RELIABILITY WEEKLY 2 MACHINE RESONANCE & VIBRATIONS

RELIABILITY WEEKLY 2 MACHINE RESONANCE & VIBRATIONS

It's no secret that severe vibration can destroy bearings, ruin shafts and potentially disrupt production. What's less well known is that resonant machine components and supporting structures can magnify even small vibration problems enough to damage connected equipment or cause catastrophic machine failure. Resonant vibration in mechanical structures such as pumps, turbines and motors occurs when a natural frequency is at or close to a forcing frequency such as rotor speed. When present, this condition can cause severe vibration levels by amplifying small vibratory forces from machine operation. Such problems often develop after a speed change has been implemented, as with retrofitting a machine with an adjustablespeed drive (ASD) or operating a 50 Hz motor on 60 Hz power. The solution frequently depends on the ability to distinguish between structural resonance and a rotor critical speed. Structural resonance: Structural resonance refers to excessive vibrations of non-rotating components, usually machine parts or supporting structures. Due to the complexity of these components, it is the more common resonant condition and usually occurs at or near the rotating speed of the machine. Even slight vibratory forces from residual unbalance and misalignment effects of the machine can excite the resonant base structure, resulting in severe vibration. Rotor critical speed: A rotor critical speed exists when a machine's rotating element is the resonant component and its speed matches the natural frequency of the rotor. This is common with centrifugal pumps, gas and steam turbines, and large, two-pole electric motors. While the result is similar to structural resonance (high vibration at a certain operating speed), a rotor critical speed is a more complex phenomenon. When the operating speed reaches the resonant frequency of the rotating element, the rotating element distorts and the vibratory forces increase significantly. It is important to properly distinguish between structural resonance and rotor critical speed. The term "critical speed" (without the word "rotor") is somewhat ambiguous. Technically, a critical speed could be either a structural resonance or a rotor critical speed. For the sake of clarity it's best to avoid using that term. The simple term "resonance" can be applied to both conditions to avoid confusion.

How Do We Correct for Resonance If the source vibration that is exciting the natural frequency is low in amplitude, the resultant amplified vibration will be lower in amplitude and it will do less damage. That is one way to solve a resonance problem. But in most cases we take a different path. The natural frequencies of a structure are related to its design and are greatly affected by the mass and stiffness of the structure. If we made the tines of a tuning fork stiffer, the note made by the tuning fork would increase in frequency. And the same is true for the structure supporting our machine; which is normally what is resonating. If we know that we have a resonance problem we can consider either adding mass to reduce the natural frequency or increasing stiffness to increase the natural frequency. The aim is to change the natural frequency so that it is no longer excited by the machine. This is a very sophisticated process and there are a number of issues that must be considered. In essence this is what we are trying to do modify the structure so that the vibration generated by the machine is no longer amplified and therefore harming the machine or the structure or the product being manufactured or generating noise that affects workers or homes located near the plant. An famous example of resonance from the History One of the most famous examples of a structure which was NOT designed correctly is the Tacoma Narrows Bridge, which was dedicated on July 1, 1940. At the time, it was the third-longest bridge in the world. Alas, it had a strong response to frequencies of a few tenths of a Hz, which were on occasion supplied by winds blowing down the river's channel. Locals quickly nicknamed the bridge "Galloping Gertie" after its gentle swaying. On November 7, 1940, however, a violent windstorm excited very large amplitudes of motion...

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