by Chuck Yung

Modern vibration analyzers and software make it possible to spot emerging problems and avoid costly failures. That said, a basic understanding of vibration analysis is still necessary to recognize misalignment, defective bearings, or bent or loose parts. What follows are basic concepts and tips for interpreting the vibration signature of rotating plant equipment.

What data is important?
The first step to predicting problems 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 driven equipment (see figure graph).

Because not all plant equipment operates at the same speed, it helps to think in terms of multiples of rotating speed (Item A in graph). 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 two times rotating speed (Item B).

Vibration frequencies at higher multiples of the rotating speed correspond with the number of components in a specific rotating part — e.g., 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 of these sources.

The nameplate speed probably is not the exact running speed of the motor. At rated frequency, an induction motor’s actual speed is always lower than its synchronous speed. This is true especially for higher frequency ranges. A synchronous speed of 1,800 revolutions per minute might prompt a technician to look for something with 53 components for a peak at 95,400 cycles per minute (95,400/1,800 = 53). 

If the actual running speed were 1,766 rpm, a peak at 95,400 cpm would really indicate 54 rotor bars (95,400/1,766 = 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 bearing. Be aware, though, that one manufacturer may use eight balls in a particular bearing size, while another uses nine. Watch, too, for increased load capacity max bearings. These have more elements than standard bearings.

Figure 1                                                                                          Source:EASA

Inner and outer race defects also show up at specific frequencies (Items D and E). The outer bearing race has a larger circumference than the inner race, so the rate at which balls pass a race defect will differ. The frequency at which bearing defects manifest themselves also depends on bearing rotation speed and the number of balls in the bearing.

Aerodynamic or hydraulic forces occasionally show up in a vibration signature at the rpm times the number of pump or fan blades. If the amplitude significantly increases from one reading to the next, a problem may be developing.

Resonance becomes a problem when the natural frequency of the entire assembly is close to the vibrating frequency of one part of the rotating system. With existing equipment, this occurs only if something changes. Installing a lighter-weight replacement pump or motor may alter the natural frequency of the assembly. A replacement shaft with a different diameter could change the resonant frequency of the package (motor/base/driven equipment). The change also could be structural — e.g., a cracked weld that reduces overall stiffness.

In the graph, the high readings show a resonance problem at 1,200 cpm (the motor rpm) that developed after the installation of a new sole plate. A bump test verified this. The motor’s supporting structure literally rang like a bell, resonating at 1,200 cpm.

Very high frequencies usually correspond to a rotor bar- or slot-passing frequency (Item G). These frequencies are multiples of the running speed, so they are much higher for high-speed motors than for low-speed motors. A 150-horsepower motor with 65 rotor bars that operates at 3,565 rpm has a bar-passing frequency of 231,725 cpm (3,565 x 65 = 231,725). 

By contrast, a 65-bar rotor running at 300 rpm exhibits a bar-passing frequency of 19,500 cpm (300 x 65 = 19,500).

Defective rotor bars generally increase the amplitude of the motor’s bar-passing frequency, usually producing a pair of symmetrical 60-hertz (Hz) side bands bracketing the rotor-passing frequency. 

On 60-Hz systems, electrical problems usually show up at 7,200 cpm (60 cycles per second x 60 seconds x 2 = 7,200 cpm), as in Item C in Figure 1. 

Possible causes include:
– voltage unbalance;
– eccentric air gap resulting from an out-of-concentric rotor body or stator bore;
– a rotor shaft bent between the bearings; or
– defective rotor bars.

Chopping the power while monitoring the vibration signature is an effective check for electrical problems. The electrically induced portion of the vibration disappears instantly when the power is cut.

Variable-frequency drives make interpreting data even more challenging, since inverter-driven induction motors may operate far below nameplate speed. Even electrical problems are more difficult to diagnose because the frequency varies. 

As a point of reference, record the actual speed when collecting the vibration data.

Axis-specific problems sometimes appear. A high vertical reading, for example, is usually caused by a base-related problem (soft feet, loose base bolts, structural looseness or a disbonded sole plate). High horizontal readings are commonly associated with unbalance in the rotating elements or mechanical looseness (worn coupling, oversized bearing housing or, in the case of a sleeve bearing motor, excess bearing-to-shaft clearance). 

High axial readings generally indicate misalignment — either external, like coupling misalignment; or internal, like mechanical looseness or a bent shaft. Normally, the magnitude of axial readings should not exceed half of radial readings.

Making it happen
Even the most sophisticated analyzer is no substitute for a seasoned vibration technician. Those with less experience can obtain good basic results by remembering a few simple steps:

– Gather data in three axes at both ends of the motor and driven equipment.

– Look for the highest peaks on the frequency spectrum, paying close attention to those showing up only in one or two axes.

– Factor in the history of the machine and identify probable sources of vibration for each peak.

– Match up the likely causes of vibration in each axis and see how they relate to each other.

Chuck Yung is a technical support specialist at the Electrical Apparatus Service Association. EASA is a trade organization consisting of more than 2,500 electromechanical sales and services firms in 59 countries. For more information, call or visit www.easa.com.

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