Test rig eigenfrequency analysis

By 5.3.2020


Analyzing eigenfrequencies of a structure is on a common to-do list when designing a component subjected to known excitations / vibrations, especially in rotating machines. However, if the structure is modifiable it can be very time consuming to study all the variations.

If the eigenfrequencies are left unstudied, unexpected vibration can occur when the structure is subjected to excitation close to natural frequency. In addition, sometimes operating the machine at the range of existing natural frequencies is intentional and thus occasional resonances are inevitable. Most of all vibration is disturbing and may also lead to fatigue failure.

Operators at the Moventas Gears Oy Ikola test rig had observed vibrations of the test setup while running tests for certain type of windmill gearboxes. The most severe vibration occurred at the test rig itself. Moventas Gears Oy decided to study the test rig fatigue due to vibration in order to avoid any damage to the test rig that would lead to interrupts of the tests. In addition to study current test setup, also the upcoming test setup configuration was studied.


Results for the natural frequency mode shape from the eigenfrequency analysis were used as the loading for the fatigue study. Natural frequency mode shapes 1 and 2 for test setup configuration 1 animations are shown in Videos 1 and 2. Since the deformation shape of the result for natural frequency matched the measured vibration, the amplitude of the displacements was scaled to match the measured values in order to obtain the stress ranges. The most critical location based on the stress levels was then recognized and studied more precisely, as shown in figures 1, 2 and 3. The estimated lifetime was obtained from the stress ranges for the current test setup.

For the upcoming test setup, allowed displacement amplitude was solved. It was assumed that vibration will occur at first or second natural frequency, as in the first test setup configuration.

First natural frequency for the test setup configuration 1
Second natural frequency for the test setup configuration 1


The results indicated that failure due to fatigue is not likely to happen when performing tests with the first test setup configuration. The allowed displacement amplitude for the second test setup configuration is something that the operators can keep an eye on when the actual tests start.

Figure 1. Finding the most severe location.


Figure 2. Studying the most critical toe line of the welds.


Figure 3. SN-curve for the effective notch stress method analysis.