# Design of a Spin Test System for Burst Phenomen Considering Nonlinear Rotordynamic Effects
## 4 Discussion  
In spin test rigs, plastic deformation, crack propagation, and burst phenomena can occur on the test specimen. For accurate results, tests must be conducted without external influences such as imbalance, vibration, and misalignment. A critical speed should not exist within \(\pm\)20% of the test speed. Vibrations can lead to inaccurate results by affecting stress variations, potentially causing High Cycle Fatigue (HCF) and resulting in damage to the test specimen and associated components. It is crucial to ensure that the test component is operated without exposure to vibrations. Detailed rotor dynamics calculations are essential to ensure test accuracy and design reliability, especially in high-speed systems where bearing stiffness must be modeled nonlinearly in both axial and radial directions. Radial stiffness should be based on allowable imbalance forces, while axial stiffness should account for assembly loads to create an accurate mathematical model incorporating nonlinear bearing stiffness. Performing spin tests under vacuum removes aerodynamic damping, allowing the direct use of hammer test-derived damping ratios in alternating stress calculations. Since damping values from hammer tests are below 3%, their effect on natural frequencies is negligible. However, systems with squeeze film dampers (SFD) and aerodynamic damping require consideration of damping-related critical speeds and appropriate coefficients in harmonic response calculations. In this study, a discrete model approach was applied to the equations of motion, utilizing a rotordynamic tool established in MATLAB Simulink to achieve the rotordynamic layout during the preliminary design phase. Furthermore, in the created model, bearing \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{3}{*}{**Analysis Type**} & \multirow{3}{*}{\begin{tabular}{c} **1 st Natural** \\ **Frequency** \\ **[Hz]** \\ \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} **2 nd Natural** \\ **Frequency** \\ **[Hz]** \\ \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} **3 rd Natural** \\ **Frequency** \\ **[Hz]** \\ \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} **Relative Error** \\ **Percentage with** \\ **Respect to Impact Test** \\ **Result for 1 st Natural** \\ **Frequency** [\(-\)] \\ \end{tabular} } & \multirow{3}{*}{\
\begin{tabular}{c} **Relative Error** \\ **Percentage with** \\ **Respect to Impact Test** \\ **Result for 3 rd Natural Frequency** \\ **[\(-\)]** \\ \end{tabular} } & \multirow{3}{*}{
\begin{tabular}{c} **Relative Error** \\ **Percentage with** \\ **Respect to Impact Test** \\ **Result for 2 nd Natural Frequency** \\ **[\(-\)]** \\ \end{tabular} } \\ \hline General & 241.08 & 448.49 & 646.79 & \(-5.49\%\) & \(0.27\%\) & \(1.86\%\) \\ \hline
3D & 262.44 & 466.09 & 632.13 & 2.88\% & 4.2\% & \(-0.45\%\) \\ \hline Simulink & 268.12 & 439.96 & 665.14 & 4.85\% & \(-1.64\%\) & 4.75\% \\ \hline Impact Test & 255.1 & 447.3 & 635 & - & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Modal analysis and test results for 4 kg test specimen. Figure 18: Equivalent stress distribution for shaft for 4 kg test specimen under 60,000 rpm. stiffness values were not assumed to be constant; instead, radial stiffness values based on unbalance forces and axial stiffness values based on assembly loads were integrated into the system. To validate the accuracy of this study, tests were conducted on two different specimens weighing 10 kg and 4 kg, respectively. For the 10 kg test specimen, the maximum natural frequency deviation between the results and the Simulink-based rotordynamic calculations was found to be 4.2%. A similar approach was applied to the 4 kg test specimen, with the maximum frequency deviation calculated as 4.85%. The primary factors contributing to the discrepancies between the test results and Simulink outcomes include contact damping, Multi-Point Constraint (MPC) connection assumptions, and differences in mass and inertia tensor that disrupt axial symmetry.