Coupled imbalance

Couple imbalance is caused by two equal non-colinear imbalance forces that oppose each other angularly (i.e., 180° apart). Assume that a rotor with pure couple imbalance is placed on frictionless rollers. Because the imbalance weights or forces are 180° apart and equal, the rotor is statically balanced. However, a pure couple imbalance... 

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. 

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. 

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). 

It is possible that the driven equipment to which the motor is coupled has a higher vibration level than the motor, resulting in quantum imbalance and more vibrations than when the motor was tested. All attempts must be made, to bring the vibration level of the drive and the driven system within the limits as prescribed in Table 11.3. 

Balance. If the center of mass of the projectile is not located extremely close to its longitudinal centerline, the center of pressure can form a couple with the inertial reaction through the center of mass, making the projectile tend to tumble in the barrel (Barker, 1985). Tumbling is prevented by the gun barrel, which exerts counteracting asymmetric forces on the projectile during its acceleration. These forces can lead to asymmetric wear of the projectile. If the center of mass is closer to one end of the projectile, which it nearly always is, the situation is unstable the wear increases the imbalance, which increases... 

In order to prevent an imbalance from occurring, do not allow the balance operator to guess the key length. It is strongly suggested that the actual key length be recorded on a tag that is attached to the rotor to be balanced. The tag should be attached in such a way that another device (such as a coupling half, pulley, fan, etc.) cannot be attached until the balance operator removes the tag. 

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. 

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. 

An important point to remember is that static imbalance is always removed first. In static and couple balancing, remove the static imbalance first and then remove the couple. 

Most balancing standards are based on a residual imbalance and do not include multi-plane imbalance. If they are approximately 180° to each other, they form a couple. If the distance between the planes is small, the resulting couple is small if the distance is large, the couple is large. A couple creates considerably more vibration than when the two residual imbalances are in-phase. Unfortunately, there is nothing in the balancing standards that considers this. 

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

The potent antidiuretic hormone AVP orchestrates the regulation of free water absorption, body fluid osmolality, cell contraction, blood volume, and blood pressure through stimulation of three G-protein-coupled receptor subtypes Vi-vascular types a and b, V2-renal, and V3-pituitary. Increased AVP secretion is the trademark of several pathophysiological disorders, including heart failure, impaired renal function, liver cirrhosis, and SIADH. As a consequence, these patients experience excess water retention or inadequate free-water excretion, which results in the dilution of sodium concentrations, frequently manifesting as clinical hyponatremia (serum sodium concentration <135mmol/L). This electrolyte imbalance increases mortality rates by 60-fold. Selective antagonism of the AVP V2 receptor promotes water... 

Sometimes interdiffusion between two metals is uneven and may lead to the creation of vacancies or voids. This type of imbalance is the result of possible unequal mobilities between a metal couple. These voids occur individually near the common interface. The voids, like bubbles, coalesce, resulting in porosity and loss of strength. Many thin-fihn couples exhibit this phenomenon, which is referred to as Kirkendall void creation. Al-Au, Cu-Pt, and Cu-Au are just a few examples. To be specific, it has been found (7), for instance, that in the case of Au-Ni, about five times more Ni atoms diffuse into Au than Au atoms diffuse into Ni. 

Gurtovenko, A.A., Vattulainen, I. Pore formation coupled to ion transport through lipid membranes as induced by transmembrane ionic charge imbalance atomistic molecular dynamics study. J. Am. Chem. Soc. 2005, 127, 17570-1. 

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