ISO 12129-2:2019

Overview

ISO 12129-2:2019, titled Plain bearings - Tolerances - Part 2: Tolerances on form and position and surface roughness for shafts and thrust collars, specifies essential tolerance requirements for shafts, flanges, and thrust collars used in plain bearing units. This international standard is developed by ISO/TC 123 (Plain bearings), focusing on ensuring optimal bearing performance through precise control of shaft and collar geometries and surface finishes.

This standard applies to journal and thrust plain bearing units, including combinations thereof, used in either horizontal or vertical installations. However, it explicitly excludes crankshaft bearing units in combustion engines.

Key Topics

• Form and Position Tolerances
  ISO 12129-2:2019 defines strict parameters for roundness, straightness, parallelism, runout, flatness, and concentricity of shafts and thrust collars. These tolerances are calibrated according to different accuracy grades linked to the specified lubricant film thickness, ensuring reliability and functionality in bearing operation.

• Surface Roughness Specifications
  Surface finish requirements are given for shafts and thrust collars, using roughness average (Ra) and maximum height (Rz) parameters. The values correlate with the operating conditions and lubrication film thickness to reduce wear and friction within the bearing assembly.

• Accuracy Grades Based on Lubricant Film Thickness
  The standard categorizes tolerances and surface roughness into accuracy grades (5, 10, 20, 30), aligned with the calculated minimum lubricant film thickness (h_min), measured in micrometers. This classification tailors manufacturing precision to specific bearing performance demands.

• Shaft and Thrust Collar Designs
  Illustrations included in the standard show various shaft configurations, such as shafts with one or two thrust collars, designed for journal-thrust bearing units. These designs influence tolerance assignments and manufacturing approaches.

• Testing and Quality Control
  Quality inspection protocols adhere to ISO 12301, which governs the geometrical and material quality characteristics of plain bearings. This reinforces traceability and conformity in bearing components.

Applications

ISO 12129-2:2019 is crucial for manufacturers and designers of plain bearing systems, particularly where precise dimensional control affects bearing life and efficiency. Typical applications include:

• Machinery with journal and thrust plain bearings requiring controlled tolerances for shafts and collars.
• Industrial equipment where bearings operate under mixed load conditions.
• Bearings installed in various orientations, ensuring consistent performance horizontally or vertically.
• Quality assurance and inspection processes for bearing shaft components.
• Engineering design of bearing units that require specification of form, position, and surface roughness tolerances to match lubricant film requirements.

This standard supports improved bearing reliability, reduced operational wear, and optimized lubrication retention, which are critical in reducing maintenance costs and enhancing machine uptime.

Related Standards

• ISO 12301 – Plain bearings: Quality control techniques and inspection of geometrical and material quality characteristics. This standard outlines testing procedures closely referenced in ISO 12129-2 for verifying compliance.

• ISO 7902 (Parts 1, 2, 3) – Hydrodynamic plain journal bearings: Calculation procedures and permissible operational parameters for lubricant film thickness. Although ISO 12129-2 no longer mandates ISO 7902 exclusively, it remains a key reference for calculating minimum lubricant film thickness (h_min).

• ISO/IEC Directives, Part 1 and 2 – Editorial and procedural rules governing the development and maintenance of ISO standards like ISO 12129-2.

Keywords: ISO 12129-2, plain bearings tolerances, shaft tolerances, thrust collar surface roughness, journal bearing standards, bearing shaft roughness, bearing accuracy grades, bearing manufacturing standards, plain bearing quality, lubricant film thickness.