ISO 1971:1975

Overview

ISO 1971:1975 - Aircraft - Accessory drives and mounting pads defines a standardized design for quick‑release couplings with involute spline drive used on aircraft engine and gearbox accessories that must be removable while the engine or gearbox remains in service. The standard covers recommended geometries, assembly sequence, sealing arrangements, coupling clamps, transfer tubes/locating pins and selection of involute splines for spigot references up to and above spigot reference 250.

Key topics and requirements

• Quick‑release coupling design: guidance for coupling geometry and mating flanges to permit safe, repeatable removal/installation of accessories while minimizing assembly difficulty.
• Involute spline selection: specified spline types, pressure angle and recommended clearances/backlash, together with guidance on spline length, engagement and tolerance control.
• Mounting pad and flange dimensions: indexed figures and tables for accessory flanges and engine/gearbox pads (separate sets for up to and above spigot ref. 250).
• Sealing rings and grooves: recommendations for toroidal sealing rings and groove/land dimensions based on common elastomers (nitrile, fluorocarbon, silicone) and hardness/stretch characteristics.
• Transfer tubes / locating & torque reaction pins: retention, angular positioning and responsibility for supply assigned to the engine/gearbox manufacturer.
• Coupling clamps & tightening: clamp design responsibilities, fail‑safe locking, tightening procedures (torque wrench use, bolt/nut sequence and re‑tightening), plus relief of contact edges to avoid “dig‑in”.
• Lubrication & sealing of splines: obligation for engine/gearbox manufacturer to define lubrication arrangements and sealing to prevent oil ingress/egress.
• Assembly sequence & safeguarding: requirements that locating pins engage before spigot seals prevent rotation, and measures to avoid hanging heavy accessories on splines; joint responsibility rules for safety and failure containment.
• Identification: example of marking format combining ISO number, spigot reference and selected spline drive (e.g., “ISO 1971, MOUNTING PAD AND DRIVE SPIGOT REFERENCE 412 - DRIVE: 11 SPLINES 24/48 PITCH”).

Applications and users

ISO 1971 is intended for:

• Aerospace OEMs: engine and gearbox manufacturers specifying accessory drive interfaces.
• Accessory manufacturers: producers of pumps, generators, starters and other engine‑mounted units that require quick removal.
• MRO organizations and assembly engineers: maintenance teams following standardized coupling, clamp and lubrication practices.
• Aerospace design & standards engineers: ensuring interchangeability, correct tolerancing and safe assembly methods.

Practical benefits include reduced assembly time, improved interchangeability of accessories across platforms, controlled spline life and clearer allocation of design/supply responsibilities.

Related standards

• ISO 48 - Vulcanized rubbers: determination of hardness (referenced for sealing material hardness)
• ISO/R 468 - Surface roughness
• ISO/R 1078 - Dimensions of elastomeric toroidal sealing rings (inch series)