Identifying A Structure's Natural Frequency Using Bump Tests
All structures have a natural frequency. Resonance is the excitation of a component’s, or combination of components (assembly), natural frequency. Why do we care about resonance? Because resonance amplifies vibration.
Resonance problems occur in two primary forms, which often lead to vibration. The first occurs at a critical speed – A component rotates at a speed that coincides with its own natural frequency. The smallest amount of residual unbalance (something that is always present) is enough to excite the natural frequency, causing large amounts of vibration. Rotors with slow run-up and coast-down procedures are susceptible to critical speed resonance.
The second, and most common, is structural resonance. Structural resonance becomes an issue when some forcing frequency comes within +/- 20% of the natural frequency of a structure. This vibration can be observed on the machinery housing itself, or some nearby structure, such as a handrail or I-beam.
Whether it’s a critical speed or structural problem, testing for natural frequency is critical. To identify a structure’s natural frequencies, AME performs specialized “Bump Tests.”
On vessels, with shafting removed, AME mounts accelerometers to the strut barrels. Once the connection is confirmed to be rigid, measurement is started and the strut barrel is struck with a heavy, yet soft, mallet to excite the natural frequencies. Our analyzers will then identify the strut’s natural frequencies and AME will determine if this falls near a forcing frequency of the propulsion system – Shaft speed, propeller blade pass frequency, etc. This will also be an indication of the strut’s rigidity to the hull. We also use a RDI motion amplification camera, which will identify natural frequencies as well as any/all movement of the strut and its connection to the hull.