This standard defines classes of flexible coupling potential unbalance, one of which the user must select in order to meet the needs of their system. The classes are established using weight and speed and system sensitivity to arrive at a mass displacement value that defines the potential unbalance. The standard defines types of unbalance, provides a method of selecting balance class, identifies contributors to potential unbalance, and provides a method of determining potential coupling unbalance. The balance classes are derived from consideration of the potential unbalance of the coupling.

The balancing requirements for a flexible coupling depend upon the rotating system into which it is mounted. Each half of the coupling is mounted on a separate rotor with the whole coupling providing the connection. Each of the connected rotors is balanced independently of the coupling and the coupling is added when the rotors are installed.

This standard is used with ANSI S2.19-1999 or ISO 1940-1:2003 which apply to balance quality requirements of rigid rotors. If ANSI S2.19-1999 or ISO 1940-1:2003 is used for balancing coupling components and assemblies in the balancing machine, then potential unbalances are introduced after the coupling is disassembled and reassembled either in the balancing machine or the rotor system. These potential unbalances are primarily the result of:

- balancing mounting fixture inaccuracies;

- displacement of coupling components with respect to the axis of rotation of the rotor system during disassembly and reassembly of the coupling.

Application

This standard is applicable to couplings and addresses potential unbalance which could be expected of a coupling in service. This standard accounts for issues of runout and clearances in the calculation of potential unbalance and resulting balance class. It should be noted that a flexible coupling is generally an assembly of several components having diametral clearance and eccentricities between the pilot surfaces. ANSI S2.19-1999 (ISO 1940-1:2003) addresses residual unbalance as measured in the balancing machine.

Exclusions

This standard does not take into account arbitrary balance standards developed by other standards organizations (e.g., American Petroleum Institute). In addition, this standard does not address the unbalance effects caused by:

- shaft runout;

- keys that protrude beyond the hub or shaft;

- unfilled keyways or keyseats;

- coupling mounting surface clearance;

- non-homogeneous materials;

- curved datum.

Additional considerations

Balancing a coupling does not assure one of great gains in balance. The greatest gains in balance comeduring themanufacturing of the coupling. Controlled runouts of pilots and pilot fit clearances, provided by the coupling supplier during manufacturing, give the greatest results. Balancing of various components results in minimal gain over controlling the bore runout during re-boring. It should be noted that two perfectly balanced parts mounted eccentrically would still shake. An assembly balanced coupling, re-assembled to runouts different than those when in the balance machine, will still be out of balance (the difference between residual and potential unbalance).

Calculations of system unbalance cannot be used to determine system vibrations as stated in the introduction to ISO 1940-1:1986 and ANSI S2.19-1999, which reads:

"It is not readily possible to draw conclusions as to the permissible residual unbalances from any existing recommendations on the assessment of the vibratory state of machinery, since there is often no easily recognizable relation between the rotor unbalance and themachine vibrations under operating conditions. The amplitude of the once-per-revolution vibrations is influenced by characteristics of the rotor, of themachine, of the structure and of the foundation, and by the proximity of the service speed to the various resonance frequencies, etc. Moreover, the machine vibrations may be due only in part to the presence of rotor unbalance."