“Introduction:

Rotor dynamics applies the general laws of dynamic motion to rotating equipment. Rotating equipment includes electric motors, pumps, paper mill rolls, turbines, compressors, and many other types of specialized equipment that rotates while contained in some form of support system. The parts that turn are called the rotor and the supports are called bearings. The two properties that determine the dynamic behavior of a rotor are its mass and stiffness. Thus, the geometry of a system to be analyzed (rotor and bearings) must be known fairly precisely.

The importance of rotor dynamic analysis has increased as machine speeds have increased and higher flows and efficiency requirements have had the side effect of sometimes introducing problems with critical speeds, excessive vibration and rotor stability. Some of these problems are related to the economics of the capital expenditures. A machine bought on a cost basis may not have had as through a rotor dynamics analysis as one that is purchased conforming to API specifications or other rigorous requirements. Machines with rotor dynamic design deficiencies often result in excessive vibration problems. This can lead to frequent expensive rebuilds and costly lost production. This paper will introduce the basics of rotor dynamics and lead into some of the more advanced concepts. The mathematics will be kept to a minimum and as many helpful “rules of thumb” will be included as this subject allows.

This paper is written with English units. However, this choice is irrelevant when the purpose is to teach fundamentals. In SI units the equations are essentially identical. One must always be aware of nomenclature. It is quite common to see an analysis done by an OEM or an independent analyst that is unclear about which symbols relate to which properties and what the specific units are. Unless symbols and units are fully defined an analysis becomes confusing or even deceptive.

The question of accuracy also becomes an issue at times. This can be very controversial since model simplification is almost always possible. However, the analyst deciding what to simplify and how has to have a firm grip on what can and what cannot be simplified without compromising the results. My personal approach is to start by including as much detail as possible. My reasoning is that it hardly takes any additional time, the model looks like the actual rotor and it is easier to simplify later than to add complexity to a simple model.”