Impeller balance grade sets the residual unbalance a fan can carry; G2.5 is tighter than G6.3 and protects bearings and structure.
What the Grade Means
Balance grade G expresses the allowable residual unbalance as a velocity, in millimetres per second, at the operating speed. A lower number means a better-balanced rotor and less vibration at the same rpm, which is the practical meaning behind the letter and digit.
A centrifugal impeller spins fast and any mass off its centre pulls on the shaft and the bearings, so balancing is the step that turns a rough casting into a smooth-running wheel. The balance grade states how much residual unbalance is allowed at the service speed.
Grades G2.5 and G6.3 come from the ISO 1940 balancing standard, where the number is a measure of permissible vibratory severity; G2.5 is the tighter grade for high-speed or precision fans and G6.3 the looser for slower, robust units. The chosen grade reflects the speed and the consequence of vibration in the application.
Unbalance has two basic forms: static, a heavy spot that can be corrected in one plane, and couple, which needs correction in two planes because the heavy ends oppose. A proper balance run measures the wheel on a balancing stand and adds or removes mass at the right radius so the residual falls inside the grade.
The payoff is bearing life and noise. An unbalanced wheel wears bearings early, transmits vibration into the casing and the structure, and can trip a VFD on current variation; a well-balanced impeller to G2.5 or G6.3 runs quietly and lets the bearings reach their rated hours, which is why balancing is checked, not assumed, before shipment.
The company's 80,000 m² production site builds centrifugal fans under ISO 9001:2015 with CCC certification, and its in-house performance laboratory checks balance and run-out so the quoted grade is met on the wheel that ships, with engineers responding to selection enquiries within 24 hours.
The grade is checked at the factory but can be lost in the field if the wheel is repaired or a weight falls off, so a field rebalance uses the same two-plane method and the residual is confirmed against the original grade before the unit is returned to service. Balance weights are fixed, not taped, and the impeller is re-checked after any welding or scraping so the smooth running the bearing life depends on is restored rather than approximated.
G6.3 is adequate for many general industrial fans running at moderate speed, where small vibration is tolerable and the duty is not sensitive to wear or noise, such as a robust extract fan in a tolerant plant environment.
G2.5 is the tighter grade used where vibration must be low, such as fans near occupied spaces, high-speed machines, or systems where bearing life is critical to uptime and an unexpected failure would stop a process or a building service.
Balancing is done on a balancing machine or in situ, adding or removing mass at set radii until the residual unbalance falls within the grade's limit for the rotor's mass and speed, a measured procedure rather than a visual judgement.
Why It Matters in Service
Excess unbalance raises vibration that fatigues bearings and the casing, shortening life and risking failure; a better grade directly extends the service interval and reduces the chance of an unplanned stop during peak demand.
Vibration transmits through the structure, so a poorly balanced fan in a plant room can annoy occupants or trigger protective trips on adjacent sensitive equipment, and it makes the unit harder to commission to an acceptable signature.
At higher speeds the forces grow with the square of rpm, so the same unbalance that is harmless at 1,000 rpm becomes serious at 3,000 rpm, pushing the choice toward G2.5 for high-speed cleanroom or laboratory supply fans.
A balanced impeller also runs more efficiently and quietly, because the energy spent shaking the frame is instead delivered to the air, and the bearing temperature stays lower, which further supports a long, stable service life.
| Item | Specification | Note |
|---|---|---|
| G6.3 | coarser | General duty |
| G2.5 | tighter | Sensitive / high speed |
| Unit | mm/s | Residual velocity |
| Speed effect | force ~rpm² | Worse at high rpm |
| Life | better grade | Longer bearing |
| Method | balance machine | Add/remove mass |
Selection Checklist
- Use G6.3 only for robust general-duty low-speed fans.
- Specify G2.5 near occupied spaces or at high speed.
- Match the grade to the operating rpm, not just the duty.
- Require a balance report with the supplied fan.
- Re-balance after any impeller repair or rebuild.
- Isolate the fan so residual vibration is not transmitted.
About the Manufacturer
Keruite Air Conditioning Group, headquartered in Dezhou, Shandong, has more than 30 years of HVAC manufacturing experience and operates an 80,000 m² production site with its own performance testing laboratory. Products are built under ISO 9001:2015 quality management and carry CCC certification, and are supplied to customers in over 50 countries.
Our engineers respond to selection enquiries within 24 hours and can provide project-specific sizing, performance curves, and documentation to support specification and procurement.