ISO 21940-14:2014 pdf free
ISO 21940-14:2014 pdf free.Mechanical vibration一Rotor balancing一Procedures for assessing balance errors
In some cases, rotors are in balance by design, are uniform in material and are machined to such narrow tolerances that they do not need to be balanced after manufacture. Where rotor initial unbalance exceeds the permitted values given in ISO 1940-1 or ISO 11342, the rotor should be balanced.
Balance errors caused by balancing equipment and instrumentation can increase with the magnitude of the unbalance present. By considering unbalance causes during the design stage, some error sources can be completely eliminated (e.g. by combining several parts into one) or reduced (e.g. by specifying decreased tolerances). It is necessary to weigh the cost due to tighter specified tolerances against the benefit of decreased unbalance. Where the causes of unbalance cannot be eliminated or reduced to negligible levels, they should be mathematically evaluated.
If significant randomly variable errors are suspected to exist it is necessary, where practical, to carry out several measuring runs to assess their magnitude.
When carrying out measuring runs, it is important to ensure that the random errors are themselves produced randomly in each run (e.g. by ensuring that the angular position of the rotor is different at the start of each run).The random error magnitude can be evaluated by applying standard statistical techniques to the measurement results obtained. However, in most cases, carrying out the procedure described in 5.5.2 is adequate.If rlling element bearings are ftted for a balancing operation, they introduce an error proportional to the eccentricity or angular misalignment of their rotating races (and their tracks) and rotor mass. This error can be determined by indexing the bearing races 180° on their mounting surfaces.
NOTE Eccentricity is assumed to result from radial and/or axial runout.
Mechanical fit can be a potential source of error (e.g. a change of unbalance can result from reassembly of parts), and there are many possible sources (e.g. if there is radial clearance or if the interference is too great or if the connecting bolts interfere with the spigot location).
The scatter caused by fit non-repeatability should be determined by repeated reassembly, with clearances taken up at different angles. For each reassembly, unbalance or vibration readings are taken and a mean value can be obtained.ISO 21940-14 pdf free download.
