ISO 21940-13: Criteria and Safeguards for In-Situ Balancing of Medium and Large Rotors

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ISO 21940-13 is the specialised international standard governing the practical art of balancing a rotor in its own bearings and support structure, right where the machine lives — that is, in-situ or field balancing. Its full title is “Mechanical vibration — Rotor balancing — Part 13: Criteria and safeguards for the in-situ balancing of medium and large rotors.” Where a dedicated balancing machine cannot be used — because the rotor is too large, too costly to remove, or only misbehaves under real operating conditions — this part lays out when field balancing is the right choice and how to carry it out safely. It complements the tolerance-focused ISO 21940-11 (rigid rotors) and ISO 21940-12 (flexible rotors) by addressing the realities of working on a running, installed machine.

1. Scope and Applicability

The standard provides guidelines and safeguards for the in-situ balancing of medium and large rotors, performed while the rotor remains in its own bearings and support structure — usually in its final operational location. In practice the same in-situ principles are applied whether the rotor behaves as rigid or flexible in its installed state: it is the dynamics of the whole rotor-bearing system, not the rotor in isolation, that dictates the approach. The document is written for the technicians, engineers and managers who must decide upon, plan and safely execute a field balancing campaign.

2. Criteria: When In-Situ Balancing Is Justified

Field balancing is not the automatic answer to every case of high vibration, and this chapter supplies a decision framework. The standard identifies several scenarios where in-situ balancing is the appropriate course:

• Removal is impractical or uneconomic: stripping out a large turbine, generator or fan rotor for a shop balance may be prohibitively expensive or simply not feasible.
• Unbalance only appears in service: some unbalance is created by conditions that exist only when the machine runs — thermal distortion, aerodynamic forces, or process build-up such as debris and product caked onto a fan blade. A shop balance cannot reproduce these.
• Final trim after reinstallation: a rotor that was shop-balanced may still need a trim balance once reassembled into the machine, to absorb the small shifts that assembly introduces.

Critically, the standard insists on confirming first that the high vibration really is caused by unbalance — and not by misalignment, resonance, or mechanical looseness, which mimic or compound an unbalance signature. Adding weights to a misaligned or resonant machine wastes time and can make matters worse.

3. Procedures and Methodology

This section is a step-by-step guide to executing the job. It first sets the instrumentation requirements: a multi-channel vibration analyzer able to measure amplitude and phase, one or more vibration transducers (shaft-relative proximity probes and/or casing-mounted accelerometers), and a phase-reference sensor — typically a photo-tach or laser tachometer — to put a once-per-revolution timing mark on the shaft.

Notably, ISO 21940-13 sets out the criteria, instrumentation and safeguards but deliberately does not prescribe the method used to calculate the correction masses from the measured vibration data, leaving the choice of algorithm to the practitioner. In practice the universally used technique is the influence coefficient method: the analyst records the initial vibration vector (amplitude and phase), attaches a known trial weight at a known angular position, measures the new “response” vector, and then uses vector mathematics to compute the mass and angle of the required correction weight, applied in a single plane or in two planes as the machine requires. This is exactly the workflow a portable instrument automates: the Balanset-1A, a two-channel field balancer and analyser, measures 1× amplitude and phase in the machine’s own bearings at operating speed, computes the influence coefficients, and reports the correction mass and angle for each plane — letting an engineer balance and verify without removing the rotor. A Trial Weight Calculator helps size that first test weight sensibly.

4. Balance Quality Evaluation — Vibration, Not Residual Unbalance

Here the standard draws its most important distinction from shop practice. Shop balancing aims to meet a specific residual unbalance tolerance derived from a G-grade. Field balancing has a more pragmatic objective: to reduce the machine’s operational vibration to an acceptable level. Accordingly, acceptance is judged not on residual unbalance in g·mm but on the final vibration amplitudes. The standard directs that this assessment use the in-service vibration limits defined in the companion standards it references — ISO 7919 for shaft vibration and ISO 10816 for vibration on non-rotating parts (both since consolidated into the modern ISO 20816 series). The practical aim is to drive the 1× running-speed component down until the machine’s overall level falls into an acceptable evaluation zone — Zone A or B — for long-term operation. You can check a reading against those bands with the ISO 20816-1 Vibration Zones calculator.

5. Safeguards and Safety Precautions

This chapter is arguably the reason the standard exists, because field balancing carries hazards absent from a controlled workshop — chiefly, deliberately running a machine with added trial weights that could be flung free. It mandates a rigorous, documented safety approach:

• Mechanical inspection first: verify before any run that all fasteners are tight and every guard is in place.
• Positive weight attachment: trial and correction weights must be securely fixed — welded, bolted, or seated in dedicated holders — so they cannot become projectiles.
• Controlled access zone: a cordoned exclusion area around the machine during every test run.
• Clear communication: unambiguous protocols between the balancing analyst and the machine operator.
• Emergency stop: a pre-defined, rehearsed shutdown procedure ready before the first start.

This emphasis on safety is paramount: at the speeds and masses of medium and large rotors, a thrown weight or an unguarded coupling can cause serious injury and catastrophic equipment damage.

6. Key Concepts to Carry Away

• Field vs shop balancing: the standard is entirely about balancing a rotor in the machine, correcting the whole assembly in its true operational state, rather than on a balancing machine in a workshop.
• Vibration reduction is the goal: success is measured by acceptable in-service vibration per ISO 7919 / ISO 10816 (now consolidated as ISO 20816), not by a residual-unbalance figure.
• Safety first: the deliberate addition of weights to a running machine makes documented safeguards non-negotiable.
• Influence coefficient method: the universal in-situ technique — measure the initial vector, add a known trial weight, measure the response, and solve with vector maths for the correction.

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