Overview
ISO 12129-1:2024 - Plain bearings - Tolerances - Part 1: Fits specifies a system of fits for metallic plain (journal) bearings used in general engineering. It defines how to dimension and indicate shaft/bore fits to achieve target mean relative bearing clearances (ψm) in the range 0.56 ‰ to 3.15 ‰ (other ranges allowed for specific applications). The standard is intended primarily for bearings in rotating machines and transmissions but can be applied elsewhere. It excludes half‑bearings, bushes measured by wall thickness, profile bore and tilting‑pad bearings, and cases with bespoke clearance tolerances.
Key topics and requirements
- Mean relative bearing clearance (ψm): Defined as diametral clearance relative to nominal diameter (‰). Standard provides formulae and worked examples to calculate ψm from C and Dm.
- Prescribed ψm values: Standard lists preferred mean clearances: 0.56, 0.8, 1.12, 1.32, 1.6, 1.9, 2.24, 3.15 ‰.
- System of fits: Uses a normal fit system where the bearing bore fundamental deviation is H (per ISO 286‑2) and the shaft tolerance interval is correlated to give the intended ψm. The shaft tolerance interval is typically one IT grade smaller than the bore tolerance.
- Tolerance intervals & limit deviations: Tables provide limit deviations for shafts and tolerance intervals for bearing clearances across stepped nominal dimension ranges so that maximum deviation from ψm is controlled.
- Notation & symbols: Key symbols (C, D, ψ) and index usage are specified. Preferred form for fit indication example: ∅200 ψ1,12 (alternatives: ∅200 C1,12 or ∅200 (C 1,12)).
- Design considerations: When selecting ψm the standard recommends considering bearing material, load, speed, operating temperature, and required mechanical accuracy.
Applications
- Specifying shaft and bearing-bore fits for journal bearings in:
- Rotating machinery (motors, pumps, compressors)
- Gearbox and transmission design
- General engineering where controlled bearing clearance is critical to lubrication and performance
- Engineering drawings, procurement specifications, manufacturing tolerancing, and quality inspection of plain bearings.
Who should use this standard
- Mechanical designers and rotating‑equipment engineers
- Bearing and transmission manufacturers
- Metrology and quality‑control personnel
- CAD/drawing authors and procurement/specification writers requiring standardized fit notation
Related standards
- ISO 286‑1 / ISO 286‑2 (general ISO deviation and tolerance system) - ISO 12129‑1 references ISO 286‑2 for fundamental deviation H.
- Other parts of the ISO 12129 series (for complementary tolerancing and construction details) - see ISO catalogue for the full list.
Keywords: ISO 12129-1:2024, plain bearings tolerances, bearing fits, mean relative bearing clearance, ψm, shaft fit, limit deviations, tolerance intervals, rotating machines, transmissions.