives. The graph of sound pressure levelvs tip relief illustrates how tooth profile modifications can affect overall drive noise. If the tip relief is less than this optimum value, drive noise increases because of greater tooth interference; a greater amount of tip relief also increase noise because the contact ratio is decreased. Tighter manufacturing tolerances also produce quietier gears. Tolerances for such parameters as profile error, pitch AGMA quality level. For instance, the graph depicting SPL vs both speed and gear quality shows how noise decreases example, noise is reduced significantly by an increase in accuracy from an AGMA Qn 11 quality to an AGNA Qn 15 quality. However, for most commercial drive applications, it is doubtful that the resulting substantial cost increase for such an accuracy improvement can be justified simply on the basis of reduced drive noise. Previously, it was mentioned that gears must have adequate clearance when loaded to prevent tooth interference during the course of meshing. Tip and flank relief are common profile modifications that control such interference. Gears also require adequate backlash and root clearance. Noise considerations make backlash an important parameter to evaluate during drive design. Sufficient backlash must be provided under all load and temperature conditions to avoid a tight mesh, which creates excessively high noise level. A tight mesh due to insufficient backlash occurs when the drive and coast side of a tooth are in contact simultaneously. On the other hand, gears with excessive backlash also are noisy because of impacting teeth during periods of no load or reversing load. Adequate backlash should be provided by tooth thinning rather than by increase in center distance. Tooth thinning dose not decrease the contact ratio, whereas an increase in center distance does. However, tooth thinning does reduce the bending fatigue, a reduction which is small for most gearing systems. 齿轮和齿轮传动 在所有的机械传动?#38382;?#20013;,齿轮传动是一种最结实耐用的传动方式。