Checking and correcting rotor unbalance

Electrical Apparatus, Jan 2001 by Nailen, Richard L

Vibration pickups for the balancing machine

WHAT'S COMMON TO ALL ROTATING machines? Electric motor or generator, gasoline or diesel engine, fan, pump, turbine-all are subject to potentially destructive mechanical unbalance. No matter how precisely machined or assembled, rotating components inevitably contain some dissymmetry of weight distribution. The result of an out-of-balance condition can be mere annoyance, or-through loss of a turbine blade-an cause a jetliner to crash. In the electrical apparatus service shop, then, checking and correcting rotor unbalance is a basic maintenance and repair technique.

For that purpose, most shops use dynamic balancing machines to quickly determine the magnitude and location of unbalance so that it can be corrected insofar as possible.

We know that unbalance isn't always the cause of machine vibration, but that vibration always results when unbalance is present. Our only means of evaluating-and correcting-unbalance is by measuring the vibration it produces.

For motors or generators in the field, or for "trim balancing" any assembled machine in the factory or service center, we measure and analyze vibration. So-called field balancing equipment is one of the available tools.

Dynamic balancing

During assembly or repair, however, the more usual tool is the dynamic balancing machine. Designed solely to locate and quickly define unbalance of a rotating component, the balancer tells us how that unbalance should be corrected.

What the balancer responds to, however, is the vibration, sensing it in the same way as a field balancer or other vibration analyzer does-by reading the magnitude and frequency of the mechanical movement caused by vibratory force.

What do you need to know when choosing a balancing machine--or, as some shop people have done, building one of your own? Buzzwords abound in the literature: digital readout, plane separation, polar graphics, tracking filter, hard bearing versus soft bearing, prompting display-and on and on. Seldom mentioned, however, is the one feature essential to all balancing equipment: the transducer or pickup that provides the vibration signal (Figure 1). Everything depends upon that little device. What types are available? What governs the choice?

Choosing a transducer

The type of balancing machine, and the way it is used, may dictate transducer selection. More is involved than just the workpiece weight to be handled. The user needs to conROTOR UNBALANCE continued from previous page sider these other interrelated limitations as well:

* Balancing speed versus workpiece operating RPM.

* Balance standards to be met.

* Type of workpiece suspension system.

* Characteristics of software programs to be used.

As the terms hard bearing and soft bearing imply, the soft machine suspension permits an unbalanced rotor to swing through a much wider range of motion. Several types of support systems are used. Each has its own natural frequency of oscillation, often within the balancer's useful speed range. At that frequency, when excited by unbalance, some systems may allow workpiece movement up to two inches. Not all transducers can tolerate such an extreme. Other soft bearing suspensions may allow excursions of half an inch or less, which is acceptable for those transducers.

A hard bearing balancer normally differs in two ways. First, supporting head movement doesn't exceed 0.2". Second, its natural frequency is normally above balancing speed.

If you go through the extensive literature published by suppliers of balancing and vibration measurement equipment, including the "how to" manuals, you find little information on the sensors or "pickups" themselves. Under what conditions is one type preferred over another? Answers aren't readily apparent.

For example, one 149-page user's manual titled Dynamic Balancing-What It Means to You and Your Customer, makes no mention of transducer types or functions in either text or illustrations.

You will come across many different names for these devices. Some examples:

* Low impedance voltage mode accelerometer

* Absolute displacement transducer

* Internally amplified accelerometer

* Integrated accelerometer

Many of these are simple enhancements or modifications of a few basic devices.

An appropriate reminder here is that measuring vibration magnitude (displacement) alone is insufficient. A displacement of one mil, for example, is of no importance at a low frequency-say, 100 cycles per minute. But at 10,000 cycles per minute, it becomes dangerous.

We can readily grasp this without a lot of theory. The faster an object moves back and forth through a distance, the more force it exerts on its surroundings, and the more energy is involved in reversing the motion at the end of each half cycle. A basic equation of mechanics is that force equals mass times acceleration, or F=MA. The greater the acceleration (which a vibrating mass undergoes each time its rate or direction of movement changes), the greater the force involved.

Ideally, then, we want to judge vibration severity by measuring acceleration. That's done by means of an accelerometer-a type of transducer that produces a voltage output proportional to the acceleration it senses.