The bolts used to mount an engine on an industrial machine must each carry a static tensile load of 5600 lb. a.) What is the minimum acceptable size of a SAE grade 5 UNC bolt required to support this load with a "Safety Factor" of 2.5 and the failure of the bolt based on the "proof strength"? b.) If the bolts from "Part a" are used with nuts made from steel having the SAE grade 2 specification, calculate the minimum thickness of the nut "t" and the number of threads that must be engaged to support the load if: i.) The shear strength of the threads must be sufficient to support the given load. ii.) (Preferred Engineering Solution) The force needed to "yield" the threads in the nut due to shear must be equal to the force needed to "yield" the bolt. Ans. (Part a) -12 UNC, 16 Ans. (Part b) i.) t = 0.3311 in, #Threads = 3.973 Ans. (Part b) ii.) t = 0.3659 in, #Threads = 4.391
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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