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.

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).

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-

Ance e = — is equivalent to the displacement of the centre of m Gravity where this coincides with the plane of the static unbal­ance. Practical experience shows that e varies inversely with the speed N over the range 100 to 30000 rev/m in for a given bal­ance quality. It has also been found experimentally that eN = constant (see Figure 15.6).

K i ± i____ ±— i—i » ! I it Rev/S *■ *■ Maximum service speed of rotation Figure 15.6 Balance quality grades to ISO 14694 and ISO 1940

1 Pickup case 4 Spring 2 Wirt cod 5 Damper 3 Mass 6 Permanent magnet

Figure 15.7 Cross-section of a velocity pickup

By springs of low stiffness remains stationary in space. Thus the conductor is moving through a magnetic field and a voltage is therefore induced. The voltage generated is directly propor­tional to the velocity. Piezoelectric accelerometer This consists of a mass rigidly attached to certain crystal or ce­ramic elements which when compressed or extended produce an electrical charge (see Figure 15.8). The voltage generated by the element is proportional to the force applied and since the mass of the accelerometer is a con­stant, is proportional to the acceleration. As acceleration is a

Example: For an impeller of 40 kg mass the recommended value e = 20 i^m is found from the graph for a maximum service speed of 3000 rev/min. If this is of the DIDW pattern and the centre of gravity is located within the mid third of the distance between the bearings, then one half the recommended permis­sible residual unbalance should be taken for each correction plane, i. e., 400 g. mm. The balancing machine used must be capable of determining the magnitude of the unbalance forces: in other words, it must be objective. It is insufficient for the machine to be subjective in approach, relying on the centring of a “spot” on, a screen.

Preloading

Seismic Hass

Element in compression

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