CRM Engineering » Fatigue http://www.crmeng.com Engineering Design & Analysis Wed, 02 Jan 2008 04:27:16 +0000 http://wordpress.org/?v=2.8.5 en hourly 1 Random response analysis of avionics tray http://www.crmeng.com/2007/12/14/random-response-analysis-of-avionics-tray/ http://www.crmeng.com/2007/12/14/random-response-analysis-of-avionics-tray/#comments Fri, 14 Dec 2007 16:40:14 +0000 Charles McCreary http://www.crmeng.com/2007/12/14/random-response-analysis-of-avionics-tray/ A client approached us with a difficult electronics packaging problem. Their customer required a commercial, off-the-shelf (COTS) board to be included in a device that must pass the MilSpec random response envelope for a particular aircraft. Since the COTS board could not survive the vibration unless the enclosure provided shock and vibration isolation, the objective of the analysis was to specify the required properties for the enclosure supports as well as to ensure that the enclosure itself had sufficient fatigue life to survive the vibration spectrum.
Single degree of freedom random response prediction
Center of mass response from FEA random response analysis
RMS S11 component

]]> http://www.crmeng.com/2007/12/14/random-response-analysis-of-avionics-tray/feed/ 0 Machine Design – Fatigue http://www.crmeng.com/2007/12/13/machine-design-fatigue/ http://www.crmeng.com/2007/12/13/machine-design-fatigue/#comments Fri, 14 Dec 2007 03:09:16 +0000 Charles McCreary http://www.crmeng.com/2007/12/13/machine-design-fatigue/ A client was experiencing failure in a cantilevered shaft which supported a heavy spool that was subject to frequent rapid starts and stops. The initial design incorporated a a 3″ diameter shaft that was supported by two bearings with a drive gear halfway between the bearings. A larger, 5″ diameter shaft was fillet welded onto the 3″ shaft just outboard of the outer bearing.

The massive spool was accelerated from rest to 60 rpm in a fraction of a second and braked several times an hour, 24/7. Thus the number of stress reversals seen in a year required a design in which the peak stress is well below the fatigue limit. The shaft would break at the fillet weld within a few weeks of service at the fillet weld (no surprise).

Examining the problem from first principles (beam theory and stress concentration factors) showed that the expected fatigue life was measured in hours. A detailed finite element analysis of the system corroborated the diagnosis.

Our recommendations:

  • Machine the shaft in one piece with a generous shoulder fillet. This eliminates the fillet weld and greatly reduces the stress concentration at the shaft diameter transition
  • Specify a generous root fillet in the gear keyway to reduce the peak stress
  • Use a material with a fatigue limit much higher than the mild steel used in the original design
    Stress concentration in a stepped shaft
    FEA

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