ISO 21940-11, the standard that replaced ISO 1940-1, defines balance quality grades for rigid rotors: the familiar labels like G2.5 and G6.3 that tell a balancing shop how good is good enough. The grade sets the permissible residual unbalance, and picking the right one keeps a machine smooth without paying for precision it does not need.
A balance grade is written as the letter G followed by a number, from G0.4 for the most demanding rotors up to G4000 for the crudest. The number is a velocity in millimetres per second: it is the product of the permissible residual specific unbalance and the maximum service speed, so a single grade represents a constant orbital velocity of the rotor's mass centre. That is why a faster rotor must be balanced more finely to hold the same grade.
The relationship is G equals the permissible specific unbalance (e-per, in micrometres or g-mm per kg) multiplied by the service angular velocity (in radians per second), divided by 1000. Rearranged, the permissible specific unbalance is e-per = G times 1000 divided by the angular velocity.
Worked example: a rotor to be balanced to G2.5 running at 3000 rpm has an angular velocity of about 314 rad/s, so e-per = 2.5 times 1000 divided by 314, which is about 8 g-mm per kg. Multiply by the rotor mass to get the total permissible residual unbalance in g-mm.
The standard gives a table of typical grades by machine type. Common choices are:
Once the grade sets a target, the rotor is balanced, usually in one or two planes, until the residual unbalance sits under the permissible value. A rotor balanced to grade will run within the vibration limits of ISO 10816-3 vibration severity, because unbalance is the single most common source of machine vibration. The practice of correcting it is covered in dynamic balancing.
Balance is not permanent: eroded fan blades, product build-up and bent shafts all reintroduce unbalance, and the first sign is a rising vibration at running speed. A platform that trends that signal and raises a work order when it crosses a limit turns a slow drift into a planned rebalance rather than a failure. Fabrico closes that loop from the vibration reading to the routed job. Book a Fabrico demo to see it, or compare detection methods in thermography vs vibration analysis.
It is a velocity in millimetres per second that combines the permissible unbalance with the rotor's service speed. A lower G number means a finer, more demanding balance.
Standard electric motors are typically balanced to G2.5. Precision spindles go finer, to G1 or G0.4, while general fans and process machinery are often G6.3.
Yes. Because the grade is a fixed velocity, holding the same grade at a higher speed requires a smaller permissible unbalance, so high-speed rotors must be balanced more precisely.
ISO 21940-11 replaced ISO 1940-1. The grade system and the permissible-unbalance method carry over, so a G2.5 target means the same thing under either number.
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