Overview
SIST ISO 4156-1:2021 – Straight cylindrical involute splines - Metric module, side fit - Part 1: Generalities - defines the general principles, terminology and essential data for the design, manufacture and inspection of straight (non‑helical) side‑fitting cylindrical involute splines. It specifies limiting dimensions, symbols, tolerances, and the effects of manufacturing deviations on the fit between coaxial spline elements. Unless stated otherwise, linear dimensions are in millimetres and angular dimensions in degrees.
Key topics and technical requirements
- Terminology and symbols: definitions for pitch circle, module, pressure angle, form diameter, effective and actual space width/tooth thickness.
- Pressure angles and module series: supported nominal pressure angles are 30°, 37.5° and 45°, with prescribed metric module increments for each angle (e.g., 0.5, 0.75, 1 … up to 10 mm for 30°/37.5°, and 0.25–2.5 mm for 45°).
- Concepts of side fit and effective fit: how clearance or engagement occurs between mating internal and external splines, including definitions used for design and inspection.
- Profile and pitch control: basic rack profiles, total profile/pitch deviations and their influence on fit and torque transmission.
- Spline fit classes and tolerancing: classes for different fit conditions and the formulae/tables for space width, tooth thickness tolerances, total and machining tolerances (T, λ, F etc.).
- Diameters and geometry control: minor, major and form diameters, their tolerances and adjustments for external splines.
- Manufacturing considerations: recommended radii, profile shifts, eccentricity/misalignment effects and practical measurement points.
- Drawing and designation data: recommended data and examples (Annex A) to ensure unambiguous specification on engineering drawings.
- Normative references: includes ISO 1101 for geometrical tolerancing.
Practical applications
- Transmission of torque using shafts and hubs in powertrain, gearboxes, machine tools and heavy machinery.
- Use cases include clearance, sliding, and interference spline connections where controlled side fit is required.
- Ensures repeatable, economical production and reliable assembly for replacement parts, high‑volume manufacturing and interchangeable assemblies.
Who should use this standard
- Mechanical designers and drivetrain engineers specifying spline joints.
- Manufacturing engineers and toolmakers producing splines (broaching, hobbing, grinding).
- Quality and inspection personnel verifying spline dimensions and fits.
- Procurement and standards managers ensuring component compliance and interchangeability.
Related standards
- ISO 4156 series (other parts covering additional specifics)
- ISO 1101 (geometrical product specifications - tolerancing)
Keywords: ISO 4156-1:2021, straight cylindrical involute splines, metric module, side fit, spline tolerances, pressure angle, spline fit classes, shaft and hub splines.
