ICS 21.120.40                          MED 28

Reaffirmed 2019

NATIONAL FOREWORD

This Indian Standard which is identical with ISO 21940-31 : 2013 ‘Mechanical vibration — Rotor balancing (Part 31) Susceptibility and sensitivity of machines to unbalance’ issued by the International Organization for Standardization (ISO) was adopted by the Bureau of Indian Standards on the recommendation of the Mechanical Vibration and Shock Sectional Committee and approval of the Mechanical Engineering Division Council.

This part is only concerned with once-per-revolution vibration caused by unbalance; however, it should be recognized that unbalance is not the only cause-per-revolution vibration.

Rotor balancing during manufacture (e.g. as described in IS/ISO 1940 and IS/ISO 11342) is normally sufficient to attain acceptable in-service vibration magnitudes if other sources of vibration are absent. However, additional balancing during commissioning may become necessary and after commissioning, some machines may require occasional or even frequent rebalancing in situ.

If vibration magnitudes are unsatisfactory during commissioning, the reason may be inadequate balancing or assembly errors. Another important cause may be that an assembled machine is especially sensitive to relative small residual unbalances which are well within normal balancing tolerances.

If vibration magnitudes are unsatisfactory, the first step often is an attempt to reduce the vibration by balancing in situ. If high vibration magnitudes can be reduced by installing relatively small correction masses, high sensitivity to unbalance is indicated. This can arise, for example, if a resonance rotational speed is close to the normal operating speed and the damping in the system is low.

A sensitive machine which is also highly susceptible to its unbalance changing may require frequent rebalancing in situ. This may be caused, for example by changes in wear, temperature, mass, stiffness, and damping during operation.

If the unbalance and the other conditions of the machines are essentially constant, occasional trim balancing may be sufficient. Otherwise it may be necessary to modify the machines to change the resonance speed, damping or the other parameters to obtain acceptable vibration magnitudes. Therefore, there is a need to consider permissible sensitivity values of the machine.