Fundamentals of Rotating Machinery Diagnostics — Foreword

Rotating machinery vibration analysis requires the use of principles that are still quite unfamiliar to many mechanical engineers. These principles are probably the least understood of those in any other field, yet are critical to the design, operation, and diagnosis of high-speed, high-power machinery. Over the past 100 years, misconceptions, misstatements, and mistakes in the description of rotor dynamics have compounded the problems.

In this age of detailed mathematical study of shaft dynamics, the rapidly advancing technology is not being properly communicated to the practicing engineers and engineering students in straightforward, compelling terms. Certainly, these days, most engineers do not have the time to digest all the published material. One of the most powerful new ideas is Dynamic Stiffness.

The vibration we measure is a ratio, the ratio of the dynamic force to the Dynamic Stiffness of the machine. This book clearly shows how to use Dynamic Stiffness to understand and recognize malfunction behavior. It is also a single source for the description of the fundamental principles of rotor dynamics and how machinery behaves. It corrects the misconceptions that have plagued the discipline and opens new territory and routes to understanding the dynamics of rotating machinery.

For example, in existing literature, the cross stiffness terms, Kxy and Kyx, are treated as independent variables. We call these terms quadrature terms, which have a very simple relationship. The “cross stiffness” is actually a tangential stiffness term (quadrature term) that acts perpendicular to the direction of displacement. The tangential stiffness term, DλΩ, is defined in basic rotor dynamic parameters, which are much more useful when you’re trying to diagnose machinery operation.

Exploring new territory is always a fantastic adventure, and never without problems. In exploring the basic nature of rotating machinery, I regularly hit unforeseen cliffs, swamps, or other impediments. Looking back, having solved the problem, these pitfalls are interesting.

Crossing into new territory, it sometimes was necessary to tread on old traditions where these traditions were wrong, or were nearly correct but had been slightly misinterpreted. Great resistance to progress was, therefore, encountered from people who had an incorrect view of the theory.

Since the invention of rotating machines, the pursuit of higher power output has driven machine speeds higher and higher. With the breaking of the first balance resonance “barrier” (achieved by De Laval with a steam turbine in 1895), rotating machines were shown to be able to operate above the first balance resonance. However, with this new capability came a new problem for machines using fluid-lubricated journal bearings: fluid-induced instability. Over the years, many different methods have been developed by researchers to identify and understand the important parameters that influence rotor stability and, so, increase the reliability of the machinery.

Reliability is often thought to be synonymous with long, trouble-free life, and improved reliability to mean a longer, trouble-free life. But these are not acceptable definitions. A machine or component becomes reliable when its operation and actions are predictable. The accuracy with which these actions may be predicted is a true measure of its reliability. It follows, then, that reliability can best be improved by learning as much as possible about equipment operation and using this knowledge to reduce or eliminate as many unpredictable items as possible. Accurate predictions require accurate, meaningful data from which analysis can be made. When you have the data necessary to make accurate predictions of machine operations, you also have the data to improve designs, extend the life of components, probably even reduce its cost and increase its safety.

Meaningful information is the key. This book is a major step in assuring that good data can become meaningful information through the increased knowledge of the machinery specialist. It is a well-constructed foundation of the bridge to the future.

Machinery technology is rapidly changing, and new developments are always making their way into machines. One very promising new technology is the externally pressurized bearing, which Bently Nevada is developing. This bearing is an externally pressurized (hydrostatic), fluid-film bearing that can be operated in a passive mode, a semi-active mode, or in a fully active mode. In the passive mode, the bearing operates with a fixed design pressure and, by extension, fixed-by-design spring stiffness and damping. In the semi-active mode, the external supply pressure can be adjusted under operator control to change the values of stiffness and damping while the machine is operating. In its active mode, it is capable of producing fully automatic, instantaneous changes in stiffness and damping to control the rotor position in real time.

In June 2001, we demonstrated suppression of oil whirl by increasing bearing pressure at the International Gas Turbine Show in Munich, Germany. In August 2001, we demonstrated the suppression of oil whip. This was the first demonstration of a supplementary bearing in the central span of a rotating machine.

These two successful innovations, never performed before in history, do not solve all instability problems, but they certainly make it possible to control two obvious problems that have presented challenges for rotating engineers for many decades.

This new technology promises to change the way machines respond dynamically and will require changes in the way we interpret and apply machinery data.

For example, the balance resonance is usually thought of as occurring at a fixed operating speed, where running speed coincides with a fixed rotor system natural frequency. With a semi- or fully active bearing, the natural frequency and balance resonance speed now become variables under the machine operator’s control. By changing the bearing spring stiffness in semi-active mode, the balance resonance can be quickly moved to another speed, enabling the operator or machine control system to jump the resonance rapidly through the machine during startup or shutdown. This behavior will greatly alter, even eliminate, the usual balance resonance signature in a polar or Bode plot.

Changes in the balance resonance speed will also affect balancing. Active shifting of resonances will make polar plots look different, changing the way we identify the heavy spot. If a resonance is shifted to a different speed, then heavy spot/high spot relationships may change. For example, what was above a resonance might now be below, or vice versa. Response that was out of phase might now be in phase. Influence vectors may depend on bearing settings, and repeatability will require similar bearing settings.

Changes in bearing stiffness can also change the rotor mode shape. A mode associated with low bearing stiffness, for example, a rigid body mode, could be modified by higher bearing stiffness to a bending mode. This change in mode shape could change the match to the unbalance distribution, producing a change in balance state. It is possible that the existing unbalance distribution would become a better or poorer match to the new mode shape, and that the rotor would have to be balanced specifically at particular bearing settings.

Some malfunctions manifest themselves as a self-excited vibration at a system natural frequency. Because of the new, variable nature of the balance resonance, this natural frequency will exist somewhere in a frequency band, which will depend on the range of bearing settings and their effect on rotor modal stiffness. Under some circumstances, the bearing will allow the operator to move the natural frequency to a place where the malfunction vibration cannot occur. The diagnostician will need to understand how this kind of variable-parameter bearing operation will affect his or her interpretation of the data, and how it can be used to suppress unwanted vibration.

New technology will give us awesome new opportunities and new challenges. No matter what new developments occur, the fundamental principles of rotor dynamics presented in this book will remain the same. The machinery diagnostician who has a solid foundation in the fundamentals will be able to apply the basic principles presented in this book and solve machinery problems.

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