ISO/TR 27507:2010 — Plain Bearings — Recommendations for Automotive Crankshaft Bearing Environments

1. Technical Report Overview

ISO/TR 27507:2010 ‘Plain bearings — Recommendations for automotive crankshaft bearing environments’, published in July 2010, was prepared by ISO/TC 123 (Plain bearings), Subcommittee SC 3. This Technical Report details the various dimensions and conditions that most engine manufacturers can achieve with current production machinery to produce crankshaft bearing environments that generally do not lead to bearing problems.

It is important to note that this is a Technical Report, not an International Standard. It provides informative guidance rather than mandatory requirements. Nevertheless, it holds significant reference value in the automotive engine engineering field, as its recommendations are based on extensive production experience.

2. Crankshaft Technical Requirements

ISO/TR 27507 specifies recommended values for multiple critical crankshaft parameters that directly affect bearing lubrication performance and service life:

2.1 Surface Finish

Crankpins and journals should be no rougher than 0.25 μm Ra. Thrust faces should never be rougher than 0.4 μm Ra, though the load a thrust washer can carry is inversely proportional to the mating surface finish value, often necessitating much lower values for thrust cheeks.

2.2 Geometric Tolerance Requirements

2.3 Grinding and Polishing Process

For modular cast iron shafts, grinding exposes graphite nodules, and filaments or tongues of the iron matrix material form at these sites. Tests indicate the optimal procedure is to grind with the crankshaft rotating in the same direction as in service, followed by polishing in the same direction. Filament orientation should point opposite to shaft rotation during operation to minimize bearing performance effects. Control of the polishing operation is critical — both insufficient and excessive polishing can be detrimental.

3. Housing Technical Requirements

For thin-walled automotive bearings, the housing dictates the bearing bore profile and size, making housing machining critically important for overall bearing performance.

3.1 Surface Finish and Bore Tolerance

Connecting rod housing bore surface roughness should not exceed 0.8 μm Ra. Crankcase housing bores should not exceed 1.6 μm Ra. Higher roughness reduces heat transfer from bearing to housing, causing overheating in the bearing clearance space. Main bore dimensional tolerance is typically 25 μm.

3.2 Bore Alignment and Cap Location

Overall misalignment should not exceed 50 μm (25 μm for heavy duty). Misalignment between adjacent bores should be limited to 25 μm (12.5 μm preferred). Positive location of housing caps is paramount — even slight mismatch creates a step at bearing joints, potentially causing uneven loading and lubricant film rupture.

4. Engineering Design and Implementation Insights

A key insight from ISO/TR 27507 is that bearing performance depends on complex interactions among numerous parameters. While modern computer techniques enable calculation of basic operating conditions (load and film thickness), the bearing designer has no knowledge of assembly precision, lubricant contamination levels, or component distortion during operation — all of which significantly influence actual bearing performance.

The report therefore emphasizes that, strictly speaking, each case requires individual consideration with reference to the loading and lubrication characteristics peculiar to that engine design. Relaxation or tightening of recommended values should be discussed with the bearing supplier based on the specific application.

With engine operating conditions becoming increasingly arduous, crankshaft bearing conditions are becoming more critical. The recommendations in ISO/TR 27507 represent achievable levels with current production machinery that generally prevent bearing problems, balancing reliability with mass-production economics.

5. Frequently Asked Questions