Expert Review Note & Disclaimer:
Technical knowledge sharing and engineering reference only. All conversion values (roughness and hardness) are approximate empirical figures based on historical and current international standards. Hardness conversions vary significantly by material and heat treatment state; physical testing using designated methods is always required for quality acceptance. This guide does not guarantee specific performance outcomes.

1. Overview of Engineering Data

This report provides an in-depth analysis of two fundamental engineering reference tools: the Surface Roughness International Conversion Table and the Hardness Conversion Table. These parameters serve as the quantitative foundation for the design, material selection, and quality control of plain bearings (sliding bearings) across global supply chains.

Key Data Scope Analyzed:

• 9 National/Regional Standard Comparisons

• 4 Major Hardness Testing Systems

• 50+ Roughness Grade Equivalents

• 80+ Hardness Conversion Data Sets

Core Engineering Value:
The surface roughness table resolves the challenge of aligning machining specifications across international drawings. The hardness table bridges the gap between different testing methods, ensuring proper material validation. Together, they define the tribological performance of bearing systems.

2. In-Depth Analysis: Surface Roughness Conversion

2.1 Definition of Core Roughness Parameters

Surface roughness directly impacts a bearing’s friction coefficient, lubricant film formation, and wear rate. The standard parameters include:

• Ra (Arithmetic Mean Roughness): The most universally used parameter, representing the arithmetic average of the absolute values of the profile deviations. Defined by ISO 4287.

• Rz (Maximum Height of Profile / Ten-Point Mean): ISO defines it as the vertical distance between the highest peak and lowest valley within a sampling length. Older JIS/DIN definitions used the average of the 5 highest peaks and 5 lowest valleys. It is highly sensitive to surface defects.

• Ry: The distance between the profile peak line and valley line. Equivalent to the ISO Rz definition.

• Rmax (Upper Limit of Profile Valley Depth): Primarily used in DIN/JIS standards to identify the single most significant defect (like a deep scratch) within the evaluation length.

Empirical Relationship (Ra vs. Rz):
For most machined surfaces, Rz ≈ 4 to 10 × Ra. For instance, grinding typically yields an Rz/Ra ratio of 45, while turning yields 57. Ratios exceeding 10 usually indicate abnormal surface damage.

2.2 Global Standards Comparison (9 Systems)

Navigating cross-border manufacturing requires understanding the alignment of global standards:

Crucial Engineering Differences:

• Imperial vs. Metric: US/UK standards often use micro-inches (μin). Conversion: 1 μin = 0.0254 μm. (e.g., 32 μin ≈ 0.8 μm Ra).

• The Rz Discrepancy: Older DIN/JIS standards evaluate Rz using the “ten-point method,” whereas modern ISO uses the upper limit of peak-to-valley height.

2.3 Role of Roughness in Plain Bearings

Selecting the right surface finish is not about making it “as smooth as possible,” but finding the appropriate balance for the specific operating conditions.

Recommended Roughness for Mating Shafts:

• Heavy-Duty/Construction: Ra 0.4 – 1.25 μm (Hardness > 55 HRC)

• Precision/Oscillating: Ra 0.2 – 0.4 μm (Hardness 52 – 58 HRC)

• General Industrial: Ra 0.4 – 0.8 μm (Hardness > 50 HRC)

The Dual Traps of Surface Finish:

• Overly Smooth (Ra < 0.1 μm): May cause lubricants to be squeezed out completely, leading to stick-slip phenomena and adhesive wear.

• Overly Rough (Ra > 3.2 μm): Acts as an abrasive surface, rapidly increasing heat generation and reducing the wear life of polymer or bronze bearing layers. Industry practice suggests polymer-composite mating surfaces should generally not exceed Ra 0.3 μm.

3. In-Depth Analysis: Hardness Conversion

3.1 The Four Major Hardness Systems

Hardness dictates the load-bearing capacity and wear resistance of the bearing materials.

• Brinell (HB): Uses a 10mm hard alloy ball under a 3000kgf load. Creates a large indentation. Suitable for macroscopic average hardness of castings, bronzes, and unhardened steels.

• Rockwell (HR): Measures depth of penetration.

  • HRA: For extremely hard thin layers.

  • HRB: For softer steels and copper alloys.

  • HRC: For hardened/quenched steels (typical for bearing shafts).

• Vickers (HV): Uses a diamond pyramid. Leaves a microscopic indentation. Suitable for thin coatings, nitrided layers, and precision parts.

• Shore (HS): Rebound method. Portable and non-destructive, suitable for massive, installed workpieces.

3.2 Key Hardness Zones & Rules for Bearings

Based on standard conversion tables (e.g., ASTM E140), here are the critical engineering benchmarks:

Important Conversion Rules:

1. Approximation Only: Different testing methods rely on different physical principles (area vs. depth vs. rebound). Conversions are highly material-dependent.

2. The Transition Zone: Around HB 220-240, HRC drops below 23, making it inaccurate. Testing should shift to the HRB scale.

3. Tensile Strength Estimation: For carbon/low-alloy steels, tensile strength (σb) can be estimated as σb ≈ 3.4 to 3.6 × HB (MPa).

4. Collaborative Application: Bearing System Design

In tribology, roughness and hardness must be designed collaboratively. A proper match ensures reliable operation.

4.1 The Roughness-Hardness Matrix

• Low Hardness + Low Roughness: Not Recommended. Soft materials with overly smooth surfaces are prone to adhesive wear and galling.

• High Hardness + Low Roughness: Highly Suitable. (e.g., Ra 0.2-0.8μm paired with >50 HRC). Promotes stable lubrication films and lower friction.

• Low Hardness + High Roughness: Not Recommended. Leads to rapid abrasive wear and thermal failure.

• High Hardness + High Roughness: Acceptable with limits. Can be tolerated in low-speed, heavy-duty applications, though friction will be elevated.

4.2 Material Matching Guidelines

5. Summary and Engineering Recommendations

The integration of surface roughness and hardness conversion tables is essential for cross-border mechanical design and quality assurance.

5.1 Practical Recommendations for Engineers

1. Prioritize Physical Testing: Never rely solely on hardness conversion tables for critical part acceptance. Physical testing using the drawing’s specified scale is mandatory.

2. Specify the Standard: When denoting roughness (especially Rz), always indicate the standard year (e.g., ISO 4287) to avoid confusion between profile height and ten-point definitions.

3. Standardized Series: Prefer ISO priority numbers for Ra specifications (e.g., 0.4, 0.8, 1.6, 3.2 μm) to align with global measuring equipment.

4. The Golden Rule for Shafts: To protect plain bearings, ensure the mating shaft has a surface hardness of > 50 HRC and a roughness of Ra 0.2–1.6 μm.

5.2 Note on Standard Evolutions

While historical standards (like ISO 468, GB 1031-83) are still frequently encountered in legacy drawings, modern engineering relies on updated frameworks. Note that ISO is currently transitioning from the ISO 4287 profile system to the next-generation ISO 21920 series. However, the foundational conversion principles detailed in this guide remain standard practice for legacy conversions and global procurement.