Rotor balancing: when to do it and what grade G2.5 means
Unbalance is the uneven distribution of a rotor's mass around its axis of rotation — and the most common cause of vibration in pumps. The force it generates grows with the square of speed and silently destroys bearings, seals and shafts. This guide explains where unbalance comes from, the difference between static and dynamic balancing, and what the G2.5 quality grade referenced by every serious report actually means.
Updated on July 8, 2026 · Reviewed by Hydro Pumps engineering
What unbalance is
Picture a bicycle wheel with a lead weight glued to one point of the rim. Spinning slowly, nothing much; spinning fast, the whole wheel shakes. That is unbalance: the rotor's center of mass does not coincide with its geometric center of rotation, and every revolution generates a centrifugal force pulling the shaft toward the heavy spot — a rotating force the bearings absorb thousands of times per minute.
The physics is unforgiving: centrifugal force grows with the square of rotational speed. Doubling the speed quadruples the force. An imbalance of a few grams on an impeller spinning at 3,500 rpm generates forces of dozens of kilograms hammering the bearings continuously.
Unbalance does not fix itself — it only worsens: vibration accelerates wear, wear increases unbalance, and the cycle closes until the bearing, the seal or the shaft itself fails.
Where it comes from: the real causes
Rotors leave the factory balanced. Unbalance appears — or reappears — through the predictable paths of service life:
Uneven wear. Abrasion and erosion never remove material symmetrically — every hour pumping abrasive fluid redistributes mass.
Cavitation. Pitting tears material off the vane inlets irregularly — a cavitated impeller is an unbalanced impeller.
Fouling and deposits. Mass adhering unevenly unbalances just as much as mass removed — common with solids-laden fluids or depositing processes.
Repairs and welding. Every intervention that adds or removes material — weld recovery, machining, wear ring replacement — changes the mass distribution. A repair without subsequent balancing is half a job.
Mechanical damage. A vane chipped by a foreign object, bending from a seizure — unbalance appears overnight.
Static vs dynamic: one plane or two
Static unbalance is the simple case: the heavy spot sits in a single plane, and the rotor resting on free supports rolls until the heavy spot faces down. It is corrected by adding or removing mass in one plane — sufficient only for narrow, slow rotors, like a thin disc.
Real pump rotors have length: the heavy spot on one side can sit 180° from the heavy spot on the other side. At rest, the rotor looks balanced; spinning, the two spots create a couple that makes the shaft 'wobble' — that is couple (moment) unbalance, invisible to the static test.
That is why the professional standard is dynamic balancing in two planes: the rotor spins on an instrumented bench that measures magnitude and phase of unbalance at each correction plane, and the balancer corrects both. Only dynamic balancing guarantees a smooth rotor in real operation.
What grade G2.5 means
Balancing 'until it feels right' is not a criterion. The international balancing standard — ISO 21940-11, successor to the former ISO 1940-1 — defines balance quality grades, written as G followed by a number: G6.3, G2.5, G1.0. The number expresses, in millimeters per second, the velocity at which the rotor's center of gravity orbits at service speed — the smaller, the finer the balance.
Each machine class has its recommended grade in the standard. For industrial pump rotors, established practice is G2.5 — the same grade applied to turbines and compressors. The grade defines the admissible residual unbalance: the standard accepts that perfection does not exist, and states exactly how much imperfection is acceptable for each rotor mass and service speed.
In practice, this means a serious balancing report states: the initial unbalance found, the final residual in each plane, the admissible limit calculated for that rotor at the specified grade — and the proof that the residual stayed below the limit. Without those numbers, 'balanced' is just a word.
When to balance
The objective triggers for sending a rotor to the bench:
After any rotor repair. Welding, thermal spray, machining, ring replacement — every intervention touching mass demands rebalancing. No exceptions.
Diagnosed 1×RPM vibration. A spectrum with vibration dominant at running speed is the classic signature — vibration analysis points, the bench confirms and corrects.
Repeated bearing or seal failures. Components that don't last are frequently victims of chronic untreated unbalance.
Rotating assembly build. In assembling a new or reconditioned rotating assembly, documented balancing is part of the quality standard — not an optional.
A new rotor without a certificate. A replacement impeller without a balancing report is a wildcard: balancing before assembly costs a fraction of discovering the problem with the pump installed.
Bench or field?
Bench balancing is the gold standard for pump rotors: the disassembled rotor spins on a dedicated machine that measures with gram precision, allows correction in both planes and issues a report against the specified grade. It is the correct method after any repair — the rotor is already disassembled anyway.
Field (in-situ) balancing corrects the assembled machine, measuring vibration at its own bearings and adding correction masses without disassembly. It is valuable for large machines where disassembly is expensive, or to correct residual assembly unbalance. But it does not replace the bench when the rotor needs repair — correction mass on top of an eroded impeller is a patch, not a solution.
At Hydro Pumps, every rotor passing through the workshop — recovered, new or part of an assembly build — leaves bench-balanced to grade G2.5 of ISO 21940-11, with a residual unbalance report. It is part of the process, not a catalog item.
Repaired rotor, or vibration at 1×RPM?
Dynamic balancing on an instrumented bench, grade G2.5, with a residual report — for rotors up to 5 tonnes.
Balancing FAQ
From the vibration spectrum: unbalance shows as vibration dominant at running speed (1×RPM), mainly in the radial direction, with little energy at other frequencies. Misalignment, looseness and bearings have different signatures. A vibration analysis measurement distinguishes the causes before any disassembly.
Yes, without exception. Welding, thermal spray, machining and even wear ring replacement change the rotor's mass distribution. A perfect hydraulic repair with balancing skipped returns a vibration generator to the pump — and the bearings pay the bill within months.
It is the ISO 21940-11 quality grade applied to pump rotors: it defines the maximum admissible residual unbalance as a function of rotor mass and service speed. A complete G2.5 report shows the measured residual in each correction plane and the calculated limit — proving the rotor stayed within tolerance.
Static balancing corrects in a single plane and only sees force unbalance — adequate only for thin, disc-like rotors. Dynamic balancing spins the rotor on an instrumented bench and corrects in two planes, also eliminating couple unbalance (opposite heavy spots on different sides), which the static test cannot detect. For pump rotors, dynamic is the standard.
No — it solves vibration caused by unbalance. If the cause is misalignment, structural looseness, a damaged bearing or cavitation, balancing will not correct it (and the vibration spectrum differentiates each case). That is why diagnosis comes before correction: treating the wrong cause costs two interventions.
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