What is resonant vibration? Part One
Everything has a natural frequency. This frequency is effected by two properties: Mass and Stiffness. This "natural frequency" is the cause of many vibration problems in HVAC equipment. If you strike an object (say a tuning fork or a bell) it will continue to vibrate at its natural frequency until damping extinguishes the vibration.
Almost everyone has heard of the crosswalk that collapsed in Kansas City a number of years ago. It collapsed as a result of resonant vibration.
When the natural frequency of an object is excited by a vibration of the same frequency the objects natural frequency escalates to a point much greater than the exciting force or vibration. Hence, the singer breaks the glass, or the walls of Jerico come falling down.
Believe it or not we had equipment in Viet Nam that generated a vibration approximate to the natural frequency of the human colon. The result was a lot of dirty shorts.
With so many different vibrations inherent in a piece of HVAC equipment (say an air handling unit) it is easy to see why resonance is such a common problem. The real question is: Why is it so often mis-diagnosed.
The answer is: Very few of us understand resonance, and so, understanding something about balance and vibration, we try to solve the problem by replacing blowers, sheaves, belts, etc.
Some of you suggested turbulence problems in the original hypothetical--there is a good reason for this. Turbulence being the exciting force often excites resonance in the ductwork. In this discussion I am going to avoid this aspect because it is so closely related to the problem that to explain one would explain the other. Case and structure resonant problems narrows the field of discussion.
Part Two will cover diagnosis and Part Three will cover solutions. I would like to stop here for now so that others can contribute their experience and those with questions will have a chance to respond. I suggest that we wait to discuss actual problems until all three parts have been presented--hopefully by that time we will all know how to solve these *real* problems.
I recieved this email from Duane and got his permission to post it. It makes some very good points.
Dave... a few clarifications are needed or you'll confuse your audience. Perhaps you should include definitions, in laymans terms, of the jargon you'll be using. Frequency and amplitude, for example (how fast and how "far" something vibrates). I've learned never to overestimate the experience level of an audience.
I encourage your effort and look forward to the discussion.
The object's amplitude escalates... not the frequency.
> Believe it or not we had equipment in Viet Nam that generated a vibration approximate to the natural frequency of the human colon. The result was a lot of dirty shorts.
Believable. One of the concerns during shuttle launches, noise wise, is the natural frequency of internal organs and the launch sound spectrum. Other things, like explosions, keep folks far enough away for this to be a real concern though.
> Some of you suggested turbulence problems in the original hypothetical--there is a good reason for this. Turbulence being the exciting force often excites resonance in the ductwork. In this discussion I am going to avoid this aspect because it is so closely related to the problem that to explain one would explain the other. Case and structure resonant problems narrows the field of discussion.
Probably a good idea. Turbulence by it's very definition is a random chaotic disturbance and doesn't cause consisitent natural frequency excitation of a structure. It'll cause other noises, like rumble, for sure. Things like vortex shedding can cause excitation. Whole 'nother topic. General rule is to keep the flow path smooth.
Frequency is the time it take for the vibration to complete one cycle. It is usually expressed in cycles per minute (CPM). For example: You are examining the vibration of a blower that is rotating at 800 RPM--The 1X frequency of vibration would be 800 CPM. Every rotating object has a heavy spot, as this heavy spot moves 180 degrees, the force causes radial movement of the rotating part eg. the shaft moves in a radial direction as examined at the bearing. This is the object of a simple balance. Examine the amount of vibration (Amplitude) at the 1X frequency and reduce this by locating the heavy spot and either remove it or add weight 180 degrees to it. This of course is describing simple not dynamic imbalance.
Amplitude is simply the amount of vibration. One simple measurement of amplitude is Displacement. This is simply the amount of movement of the vibrating part. It is normally expressed in mils (0.001 in.) or Microns (0.000001 meter) (0.001 millimeter)
Displacement was used for years as the primary measurement of Vibration Amplitude. It is okay for primary balance but it has problems when examining wear due to vibration.
The following illustration may help: If we bend a coat hanger, the distance the bend travels is displacement, the speed at which it travels is velocity, and the time it takes to get from one extreme to the other is frequency. Obviously the faster we bend the coat hanger the quicker it will fail. This is why Velocity as a measurement of Amplitude is so valuable. It is a better indication of wear than Displacement. EX: bend the hanger five times as fast and half as far and see how much less time it takes to fail.
This is important, because Displacement can often be noticed where Velocity can't. In simple terms you could have a very stressful vibration problem occurring and not know it because of low Displacement Amplitude with high Velocity Amplitude.
How does this all relate to our Resonant problem?
Being able to determine Frequency is critical in finding the source of the problem. In order for Resonance to be a problem there has to be an exciting force at some related frequency to the natural frequency of the part that is resonating.
Please let me know if I can clarify anything at this point.