“Introduction:

This paper continues the series begun in 2001 for the Vibration Institute annual meeting proceedings. The two companion papers, Understanding Journal Bearings published in 2001 and Practical Rotor Dynamics, published in 2002, discussed the theory behind lubrication, fluid film bearings and rotor dynamics. This paper combines the theory of those two works into a guide for implementing rotor dynamics into the process of machinery evaluation. The two areas that can be resolved with rotor dynamics evaluation and modification are critical speed response and rotor system stability.

In order to use rotor dynamics as a tool to solve machinery problems, one must first look at what factors can be altered to make the machinery operate with less vibration and stress. In most cases this means changing or altering different machine components since replacing a machine is often too costly or impractical. For example, if a shaft keeps breaking, a change to high strength material may eliminate the failures. However, changing the shaft diameter may require significantly more effort and expense.

This rotor will be used throughout this paper as a benchmark that will be evaluated as various bearing factors are adjusted. The rotor is exactly symmetrical with negligible overhung weight. The rotor weighs 1000 pounds, has a principal diameter of 5.0 inches and a bearing span of 50 inches. The bearing journals are 3 inches in diameter. Speeds from 1,000 to 10,000 RPM will be evaluated. 

So, for this case the shaft stiffness is 348,000 LB/IN which is reasonably flexible for this type of machine. The guideline previously expressed by the author is that a span-to-diameter ratio of 10 or more usually signals a potential problem machine with high amplification factor and low stability.

Let’s assume that this is a problem machine. What could be done to make it run better with the least effort and cost? In order of increasing difficulty, possible modifications would be:

Unfortunately, only the first 3 items can be done relatively inexpensively and reversibly. The other changes can be very costly and may be irreversible except at additional cost.”