Vibration analysis: what does it mean?

Interpreting the vibration signature of plant equipment.

By Chuck Yung

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Vibration analysis, properly done, allows the user to evaluate the condition of equipment and avoid failures. Maintenance personnel can minimize unplanned downtime by scheduling needed repairs during normal maintenance shutdowns. How can you interpret — at a glance — the comprehensive spectrum information available? In layman's terms, here is how to interpret the vibration signature of rotating equipment. You can learn to recognize misalignment, a defective bearing, bent or loose parts — and tell them apart.

Unscheduled downtime may cost tens of thousands of dollars per hour. Fortunately, modern vibration analysis equipment and software predict developing problems so that repair happens before disaster strikes. While these sophisticated tools offer many automated features and capabilities, it still takes a basic understanding of vibration analysis to use them effectively. Plant maintenance personnel having this knowledge also have fewer emergencies — and happier bosses.

In the past, vibration analysis required dialing an instrument through the full spectrum to identify frequencies at which vibration was prominent. The operator then compared the peak frequencies with the operating speed and consulted a chart for likely causes. One advantage of that method was that the operator gradually developed a sense of how equipment vibrates and why certain problems occur at the same multiples of the rotating speed.

The latest generation of vibration analyzers has more capabilities and automated functions than their predecessors had. Many units display the full vibration spectrum of three axes simultaneously — providing a snapshot of what is going on with a particular machine. But despite such capabilities, not even the most sophisticated equipment successfully predicts developing problems unless the operator understands and applies the basics of vibration analysis.

What follows are the basic concepts of vibration analysis that operators in the past learned the hard way through 20 years of hands-on experience. Besides tips on how to record and interpret vibration readings, there is also an example that shows how some of these principles might apply in a typical situation.

Collecting the data — what is important?

First things first. The first and most important step is to gather complete data. That means obtaining a full-spectrum vibration signature in three axes (horizontal, vertical, and axial) on both ends of the motor and the driven equipment. Busy predictive maintenance personnel take only one reading and hope to spot emerging problems. Unfortunately, some problems show up in only one axis. Unexpected machinery failure damages the credibility of vibration analysis whereas comprehensive data should have predicted the problem.

When looking at a vibration signature, it helps to think in terms of multiples of the rotating speed. Because not all plant equipment operates at the same speed, this simplifies analysis. Rotor unbalance, for instance, usually shows up at rotating speed. Mechanical problems — such as a bent shaft, bad coupling, or oversized bearing housing — tend to appear at 2 x rpm.

Vibration frequencies at higher multiples of the rotating speed correspond with the number of components in a specific rotating part, such as the number of balls in a bearing. Other sources of vibration frequencies at multiples of the rotating speed may include fan blades, impeller vanes, rotor bars or stator slots, or some combination thereof.

The nameplate speed probably is not the exact running speed of the motor. It is based on the manufacturer's average speed for that particular motor design. The actual speed of an induction motor is always lower than the synchronous speed. Be sure to consider this, especially when looking at higher frequency ranges. Using a synchronous rpm of 1,800, for example, might prompt a technician to look for something with 53 components if there is a peak at 95,400 cpm (95,400/1800 = 53). If the actual running speed was 1,766 rpm, the same peak at 95,400 cpm would really indicate 54 rotor bars (95,400/1766 = 54).

Ball or roller bearings have several specific frequencies associated with them. The ball-passing frequency, for instance, depends on the number of rolling elements in a particular bearing. Be aware of differences among manufacturers in the ball complement of a particular bearing size. One manufacturer may use 8 balls in a particular bearing; another may use 9 in the same bearing. Watch out for max bearings, too. These are designed with more balls than standard bearings to increase load capacity.

The number of balls and the actual speed of the motor are factors in determining the frequency at which to expect rolling element problems. Inner and outer race defects show up at specific frequencies as well. Because the circumference of the outer bearing race is greater than that of the inner race, the rate at which balls pass a race defect will differ. The frequency at which these defects manifest themselves depends on several things--the speed at which the bearing is rotating; the number and diameter of balls in the bearing; and the circumference of the inner and outer races.

The programs for many vibration analysis equipment and data collectors store this bearing information in a database. To make the most of these programs, though, it helps to work with a service center that provides detailed repair reports, including bearing size and manufacturer. This eliminates guesswork when a problem arises.

Aerodynamic or hydraulic forces occasionally show up in a vibration signature. If a 1,775-rpm motor has 7 fan blades, a peak may occur at 12,425 cpm (1775 x 7 = 12,425). A 5-bladed pump impeller rotating at 1,450 rpm could have a peak at 7,250 cpm. Normally, these peaks are of a low magnitude. While interesting, they are of no concern unless resonant frequency problems occur. If the amplitude — the height — of the peak increases significantly since the last reading, it may indicate a developing problem like a high fan blade passing a fixed protrusion or a damaged impeller blade.

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