机械制造专业英语4B原文及翻译(章跃主编)

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CHAIN DRIVES

Chain drives occupy a unique position in the mechanical field.In a sense,they are similar to belt drives;a chain connects sprockets on the driving and driven shafts.The velocity ratio transmitted from one shaft to the other depends on the size of the two sprockets(at the pitch line);unlike that found with belts,the ratio is positive.In belt drive ,creep and slip play important roles and must be considered;in a chain drive,which is made up of numerous links,there is a small amount of play in the total length of the chain.This may be desirable in case of small overloads.A chain drive is similar to the open-belt type of dirve in that the driving and driven shafts rotate in the same direction.

There are also similarities between chain drives and gear drives.Both types transmit a positive velocity ratio.In a regular spur-gear drive,the driving and driven shafts turn in opposite directions unless an idler is employed.In gear drives employing an annular or ring gear,however,the input and output shafts rotate in the same direction.In a spur-gear drive there is very little contact between meshing gears;therefore,tooth loads are excessive.Since the connection between a chain and its sprocket extends over several teeth,no one tooth is subjected to heavy loads.

Chain drives are often noisy.The silent chain type of drive was developed to counteract this undesirable characteristic.Belt,chain,and gear drives are all dangerous.Belt and chain speeds are high,as is the pitch-line velocity of most gear drives.All should be adequately covered by suitable guards(or completely enclosed) so that no boby or clothing parts can possibly be caught in the moving components.

Other comparisons can be made among chain,belt,and gear drives.Gear drives can operate at higher speeds than the others,and are usually more compact.However,chain drives do not require the mounting precision that is so essential for gearing.An advantage of chain drives in comparison to belt drives is that the former do not require tension on the slack side;this leads to better bearing life.In general,chain drives are more compact than the belt type;for a given ratio,the sprockets can be smaller than belt pulleys.For a given load,chain widths are narrower than comparable flat belts.The connecting link on chain drives makes them easy to install—the chain is merely placed over the sprockets and the proper pins are placed in position.In most belt installations,force is needed to place the belt in the proper position;such action is detrimental to mounting bearings.The angle of contact for the driving sprocket can be smaller than the driving pulley for a belt drive.For a chain drive,this angle can be as small as 120 degrees.The center distance can also be small;if a 120-degree angle of contact is provided,the center distance need be no more than just enough to provide clearance for the sprocket teeth.

An essential part of any chain drive is the sprocket.A chain drive requires a sprocket on the driving shaft and another on the driven shaft.Chain drives sometimes contain additional sprockets,used for additional power take-offs or for taking up chain slack.When used to take up chain slack,they are known as idlers.Idlers should be mounted on the slack side,not on the tight side.It is desirable to have an adjustment for idler positioning where possible.Fig.4-5 shows

a roller-chain sprocket.

Sprocket teeth are standardized to accommodate the roller chain.Various sizes of sprockets may be used,and several types are available.The simplest type is one without a hub.Sprockets are also available with hubs on one side or on both sides.Special arrangements are also possible.Certain of these are used in conjunction with a shear-pin hub;the shear pin is designed to fail in case of an accidental or unexpected overload.Some types can be installed on large lineshafts;these sprockets are made in two parts so that they can be installed without disconnecting the long length of shafting.Smaller sprockets are usually made solid;large ones are sometimes constructed with arms or spokes to reduce weight.In general,the selection of a sprocket depends on the type of application.

链传动

链传动在机械领域中占据了独特的地位。在某种意义上，他们跟皮带传动有点相似；链条将主动轴和从动轴上的链轮连接起来。从一个轴传送到另一个轴的速度比率取决于两个链轮（节线）的大小；跟皮带传动的认知不同，链传动的比率是恒定的。在皮带传动中，蠕动和滑动扮演了重要的角色并且必须加以考虑；而在由众多链接组成的链传动中，整个链条长度上只分布了少部分的蠕动和滑动。在负载比较小的情况下才会去考虑它们。链传动和开口皮带传动的相似之处在于它们的主动轴和从动轴回转方向相同。

链传动和齿轮传动之间也有相似之处。这两种传动类型都传输一个恒定的速度比率。在常规直齿圆柱齿轮传动中，主动轴和从动轴的旋转方向是相反的（除非采用惰轮）。然而在采用内齿圈或环形齿轮的齿轮传动中，输入和输出轴的旋转方向是相同的。在直齿圆柱齿轮传动的啮合齿轮之间有一点点的接触；因此，齿载荷是过高的。因为链和链轮之间的连接遍及多个齿，所以没有一个齿会受到过高载荷。

链传动往往是嘈杂的。无声传动链被开发出来用来抵消这个不受欢饮的特征。皮带，链条和齿轮传动装置都是危险的。皮带和链条的速度都很高，和大多数齿轮传动的节线速度差不多。它们都应该用合适的防护物进行足够的覆盖（或完全封闭），这样才能保证不会有身体或服装部件可能被卷入运动部件中。

链条，皮带和齿轮传动之间还有别的不同点。齿轮传动相比较另外两个而言可以运行更高的速度而且通常更加紧凑。但是链传动不需要对齿轮传动来说至关重要的装配精度。链传动跟皮带传动相比的优势在于前者不需要在松边上的张紧力，因而增强了轴承寿命。在一般情况下，链传动比皮带传动更紧凑。对于一个给定的比率，链轮通常要比皮带轮更小。对于一个给定的载荷，链条宽度要比同类型的平皮带更窄。链传动上的连接形式使得它们易于安装——链条只是被放置在链轮上并且套在合适的链轮齿上。在大多数皮带装置中，需要用压力将皮带放置到正确的位置，而这样做对安装轴承是不利的。主动链轮的接触角度要比皮带轮的小。对于链传动来说，这个角度最小是120度，而且它的中心距也是同样小的，如果提供一个120度角的接触，那么该中心距对于提供链轮齿径向间隙来说是远远足够的。

任何链传动不可或缺的组成部分是链轮。链传动需要主动轴和从动轴上都装有链轮。链传动有时包含额外的链轮，用来提供额外的力来松弛或拉紧松边链条。当用于拉紧松边链条时，它们被称为惰轮。惰轮应装在松弛端，而不是拉紧端。理想的情况是在可行的位置放置一个可调整的惰轮。图4-5所示为一个滚子链链轮。

链轮齿都被标准化以适应滚子链条。各种规格的链轮可以被使用，但只有几种是有效的。最简单的类型是没有轮毂的链轮。单边或双边有轮毂的链轮同样是可用的。当然特别的类型

也是可能的。这其中的某些被用来连接剪刀销轮毂，剪刀销被设计用来打断偶然或意想不到的过载荷情况。一些类型可以安装在大型的总轴上，这些链轮被制造成两部分，这样它们可以在不分离长轴的情况下安装上去。小的链轮通常被做成实心的，大型的链轮则有时候由轮臂或轮辐构成，以此来减轻重量。一般来说，链轮的选择取决于其应用类型。