ISO 1940 Mechanical Vibration - Balance Quality Requirements for Rigid Rotors

The “ISO 1940 Mechanical vibration - Balance quality requirements for rigid rotors” standard, developed by the International Standards Organization (ISO), consists of the following sections under the same title:

ISO 1940-1 Part 1: Determination of allowable residual imbalance
ISO 1940-2 Part 2: Balance errors

The ISO 1940-1 standard contains recommendations for determining unbalance and the associated quality requirements for rigid rotors. This standard specifies:

Display of imbalance in one or two planes
Methods for determining the allowable remaining imbalance
Methods of allocating this to correction planes
Methods of determining the residual unbalance condition of a rotor by measurement
Summary of errors associated with residual imbalance identification

The table and figure provided in the standard provide recommendations for the balance quality requirements of rigid rotors according to their types, masses and maximum service speeds, based on worldwide experience. This standard also aims to facilitate the relationship between the machine manufacturer and the user. The terminology specified in this standard can be used in determining technical specifications.

A detailed evaluation of the errors associated with the determination of the residual unbalance is not included in this standard. ISO 1940-2 deals with these errors. ISO 1940-1 does not define the residual unbalances allowed for flexible rotors. These are discussed in ISO 5343 Criteria for the evaluation of flexible rotor balance. Balancing methods are not described.

The recommended balance quality ratings are not intended to serve as acceptance specifications for any particular rotor assembly. Instead, they are intended to give indications of how to avoid gross deficiencies as well as exaggerated or unattainable requirements. They may also serve as a basis for more extensive investigations, for example, when more precise determination of the required balance quality is required by laboratory or field measurement. If due regard is paid to the recommended limits, satisfactory operating conditions can probably be expected. However, there may be cases where deviations from these recommendations are necessary, for example, because of unusual construction or geometry.

The ISO 1940-2 standard mainly covers the determination of errors in the balancing process of rigid rotors, the evaluation of errors, guidelines for accounting for errors and the evaluation of permanent unbalance in any two correction planes.

This standard was later replaced by the standard “ISO 21940-14 Mechanical vibration - Rotor balancing - Part 14: Procedures for evaluating balancing errors”. This standard also specifies requirements for the following cases:

Determination of errors in the unbalance measurement process of a rotor
Evaluation of identified errors
Taking errors into account

This new standard specifies the balance acceptance criteria for the remaining unbalance, both immediately after balancing and for subsequent checking of the balance quality by the user. The standard lists the main typical errors in an informative annex, along with methods for their reduction.

Balancing machine balance errors can be classified as follows:

Systematic errors in which magnitude and angle can be assessed by calculation or measurement.
Random variable errors in which the magnitude and angle vary unpredictably across a series of measurements performed under the same conditions
Scalar errors, where the maximum magnitude can be evaluated or estimated, but the angle is uncertain

Depending on the manufacturing processes used, the same error may be placed in one or more categories. Examples of sources of error that may occur are listed in the standard. Some of these errors are discussed in more detail in the annex to the standard (Annex A).

Examples of balancing machine systematic error sources include:

Internal imbalance in the drive shaft
Internal imbalance in the mandrel
Radial and axial misalignment of the drive element from the rotor shaft axis
Radial and axial misalignment in the fit between the component to be balanced or in the balancing machine mandrel
Lack of concentricity between the shafts and support surfaces used for balancing
Radial and axial runout of rolling element bearings used to support the rotor and not service bearings
Radial and axial runout of rolling housings (and tracks) of rolling element service bearings installed after balancing
Imbalance caused by keyways and keyways
Residual magnetism in the rotor or mandrel
Reassembly errors
Balancing equipment and instrumentation errors
Differences between service shaft and balancing mandrel diameters
Universal joint defects
Temporary bending of the rotor during balancing
Permanent bending of rotor after balancing

Examples of random variable error sources of the balancing machine are:

Loose parts
Trapped liquids or solids
Deterioration due to thermal effects
Wind effects
Use of a loose coupling as the driving element
Temporary bending of the horizontal rotor caused by gravitational effects when the rotor is stationary

Examples of balancing machine scalar error sources include:

Changes in the gap in the interfaces that will be removed after the balancing process
Excessive play in universal joints
Excessive play in mandrel or shaft
Design and manufacturing tolerances
If the diameters of the balancing machine support rollers and the rotor shaft diameter are the same, almost the same, or have an integer ratio, their misalignment

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