September 2011

Engineering Case Histories

Case 64: Averages can be misleading on service life data

Take care when making important decisions on data

Sofronas, T., Consulting Engineer

We have all used averages to determine average stresses, forces, torques, service life and other quantities. But care must be taken when using averages. What if a shaft failed with an average torque meter value of 360 ft-lb. Spot values show torques of: (300 + 800 + 200 + 400 + 100) / 5 = 360 ft-lb Obviously, it was the 800 ft-lb torque that probably caused the failure, but if the torque meter took hourly averages, it would miss the peaks. Example. Here is an actual case involving the wear of a gas-engine compressors power cylinder.1 Excessive warping of a lined cylinder can result in expensive downtimes and piston wear, such as shown in Fig. 1. This was occurring on cylinders that had

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