Lesson 5 Shafts and couplings 机械专业专业英语教学用书.ppt

Lesson 5 Shafts and couplings 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减轻重量)．Rectangular shafts are sometimes used，as in screwdriver blades，socket wrenches and control knob stems（螺丝起子的头、套筒扳手和控制旋扭的杆）． Lesson 5 Shafts and couplings A shaft must have adequate torsional strength（抗扭强度）to transmit torque and not be overstressed．It must 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（扭转角）should not exceed one degree in a shaft length equal to 20 diameters． Lesson 5 Shafts and couplings Shafts are mounted in bearings and transmit power through such devices as gears，pulleys，cams and clutches（齿轮、皮带轮、凸轮和离合器）．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 bearings supports． Lesson 5 Shafts and couplings In addition，the shaft must be able to sustain a combination of bending and torsional loads（弯矩和扭矩）．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（交变）occur． Lesson 5 Shafts and couplings For diameters less than 3 in., the usual shaft material is cold-rolled steel containing about 0.4 percent carbon．Shafts are either cold-rolled or forge（锻造）in sizes from 3 in. to 5 in. For sizes about over 5 in.，shafts are forged and machined to size．Plastic shafts are widely used for light load applications．One advantage of using plastic is safety in electrical applications，since plastic is a p