它们可以传递很大的功率,效率可?#28304;?#21040;98,并且服务年限长。由于具有以上优点,齿轮传动比皮带装置等其它传动方式更常见于自动式传动机构和重载机构中。在另?#29615;?#38754;,齿轮比其它传动方案贵得多,特别是精加工齿轮和合金钢材料的。齿轮的制造成本会随便着精度和公差的要求?#26412;?#22686;加。因此,在合?#23454;?#33539;围内选一个合理的公差带就显得尤其重要。用于大功率传递和高速传递的齿轮传动系统不是特别的贵,但是用合金钢材料和精加工的齿轮成本比较高。 低噪声齿轮机构也很昂贵。精密仪器和电脑里用的齿轮机构住住是相当昂贵的,因为它们对速度和传动比的要求很高。低速?#30446;?#24335;传动的?#27426;?#20041;为非临界状态,并且?#28304;?#20316;为齿轮的最小标准。齿轮的形状、尺寸、性质和工业用途都遵循美国齿轮制造协会所制定的标准。 美国齿轮制造协会发布的标准说明齿轮?#26723;?#20256;动比分配比例和齿的轮廓。齿的几何形状主要是由节距、齿高和压力角来?#33539;?#30340;。 节距标准节距通常都是整数。大节距齿轮的节距?#26412;侗人?#30340;节距的二十倍还大,一般在0.5~19.99之间。小节距齿轮的节距?#26412;?#19968;在20~200之间。 齿高以节距为标准,齿轮的工作齿面高度是全齿高的一半。如果齿轮有相同的齿高那么齿高是节距的倒数。变位齿轮它的工作时的啮合深度通常?#20154;?#30340;全齿高少20,以?#20048;共?#29983;根切身。不变位齿轮比变位齿轮的传动比更大。齿数较少的齿轮可能会产生根切,所?#28304;?#20999;削深度的齿轮比起它们来在啮合时候齿轮互不影响。减少齿轮的有效齿廓会使齿轮的强度削弱。让变位齿轮和不变位齿轮相啮和能传递比标准齿轮更大的功率。两个个啮合的齿轮当变位齿轮齿高减小?#20445;?#19981;变位齿轮向变位后的齿轮深入一些,保证啮合高度不变。这就?#20405;?#25152;周知的间歇性齿轮。 压力角压力角通常取和。早期的压力角还包括14-1/2,现在仍然在使用。压力角的大小会影响相啮合齿轮的强度。大的压力角可以减少齿轮在啮合时的齿数,而且利用不变位齿轮还能够传递更大的功率。 齿侧间隙在两个啮合的齿之间非接触最小的那个间隙。齿轮传动系统都严格按照美国齿轮制造协会所制定的等级制造,每个指标?#24613;?#31034;齿轮的一项重要性能。特性指数表示齿轮元素的公差,等级数目越高,它越接近于公差。等级3~5应用于大节距齿轮,8~16应用于小节距齿轮。 齿轮通过热处理提高强度,?#28909;?#34920;面?#19981;?#28140;火、氮化、回火。一般而言,硬齿面的齿轮系统比软齿面的齿轮系统使用寿命更长更坚固。因而,淬火可以减小齿轮的尺寸和重量。有些处理方式,例如表面淬火可以提高齿轮的使用寿命但是没有必要提高它的强度。 齿轮传动系统的校核项目 在?#27426;?#30456;啮合的齿轮中,大的那个是从动轮,小的?#20405;?#21160;轮。 齿数比大齿轮的齿数除以小齿轮的齿数。同样也是小齿轮的线速度除?#28304;?#40831;轮的线速度。在齿轮减速机构中,是输入速度与输出速度的比值。 齿轮传动的效率齿轮输出功率与输入功?#23454;?#27604;值。(包括考虑传动时的功率损失,轴?#23567;?#32852;轴器、和润滑的功率损失) 在一些给定的齿轮中,节圆线速度是限定的。齿轮传动速率可以通过提高齿轮制造精度、增加回转件的平衡性来提高。负荷速度和传递功率大小受齿轮尺寸和齿轮类型的限制。斜齿轮和斜齿轮系所能传递的功?#39318;?#22823;,可以近似达到30000马力、弧齿锥齿轮一般限制在5000马力、蜗轮蜗杆传动限制在大约750马力。 工艺要求 齿轮配合在工艺上要求比较高精度的齿轮系?#25345;校?#23545;于?#20048;?#38169;齿、齿廓与齿廓接触和从动齿轮的啮合,不会超过规定的范围是很有必要的。 齿间隙有些齿轮对齿廓的精度要求相当高,因此,齿轮的规格等级必须符合所规定的精度等级。 无声传动装置将齿轮传动系?#25345;?#36896;得尽可能的静音。为了达到此目的可以有以下多种方法供选择,选择小螺距齿轮来满足负荷状态的要求;在某些特定情况下,可以改变齿轮的啮合?#38382;?#26469;使传动声音减小,或者使声音更加?#32479;烈源?#21040;静音的目的;用压力角较小和对齿轮根尖都进行过修正的齿轮;允许足够大的齿间隙;采用高的特性指数;保证表面粗糙度在20或者更小;合理分配齿轮?#26723;?