带。平型和圆形皮带适用于皮带传动中中心距较远的两个皮带轮之间。同时，v型和同步皮带比较适合于较短中心距的情况。除了同步皮带在皮带和皮带轮之间也会有铸铁或者钢做成的滑轮。
A widely used type of belt, particularly in industrial drives工业传动 and vehicular 车辆applications应用, is the V-belt driveV型皮带传动. The V-belt causes the belt to wedge楔入 tightly紧紧的 into the groove of the sheave滑轮的槽, increasing friction摩擦力 and allowing high torques力矩 to be transmitted传输 before slipping滑动 occurs.
在工业传动和车辆应用中v型皮带被广泛的运用。v型皮带紧紧的楔入滑轮的槽中，提供了大的摩擦力和力矩用于传输，以避免滑动的发生。
Flat belt drives produce very little noise and absorb more vibration from the system than either V-belt or other drives. A flat belt drive has an efficiency效率 of around 98%, which is nearly the same as for a gear drive. A V-belt drive can transmit more power than a flat belt drive. However, the efficiency of a V-belt drive varies between 70% and 96%.
平带比其他各种传动发出的噪声都要少，而且能够吸收更多的震动。平带的效率大约是98%，和齿轮传动的效率差不多。v型皮带传动可以传输更多的能量，但是他的效率只有70%到96%。
A synchronous belt, sometimes called timing belt, has
evenly均匀的 spaced teeth on the inside circumference圆周 (see Fig.8/2). A synchronous belt does not slip and hence transmits power at a constant恒定的 angular velocity ratio角速度比.
同步皮带也成为同时皮带，在内圆周有均匀的齿轮空间。同步皮带传动不会打滑并且传递着恒定的角速度比。
However, if very large ratios比率 of speed reduction减速 are required in the drive, gear reducers are desirable理想的 because they can typically代表性的 accomplish large reductions in a rather small package很小的空间. The output shaft of the gear-type speed reducer is generally at low speed and high torque. If both speed and torque转矩 are satisfactory满足 for the application应用. It could be directly直接 coupled to耦合 the driven machine从动机.
但是如果要求传递大比率的减速那么齿轮传动是代表性的理想方式。他可以在很小的范围内完成大范围的速度减少。输出轴如果能满足速度和转矩要求，一般是低速高转矩。还可以直接耦合从动机。
Since gear reducers齿轮减速箱 are available only at discrete离散的 reduction ratios减速比, the output must often be reduced more before meeting the requirements of the machine. At the low-speed, high-torque condition, chain drives链传动 become desirable. The high torque causes high tensile forces拉力 to be developed in the chain. The elements of the chain are typically metal, and they are sized to withstand抵挡 the high forces. When transmitting power between rotating shafts
回转轴, the links of chains are engaged in从事于 toothed wheels齿轮 called sprockets齿轮链 to provide positive (no slip) drive. Fig.8.3 shows a typical chain drive.
由于齿轮减速箱只适用于离散的传动比，输出常常会达不到机器的要求。在低速高转矩条件时，比较适用链传动。大的转矩在传动链中造成了大的拉力。在回转轴间的传动，如果链连接作用与齿轮就称为是齿轮链用以提供正作用。
Chain drives combine some of the more advantageous features特色 of belt and gear drives. Chain drives provide almost any speed ratio速度比. Their chief主要，首席 advantage over gear drives is that chain drives can be used with arbitrary任意的 center distances. Compared with belt drives, chain drives offer the advantage of positive drive强制驱动 (no slip) and therefore greater power capacity.动力能量
链传动比皮带传动和齿轮传动有更多的特色。链传动基本上可以提供任何的传动比。相对于齿轮传动，链传动最大的好处是可以在任何中心距上工作。和皮带传动比较，链传动可以提供强制驱动并从而得到更大的动力能量。
The most common type of chain is the roller chain滚子链, in which the roller on each pin provides exceptionally特别的 low friction摩擦 between the chain and the sprockets. Of its diverse不同的 applications, the most familiar is the roller chain drive on a bicycle. A roller chain is generally made of hardened steel硬化钢 and sprockets链
齿轮 are generally made of steel or cast iron. Nevertheless然而, stainless steel不锈钢 and bronze青铜 chains are obtainable能得到的 where corrosion resistance抗腐蚀性 is needed.
One of the important considerations注意事项 in material selection材料选择 is their physical properties物理性质 (that is, density密度, melting point, specific heat比热, thermal conductivity导热性, thermal expansion热膨胀, and corrosion resistance抗腐蚀性). Physical properties化学特性 can have several important influences on manufacturing and the service life of components零件 .For example, high-speed machine高速钻机 tools require lightweight components to reduce inertial forces惯性力 and, thus, keep machines from excessive过多的 vibration.震动
在材料选择中，要注意材料的物理性质和化学性质。物理性质包括：密度，熔点，比热，导热性，热膨胀和抗腐蚀性。化学性质对零件的寿命和工作能力会有很大的影响。例如，高速钻机就要求使用轻质的零件来减少惯性，从而消除多余的震动。
1. Density密度 The density of a material is its mass per unit volume单位体积的质量. Another term is specific gravity比重, which expresses a material’s density in relation to that of water, and thus, it has no units. Weight saving is important particularly for aircraft and aerospace structures, for automotive bodies车身 and components, and for other products where energy consumption能量损耗
and power limitations局限性 are major concerns. Substitution代替 of materials for the sake of为什么的利益 weight saving and economy is a major factor主要因素 in the design both of advanced先进的 equipment and machinery and of consumer products消费品, such as automobiles.
材料的密度就是材料单位体积的质量。另一种叫法是比重，表示金属密度与水密度的比值，因此是没有单位的。对于飞行器和航空器，减少重量是异常重要的。对于车身零件，和其他一些主要关心能量损耗和能量局限性的装置时，设计中主要用减轻重量的材料代替。不论是在先进装备的设计还是消费品的设计。
2. Melting Point The melting point of a metal depends on the energy required to separate its atoms原子. The melting temperature of a metal alloy金属合金 can have a wide range (depending on its composition) and is unlike that of a pure metal, which has a definite melting point. The temperature range within which a component or structure is designed to function is an important consideration in the selection选择 of materials.
材料的熔点与材料融化需要的能量有关。金属合金的溶解温度可能是在一个范围内。与纯金属有恒定的溶解温度不同。零件的允许工作范围对于零件材料的选择是非常重要的指标。
The melting point熔点 of a metal has a number of indirect effects间接效应 on manufacturing operations生产过程. Because the recrystallization temperature再结晶温度 of a metal is