D=45mm R2mm d=30mm Tx=80NM M=100NM Tx=320NM M=250NM
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.
The stepped and surface-ground steel bar in the figure is subjected to a bending moment varying between Mz=100 Nm and Mz=250 Nm and a torsional moment varying between Tx=80 Nm and Tx=320 Nm in opposite directions. The tensile strength of the rod material is Sut=800 MPa and it works under room conditions. For 50 percent reliability, check the fatigue safety of the rod.
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