Balance tolerances

A very low-production balance tolerance is needed to meet rigorous vibration specifications. Vibration levels below those associated with a standard production-balanced rotor are often best obtained with a multiple-plane balance at the operating speed(s). 

Precision balancing requires a G2.5 guide number, which is based on 2.5mm/sec (O.lin/sec) vibration velocity. As can be seen from this, 6.3mm/sec (0.25 in/sec) balanced rotors will vibrate more than the 2.5mm/sec (O.lin/sec) precision balanced rotors. Many vibration guidelines now consider 2.5mm/sec (0.1 in/sec) good, creating the demand for precision balancing. Precision balancing tolerances can produce velocities of 0.01 in/sec (0.3mm/sec) and lower. 

The data in Table 4.1 were obtained using a calibrated balance, certified by the manufacturer to have a tolerance of less than 0.002 g. Suppose the Treasury Department reports that the mass of a 1998 U.S. penny is approximately 2.5 g. Since the mass of every penny in Table 4.1 exceeds the reported mass by an amount significantly greater than the balance s tolerance, we can safely conclude that the error in this analysis is not due to equipment error. The actual source of the error is revealed later in this chapter. 

CI-979 (29) is a balanced muscarinic agonist having equal affinities for cloned ml and m2 receptors (144). However, unlike prototypical muscarinic compounds such as (25), (29) increases central muscarinic tone, as indicated by behavioral and electroencephalogram (EEG) parameters, at doses lower than those requited to produce gastrointestinal effects (144). CI-979 is well tolerated in humans up to a dose of 1 mg. Dose-limiting side effects such as stomach pain and emesis were observed at a dose of 2 mg. 

To find the best a priori conditions of analysis, the equilibrium analysis, based on material balances and all physicochemical knowledge involved with an electrolytic system, has been done with use of iterative computer programs. The effects resulting from (a) a buffer chosen, (b) its concentration and (c) complexing properties, (d) pH value established were considered in simulated and experimental titrations. Further effects tested were tolerances in (e) volumes of titrants added in aliquots, (f) pre-assumed pH values on precision and accuracy of concentration measured from intersection of two segments obtained in such titrations. 

A bent shaft is physically bent and distorted. Placing the shaft into a lathe or dynamic balancer and rotating it will reveal the distortion. If a bent shaft is installed into a pump and run, it will fail prematurely, leaving evidence and specific signs on the eircumferenee of close tolerance stationary parts around the pump s volute circle. The shaft will exhibit a wear spot on its surface where the elo.se tolerance parts were rubbing. 

A deflected shaft is absolutely straight when rotated in a lathe or dynamic balancer. The deflection is the result of a problem induced either by operation or. system design. The deflected shaft also will fail prematurely in the pump, leaving similar, but different evidence on the elo.se tolerance rubbing parts in the pump. The next two pictures show how a bent shaft appears when rotated 180 degrees (Figure 9-9 and Figure 9-10). 

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. 

The subtle interaction of air pollutants with these other stressors to plants and vegetation is the subject of ongoing research. For some plant systems, exposure to air pollutants may induce biochemical modifications which interfere with the water balance in plants, thereby reducing their ability to tolerate drought conditions. 

The most probable distribution of unbalance in the finally installed rotor, considering manufacturing tolerances, balancing residuals after low-speed balance, assembly tolerances, etc. 

The set pressure of a conventional valve is affected by back pressure. The spring setting can be adjusted to compensate for constant back pressure. For a variable back pressure of greater than 10% of the set pressure, it is customary to go to the balanced bellows type which can generally tolerate variable back pressure of up to 40% of set pressure. Table 2 gives standard orifice sizes. 

For new rotors, where the elements have not yet been put on the rotor, other techniques can be used. First, the components can be individually balanced on a precision mandrel. Precision means that the runout is a few tenths of a mil (.001 inch). The runout high spot should be scribed on the mandrel. The new component now can be reasonably well-balanced. As the component is removed from the mandrel, the mandrel mark should be transferred to the component. When all the components are completed, the shaft is checked for runout. The high spot should be marked. As the components are stacked onto the shaft, the marks on the shaft are aligned with those transferred to the component. This works well with keyless rotors (no key between shaft and component). Experience has shown ihat in most cases with keyless rotors when the stacked rotor is put in the balance machine and checked, the residual unbalance is within the acceptable tolerance. If not, the rotor must be unstacked and the problem located. It must be remembered, however, if the components were properly balanced and the rotor comes out with unbalance, there must be a proh-... 

Feldman, S., Unbalance Tolerances and Criteria, Proceedings of Balancing Seminar, Vol, IV, Report No. 58GL122, General Engineering Laboratory, General Electric, Schenectady, N.Y., 1958. 

Balanced bellows valves need no reduction in spring pressure to compensate for superimposed back pressure, and they can tolerate variable superimposed back pressure without an effect on opening pressure. 

In contrast, most equipment can safely tolerate higher degrees of heat density than those defined for personnel. However, if anything vulnerable to overheating problems is involved, such as low melting point construction materials (e.g., aluminum or plastic), heat-sensitive streams, flammable vapor spaces, or electrical equipment, then the effect of radiant heat on them may need to be evaluated. When this evaluation is required, the necessary heat balance is performed to determine the resulting surface temperature, for comparison with acceptable temperatures for the equipment. 

The advantages of low-level hoods are listed in Table 13.17. The first step is to verify that the general principle of local capture of emissions is acceptable and feasible for the process. The next step is to establish the most efficient hood geometry. In most cases, this involves a balancing of the degree of process interference tolerable against the degree of emission source enclosure required. 

The set points for pilot-operated and balanced-bellows relief valves are unaffected by back-pressure, so they are able to tolerate higher backpressure than conventional valves. For pilot-operated and balanced-bellows relief valves, the capacity is reduced as the back-pressure goes above a certain limit. 

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. 

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. 

The most common rule of thumb is that a disc-shaped rotating part usually can be balanced in one correction plane only, whereas parts that have appreciable width require two-plane balancing. Precision tolerances, which become more meaningful for higher performance (even on relatively narrow face width), suggest two-plane balancing. However, the width should be the guide, not the diameter to width ratio. 

Tolerance The penalty for having an unbalanced wall is the reduction of tolerance control. Tolerance limits are usually at least doubled. Also, with certain plastics it is more difficult to process them, such as those with low melt strength. Although the balanced wall is the ideal, having it is not always possible. Recognize that the unbalanced wall can be extruded with proper die design and control of the extruder line from upstream to downstream equipment. 

The permit allows Stahlwerke Bremen to use 500 tonne MPW per day with a chlorine content of up to 1.5% (= ca. 3% PVC) on a daily average. This level seems to be a balance between the need to allow for a reasonable PVC tolerance in MPW (lower values are rare in MPW), and the desire of Bremen Stahlwerke to use a material that is as free of impurities as possible. After all, chlorine has no added value in the process, and may only contribute to problems like corrosion in the blast furnace, etc. In sum, the 1.5% level seems to be a balance between commercial reality and a technical ideal. 

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