ISO 12129-1:2019

Overview

ISO 12129-1:2019 - Plain bearings - Tolerances - Part 1: Fits specifies a system of fits and tolerances for metallic plain bearings used in general engineering. The standard defines how to achieve targeted mean relative bearing clearances (ψm) in the range 0.56 ‰ to 3.15 ‰ (other ranges may be used for specific applications). It is intended primarily for rotating machine parts and shafting and provides tables and rules for selecting limit deviations and tolerance intervals for shafts and bearing bores.

Key topics

• System of fits: A normal system in which the fundamental deviation H for the bearing bore follows ISO 286‑2. Shaft tolerances are correlated to provide the desired mean relative bearing clearance.
• Mean relative bearing clearance (ψm): Defined in per thousand (‰) using mean diametral clearance and arithmetic mean of nominal dimensions. The standard gives the accepted ψm values: 0.56, 0.8, 1.12, 1.32, 1.6, 1.9, 2.24 and 3.15 ‰.
• Tolerance intervals and IT grades: Shaft tolerance intervals are one IT grade smaller than the correlated bearing-bore tolerance. Table 1 provides limit deviations for shafts and maximum/minimum clearances across stepped nominal-dimension ranges.
• Stepped nominal ranges: Nominal dimension ranges are more closely stepped than in ISO 286‑2 to keep deviations from the mean clearance tightly controlled.
• Symbols and notation: Uses ψm for mean relative bearing clearance; alternative notation C* (or (C* 1,12)) is allowed when Greek letters or subscripts are unavailable.
• Scope limitations: Not applicable to half-bearings and bushes (measured by wall thickness and dimensionally changed on assembly), profile-bore or tilting-pad bearings, or situations where tolerances are set by performance at both clearance extremes.

Applications

• Design and manufacture of metallic plain bearings and mating shafts for rotating machinery (e.g., industrial motors, pumps, gearboxes).
• Selection of shaft/bore fits to achieve predictable hydrodynamic or boundary lubrication clearances.
• Engineering drawing specification: standard fit notation (e.g., ∅200 ψ 1,12) for procurement and quality control.
• Use by mechanical designers, bearing manufacturers, machine builders, maintenance engineers and standards committees who need consistent clearance criteria across nominal size ranges.

Related standards

• ISO 286‑1 / ISO 286‑2 - Geometrical product specifications: ISO deviations and IT grades referenced for fundamental deviation H and tolerance grading.
• Other parts of the ISO 12129 series covering additional plain-bearing tolerances and construction details.

Keywords: ISO 12129-1:2019, plain bearings, tolerances, fits, mean relative bearing clearance, ψm, metallic plain bearings, shaft tolerances, limit deviations, IT grades, rotating machinery.