Ask that question to any vibration specialist worth his salt and he would immediately tell you that it is easy.
He might start something like this: “The frequencies tell us what the problems are and the corresponding amplitudes inform us how serious are the problems”.
He would then fish out a chart that shows us what might be the possible faults at the various frequencies and another chart that might give some indication about the severity of the problem by indicating the possible amplitudes up to which the problem can be tolerated.
This is the easy way but often proves difficult to apply to understand vibration signatures. At one point or the other analysts do get into tremendous difficulties to make the right sense of the situation and find the right way to improve the system. Why is that? This is because there are no universal laws that might be applied to all vibration signatures. It is important to understand that all signatures reflect the general and the particulars. It means that every system and therefore every signature from a system is unique. Hence trying to make sense of a signature through some universal rules is bound to make things very difficult.
Is there any other way to get out of such difficulty?
Yes, the other way is based on the following premises:
1. The vibration signature shows a snap shot of the patterns the system generates. This pattern represents the present state of the system. Hence the frequencies and the amplitudes are all related to each other and have developed over time. Therefore it makes little sense to see or examine them in isolation.
2. What we are looking at are not looking at data (like frequencies and amplitudes) but observing movement and motion of the system. This is an important point that is often overlooked by analysts. The pattern would tell us about the movement of the system. From studying the movements we would understand why such movements are taking place and what is to be done to stabilize or improve the system.
3. Such movement might be noticed in 3 distinct ways. This is because essentially three types of movements are taking place simultaneously. They arise and abate simultaneously. These are — the slow cycles (characterized by the low frequencies), the cycles that move at a medium speed (understood by the medium frequencies) and the cycles of movement that are quite fast, where changes take place quickly (reflected by the high frequencies).
4. The slow movement determines and affects the movement of the other two movements, namely the medium and the rapid. This slow movement is generated by the rotation of the machine/system. Hence such a frequency is always known as the ‘fundamental frequency’.
5. Now comes the crucial law — Interdependence and the Principle of Opposites. Any movement is only possible if there are two opposing forces or aspects. One is always trying to win over the other or their interaction would produce an in-between state. Such opposition or contradiction of the opposites not only produces the movement or breakdown but also affects the medium and fast movements. This is the way they are all related.
Let us take an example to illustrate the principle. For our purpose we take a fan, say a ID fan. As the blades rotate two forces immediately come into play — the force that makes the blades move (the primary motive force) and the wind resistance that opposes it (called the aerodynamic force). So long the primary motive force is stronger or greater than the aerodynamic force the fan would continue to do its job effectively. As soon as the aerodynamic force starts winning over the primary force, the effectiveness of the fan starts to decreases causing problems and wastage of energy reflected in the medium frequency movements and extremely slow movement (slower than the primary one).
Now let us consider the movement of the bearings (assume anti-friction bearings). As soon as a bearing starts moving two forces immediately come into play. The balls or rollers want to move in a particular direction (decided by the geometry of the bearing and the primary motive force which gives rise to the centripetal acceleration acting towards the center) as opposed to trying to fly off in a tangent but bounded by the outer race of the bearing. As the balls move against the inner race the opposing force that comes into play is frictional force, which we try to reduce through proper lubrication. And as the balls tend to fly off (which it actually does when the balls enter the non-loaded zone) it hits the walls of the outer race and tries to damage it. The only thing that prevents the damage is the extent of the space provided as running and radial clearance and the relative wear of the bearing running surfaces. Hence if the radial clearance is more than necessary the outer race of the bearing would tend to get damaged faster. This would be reflected in the medium and high frequency movements. Higher the rate of damage to either the inner or the outer race more would be the reflection we would find in the rapid cycle movement of the signatures. Similarly as the ball or the roller presses on the inner or outer races as the case might be the material or the lubricant film opposes the movement. If the centripetal force wins over the upward force exerted by the lubricant film or the bearing material itself then the bearing surface gets damaged in no time, which is then reflected immediately in the rapid movement phase (the high frequency signals).
Now let us consider the interactions between the fan blades and the bearing. As the blades develop unbalance the centripetal force in the bearing becomes higher and exerts more force than the lubricant film can exert back thus damaging the bearing. This would be immediately reflected both in the slow movement (the fundamental) and the rapid movement part of the signature (the high frequency zone). Such development of relationship through interactions is governed by another principle. It is called the Quantity to Quality. That is as the quantity (say for example, the amount of unbalance) increases the quality changes accordingly (the degree of damage). Beyond a certain point the system would suddenly collapse.
Hence we can summarize the two principles involved in reading movements in Vibration Signatures as:
a) The Principle of Unity in Opposites
b) The Principle of Quantity to Quality
The change in state can be aptly described by the following diagram that reflects ‘movement’.
Hope we are now clear about how the three types of movement (slow, medium and fast) are related to each other and how the interaction of the movements produce the pattern of the system, which we call the signature.
In the next parts we would develop the ideas in more details with examples and add more principles as we go along.