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  • Shaft loads static or fatigue calculation ?

    Discussion in 'The main mechanical design forum' started by Yuri, Dec 11, 2013.

    1. Yuri

      Yuri Member

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      Hi

      I have a new question. I need to calculate shaft strenght.

      the configuration is shaft with two self align bearings and a pulley in the midlle.
      Loads are radial 26 kN and torgue 200Nm
      the radial load have constant direction, the torgue change direction (spining is reversed)

      Now the question: is the shaft static loaded or i have to consider and calculate for fatigue failure ??
      I am using Ivertor design calculator will do and FEA analysis.

      The main thing is i have to choose safety factor which is different for the two types of loading

      I hope it makes sense.
       
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    3. Erich

      Erich Well-Known Member

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      Your shaft is subject to fatigue loading. Consider the radial load from the viewpoint of the shaft. As the shaft spins it appears that the radial load reverses direction. You have fatigue. How frequently does the shaft change directions?
       
    4. Yuri

      Yuri Member

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      I also find out that is subject to fatigue :( now trying to understand the calculations. I think i have Alternating /Reversing Stress due to bending of the shaft compresion/tension on every 180 degrees. I think i have to find Se-modified fatigue strenght ?

      I will post all the data I have and hope somebody will help.

      direction of rotation is changed normaly every 30-40 seconds. Using VFD for smooth change CW to CCW
       
    5. Yuri

      Yuri Member

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      Fatigue calculation

      Se´=0,5.Sut
      Se=Ka.Kb.Kc.Kd.Se´

      Sut- Ultimate tensle strength
      Se´- Endurance limit stress
      Se- Endurance limit stress of component
      Ka- surface finish factor = 1,58. 0,57 = 0,89 (ground)
      Kb- Size factor = 0,85
      Kc- Reability factor (90% survival ) = 0,897
      Kd- modifyin factor to account for stress concentration = 1/Kf
      Kf- stress concentration factor


      I need to find out stress concentration factor Kf ? D/d=1,28 r/d= 0,03 material steinles steel 420
       
    6. Yuri

      Yuri Member

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      Update

      using eAssistant http://www.eassistant.eu/en/startseite.html for the fatigue calculation according DIN 743. Keyway section in the middle fail.
      Can somebody confirm if the shaft is rotating and have constant radial load in one direction is it subject to alternating bending or constant bending ?
       
    7. Erich

      Erich Well-Known Member

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      See reply #2 in this thread. Yes you have alternating bending.
       
    8. JonGreene

      JonGreene New Member

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      I agree with Erich, you certainly have alternating/fully reversing bending. I would start with a bending moment diagram, torque diagram, and angle of twist diagram; These diagrams will help you define the stress within the shaft:

      Bending moment diagram will help you define the moment at any point along the shaft. If you take an element at the surface of the shaft you can use M(x) to calculate stress in the axial direction of the shaft (Axial Stress = Mc/I).

      Torque diagram will help you define the shear stress at any given point in the shaft (Shear Tau = Tc/J). The shear stress is felt by the same element as above; Your element now possesses shear and axial stresses.

      Be careful as your shaft is certainly in combined loading; the pulley produces both a torque and a static force etc.

      Once all the stresses on an element are known, you can convert them to principle stresses and use the octahedral stress equation to obtain the Von Mises stress or single equivalent stress on the element. Compare this to your yield stress and you can understand the factor of safety built into the shaft.

      The rule of thumb when it comes to twist is your shaft should not twist more than 1 degree within a length 20 times it's diameter. An angle of twist vs shaft length diagram will help you see if you are meeting this criteria with the current material (angle of twist = TL/JG)

      Hope this helps.
       
    9. vyasang

      vyasang New Member

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      Design of shaft subjected to steady bending load

      Should we design a rotating shaft with steady bending loading only for fatigue failure or should we design it for static and fatigue failure both?
       
    10. Erich

      Erich Well-Known Member

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      As with all things engineering, it depends.

      It depends on your specific design requirements.

      Is this shaft going to used in a piece of equipment that will turn many millions of revolutions? Yes, design for fatigue.

      Is this shaft in a disposable toy being given away for free? Static is good enough.
       
    11. PierArg

      PierArg Well-Known Member

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      Hello vysang and welcome to the forum,

      if the shaft rotates it is subject to fatigue loads even if the load appears static...unless it is loaded in the axial direction (but this would be discordant with the real function of a shaft).
       

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