Balancing
Balancing is the process of improving the distribution of mass in an impeller so that it can rotate in its bearings without producing unbalanced centrifugal forces. Perfection is impossible and even after balancing there will be residual unbalance, its magnitude being dependent on the machinery available and the quality necessary for the application.
Fan application category |
Balance quality grade for rigid rotors/impeller |
BV-1 |
G 16 |
BV -2 |
G 16 |
BV -3 |
G 6.3 |
BV -4 |
G 2.5 |
BV -5 |
G 1 |
Table 15.4 Balance quality grades
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The relevant grades are specified in IS014694:2003. Recommendations are given for various types of fan impeller to avoid gross deficiencies or unattainable requirements. If the balance quality grades shown in Table 15.4 are adopted according to the fan application categories shown in Table 15.5 then satisfactory running due to this cause should result. There may however be vibration resulting from other faults. Large fans for public utilities are included with ISO 10816-3.
An unbalanced impeller will create forces at its bearings and foundations and the complete fan will vibrate. At any given speed the effects depend on the proportions and mass distribution of the impeller as well as the stiffness of the bearing supports. In the past residual unbalance has been resisted by massive supports. Now, it is recognised that a preferable solution is to reduce this unbalance so that unnecessary weight need not be added to the bearing pedestal.
For narrow impellers (width less then 20% of diameter) the static unbalance is of primary importance. Two unbalances (in different planes) in the same direction usually cause a greater disturbance than two equal unbalances in opposite directions. With wider impellers (width up to 50% of diameter) couple effects become of importance.
Static unbalance, sometimes called force or kinetic unbalance, can be detected by placing the impeller on parallel knife edges. The heavy side will swing to the bottom. Correction weight can be added or removed as required and the part is considered statically balanced when it does not rotate on knife edges regardless of the position in which it is placed (see Figure 15.4).
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Figure 15.4 Static unbalance
Dynamic unbalance is a condition created by a heavy spot at each end of the impeller but on opposite sides of the centreline. Unlike static unbalance, dynamic unbalance cannot be detected by placing on knife edges. It becomes apparent when the impeller is rotated and can only be corrected by making balance corrections in two planes (see Figure 15.5).
An impeller which is dynamically balanced is also in static balance. Thus there is no need for the two operations where a dynamic balancer is used, despite the many specifications calling for both.
In general, the greater the impeller mass, the greater the permissible unbalance. It is therefore possible to relate the residual unbalance U to the impeller mass m. The specific unbal-
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A |
Extension probe |
Spring |
Piezoelectric Element Base — Fixing Thread |
— Preloading — Mass |
I |
ElKtrical Output Acceleration |
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