Natural frequency

When one fork was struck, both forks would vibrate. Why? Well, the fork that was struck had to vibrate at its natural frequency. This followed with a pulsation that was transmitted to the surrounding air. The pulsation reached the second tuning fork and since the air-disturbance frequency was the same as the second tuning fork’s natural frequency, the second fork vibrated in spite of the fact that it was not physically struck.

Note the words “natural frequency.” If an object is struck, the noise heard is the natural or base frequency of the object. Multiples, or near multiples, of this frequency are called harmonics.

The 'swing' of harmonics

Consider a person on a swing. (The pretty girl at an Old San Francisco steak house will do nicely.) Watch her motion; she will add an impulse precisely at the moment that her velocity reaches zero and her direction changes. By doing this, she permits herself to travel a little bit further than on the previous swing.

The time for her to complete a cycle each time is the same. What changes is her speed, not her natural frequency. Imagine her applying a force at any other time in the cycle; the result would be a disruption and a reduction in the length of the swing.

The point is that a small force, timely applied, provides a large result if sufficient time is available. This is not always desirable.

What about an unbalanced tire? As one accelerates to, say, 60 mph, everything is fine until a vibration is felt. Some imbalance causes the wheel to vibrate at the wheel’s natural frequency or at some multiple of this frequency, the harmonic. If we balance the wheel, we remove the exciting force and consequently the vibration.

As a point of information, balancing a wheel has to be done in two planes, which means that a static balance is useless. As a further point of information, if and when a wheel is dynamically balanced, it is inherently in static balance.

This means that the term “statically and dynamically balanced” is redundant. If an object is balanced at its base or lowest fundamental frequency, then it is balanced for all of the object’s harmonics. The nice part of this phenomenon is that a wheel does not have to be spun fast to be dynamically balanced.

Actually, were it not for internal dampening, the amplitude of the vibrations would increase indefinitely with disastrous results. In theory, as a resonant frequency is reached, the amplitude goes to infinity.

This applies whether the object is speeding up or slowing down. Watch your automobile engine as someone kills the ignition; as the engine comes to a stop, it has to go through a resonant rpm, either the base frequency or a harmonic. A shudder occurs and then disappears. The engine finally coasts to a stop.

Blower motor and compressor vibration

A belt-driven fan or blower may sometimes be unbalanced and provide a vibration. While balancing the assembly will cure the problem, it might be easier and less expensive to either increase or decrease the fan speed by changing pulleys.

Compressors also vibrate. This is a given. You can accept the vibration and subsequent motion or take measures to nullify the effects. If you decide to accept the motion, you provide compressor mounting springs and a vibrator. (Incidentally, a vibrator should be lined up parallel to the compressor crankshaft and never be used as an ell.)

If you decide not to accept the motion, you have to change the natural frequency of the compressor. This is done with a hard-mount installation. One technique is to provide a concrete block containing Reba and mounting bolts.

The rule of thumb is that the mass of concrete has to be a minimum of one to one-and-a-half times the mass of the compressor. The compressor is bolted to the concrete block with no isolation. Now the vibration has to try to move both the compressor and concrete block — no easy feat since the mass has greatly increased.

The result is that since the vibration energy has not changed, it cannot move the overblown mass very much, if at all.

This is the reason why some of us like to see compressors hard mounted to receivers. A receiver is a registered pressure vessel that has to comply with ASME criteria. This means that it has tremendous strength and subsequent structure.

Why not take advantage of this opportunity and permit the receiver to provide a twofold purpose?