PSI - Issue 57

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Karthik Krishnasamy et al. / Procedia Structural Integrity 57 (2024) 793–798 Karthik KRISHNASAMY/ Structural Integrity Procedia 00 (2019) 000 – 000

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The load cycle is dependent to the limiting stress and it influences its lifetime. This leads to have a damage in the machine elements and generates a Wohler curve (Fig-2) from test measurements. In these test conditions, an alternating load is applied to the component until is reached its failure and failure load cycle varies even though the load to component is same. The extracted Wohler curve statically defined for a given failure probability and from this curve, the safety factor is calculated.

Fig-2: Wohler curve with scattered measurement points at different loading condition.

Certain variables are defined in this article to avoid confusions. Target design life is to attain the requirement of the machine elements and achievable life is the machine element sustains the loads cycles until it fails. On the other hand the maximum stress level endure is limiting stress and the strength assessment for the applied loading condition is induced stress. For gears, ISO 6336 is the most widely used and technically advanced document forcylindrical gear strength rating and it will provide the calculation results based on the requested life and provided torque conditions. It calculates a safety factor for the bending and contact. For bearings, the standard are different to calculate (ISO 281 – Calculate bearing rating) and this method predicts the viable lifetime and damage. The requested safety factor for the gear components may be 1.0, but with specific applications, it has to be higher than one. Similarly, for other components such as shafts, bearings, splines, seals etc. has different validation method. In all these applied different methods, stresses are commonly utilize and it is calculated based on the applied loads compared with its allowable stress for all the machine elements to corroborate the design requirements. 2. Failure probability In all the validation methods, there is an inherent failure probability that differs from method to method and this is a major weakness (Table-1). If the gear bending has a safety factor of 1.4 and shaft has a safety factor of 1.3, it is not comparable to define the most critical component. Table 1. Probability of failure for different machine elements. Components % Shafts using DIN 743 2.5 Bearing using ISO 281 10 Gear tooth bending using ISO 6336 1 Gear tooth contact using ISO 6336 1