13.3.3 Two plane balancing (The influence coefficient method)
The influence coefficient method is also called the field balancing since the balancing can be performed at site itself woithout taking out rotor from the machine as such. The number of trial runs to obtain the residual unbalnces in two planes are also less with a minimum of three runs are required. In Chapter 2 and 8, influence coefficients in detailed, here some relevant basic concept are briefed again. In the present section, then procedure would be outlines to obtain these influence coefficients experimnetally with the help of vibration amplitude and phase measurement in a rotor system.

Definition of Influence coefficients: Figure 13.16(a) shows that when a force F₁ is applied at station 1 and due to this force the beam deflections at stations 1 and 2 are given as

 

where is the influence coefficient and its first subscript represents the displacement station and second represents the force station. Similarly for Figure 13.16(b), we have

 

Figure 13.16 Definition of influence coefficients

In Figure 13.16(c), we have

Figure 13.17 Bearing response measurements and influence coefficients for a rigid rotor balancing