TABLE OF CONTENTS In the previous chapter, spectrum analysis was shown to be a powerful tool in determining the condition of various components inside a given machine. In Chapter 6, we will show the advantage of monitoring these levels on a regular periodic basis. This chapter discusses techniques that will greatly enhance the diagnostic capabilities of spectrum analysis, but do not lend themselves to periodic monitoring.
It has long been known that the measurement of the vibration levels generated by rotating machinery, such as pumps, gears, turbines, and so on, is a cost-effective endeavor. Maintenance personnel are beginning to learn that spectrum analysis for problem diagnosis enhances these benefits greatly. Less well known are the benefits of using a dual-channel spectrum analyzer for problem diagnosis in the plant. This is, perhaps, due to the size, weight, and cost of early dual-channel analyzers. Improvements in electronic technology have permitted design enhancements in current analyzers to the point where dual-channel units are about the same size, weight, and cost as single-channel analyzers. The time is right, therefore, for plant maintenance and reliability personnel to learn the attributes of dual-channel analysis.
To discuss the advantages of dual-channel analysis, it is necessary to understand natural frequencies, transfer functions, coherence, and coherent output power. Although some mathematics will be used, the explanations of these properties will rest heavily on physical interpretations.
All forcing frequencies, including those described in the previous chapter, share the characteristic that they are self-generated. When you turn a machine on, the...
