机械设计_吴昌林课后复习题答案(部分).pdf

2-4 已知某钢制零件受弯曲变应力的作用，其中，最大工作应力 =200MPa，最小工作应 max 力 = －50MPa，危险截面上的应力集中系数k =1.2 ，尺寸系数 =0.85，表明状态系数=1 。 min   材料的=750MPa，=580MPa， =350MPa 。试求： s 0 -1 （1）绘制材料的简化极限应力图，并在图上标出工作应力点的位置； （2 ）求材料在该应力状态下的疲劳极限应力 ； r （3 ）按疲劳极限应力和安全系数分别校核此零件是否安全（取Smin=1.5 ） An alloy steel part suffers bending stress ： =750MPa，=580MPa， =350MPa s 0 -1 Working stress ： =200MPa， = －50MPa max min k =1.2 ，=0.85，=1 ，   (1) Make the stress endurance figure, and find the working point. (2) Find the fatigue stress limitation r. (3) Check if this part is safe, using the fatigue stress limitation and the safety factor (if Smin=1.5). Solution： 1) calculate stress amplitude、mean stress and stress ratio 2) judge where the working point is ＝0.46 ∴ working point is in AOD 3) calculate endurance limit 4) check strength A. Allowable stress Safe ！ B. Safety factor Safe ！ 3 －3、3－6、3 －8 3 －3 A transmission with two pairs of gears is shown as problems figure 3- 1. The axial forces of the two gears on the axle II, Fa2 and Fa3 are to be eliminated. The helix angle of the first pair of gears I ＝15. Try to determine the helix angle of the second pair of gears, and the helix directions for the gear 3 and gear 4 on it. Solution: (1) The torque of gear 2 and gear 3 T2=9550000*P2/n2 T3=9550000*P3/n3 n2=n3, since the two gears are on the same axle P2=P3, when ignore the transfer efficiency  So that, T2=T3 (2) The axial force of gear 2 equal to gear 3