10 Replies Latest reply on Jan 23, 2013 4:28 AM by Gopal Sharma

    Fatigue Analysis on shaft

    Elliott Tuthill

      I have a shaft that is used in a pump.

      The shaft has a belt attached to the top that transfers a torque to rotate the shaft. The torque applied is known and that value is 42,000 lbs-in. The shaft has two bearings that are located above and below the middle extrusion on the shaft. I am trying to do a fatigue anaylsis on the shaft however I don't think I am setting it up properly because it shows that damage and life seem to have no affect after 1000 runs.

      screenshot 1.png

      That is the shaft in order to replicate the two bearings I put a fixed hinge fixture in place of both bearings and a roller slider because I saw this done in a different question.

      screenshot2.png

      Now I put a torque where the belt is located and turned on use internal relief once again beacuse I saw it done in a previous question.

      screenshot3.png

      When I run it there is a warning "there is a significant external imbalance force in Z-direction which will be balanced by the application of opposing inertia forces. UNless your model is under such a force or under marginally imbalance foreces, application of Inertia Relief may alter the characteristics of your model."

      This leads me to beleive i am doing something wrong but when I continue anyway it says excessive displacements were calculated in this model. If your system is properly restrained consider using the large displacement option to improve accuracy of the calculations. otherwise continue with current settings and review the causes of these displacements.

      I click NO to solve with small displacement. I get this

      screenshot4.png

      This seems plausible but when I then run a fatigue study on this study I get this which doesnt make sense at all.

      screenshot5.png

      This leads me to believe that i have set up the static study wrong. I want to replicate a shaft that is held in place by two bearings and rotates around due to a pully attached at the top.

       

      I have attached the solid works file as well

        • Re: Fatigue Analysis on shaft
          Mikael Martinsson

          Hi Elliott.

          If I'm not missing something in your pictures above, you haven't got any restriction on rotation in your boundary conditions.

          • Re: Fatigue Analysis on shaft
            Stuart Moore

            I'm no expert on fatigue but fatigue is a measure of how long something will last under oscillating load/stress conditions.  Your load is a constant torque so that the only fatigue inducing action is during start-up and to a lesser degree, shut-down.  Also, your max vonMises is only 10% of yield.  Under these conditions your shaft will last forever!

             

            However, you are driving it with a belt so that there will be an oscillating lateral load on the shaft at the belt pulley (bending moment between the belt pulley and the bearing).  This is where your fatigue analysis should be targeted.

              • Re: Fatigue Analysis on shaft
                Elliott Tuthill

                     This is exactly what I think, that the forces acting on the shaft due to the pully and then also the bearings will cause the fatigue. I know the values of the forces at each bearing and at the belt pulley I just am not sure how to properly put all of this information into SolidWorks to get the correct analysis. Should I treat it as a beam? is there a way I can factor both the torque and the forces and do a fatigue analysis based on rotations? I have never had to use SolidWorks to do this type of analysis so I am just not sure how to utilize the program to do this study.

                  • Re: Fatigue Analysis on shaft
                    Chris Michalski

                    Elliott -

                    do you know the radial force on the shaft due to the pulley belt tension?  Because it rotates without other forces it is a completely reversed loading (at 0 degrees you have tension on one side, compression on the other, when it rotates 180 deg those loads are reversed).

                     

                    I can't recall precisely from fatigue lab in college, but... you should be able to just simulate the reversed loading of the pulley radial force as this is the fatigue causing force.  This would be a simple radial force that oscillates + and -, then the reaction forces from the bearings are also completely reversed.

                • Re: Fatigue Analysis on shaft
                  Henry Kurniadi

                  Hi Elliott,

                  For fatigue analysis in SolidWorks, you need to define the MAXIMUM loading (in your case the tension)

                  Use it as an input load in STATIC simulation.

                  The static simulation then refered as an EVENT into FATIGUE analysis.

                  In the fatigue analysis, you need to know the material properties, (or just use one of the default materials provided, I am assuming, the shaft is made from steel?, then use structural  steel with endurance stress of 200 MPa)

                  Input number of cycles...(you said 1000?)

                  Because your load is fluctuating + and - you need to put FULLY REVERSED as the loading type.

                  Then change the properties of the fatigue simulation into Equivalent Stress (von Mises) and Gerber (for ductile material, i.e. steel).

                   

                  then finally RUN the simulation.

                  You could reduce the fatigue strength reduction factor (Kf) if you know the factor from calculation/assumption.

                  0.5 would be a very conservative value.

                  0.7 would be the most likely.

                   

                  RUN the simulation.

                  You should get a very low damage because the cycles is very little.

                  • Re: Fatigue Analysis on shaft
                    Gopal Sharma

                    Thanks for sharing this informative post with us. It will really help us alot.