Rotor Balancing Quality Grade
ISO 1940 standard (updated in 2016 and renamed ISO 21940-11:2016) is well-known for classifying the quality grade G of rotors balancing and as a measure of the expected vibration levels from rotating equipment/machinery under specific conditions after rotor balancing activity (the level is expressed in mm/s, that is, vibration velocity). To clarify, the quality grade of balancing, for example, G2.5, aims to achieve a mechanical imbalance of the rotor that results in a vibration velocity level of 2.5 mm/s during operation under certain conditions.
However, because, in addition to mechanical imbalance, other forces and factors also act on the rotor, influencing the vibration level, it becomes a bit more complicated to estimate the vibrational behavior during the operation of the equipment when only the quality of the rotor’s balancing is known.
Among the factors influencing the vibration levels of equipment (aside from the existing mechanical imbalance) are shaft misalignment, hydraulic influences caused by issues with the pump/turbine/compressor rotor, mechanical defects (poor bearing tolerances, mechanical looseness), electrical problems (uneven rotor winding, rotor defects, etc.), the rigidity of the bearing/support foundation, etc. We will address each factor in a series of dedicated articles.
The 21940:11-2016 standard provides criteria for classifying acceptable vibration levels, recommending the quality grade of balancing such that a rotor with a G4000 grade (for example, a crankshaft used in low-speed diesel engines that power various ships) has a significantly greater mechanical imbalance than a motor rotor balanced to G2.5, even if both rotors have the same nominal speed and rotor mass (which implies that balancing quality is better for the rotor with the lower grade and, correspondingly, requires more attention from the balancer, delving into the art of rotor balancing).
| Quality Grade of Balancing, G (mm/s) |
Types of Rotors
|
|
G 4000 |
Crankshafts for large, slow marine diesel engines (piston speeds below 9 m/s) |
|
G 100 |
Internal combustion engines for cars, trucks, and locomotives |
|
G 40 |
Passenger cars: wheels, rims, wheel sets, crankshafts |
|
G 16 |
Agricultural machinery: crankshafts, grinding mill rotors, drive shafts (cardan shafts, propeller drive shafts) |
|
G 6.3 |
Flywheels
Fan rotors Gas turbine rotors for aircraft Pump rotors Centrifugal rotors (separators, decanters) Electric motor/generator rotors (with shaft heights of at least 80 mm), with nominal speeds up to 950 rpm) Electric motors with shaft heights less than 80 mm Gears Rotors used in machines in the pulp and paper industry Hydraulic turbines |
|
G 2.5 |
Turbocharger rotors
Small electric armature rotors Electric motor and generator rotors (with shaft heights of at least 80 mm), and nominal speeds over 950 r/min Gas turbine rotors and steam turbine rotors Pump rotors driven by turbines (for example, boiler feed pumps) |
|
G 1.0 |
Rotors of grinding/sawing/buffing machines
Audio and video units Textile bobbins Turbocharger/turbo-supercharger rotors for automobiles |
|
G 0.4 |
Gyroscopes
Disk/hard disk units Shafts and drives for high-precision applications |
While I’m on this topic, in general, Rotofix Solutions ensures a G2.5 quality grade of balancing for most rotors under repair in our workshop, significantly improving both reliability of your equipment by increasing the MTBR / Mean Time Between Repairs, as well as the level of operational safety through reduced vibration levels. At our workshop in Targoviste, Romania, we are always at your disposal with professional advice and the quality of repairs you expect for your rotating equipment.