Troubleshooting Rotating Mechanical Equipment Using Vibration Analysis
Vibration is technically defined as the oscillation of an object about its position of rest. These oscillations are responses to mechanical forces symptomatic of a problem. Before we discuss the technology as a troubleshooting tool, a review of terminology is important.
Frequency refers to “how many” of these oscillations in a given length of time (e.g., one minute), measured in cycles per minute (CPM), or cycles per second (Hertz) Hz, related to 1X shaft turning speed.
Displacement refers to amplitude and “how much” the object is vibrating measured in Mils (1/1000 inch) peak to peak. Displacement (distance or movement) is generally the best parameter to use for very low frequency measurements (i.e., less than 600 cpm), where velocity and acceleration amplitudes are extremely low. Displacement is also traditionally used for machine balancing at speeds up to 10,000 or 20,000 rpm and where clearances are important criteria.
Velocity indicates “how fast” the object is vibrating measured in inches/second or mm/second peak. Velocity is frequently used for machinery vibration analysis where important frequencies lie in the 600 to 60,000 cpm range. For most machines, mechanical condition is most closely associated with vibration velocity, which is a measure of energy dissipated and consequent fatigue of machinery components. Overall velocity is also best for detecting a wide variety of different machinery defects occurring at the mid-frequency range.
Acceleration of the object that is vibrating is related to the forces that are causing the vibration measured in “gs” (1g = 32 ft/sec2 or 9.8 m/sec2) and is reported or shown as root mean squared (RMS). Acceleration (force) is best measured when it is known that all the troublesome vibrations occur at high frequencies, that is, above 60,000 cpm. For example, in detecting high frequency turbine blade vibration in the presence of many low frequency vibrations, acceleration will assist in emphasizing the high frequencies. It is important to remember that when using various transducers to monitor vibration, velocity leads displacement by 90 degrees and acceleration leads velocity by 90 degrees and displacement by 180 degrees.
Vibration analysis should be part of any equipment reliability management program, but what must first be determined are which machines should be monitored and how often monitoring should take place using the formula:
MACHINE PRIORITY = CRITICALITY X RELIABILITY.
Where criticality is the importance of the machine to production goals; and where reliability is a result of a review of the “probability of failure” based on history of machine repair.
1. How often should your facility apply vibration analysis?
Logic: machines that are operated under proper conditions and are considered reliable with a satisfactory maintenance history may require quarterly vibration monitoring, depending upon their criticality. Others may require monitoring on a weekly basis due to high production demands, or which might create a safety or environmental hazard. Machines that exhibit serious problems may require monitoring daily or hourly, until repairs can be planned, scheduled, and executed.
2. How does your facility determine the correct selection and application of transducers?
Logic: the quality of the data gathered by vibration analysis is directly dependent upon proper selection and mounting of the transducer. If possible, vibration readings should be taken with the transducer mounted perpendicular to the surface of interest in horizontal, vertical, and axial directions. Vibration signals containing “high frequencies” must be taken with an accelerometer tightly screwed, or glued to the surface, since hand-held pressure alone cannot hold it tightly enough to the surface for it to obtain high frequency motion.
Displacement non-contact proximeters are used to look directly at the rotating shafts of machinery, and the frequencies obtained will be quite low. Accelerometers have the advantage of having adequate sensitivity over a wide range of frequencies. The low end is typically one to three Hz, while the upper range can be as high as 20 kHz. For this reason, accelerometers are the preferred device to use.
3. Do your facilities vibration analysts understand the
causes of vibration?
Logic: the primary causes of vibration at what is referred to as rotor frequency, are unbalance, misalignment, a bent or bowed shaft, or an eccentric rotor. Any of these conditions can cause a 1X shaft speed vibration or harmonics of that frequency. These conditions cause approximately 75 per cent of all vibrations in industrial plant rotating equipment.
4. Are your vibration analysts familiar with rolling element problems that can be determined using vibration analysis?
Logic: anti-friction bearings inherently have low starting friction but high running friction, and the shaft frequency for oil lubricated bearings should not exceed 9,600 divided by the shaft diameter. Grease lubricated bearings will have a shaft frequency that will not usually exceed 7,200 divided by the shaft diameter.
For example, a six-inch diameter shaft supported by grease lubricated anti-friction bearings should not be run at a frequency higher than 1,200 RPM (or 20 Hz).
Rolling element fault frequencies are caused by fatigue or running wear, incorrect or insufficient lubrication, misalignment or manufacturing flaws within the bearing. There are four fundamental frequencies in anti-friction bearings. These are: fundamental train frequency (FTF), ball-pass frequency of the inner race (BPFI), ball-pass frequency of the outer race (BPFO) and the ball-spin frequency (BSF). These defect frequencies depend upon the shaft speed and bearing geometry. The type of bearings used in the machine should always be recorded in maintenance files and manufacturer’s data can be obtained to provide the specifications.
5. Are your analysts familiar with journal bearing problems that can be determined using vibration analysis?
Logic: at low RPM, journal bearing friction is high due to boundary lubrication. The friction decreases as the shaft moves into the rotating position where there is a full oil film between the shaft and bearing’s inner surface. A vibration problem associated with journal bearings is the possibility of hydraulic instability of the rotating shaft inside the bearing.
This vibration is caused by “oil whirl” or “oil whip.” This is caused when a wedge of lubricant moves the shaft in an eccentric motion as the shaft rotates, which if severe enough will cause a vibration. The frequency will appear somewhere between 35 to 49 per cent of the shaft’s rotational frequency.
6. Are your analysts familiar with vibration problems associated with gear drives?
Logic: gear mesh vibration frequencies will be very high and can be calculated by multiplying the number of gear teeth by RPM of the shaft. When diagnosing a multiple gear train, calculate the gear ratio between the drive and driven gear(s). Then use this ratio to calculate the speed of the driven gears. Most gear box manufacturers will provide this information and may provide the various frequencies within the drive.
7. Do your analysts understand vibration problems associated with fan blade and pump impeller vibration problems?
Logic: blade pass frequencies occur as each blade on a fan or compressor rotor delivers its contribution to the process. The blades on a pump impeller create a frequency as the blade passes the outlet port. Blade or vane pass frequencies are inherent in pumps, fans and compressors and normally do not pose a problem. However, large amplitude BPF with harmonics will be generated in pumps if the “vane to diffuser” gaps are unequal, if the impeller wear ring seizes on the shaft, or if welds fail. High BPF can be caused by bends in piping, ducts, or obstructions that disturb flow.
8. Do your analysts understand the relationship between vibration frequencies (the forces that cause vibrations) and resonance?
Logic: resonance is the excitation of the natural frequency of a system or the excitation of a component within that system. Put another way, resonance is a condition where the frequency coincides with a systems natural frequency. If a resonant frequency is excited by another frequency operating at or near the same speed, a rotating machine or component can destroy itself in a very short time.
This is a brief introduction to the topic of vibration analysis as a troubleshooting tool. If understood and applied correctly, it offers early warning of many rotating machinery problems. Used in combination with other predictive maintenance technologies like thermography, lubricant analysis, and acoustic ultrasonics, the return on investment will be worth every penny. MRO