The derailment of a freight train due to the catastrophic failure of a traction motor armature bearing provided the impetus for a study reported in the article “Locomotive Traction Motor Armature Bearing Life Study” (Lubrication Engr., Aug. 1997: 12–19). A sample of 17 high-mileage traction motors was selected, and the amount of cone penetration (mm/10) was determined both for the pinion bearing and for the commutator armature bearing, resulting in the following data:
Motor | ||||||
1 | 2 | 3 | 4 | 5 | 6 | |
Commutator | 211 | 273 | 305 | 258 | 270 | 209 |
Pinion | 226 | 278 | 259 | 244 | 273 | 236 |
Motor | ||||||
7 | 8 | 9 | 10 | 11 | 12 | |
Commutator | 223 | 288 | 296 | 233 | 262 | 291 |
Pinion | 290 | 287 | 315 | 242 | 288 | 242 |
Motor | ||||||
13 | 14 | 15 | 16 | 17 | ||
Commutator | 278 | 275 | 210 | 272 | 264 | |
Pinion | 278 | 208 | 281 | 274 | 268 |
Calculate an estimate of the population mean difference between penetration for the commutator armature bearing and penetration for the pinion bearing, and do so in a way that conveys information about the reliability and precision of the estimate. [Note: A normal probability plot validates the necessary normality assumption.] Would you say that the population mean difference has been precisely estimated? Does it look as though population mean penetration differs for the two types of bearings? Explain.
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