ISO 21940-32:2012

Overview

ISO 21940-32:2012 - "Mechanical vibration - Rotor balancing - Part 32: Shaft and fitment key convention" specifies a standardized half-key convention for balancing keyed rotor assemblies. The standard ensures compatibility when shafts and fitments (e.g., gears, pulleys, couplings) are balanced separately by different suppliers so that, after assembly, the rotor meets its overall balance tolerance and vibration limits. It applies to balancing in a balancing machine, in bearings, or in situ, and covers common key types (rectangular/square), tapered keys, woodruff, gibhead, dowel and other special keys.

Key topics and technical requirements

• Half-key convention mandated: use a half-key in the shaft keyway during shaft balancing; use a complementary half-key when balancing the fitment on a mandrel (or follow the mandrel-specific methods in Annex A).
• Mandrel/fitment rules (Annex A): procedures vary if the balancing mandrel has no keyway, one keyway, or two opposite keyways - preferred mandrel designs are those inherently balanced.
• Two-opposite-keyways: if shaft/fitment have two equal keyways 180° apart, balancing without keys is permissible.
• Axial CG alignment: the centre of gravity of the half-key must align axially with the full key in the final assembly.
• Material and holding of half-keys: half-keys used for balancing should have the same density as the final key (typically steel) and be held using means that introduce negligible unbalance (e.g., fibreglass tape).
• Marking requirements: permanently mark shaft and fitment adjacent to the keyway with H to indicate the half-key convention was used. If another convention is used for compatibility with existing parts, mark with F (full-key) or N (no-key).
• Implementation: manufacturers and processors are encouraged (and expected) to adopt the half-key convention and mark components accordingly. Annexes provide specifications, practical guidance for making half-keys, and comparison of conventions.

Practical applications and users

Who uses ISO 21940-32:

• Rotor and rotating-equipment manufacturers (pumps, motors, gearboxes)
• Balancing service providers and repair shops
• OEMs and component suppliers (keys, shafts, couplings)
• Maintenance and vibration-analysis engineers performing in-situ balancing

Benefits:

• Ensures cross-supplier compatibility of separately balanced components
• Reduces residual unbalance and vibration in assembled rotors
• Prevents internal bending moments introduced by inconsistent key conventions

Related standards

• ISO 21940 series (Parts 1, 2, 11–14, 21, 23, 31) - broader rotor balancing guidance and vocabulary
• Normative reference: ISO 1925 (Balancing - Vocabulary)
• Note: Some standards such as IEC 60034-14 reference or prescribe the half-key convention for rotating machines

Keywords: ISO 21940-32, rotor balancing, half-key convention, shaft and fitment key convention, keyways, residual unbalance, mechanical vibration.