外文原文：
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 ction can be either solid or hollow (hollow shafts can result in weight savings ).Rectangular shafts are sometimes ud ,as in screwdriver blades,socket wrenches and control knob stems .
A shaft must have adequate torsional strength to transmit torque and not be overstresd. It also be torsionally stiff enough so that one mounted component does not deviate excessively from its original angular position relative to a cond 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, pullery
s, cams and clutches. The 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 ud 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 des
ign 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 ,vere vibrations can occur which can riously damage a machine. It is important to know the magnitude of the 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.
A coupling is a device for connecting the eds of adjacent shafts. In machine construction, ouplings are ud to effect a mipermanent connection between adjacent rotating shafts. The connection is permanent in the n that it is not meant to be broken during the uful life of the machine, but it can be broken and restored in an emergency or when worn parts are replaced.
There are veral types of shaft couplings, their characteristics depend on the purpo for which they are ud. If an exceptionally long shaft is required in a manufacturing plant or a propeller shaft on a ship, it is made in ctions that are coupled together with rigid couplings. A common type of rigid coupling consists of two mating radial flanges(disks) that are cttached by key-driven hubs to the eds of adjacent shaft ctions and bolted together through the flanges to form a rigid connection. Alignment of the connected shafts is usually effeted by means of a rabbet joint on the face of the flanges.
In connecting shafts belonging to parate devices (such as an electric motor and a gearbox ),preci aligning of the shafts is difficult and a flexible coupling is ud. This coupling connects theshafts in such a way as to minimize the harmful effects of shaft misalignment. Flexible couplings also permit the shafts to deflect under their parate systems of with one another. Flexible couplings can also rve to reduce the intensity of shock loads and vibrations transmitted from one shaft to another.
Virtually all shafts contain rolling contact bearings.The concern of a machine designer wit
h ball and roller bearings is fivefold as follows:(a) life in relation to load; (b) stiffness ,i.e. deflections under load; (c) friction; (d) wear; (e) noi. For moderate loads and speeds the correct lection of a standard bearing on the basis of load rating will usually cure satisfactory performance. The deflection of the bearing elements will become important where loads are high, although this is usuallyof less magnitude than that of the shafts or other components associated with the bearing. Where speeds are high special cooling arrangements become necessary which may increa frictional drag. Wear is primarily associated with the introduction of contaminants, and aling arrangements must be chon with regard the hostility of the environment.
