A solid circular shafts is made of the 650°C temper 300-M steel with properties shown in the table. The shaft has a diameter of 6 mm, and is loaded with a bending moment of 8.0 kN-m, The nondestructive inspection assures that there are no cracks longer than a = 1.0 mm. Assume a safety factor of 2.0 against yielding is required. (a) Is the current design adequate? (yes or no) (b) It has been suggested that a weight and cost savings can be realized by changing to the 300°C temper material with a higher yield strength, and then using a smaller shaft diameter. What minimum shaft diameter would you recommend for the 300°C temper material? dmin = mm. Would you recommend a change to the 300°C temper material? Briefly explain your reason(s) Yield Elong. Red. Area Toughness Kic Material 00 % RA 100% f % MPa/m MPa % 300-M 152 1070 18 56 65 1740 12 48 650°C temper 300°C temper 300-M
Design Against Fluctuating Loads
Machine elements are subjected to varieties of loads, some components are subjected to static loads, while some machine components are subjected to fluctuating loads, whose load magnitude tends to fluctuate. The components of a machine, when rotating at a high speed, are subjected to a high degree of load, which fluctuates from a high value to a low value. For the machine elements under the action of static loads, static failure theories are applied to know the safe and hazardous working conditions and regions. However, most of the machine elements are subjected to variable or fluctuating stresses, due to the nature of load that fluctuates from high magnitude to low magnitude. Also, the nature of the loads is repetitive. For instance, shafts, bearings, cams and followers, and so on.
Design Against Fluctuating Load
Stress is defined as force per unit area. When there is localization of huge stresses in mechanical components, due to irregularities present in components and sudden changes in cross-section is known as stress concentration. For example, groves, keyways, screw threads, oil holes, splines etc. are irregularities.
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