ISO 21711:2019

Overview

ISO 21711:2019 specifies requirements for chain wheels used on mobile offshore units (MOUs). The standard covers tooth profiles, nominal dimensions, dimensional and geometric tolerances, material selection and inspection requirements for chain wheels intended to work with offshore mooring chains conforming to ISO 20438. Chain wheels are classified by tooth count (Type A = 5-tooth, Type B = 6-tooth, Type C = 7-tooth) and designated using the format e.g. Chain wheel ISO 21711:2019 A56.

Key topics and technical requirements

• Profiles & dimensions: Nominal tooth profiles and dimensional tables are defined for the 5-, 6- and 7-tooth chain wheels to ensure correct engagement with stud-link mooring chains.
• Tolerances:
  • Linear dimensional tolerances for castings: conform to ISO 8062-3, DCTG 11.
  • Geometric tolerances for castings: conform to ISO 8062-3, GCTG 7.
• Materials: Selection of base materials is linked to mooring chain grades (R3, R3S, R4, R4S, R5).
• Tooth surface hardness: Tooth hardness at the meshing zone should be compatible with the chain hardness and may be up to 10% higher depending on service conditions. Hardness can be achieved by heat treatment or embedding wear‑resistant materials.
• Inspection & testing:
  • Dimensional/geometric inspection per ISO 8062-3 requirements.
  • Visual surface quality inspection referenced to ISO 11971 (under preparation at publication).
  • Tooth hardness testing per ISO 6506-1 (Brinell) or ISO 16859-1 (Leeb).
  • Material traceability: product inspection certificate from the classification society required for chain wheel blanks.

Applications and users

ISO 21711:2019 is intended for use where reliable chain‑wheel / mooring‑chain interaction is critical:

• Offshore vessel and mobile offshore unit designers and shipyards specifying deck machinery.
• Manufacturers of chain wheels, capstans, windlasses and mooring equipment.
• Classification societies, inspection bodies and procurement teams for quality control and certification.
• Maintenance and diving/anchor-handling teams ensuring safe mooring operations.

Practical benefits include improved interchangeability, predictable load transmission (pitch diameter defined for torque calculations), wear control through hardness specification, and consistent inspection criteria.

Related standards

• ISO 20438 - Offshore mooring chains (compatibility target)
• ISO 8062-3 - Dimensional and geometrical tolerances for castings
• ISO 6506-1 - Brinell hardness test (metallic materials)
• ISO 16859-1 - Leeb hardness test (metallic materials)
• ISO 11971 - Visual examination of steel and iron castings (under preparation)

Keywords: ISO 21711:2019, chain wheels, mobile offshore units, offshore mooring chains, chain wheel dimensions, tooth surface hardness, chain wheel inspection, marine structures.