ISO 21940-21: Description and Evaluation of Balancing Machines
ISO 21940-21 is the international standard that tells manufacturers how to describe a balancing machine and, crucially, how to prove that it performs as claimed. It belongs to the modern ISO 21940 family on balansiranje rotora (which superseded the older ISO 1940 series), and it sits alongside ISO 21940-12 for flexible rotors and ISO 21940-13 for in-situ work. Part 21 is the one that defines a standardized set of procedures for stating a machine’s technical characteristics and for verifying its accuracy with calibrated test rotors. Compliance gives a buyer or auditor confidence that a given machine genuinely meets internationally agreed performance criteria rather than a vendor’s marketing figures.
1. Scope and Machine Description
The standard applies to all types of balancing machine used for rigid rotors — both hard-bearing and soft-bearing designs. Its first job is to establish a formal, common vocabulary for how a manufacturer must describe and specify a machine, so that two machines from different makers can be compared on equal terms.
The mandatory description includes the machine’s physical capacity and its measuring system:
- Rotor envelope: minimum and maximum rotor mass, journal diameter, and rotor length the machine can accept.
- Speed range: the balancing speeds over which performance is guaranteed.
- Drive system: the method of turning the rotor — belt-drive, end-drive (cardan or coupling), or self-drive (air or the rotor’s own motor) — because each affects accuracy and the practical range of rotors.
- Measuring system: the type of pickups, the indication of unbalance amount and angle, and the plane-separation capability.
By forcing every supplier to publish the same data set, Part 21 ensures a purchaser has a clear, standardized basis on which to judge whether a machine actually suits their rotors — a job no amount of brochure prose can replace.
Hard-Bearing vs. Soft-Bearing Machines
The distinction matters because the two architectures are evaluated against the same criteria but behave very differently. Hard-bearing machines are far more common in modern shops: their supports are stiff, they measure centrifugal force, and they hold a permanent calibration that is independent of the rotor mounted. Soft-bearing machines run their supports below resonance, measure displacement, and must be re-calibrated for each new rotor type with a trial weight. Part 21 accommodates both, which is why its tests are framed in terms of measured results rather than a specific sensing principle.
2. Proving Rotors and Test Masses
A performance test is only as trustworthy as the artefacts used to run it. Part 21 therefore sets strict specifications for the proving rotors and test masses that drive the evaluation. Proving rotors are not ordinary production parts; they are precision-machined, dimensionally stable artefacts manufactured to an exceptionally low level of residual unbalance, with tight requirements on geometry, material, and surface finish so that the rotor itself introduces no error into the test.
The test masses — small known weights added at defined radii to create a precisely known unbalance — must be calibrated and traceable to a national standard, ideally accompanied by a calibration certificate. Standardizing the test equipment is what makes the results repeatable and comparable across different machines, operators, and locations. If you need to convert a test mass and its radius into the centrifugal force it will impose, the Centrifugal Force from Unbalance Calculator does the arithmetic in one step.
3. Performance Tests
This is the practical heart of the standard: a defined, step-by-step methodology for tests that objectively quantify a machine’s capability. Two principal tests carry most of the weight.
- Minimum Achievable Residual Unbalance (Umar / MARU): the ultimate test of sensitivity. Starting from a well-balanced proving rotor, the test measures the smallest residual unbalance the machine can repeatedly and reliably indicate. In effect this is the machine’s electronic and mechanical noise floor — the absolute limit of what it can resolve. It links directly to balancing sensitivity, and you can estimate the comparable figure for a given setup with the Balancing Machine Sensitivity Calculator.
- Unbalance Reduction Ratio (URR): a direct measure of accuracy and efficiency. A known unbalance is added to the proving rotor; the machine measures it and computes the correction; after that single correction is applied, the residual is measured again. The URR is the percentage by which the unbalance was reduced in one shot. A URR of 95% means the machine removed 95% of the initial unbalance in a single correction step — the hallmark of an accurate, efficient machine that does not force the operator into endless trial runs.
Part 21 also specifies other vital checks: plane-separation capability (confirming that a two-plane machine can correctly distinguish static and couple unbalance across the two correction planes without cross-talk) and consistency of performance across the full speed range, so that a machine that is accurate at one speed does not degrade at another.
4. Acceptance Criteria and Documentation
The final element is the definitive pass/fail framework. For the URR test the standard sets a minimum acceptable percentage — commonly 95% or higher — that a machine must reach to be considered compliant. The MARU value is treated differently: it is not a pass/fail line in itself but a declared figure that quantifies the machine’s sensitivity, allowing a user to judge whether the noise floor is small enough for their tightest balance quality grade.
Finally, the standard mandates a comprehensive test report documenting the conditions and results of every test. That report is the machine’s official certificate of performance — the end-user’s guarantee that its capabilities were verified against a rigorous, internationally recognised procedure rather than asserted by the seller.
5. Where Part 21 Fits in Practice
For a reliability engineer or a balancing-shop owner, Part 21 answers a deceptively simple question: can this machine actually balance my rotors to the tolerance I need? The URR tells you how few iterations a job will take; the MARU tells you the finest tolerance you can credibly certify.
It is worth keeping the distinction between a shop balancing machine and on-site balancing clear. A dedicated machine evaluated under Part 21 removes unbalance from a rotor in isolation, on its own bearings, in a controlled bay. Many rotors, however, are never sent to a shop — they are corrected in place in the machine’s own rotor-bearing system at operating speed. A portable two-channel analyser such as the Balanset-1A performs that field balancing by measuring 1× amplitude and phase, computing influence coefficients, and verifying the residual unbalance against the chosen ISO 21940-11 grade — capturing the true assembled, thermal, and foundation conditions a shop machine cannot reproduce. The two approaches are complementary: Part 21 governs the bench instrument, while in-situ work governs the rotor as it really runs.
