Frequency Analysis of a rotating shaft

INTRODUCTION

Frequency response analysis in Finite Element Analysis (FEA) is used to calculate the steady-state response due to a sinusoidal load applied to a structure at a single frequency. The goal of modal analysis in structural mechanics is to determine the natural mode shapes and frequencies of an object or structure during free vibration. It is common to use the finite element method (FEM) to perform this analysis because, like other calculations using the FEM, the object being analysed can have arbitrary shape and the results of the calculations are acceptable. It is also possible to test a physical object to determine its natural frequencies and mode shapes. This is called an Experimental modal analysis.

The results of the physical test can be used to calibrate a finite element model to determine if the underlying assumptions made were correct (for example, correct material properties and boundary conditions were used). Static analysis means that we are making the assumption that the system we are simulating doesn’t depend on time. Modal analysis or Natural frequency analysis is usually carried out for complex structures by 2 means. Practical testing (Shaker bed test/using an impact hammer), Using Finite element techniques (Boundary element method/Finite element method) Both of these go hand in hand. Modal analysis is performed to find the fundamental frequencies (Modes) and their associated behaviour (Mode shapes).

The material used for ordinary shafts is mild steel. When high strength is required, an alloy steel such as nickel, nickel-chromium or chromium-vanadium steel is used. A bearing is a mechanical component whose function is to guide a rotating assembly. The bearing therefore allows the rotation of one element relative to another. Bearings are therefore high-precision parts that allow equipment to move at different speed levels by efficiently transporting notable loads. They must offer high precision and durability, as well as the possibility of working at high speeds with minimal noise and vibration.

The output we get from the Modal analysis is “Natural frequencies and mode shapes”. Natural frequency is a Basic design property as stress and Deformation. which can be used to understand the Possible resonance occurrence and avoiding it. The purpose of a modal analysis is to find the shapes and frequencies at which the structure will amplify the effect of a load. why we may need this information and how to use the answers.

1. Finding loose components
2. Deciding which rotational speeds are dangerous
3. Finding out where to constrain or load a structure
4. Finding out how to move a mode
5. Modal dynamics (classical dynamics)

Constraints: For this challenge, you will have to find out the critical frequencies of a rotating shaft in Solid works. To create the model in Solid works according to dimensions that fit. Model it in Solid works according to the image shown below.Figure 1.CAD Model of rotating shaft. Conduct a frequency analysis and apply a bearing fixture on the stepped face of the rod at the two ends and perform the analysis. Find out 5 mode shapes and list the resonant frequencies. CAD model is done using solid works and drafting of the model is shown in figure 2.(Figures 1 &2 is in the attached pdf)

FEA FREQUENCY ANALYSIS:

Step 1: Select simulation and click on new study.

Step 2: Now new dialogue box will be open at the left side with frequency. Right click on part and apply material considered for the design. Here Alloy steel is selected.

Step 3: Constraints for the shaft should be given for the analysis. So, by selecting the fixture the bearing constraint is selected. Here the stepped end in the shaft is selected because to handle the load of the shaft.

Step 4: And the load is given from external loads and we need to define torque where the shaft is in rotation. The default value is kept to see the natural frequency of the designed shaft. Now we need to select the axis or surface to denote the rotation of the shaft. After giving all these click thick at the top.

Step 5: By clicking the frequency we can edit the properties. Here the tab like options, flow/thermal effect, notification and remarks can be seen. In options tab we can define the no of frequency and the different type of solvers we can use for frequency analysis. But we can select automatic in solver so that software takes the proper solver to solve the analysis.

Step 6: After giving constraints to the shaft the meshing should be done. We can make the mesh size and how should mesh (fine or coarse). After meshing we can run the simulation.

Step 7: After analysis is done, we can see the results where the amplitude is seen for hominy mode shape we select for shaft.

Step 8: By right clicking results we can compare the results obtained.

Step 9: And also, we can change the dimensions of the obtained result of chart by changing the settings. Here changing the stress value to N/mm² and ale the values to floating from scientific.

Step 10: By right clicking the result we can also see the graphs Vs frequency for the given CAD model. We can save the results.

CONCLUSION

The frequency of mode 1 is 0.26301 Hz is minimum and maximum frequency of 407.84 Hz at the mode 4 & 5. The intermediate frequency is 84.64 found in mode 2 & 3 which are seen in figure 17.From figure 18 we can see the mass participation of model along each direction at each mode.(Figures 17 & 18 is in the attached pdf)

Result video link: https://youtu.be/OP3uIfdW9gw