The derailment of a freight train due to the catastrophic failureof a traction motor

Chapter 9, Problem 74

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The derailment of a freight train due to the catastrophic failureof a traction motor armature bearing provided the impetus fora study reported in the article Locomotive Traction MotorArmature Bearing Life Study (Lubrication Engr., Aug. 1997:1219). A sample of 17 high-mileage traction motors wasselected, and the amount of cone penetration (mm/10) wasdetermined both for the pinion bearing and for the commutatorarmature bearing, resulting in the following data:Motor1 2 3 4 5 6Commutator 211 273 305 258 270 209Pinion 226 278 259 244 273 236Motor7 8 9 10 11 12Commutator 223 288 296 233 262 291Pinion 290 287 315 242 288 242Motor13 14 15 16 17Commutator 278 275 210 272 264Pinion 278 208 281 274 268Calculate an estimate of the population mean differencebetween penetration for the commutator armature bearingand penetration for the pinion bearing, and do so in a waythat conveys information about the reliability and precisionof the estimate. [Note: A normal probability plot validatesthe necessary normality assumption.] Would you say thatthe population mean difference has been precisely estimated?Does it look as though population mean penetrationdiffers for the two types of bearings? Explain.