Technical insight
Rotor Balancing Scope: Bare Component or Assembled Unit?
Before ordering rotor balancing, name the exact configuration to be assessed: the bare component, a defined subassembly or the completed rotating unit. Attach an itemized configuration record and agree what happens if that configuration changes. Do not approve an assembled unit solely on a record that identifies only one of its components.
By Geetai Precision
- Published
- Updated
Start with the configuration, not a generic instruction
An order that says only 'balance the rotor' leaves a purchasing question unresolved: which items must be present when the agreed assessment takes place? Ask the responsible technical authority to identify the target configuration before requesting comparable quotations. The workflow below is original procurement guidance, not a balancing procedure or a declaration of operational suitability.
ISO's public summary of ISO 21940-11 addresses balancing of rotors with rigid behaviour and includes the number of correction planes and residual-unbalance assessment within its scope.
ISO 21940-12 addresses rotors with flexible behaviour. Its public summary explicitly says the document is not itself an acceptance specification for an individual rotor and notes that specialized equipment may need different treatment.
Ask the technical authority to select the applicable requirements for the actual equipment. Do not infer rigid or flexible behaviour from a photograph, material name or the fact that an item is described as a turbopump rotor. Record the governing document and edition in the order; leave unresolved technical decisions open rather than borrowing a generic requirement.
Use a configuration schedule at the quotation stage
| Configuration question | Information to attach | Point to resolve |
|---|---|---|
| Bare component or assembly? | Identify the item drawing and, when applicable, the assembly drawing and included parts list. | Ask the quotation to name the exact configuration covered by its assessment. |
| Permanent fitted items? | List shafts, sleeves, spacers, fasteners and other items included in the approved rotating unit. | Identify who supplies, fits and identifies each item; do not silently omit customer-supplied hardware. |
| Temporary mounting items? | Request identification of any arbor, adapter or other temporary mounting item proposed for the assessment. | Ask the responsible reviewer to distinguish the assessed configuration from the delivered configuration. |
| Permitted corrections? | Supply the approved correction instructions or identify the person who must approve a proposal. | Do not authorize removal or addition of material by leaving the correction scope blank. |
| Handoff and later changes? | List planned removal, refitting or substitution after the assessment. | Assign the decision on whether further assessment is needed before the altered unit is released. |
Treat each row as a purchasing clarification, not an instruction to a balancing operator. Request a written included, excluded or clarification-needed response. If assembly work belongs to another party, make that boundary explicit in both orders so the two suppliers are responding to the same configuration.
Keep the evidence attached to the assessed configuration
Request that the agreed record identify the component or assembly, included item identifiers, configuration revision, applicable requirement reference and the assessment record reference. Ask how permitted corrections will be documented. Do not require a particular record format by default; settle the content and responsible approver during quotation.
Keep the configuration schedule with the shipment documentation and any subsequent assembly record. Where an identification mark or attachment would affect a controlled surface, ask for an approved identification method rather than adding one informally. Route any discrepancy between the schedule and delivered items to the named technical reviewer.
Do not carry an approval across an undocumented change
Consider a hypothetical handoff: a supplier assesses a rotor with its specified sleeve fitted, then another team substitutes a sleeve before installation. Purchasing should flag the changed configuration and request a documented disposition. Do not relabel the earlier record to cover the substitution, and do not assume that further work is automatically required; that decision belongs to the responsible technical authority.
Before closing the order, reconcile three definitions: what was ordered, what was assessed and what was delivered. Resolve differences with referenced approvals. Keep the decision to accept the supply scope separate from the decision to install and operate the equipment.
Related review paths
References
- ISO 21940-11:2016 — Mechanical vibration — Rotor balancing — Part 11: Procedures and tolerances for rotors with rigid behaviour — ISO. Accessed 2026-09-10T06:53:37.805Z.
- ISO 21940-12:2016 — Mechanical vibration — Rotor balancing — Part 12: Procedures and tolerances for rotors with flexible behaviour — ISO. Accessed 2026-09-10T06:53:37.805Z.