An insured struck a deer with his automobile, and the damage wasrepaired. Shortly afterward, the vehicle transmission failed,leading to an expensive repair. The insured claimed thetransmission failure was a result of the impact with the deer. Thisprecipitated a detailed inspection of the vehicle andtransmission.
|Figure 1 is a view of the underside of the vehicle. There was noevidence of direct damage to the transmission or neighboring partsof the vehicle structure.
|Related: Brake lock up causes fire intrucks
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Figure 2 (Photo: C. Roberts)
|Figures 2 and 3 show views of a splined driveshaft connection atthe transmission. The splines had worn to the point where powercould not be transmitted and the connection began to slip. InFigure 2, the arrow points to the worn splines. Figure 3 shows themating part of the splined connection. The upper red arrow inFigure 3 points to worn splines on the mating shaft. The originalshape of the splines is indicated by the lower blue arrow.
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Figure 3 (Photo: C. Roberts)
|The condition of the mechanical components shown in Figures 2and 3 is characteristic of fretting fatigue. Fretting fatigue iswear on a mechanical surface caused by relative motion (vibration,road condition variation) and drive train loading that causes micropitting of the metallic surfaces, increased roughness and reductionof metal fatigue strength. This occurs over a period of time and isnot the result of a single incident. This was not a result ofimpact with the deer, but a form of mechanical failure.
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|Charles C. Roberts, Jr., Ph.D., P.E., ([email protected]) ispresident of C. Roberts Consulting Engineers, Inc., which providesprofessional engineering services in accident reconstruction,failure analysis, fire causation, explosion analysis, andbiomechanics.
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