外文翻译---轴、联轴器和滚动轴承

附录二外文原文和翻译

shafts、couplings and rolling contact bearings

key words: shafts、couplings、bearings

Virtually all machines contain shafts.The most common shape for shafts is circular and the cross section can be either solid or hollow (hollow shafts can result in weight savings ).Rectangular shafts are sometimes used ,as in screwdriver blades,socket wrenches and control knob stems .

A shaft must have adequate torsional strength to transmit torque and not be overstressed. It also be torsionally stiff enough so that one mounted component does not deviate excessively from its original angular position relative to a second component mounted on the same shaft. Generally speaking ,the angle of twist should not exceed one degree in a shaft length equal to 20 diameters.

Shafts are mounted inbearings and transmint power through such devices as gears, pullerys, cams and clutches. These devices introduce forces which attempt to bend the shaft; hence, the shaft must be rigid enough to prevent overloading of the supporting bearings. In general, the bending deflection of a shaft should not exceed 0.01 in. per ft of length between bearing supports.

In addition, the shaft must be able to sustain a combination of bending and torsional loads. Thus an equivalent load must be considered

which takes into account both torsion and bending. Also, the allowable stress must contain a factor of safety which includes fatigue, since torsional and bending stress reversals occur.

For diameters less than 3 in. , the usual shaft material is cold-rolled ateel containing about 0.4 percent carbon. Shafts are either cold-rolled or forged in sizes from 3 in. to 5 in. ,shafts are forged and machined to size .Pleastic shafts are widely used for light load applications. One advantage of using plastic is safety in electrical applications ,since plastic is a poor conductor of electricity.

Components such as gears and pulleys are mounted on shafts by means of key .The design of the key and the corresponding keyway in the shaft must be prperly evaluated. For example ,stress concentrations occur in shafts due to keyways ,and the material removed to form the keyway further weakens the shaft.

If shafts are run at cirtical speeds ,severe vibrations can occur which can seriously damage a machine. It is important to know the magnitude of these critical speeds so that they can be avoided. As a general rule of thumb,the difference between the operating speed and the critical speed should be at least 20 percent .

Another important aspect of shaft design is the method of directly connecting one shaft to another. This is accomplished by devices such as rigid and flexible couplings.