#20256;动比;采用一个非整数的传动比,那么一样的齿轮就不会重复的啮合如果它们都是?#19981;?#38050;材料。如果齿轮由软钢制成且传动比为整数,则齿轮必须冷作处理以满足工作的要求,从而实现无声传动。保证速度临界点大于全速运行的20或者通过增加齿轮啮合?#38382;?#26469;成倍增加的转速。 齿轮系传动装置?#20405;?#22312;一个传动装置中有不只?#27426;?#40831;轮在啮合工作。可以是相互平行或不平行的轴,相交或不相交的轴。在?#23548;?#24212;用中,他们可?#28304;?#21040;很高的速度?#35748;?#23545;于只有?#27426;?#40831;轮啮合的传动装置。串联齿轮系,所有啮合齿轮的传动比都是将输入轴的转速降到输出轴的转速。总的传动比是所有传动比的乘积,行星轮系不适用这种计算方法。这种传动装置的传动比很好计算,就是将每?#27426;阅?#21512;齿轮的传动?#35748;?#20056;。增速器在设计和制造方面有特殊的工艺要求。他们通常包括很高的速度还可能有一些齿轮动力学里一些很极端的问题,同样,摩擦力和拉力也包含在里面,在这种情况下还可能进一步导致操作的问题。 行星轮系传动通常在一个传动装置中,齿轮轴线是固定不变的的仅仅?#20405;?#19978;的齿轮在转动。但是在一个行星轮系中,不同的齿轮轴围着太阳轮地轴线转动给特定的输出装置提供动力。行星轮传动再配合离合器和刹车装置,就可以组成一个无级变速的自动驾驶系?#22330;?#34892;星轮传动可以用直齿或者斜齿,内齿轮或者外齿轮,或者锥齿轮。在传递过程中,可以通过增?#26377;行?#36718;的个数来达到传递更大功?#23454;?#35201;求。 在许多情况下, 提高齿轮系中相啮合齿轮?#33041;?#21160;精确度可以?#26723;?#26426;构运行?#33041;?#38899;。修改齿轮渐开线齿形可以提高齿轮的精确度,用高精度的制造公差来保证高质量的齿轮啮?#29616;?#37327;;提高齿面的粗糙度。但是,如果在一个传动系统的某个地方发生振动那么一个“完美”的齿轮机构将会减少振动和噪声。修正齿轮的齿廓可?#21592;?#20813;在传动过程中由于偏差、轴的偏移、机壳的不标?#32423;?#20135;生干涉。如果齿轮干涉不能通过修正齿廓来消除那么齿轮上的载荷应该减少。当齿轮载荷很大?#20445;?#26426;构噪声会更大因为内部传递的齿轮发生了干涉。消除干涉可以通过改变齿高、齿侧间隙或者两者都做。齿轮变?#27426;?#20110;重载机构和高速传动机构尤其重要。声音压力水?#35282;?#32447;图可以很形象地说明齿轮变位可以影响齿轮机构?#33041;?#22768;。如果减少的量比最适宜量小的话,那么机构会产生更大?#33041;?#22768;,因为齿轮干涉。减少过多的齿高度噪声也会增强因为接触比例减小了。 高制造公差等级的齿轮?#37096;?#20197;实现无声传动,那样的公差等级作为齿廓的形位误差可?#28304;?#21040;美国齿轮制造协会的质量水平。这个图表描述了速度和齿轮质量对声音压力水平的影响,还有如何减小噪声的方法。当齿轮的精度等级由美国齿轮制造协会规定的11级增加到15级?#20445;?#22122;声明显的减小了。但是对于商业用的传动机构来说,花费这么大的代价在?#26723;?#22122;声上是不划算的,因为还有别的更廉价的方式来?#26723;?#22122;声。 以前有个说法,为了?#20048;?#40831;轮干涉两个相啮合的齿轮必须经过修正。齿顶高和齿侧间隙都是很常用的齿廓修正?#21592;?#35777;齿轮?#29615;?#29983;干涉。齿轮传动系统也需要有?#23454;?#30340;齿侧间隙和齿根修正。在设计齿轮机构中,齿侧间隙是评定噪声的一个重要?#38382;?#24517;须有足够的齿侧间隙和合理的载荷、温度状况来?#20048;?#40831;轮的干涉,否则会产生很大?#33041;?#22768;。干涉是由于齿侧间隙不足造成,工作的齿面和不工作齿面同时接触上了。另?#29615;?#38754;,过大的齿侧间隙也会产生噪声,因为在齿轮无载荷啮?#29616;?#26399;内或回动载荷会对齿轮产生冲击。要获得合理的齿侧间隙,减少齿的个数比增加轴的中心距效果更好。减少齿数不会减少齿轮接触比例,反之增大中心距也不会。但是减少齿数会减小齿轮的挠曲疲劳,这个减小量对一个齿轮系统来说是很小的。 9

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