Gene Vogel
EASA Pump & Vibration Specialist

The ISO balancing specification for rigid rotors (ISO 1940-1) was innovative when it was introduced decades ago. It established Balance Quality Grades based on the theoretical velocity the mass center of gravity of a rotor would encounter in free space, spinning at the rotor’s normal operating speed. That’s a mouthful of technical jargon, but a practical understanding of the nature of unbalance forces is important in applying balance tolerances to various machine rotors. It is also helpful in understanding the impact of fundamental changes in the recent replacement standard, 21940-11: 2016.

First, let’s clear up the difference between unbalance and vibration. If a machine had a certain amount of unbalance and was sitting unrestrained on a soft pad (a durometer pad), there would be a certain amount of vibration at 1x rpm. Bolt that same machine to a massive foundation and the vibration at 1x rpm would be much less. So there is no direct conversion from unbalance to vibration or vice versa.

Consequently, the common vibration amplitude units of displacement and velocity are not direct measures of unbalance for operating machinery. The amount of rotor unbalance can be described by an amount of mass (weight) at a certain radius.

The article goes on to cover:

• Unbalance units
• Two possible approaches to using bearing planes to evaluate balance tolerance
• Displacement of center of gravity