ISO 4156-1:2021

Overview

ISO 4156-1:2021 - Straight cylindrical involute splines - Metric module, side fit - Part 1: Generalities - is the international standard that defines the fundamental data, terminology, dimensions and tolerancing principles for straight (non‑helical) side‑fitting cylindrical involute splines. It provides the formulae, tables and definitions necessary for the design, manufacture and inspection of metric‑module splines used to transmit torque between coaxial shafts and hubs. Unless otherwise stated, linear dimensions are in millimetres and angular values in degrees.

Key topics and technical requirements

• Scope and terminology: Definitions for external/internal splines, module (m), pitch circle and pressure angle (30°, 37.5°, 45°), form diameter, fillet/root types (fillet root, flat root), pitch and base pitch.
• Dimensional system: Nominal modules and acceptable pressure angle increments; pitch diameter and circular pitch relationships; use of metric module units for CAD/CAM and manufacturing.
• Fit concepts: Explanation of side fit (flank centring), effective fit and the distinction between actual vs effective tooth thickness and space width.
• Tolerances and deviations: Limiting dimensions, manufacturing deviations, total tolerances (T + λ), total pitch/profile/helix deviations and their impact on mating fit and effective clearance.
• Diameters and adjustments: Minor, major and form diameters, tolerances and rules for adjustment of external spline diameters.
• Manufacturing considerations: Radii, profile shifts, eccentricity and misalignment, tooling and inspection guidance for ensuring correct engagement and torque transmission.
• Design data & drawing rules: Basic dimensions, designations and example drawing data provided (Annex A informative).

Practical applications and who uses this standard

ISO 4156-1 is essential for engineers and professionals involved in:

• Mechanical design and drivetrain development (gearboxes, powertrains, transmissions)
• Manufacturing and machining (broaching, milling, hobbing, grinding) for spline profiles
• Quality control and inspection (metrology of tooth thickness, space width, run‑out)
• CAD/CAM, tooling and fixture design
• Suppliers, OEMs and maintenance teams specifying spline fit classes, tolerances and assembly requirements for clearance, sliding and interference connections.

Using ISO 4156-1 ensures interoperability, consistent torque transmission characteristics, predictable assembly clearances and reduced rework across supply chains.

Related standards

• ISO 4156 (series) - other parts covering specific modules and applications (see ISO catalogue)
• ISO 1101 - referenced normative standard for geometrical tolerancing (GPS)
• Documentation prepared by ISO/TC 14 (Shafts of machinery and accessories).