BS ISO 21940-14:2012 pdf download – Mechanical vibration — Rotor balancing Part 1 4: Procedures for assessing balance errors
Formula (2) is valid only if the component is symmetric about its rotational axis and is therefore particularly applicable to the balancing of disks on arbors. The effects of radial runout and axial runout of a component mounted on the rotor can be calculated separately. For rotors with rigid behaviour, the separate unbalance components can be allocated to the bearing or correction planes and then combined vectorially.
For rotors with fexible behaviour, a rigid balance quality might be maintained, but accumulated axial disk runout errors (often described as skew) can lead to signifcant vibration due to the moment unbalance generated by the skewed disk(s).
5.4 Assessment of balancing operation errors
The purpose of balancing is to produce rotors that are within specifed limits of residual unbalance or vibration.To ensure that the set limits have been met, errors need to be controlled and taken into account.
When a balancing machine is used, various error sources exist, for example:
a) the type of rotor to be balanced;
b) the tooling used to support or drive the rotor;
c) the balancing machine support structure (e.g. machine bearings and cradles);
d) the balancing machine sensing system;
e) the electronic and read-out system.
However, it is important that in those cases where the error is taken into account by calculation, both themeasured unbalance before correction and the corrected value are reported.
The balancing machine used should be such that all its systematic errors are eliminated or corrected. Whenbalancing rotors that have a rigid behaviour at their balancing speed, the requirements of IS0 21940-21 apply.
5.5 Experimental assessment of randomly variable errors
lf significant randomly variable errors are suspected to exist it is necessary,where practical, to carry outseveral measuring runs to assess their magnitude.
When carrying out measuring runs, it is important to ensure that the random errors are themselves producedrandomly 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 measurementresults obtained.However, in most cases, carrying out the procedure described in 5.5.2 is adequate.
5.5.2 Procedure
Plot the measured vectors of residual unbalance or vibration and find the mean vector OA from all the runs(see Figure 2). Draw the smallest circle about centre A to enclose all the points. The vector OA represents anestimation of the measured residual unbalance or vibration, and the radius of the circle an estimation of themaximum possible error of each single reading. The uncertainty of these results is usually diminished byincreasing the number of runs carried out.
NOTE In some cases,particularly if one point is significantly different from the others, the error estimated can beunacceptably large. In this case, a more detailed analysis is necessary to determine the errors.
5.6Experimental assessment of systematic errors
ln many cases, most of the systematic errors can be found using index balancing.lndex balancing can be performed by:
a) mounting the rotor alternately at 0° and 180° relative to the item which is the source of the particular error being investigated; and
b) measuring the residual unbalance or vibration several times in both positions.
lf OA and OB , as shown in Figure 3, represent the mean vectors of measured residual unbalance or vibrationwith the rotor mounted at 0° and 180 , respectively, a diagram can be constructed for each measurement planewhere C is the mid-point of the distance AB. The vector oC represents the particular systematic error and thevectors CA and CB represent the rotorresidualunbalance or vibration with the rotor at0° and 180° , respectively.NOTE In this case, it has been assumed that the rotor has been turned relative to the phase reference. If, however,the phase reference remains fixed relative to the rotor, the vector oc represents the rotor residual unbalance or vibration;and the vectors CA and CB represent the particular systematic error with the phase reference at0° and 180 , respectively.