Structure and Machine Natural Frequency

Every machine and structure has  natural frequencies . if machines and structures are designed correctly, these natural frequencies will not affect the operation or reliability of the machines. In reality, however, a wide variety of fault condition are either caused by, or are affected by natural frequencies.

It really is important to  understand what they are, how to detect them and how to correct them.

Plant resonances

The pipes,foundation, and rotating machinery in every plant have natural frequencies. If designed well, the natural frequencies are not excited (much). However, if a machine happened to be mounted on a structure that had a natural frequency equal to speed of the motor, the vibration level would grow considerably-there would be a resonance.

Resonances amplify vibration. the measured vibration level must be 3 to 50 times higher then they would be normally. So, instead of vibrating at 0,,5 mm/sec, for example, the machine could vibrate at up to 25 mm/sec. the potential for structural failure or a catastrophic failure of the machine is high.

By definition, the natural frequency is the frequency of free vibration of a system. The frequency at which an undamped system with a single degree of freedom will oscillate upon  momentary displacement from its position.

In simple term, if energy could be injected at all frequency of the structure it will vibrate at its natural frequencies. When a natural frequency is excited, the structure resonates, and the vibration amplitudes are amplified. So the stress can be increase up to 100 times (as compered to the stress at a frequency higher or lower than natural frequency.

 Critical speed

The term critical speed is typically used regarding very large rotors such as alarge steam turbines. These are flexible rotor. As the rotor approaches its natural frequency it will begin to flex. When the machine RPM coincides with the first mode of vibration, the speed is called the “critical speed’.

Why are resonances is important ?

When natural frequency is excited, the structure resonates. The amplitude of vibration will increase significantly, thus the stress of the machine increase significantly. The increased stress reduces the life of the machine component and structures. Weld crack, metal fatigues, bearing fail, and worse.

DETECTING RESONANCE PROBLEM

Without doing any special test, there are two basic ways to tell if you have resonance problem : unusual failures,and tell-tale signs in the spectrum.

• Unusual failure :

When there machinery failures or structure failures that seem to be as a result of fatiguing and there is not any other explanation, then consider resonance as a possible cause. Structures should last a very long time, and fatigue failures should only occur after many year service.

Typically failures as a result of resonance include :

1. Broken welds
2. Cracked and leaking pipes
3. Premature machine failure
4. Broken or cracked shaft
5. Foundation cracks

• Four tell-tale signs in the spectrum :

1. Unusually high peaks in the spectrum – peaks are amplified
2. High vibration levels in one direction / axis but not in another
3. Areas in the spectrum where the noise floor and any peaks in the vicinity seem to have been raised.
4. Peak that change amplitude when machine speed changes. Resonance is only excited under certain conditions.