Tech Tip: Vibration Analysis

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How do you monitor vibration for condition monitoring?

Vibration is considered  an important attribute in the Condition Monitoring world. Characteristics to measure the vibration are ‘displacement’, ‘velocity’ and ‘acceleration’. It is essential to measure one or more of these characteristics when the equipment is new and record as a ‘baseline’ measurement. Measurements taken afterwards should be compared with the baseline to evaluate the health of the equipment.  

What is the best way to attach a transducer?

There are many methods to use a transducer to collect data. Some are better than others and most often determined by the type of data and frequency of collection required. Continuous monitoring requires a much different installation than route collection, or monthly analysis. For most continuous monitoring systems transducers will be permanently mounted in the specified axis. These systems will want to collect the widest range of data possible to target very specific failure modes and inputs. Route collection has the ability to take general data and then focus on specific faults if detected. A fixed system will most likely be on critical equipment while balance of plant with redundancies will rely on route data collection. Both approaches are used in most reliability-based maintenance systems. As you might imagine, these two systems would have much different requirements for how the transducer is affixed to the equipment. A continuous monitoring system would require a clean, flat place for installation, (spot facing), by hand or by machining equipment. A transducer would then be affixed to the flat spot using a stud or epoxy. A route data collector will most often use a magnetic transducer they can quickly affix and remove from the equipment as they move from point of collection to the next point. In terms of highest frequency response gathered by the transducer, the spot facing and stud mounting is best, followed by adhesive non-stud installed, then magnetic based. It is important to note when using the magnetic based transducer, it is still imperative to place it on as flat a surface as possible. A great many route collection programs will spot face and label the required data collection points on all of their fixed position equipment to get the most reliable, repeatable data. There are instances where a flat spot cannot be achieved through machining or other means such as the radial fins of a motor casing. In this case there are specialized tools and mounts that provide solid contact to the casing surrounding the rolling element while supplying a flat surface for the transducer.

What factors should I consider when selecting a transducer?

Transducer selection can be a bit confusing at times. The choice of transducer should be based on the type of data required, sensitivity to change in amplitude and finally, useful frequency range. The speed of the input and output shafts, or at times just the output shaft, will be a key factor in transducer selection. If you eliminate proximity probes as only being useful on slow moving shafts or applications using plain bearings, such as steam turbines, gas compressors, or large fans, you are left with accelerometers and velomitors. Velomitors are not as common due to their cost and limited frequency response window. They are, however, common when looking at very specific frequencies or special cases. The vast majority of applications encountered in a typical industrial facility can be serviced by a “general purpose” accelerometer as it covers the widest range of frequency while maintaining adequate sensitivity across the range. A general purpose accelerometer will be useful and accurate across a much broader spectrum enveloping almost all of what a velomitor could provide. Most general purpose accelerometers are sufficient in the 60-60,000 cpm, (2-10,000 Hz) frequency range. This would cover most of the fault frequencies associated with rolling element bearings, gear mesh frequencies, and misalignment. While they can be used on journal bearings, the amplitudes in vibration will be markedly higher than with a proximity probe.  

Understanding Vibration Analysis

Three tips to make vibration analysis easier:

• Bearing fault frequencies are a function of the bearing geometry (rolling element diameter, number of rolling elements, contact angle, etc...) so bearings with the same geometries will have the same frequency coefficients no matter what other differences make up the bearings (e.g., differences in materials, clearance class, precision class).  
• Most problems with rotating machinery, such as unbalance or misalignment, are synchronous to the running speed, thus they have peaks that occur at whole number multiples of the running speed (e.g. if running speed is 500 rpm and you have a peak at 1500, then that is an whole number multiple of 3 times the rpm).
• Bearing problems are non-synchronous and thus will never have a peak at an whole number multiple of the running speed (e.g., if running speed is 500 rpm and you have a peak at 165 then that is a multiple of 0.33 and could be a fundamental train frequency defect for the bearing).

Since vibration analysis is complicated, the most effective way to use it is to take repeat measurements at regular intervals and look for changes so you can catch problems early.

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