Resonance is a condition where the frequency of a vibratory force is equal to the natural frequency of the system. In rotating equipment, that resonance is called the critical speed. If the speed of the motor/pump unit is say, 3550 rpm, and the natural frequency of that equipment is 3540 rpm, the machine will experience high vibration at its' operating speed. This would be the "critical speed". Manufacturer's design engineers always try to calculate the critical speed of an equipment assembly to insure the natural frequency is +/- 20%-30% away from the critical speed. They don't always get it right.
A contractor poured a common reinforced concrete pad for four identical motor/pump units, installed them the same way, and upon startup, one of them vibrated 3 times as much as the others(.84 in/sec vs. .23 in/sec). The motor operated fine uncoupled, so the pump was returned to the factory for disassembly, testing, reconditioning, and repair. They couldn't find anything wrong with it. The pump was re-installed, and again high vibrations. A bump test was performed, and the natural frequency of the problem unit was determined to be 3750 rpm, while the operating speed was 3575 rpm. The cause of the high vibration was resonance. The operating speed was too close to the natural frequency. Correction involved adding braces to stiffen the pump/motor unit and thereby increase its' natural frequency to 4085 rpm.
All new installations should include resonance testing during their startup commissioning. The bump test is relatively easy to perform, and the corrections are usually easy: dampen the base structure to decrease the natural frequency, or stiffen the assembly, by adding braces/supports, to increase the natural frequency. Operating equipment while in a "resonant" condition will shorten the life of the bearings and lead to premature failure.
Identifying resonance can be as simple as seeing the vibration levels decrease by standing on the baseplate. The baseplate needs stiffening. It has been confirmed that the old U-Frame electric motors could have their vibrations reduced by changing the 'D' size SS shims with 'B' size shims. Installing braces to flexible mount systems such as centrifugal blowers can significantly change the natural frequency of the motor/blower unit. I once had a customer who bolted those braces to the building steel frame resulting in resonance. The motor/blower unit was exciting the natural frequency of the building! Removing those braces solved that problem.
Resonance is present in rotating equipment much more than people are aware. Learning to identify it is the first step to correcting for it. Another reason why vibration analysis is a very important tool in your preventative maintenance program. Invest in PM, it will save you money!
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