Mechanical imbalance

The no-load test is a very informative method to determine the no-load current, core and pulsation losses, friction and windage losses, magnetizing current and the no-load power factor. The test also reveals mechanical imbalance, if any, performance of the bearings, vibration and noise level of the motor. 

Mechanical imbalance is not the only form of imbalance that affects rotating elements. It is the condition where more weight is on one side of a centerline of a rotor than on the other. In many cases, rotor imbalance is the result of an imbalance between centripetal forces generated by the rotation. The source of rotor vibration also can be an imbalance between the lift generated by the rotor and gravity. 

For example, mechanical imbalance generates radial forces in all directions, but misalignment generally results in a radial force in a single direction that corresponds with the misaligned direction. The ability to determine the actual displacement direction of the machine s shaft and other components greatly improves diagnostic accuracy. 

Electric motors are susceptible to a variety of forcing functions that cause instability or imbalance. The narrow-bands established to monitor the fundamental and other harmonics of actual running speed are useful in identifying mechanical imbalance, but other indices also should be used. 

An imbalance profile can be excited due to the combined factors of mechanical imbalance, lift/gravity differential effects, aerodynamic and hydraulic instabilities, process loading, and, in fact, all failure modes. 

Mechanical It is incorrect to assume that mechanical imbalance must exist to create an imbalance condition within the machine. Mechanical imbalance, however, is the only form of imbalance that is corrected by balancing... 

Single-plane Single-plane mechanical imbalance excites the fundamental (lx) frequency component, which is typically the dominant amplitude in a signature. Since there is only one point of imbalance, only one high spot occurs... 

Because mechanical imbalance is multi-directional, it appears in both the vertical and horizontal directions at the machine s bearing pedestals. The actual amplitude of the 1X component generally is not identical in the vertical and... 

Multi-plane Multi-plane mechanical imbalance generates multiple harmonics of running speed. The actual number of harmonics depends on the number of imbalance points, the severity of imbalance, and the phase angle between imbalance points. 

Normally associated with bladed or vaned machinery such as fans and pumps, process instability creates an unbalanced condition within the machine. In most cases, it excites the fundamental (lx) and bladepass/vanepass frequency components. Unlike true mechanical imbalance. 

Mechanical imbalance is one of the most common causes of machinery vibration and is present to some degree on nearly all machines that have rotating parts or rotors. Static, or standing, imbalance is the condition when there is more weight on one side of a centerline than the other. However, a rotor may be in perfect static balance and not be in a balanced state when rotating at high speed. 

Two major sources of vibration due to mechanical imbalance in equipment with rotating parts or rotors are (1) assembly errors and (2) incorrect key length guesses during balancing. 

Even when parts are precision balanced to extremely close tolerances, vibration due to mechanical imbalance can be much greater than necessary due to assembly errors. Potential errors include relative placement of each part s center of rotation, location of the shaft relative to the bore, and cocked rotors. 

Underfill theory According to the underfill theory (S. Sherlock et al., 1963), the development of ascites is set off by mechanical factors and physical mechanisms ( imbalance of the Starling forces ). As a result, the effective plasma volume is reduced (so-called volume deficiency concept). 

Process and machine-induced shaft instability also create seal problems. Primary causes for this failure mode include aerodynamic or hydraulic instability, critical speeds, mechanical imbalance, process load changes, or radical speed changes. These can cause the shaft to deviate from its true centerline enough to result in seal damage. 

Copper deposition on turbine blades was an early problem with supercritical units. It was found that trace amounts of copper were dissolved from the condenser tubes and recirculated into the boiler section. Because of its excellent solvent properties, the superheated steam carried the dissolved copper into the turbine where, at the lower pressure, copper was deposited upon the turbine blades. This not only affected the efficiency, but threatened to destroy the turbine by mechanical imbalance due to uneven deposition. The problem has been largely eliminated by using stainless steel or titanium in the condenser. 

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