Dynamic imbalance

Next we ll discuss evidence marks and prints that are different, but to the untrained eye, they may appear the same. You may see a spot or arc of wear and gouging on the rotary elements, and a eireumferential wear circle on the bore of the close tolerance stationary elements. This is a maintenanee-indueed problem, d his is the sign of a physically bent shaft, or a shaft that is not round, or a dynamic imbalance in the shaff-sleeve-impeller assembly. The solution is to put the shaft on a lathe or dynamic balancer, verify its condition, and correct before the next installation. 

Imbalance is the condition when there is more weight on one side of a centerline than the other. This condition results in unnecessary vibration, which generally can be corrected by the addition of counterweights. There are four types of imbalance (1) static, (2) dynamic, (3) couple, and (4) dynamic imbalance combinations of static and couple. 

Dynamic imbalance is any imbalance resolved to at least two correction planes (i.e., planes in which a balancing correction is made by adding or removing weight). The imbalance in each of these two planes may be the result of many imbalances in many planes, but the final effects can be characterized to only two planes in almost all situations. 

In dynamic imbalance, the two imbalances do not have to be equal in magnitude to each other, nor do they have to have any particular angular reference to each other. For... 

Although the definition of dynamic imbalance covers all two-plane situations, an understanding of the components of dynamic imbalance is needed so that its causes can be understood. Also, an understanding of the components makes it easier to understand why certain types of balancing do not always work with many older balancing machines for overhung rotors and very narrow rotors. The primary components of dynamic imbalance include number of points of imbalance, amount of imbalance, phase relationships, and rotor speed. 

Visualize a rotor that has only one imbalance in a single plane. Also, visualize that the plane is not at the rotor s center of gravity, but is off to one side. Although there is no other source of couple, this force to one side of the rotor not only causes translation (parallel motion due to pure static imbalance), but also causes the rotor to rotate or wobble end-over-end as from a couple. In other words, such a force would create a combination of both static and couple imbalance. This again is dynamic imbalance. 

In addition, a rotor may have two imbalance forces exactly 180° opposite to each other. However, if the forces are not equal in magnitude, the rotor has a static imbalance in combination with its pure couple. This combination is also dynamic imbalance. 

Another way of looking at it is to visualize the usual rendition of dynamic imbalance - imbalance in two separate planes at an angle and magnitude relative to each other not necessarily that of pure static or pure couple. 

Note that whenever you hear the word imbalance, mentally add the word dynamic to it. Then when you hear dynamic imbalance, mentally visualize a combination of static and couple imbalance. This will be of much help not only in balancing, but in understanding phase and coupling misalignment as well. 

In order to calculate imbalance units, simply multiply the amount of imbalance by the radius at which it is acting. In other words, one ounce of imbalance at a one-inch radius will result in one oz-in of imbalance. Five ounces at one-half inch radius results in 2 oz-ins of imbalance. (Dynamic imbalance units are measured in ounce-inches, oz-in, or gram-millimeters, g-mm.) Although this refers to a single plane, dynamic balancing is performed in at least two separate planes. Therefore, the tolerance is usually given in single-plane units for each plane of correction. 

For example, a 20-inch wide rotor could have a large enough couple imbalance component in its dynamic imbalance to require two-plane balancing. (Note The couple component makes two-plane balancing important.) Yet, if the 20-inch width is on a rotor of large diameter to qualify as a disc-shaped rotor, even some of the balance manufacturers erroneously would call for a single-plane balance. 

It is true that the narrower the rotor, the less the chance for a large couple component and, therefore, the greater the possibility of getting by with a single-plane balance. For rotors over 4 to 5 inches in width, it is best to check for real dynamic imbalance (or for couple imbalance). 

Development of the steady-state model for an evaporator involves material and energy balance. A relationship between feed composition and product composition is also required. The process dynamics require that a lead-lag dynamic element be incorporated in the system to compensate for any dynamic imbalance. In some applications evaporators are operated with waste steam, in which case the feed rate is proportionally adjusted to the available steam, making the feed the manipulated variable and steam the load variable. Generally, steam is the manipulated variable. The final consideration Is feedback trim. As a... 

Each load change is followed by a period of dynamic imbalance, which makes its appearance as a transient temperature error. 

The transient deviation of the controlled variable depicted in Fig. 8.5 was attributed to a dynamic imbalance in the process. This character istic can be assimilated from a number of different aspects. 

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