Balancing rotor imbalance

Will tolerance tightening on specific balance toohng decrease rotor imbalance and increase TBOs ... 

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. 

Assembly errors are not simply the additive effects of tolerances, but also include the relative placement of each part s center of rotation. For example, a perfectly balanced blower rotor can be assembled to a perfectly balanced shaft and yet the resultant imbalance can be high. This can happen if the rotor is balanced on a balancing shaft that fits the rotor bore within 0.5 mil (0.5 thousandths of an inch) and then is mounted on a standard cold-rolled steel shaft allowing a clearance of over 2 mils. 

Shifting any rotor from the rotational center on which it was balanced to the piece of machinery on which it is intended to operate can cause an assembly imbalance four to five times greater than that resulting simply from tolerances. For this reason, all rotors should be balanced on a shaft having a diameter as nearly the same as the shaft on which it will be assembled. 

Imbalance often results with rotors that do not incorporate setscrews to locate the shaft relative to the bore (e.g., rotors that are end clamped). In this case, the balancing shaft is usually horizontal. When the operator slides the rotor on the shaft, gravity causes the rotor s bore to make contact at the 12 o clock position on the top surface of the shaft. In this position, the rotor is end-clamped in place and then balanced. 

If the operator removes the rotor from the balancing shaft without marking the point of bore and shaft contact, it may not be in the same position when reassembled. This often shifts the rotor by several mils as compared to the axis on which it was balanced, thus causing an imbalance to be introduced. The vibrations that result are usually enough to spoil what should have been a precision balance and produce a barely acceptable vibration level. In addition, if the resultant vibration is resonant with some part of the machine or structure, a more serious vibration could result. 

With a keyed-shaft rotor, the balancing process can introduce machine vibration if the assumed key length is different from the length of the one used during operation. Such an imbalance usually results in a mediocre or good running machine as opposed to a very smooth running machine. 

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. 

In static imbalance, the only force involved is weight. For example, assume that a rotor is perfectly balanced and, therefore, will not vibrate regardless of the speed of rotation. Also, assume that this rotor is placed on frictionless rollers or knife edges. If a weight is applied on the rim at the center of gravity line between two ends, the weighted portion immediately rolls to the 6 o clock position due to the gravitational force. 

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. 

The first consideration of dynamic balancing is the number of imbalance points on the rotor, as there can be more than one point of imbalance within a rotor assembly. This is especially true in rotor assemblies with more than one rotating element, such as a three-rotor fan or multi-stage pump. 

The amplitude of each point of imbalance must be known to resolve dynamic balance problems. Most dynamic balancing machines or in situ balancing instruments are able to isolate and define the specific amount of imbalance at each point on the rotor. 

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... 

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). 

The ISO has published standards for acceptable limits for residual imbalance in various classifications of rotor assemblies. Balancing standards are given in oz-ins or lb-ins per pound of rotor weight or the equivalent in metric units (g-mm/kg). The oz-ins are for each correction plane for which the imbalance is measured and corrected. 

Figure 55.1 Balancing standards residual imbalance per unit rotor weight (Ref. 5)...

This appendix describes the procedure to be used to determine residual unbalance in machine rotors. Although some balancing machines may be set up to read out the exact amount of unbalance, the calibration can be in error. The only sure method of determining residual imbalance is to test the rotor with a known amount of unbalance. 

An exact solution to the problem of dynamic balancing in a NMR double rotor system enabling high speed spinning about two intersecting axes has been presented. An exact expression for imbalance due to asymmetric distribution of weights about the rotation axes has been provided. 

Make sure that the outside of the centrifuge tubes, the sample holders and sample chambers are dry any liquid present will cause an imbalance during centrifugation (as well as potentially causing corrosive damage to the rotor). Balanced tubes must be placed opposite each other, use a simple code if necessary, to prevent errors. 

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