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Vibration Testing & Investigations

Vibration Testing & Investigations
Unexpected high levels of vibration are usually a result of large magnitudes of input forces or the excitation of a resonance. Many factors can influence the forces, such as bent shafts or out-of-balance masses in rotating machines, and there are numerous ways to investigate their source. If no excessive forces can be discovered, yet there is high vibration, then resonance must be suspected.

Resonance occurs when the excitation frequency is close to a natural frequency of the structure. Generally, a structure will have many natural frequencies. There have been many spectacular failures when resonance resulted in vibrations high enough to cause the structure to fail.

If it has been established that a resonant condition exists and a particular mode is being excited, there are a number of modifications which can lead to a reduction in the vibration levels. Increasing the damping in the system will help, but if the natural frequency can be shifted by changing the mass or stiffness, the reduction can be dramatic. Knowing what changes to make is key to avoiding a situation where there is no improvement, or the vibration levels are actually increased. The types of tests which are used in this situation are known as resonance tests.

A resonance test involves more than just quantifying the magnitude and frequency of the vibrations. Finding the presence of any natural frequencies close to the major excitation frequencies is the most important step. Associated with each natural frequency is a mode shape which must also be determined so that the most effective modifications can be identified. The testing will also reveal the level of damping in each mode, and how much vibration reduction would be expected as a result of additional damping.

If a resonance problem is suspected, Eatec would typically follow a procedure as outlined below:

  • Carry out a survey to map the vibration levels around the structure at dominant frequencies under the operating conditions. These measurements provide the operating shapes.
  • With the machinery stopped and using a controlled excitation, determine the responses around the structure. Processing of these results identifies the natural frequencies and mode shapes.
  • Compare the natural frequencies and mode shapes to the excitation frequencies and operating shapes to determine if a resonant condition exists.
  • Using the information from the tests, identify the most effective ways to attenuate the vibration. This can be carried out by hand calculation or by using finite element modelling.

Eatec has been involved with vibration testing for over 20 years, and during this period has assisted many large and small national and international companies in a range of industries which include oil & gas, defence, marine, and rail.

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