- ▸Reducing face roughness from Ra 0.4 μm to 0.05 μm can cut seal leakage by 90% — making it a mandatory outgoing inspection for carbon graphite seal rings
- ▸Mechanical seal faces require Ra ≤ 0.1 μm combined with Rz for wear resistance; bushing bores should use Ra + Rmax to detect scratches
- ▸Flank wear of VB ≥ 0.2 mm significantly worsens roughness; diamond-coated tools achieve Ra ≤ 0.1 μm
- ▸Face flatness must be ≤ 0.6 μm, verified with an optical flat showing uniform interference bands
Surface roughness of mechanical seal faces is one of the key determinants of sealing performance. Studies show that reducing face roughness from Ra 0.4 μm to 0.05 μm can cut leakage by 90%. For precision parts such as carbon graphite seal rings, bushings, and segmented rings, surface roughness is a mandatory outgoing inspection item. Huahao Sealing Co., Ltd. applies strict standards and inspection procedures; this article systematically presents the requirements.
1. Roughness Parameters Explained
1.1 Ra (Arithmetic Mean Roughness)
Ra is the most common parameter — the arithmetic mean of absolute profile deviations over the sampling length. Ra reflects overall surface smoothness but cannot distinguish peaks from valleys and is insensitive to extreme peaks. Graphite seal faces typically require Ra ≤ 0.1 μm.
1.2 Rz (Maximum Height Roughness)
Rz is the average of the 5 highest peaks and 5 lowest valleys within the sampling length. Sensitive to extreme peaks, it better reflects wear resistance. At the same Ra, Rz can vary significantly. High-end seals may require Rz ≤ 0.5 μm.
1.3 Rq (Root-Mean-Square Roughness)
Rq is more sensitive to large deviations, commonly used for statistical analysis. Typically 10~25% higher than Ra.
1.4 Rmax (Maximum Roughness Depth)
Rmax is the maximum peak-to-valley within a single sampling length. Sensitive to point defects, often used to detect scratches.
1.5 Rt (Total Roughness Depth)
Rt is the maximum peak-to-valley within the evaluation length. Reflects maximum surface unevenness.
1.6 Parameter Selection
- Seal face: Ra + Rz (overall smoothness + wear resistance)
- Sliding bushing bore: Ra + Rmax (scratch detection)
- Non-functional surface: Ra only
- Critical applications: Ra + Rz + Rmax (nuclear, aerospace)
2. Roughness Requirements per Region
2.1 Mechanical Seal Face
The face is the core working surface, demanding high flatness and finish.
- General service: Ra 0.05~0.1 μm
- High-viscosity media: Ra 0.1~0.2 μm (some roughness retains oil)
- Low-viscosity media (e.g., LPG): Ra ≤ 0.05 μm
- Face flatness: ≤ 0.6 μm (with fluorescent check)
2.2 Bushing Bore
The bore pairs with the journal — the working surface of the sliding bearing.
- General service: Ra 0.4~0.8 μm
- High-speed service: Ra 0.2~0.4 μm
- Low-speed heavy load: Ra 0.8~1.6 μm (moderate roughness retains oil)
2.3 Bushing OD
The OD pairs with the housing, mainly for location.
- Interference fit: Ra 0.8~1.6 μm
- Clearance fit: Ra 0.4~0.8 μm
2.4 Segmented Ring Split Face
The split face is a key sealing surface of segmented rings, requiring high flatness.
- Flatness: ≤ 0.01 mm
- Roughness: Ra 0.4~0.8 μm
2.5 Face-to-OD Transition Arc
Stress concentration occurs here; smooth transition is required.
- Roughness: Ra 0.4~0.8 μm
- Arc radius: R 0.5~1.0 mm
3. Measurement Methods and Equipment
3.1 Contact Profilometer
A contact profilometer (e.g., Mitutoyo SJ-210, Taylor Hobson Surtronic) uses a diamond stylus drawn across the surface. Pros: measures Ra, Rz, Rmax with stable results. Cons: stylus may scratch soft materials; use low-force (0.75 mN) stylus for graphite.
3.2 Optical Profilometer
Optical profilometers (e.g., Keyence VK-X laser confocal microscope) scan the surface with laser, no contact. Pros: non-destructive, 3D topography. Cons: requires coating for low-reflectance graphite.
3.3 Interferometric Profilometer
Interferometry measures surface via optical wave interference, with nanometer-level precision. Suitable for ultra-precision seal face inspection.
3.4 Comparator Specimen
Standard roughness specimens (GB/T 6060.2) compared visually with the workpiece. Suitable for quick shop checks, limited precision.
3.5 Fluorescent Inspection
Fluorescent dye applied to the seal face, observed under UV light. Detects microcracks and pores. Often used together with flatness inspection.
4. Huahao Sealing Inspection Workflow
4.1 First-Article Inspection
100% roughness inspection of the first part per batch to confirm process stability.
4.2 In-Process Sampling
Sampling per GB/T 2828.1 AQL 1.0. Key parameters: Ra, Rz.
4.3 Final Outgoing Inspection
100% inspection of key region roughness and appearance per part before shipment. Report accompanies the product.
4.4 Equipment
- Mitutoyo SJ-410 profilometer (0.01 μm precision)
- Taylor Hobson Form Talysurf PGI 1240 (0.001 μm precision)
- Keyence VK-X200 laser confocal microscope
- Optical flat (φ 150 mm, λ/10 precision)
5. Process Factors Affecting Roughness
5.1 Tool Factors
- Wear: flank wear VB ≥ 0.2 mm significantly increases roughness
- Geometry: rake 5~10°, clearance 8~12°, corner radius 0.4~0.8 mm preferred
- Material: diamond-coated tools achieve Ra ≤ 0.1 μm
5.2 Cutting Parameters
- Speed: higher speed, smoother surface
- Feed: lower feed, smoother surface, but lower efficiency
- Depth: 0.1~0.2 mm for finishing
5.3 Workpiece Factors
- Graphite density: higher density yields lower roughness
- Impregnation: high porosity produces "pinholes" after machining
- Anisotropy: properties differ along vs across pressing direction
5.4 Machine Factors
- Spindle runout: ≤ 1 μm
- Feed accuracy: ≤ 2 μm
- Rigidity: high rigidity reduces vibration and improves surface
6. Typical Issues and Solutions
6.1 Surface Mottling
Symptom: periodic stripes or spots.
Cause: tool vibration, insufficient machine rigidity.
Solution: reduce depth, raise speed, check tool mounting.
6.2 Edge Chipping
Symptom: fractured edges.
Cause: excessive clearance angle, impact force.
Solution: reduce clearance, use arc entry, add support.
6.3 Pinholes
Symptom: small holes on surface.
Cause: insufficient impregnation, high porosity.
Solution: improve impregnation, raise impregnation pressure.
6.4 Scratches
Symptom: deep directional scratches.
Cause: contaminated coolant, worn tool.
Solution: inspect tool, filter coolant.
Conclusion
Surface roughness control for graphite seals is a system engineering effort covering material, process, equipment, and inspection. Huahao Sealing Co., Ltd. has established a complete roughness assurance system with strict process specifications at every stage from impregnation to lapping. We provide roughness inspection reports and process consulting; please contact our engineering team.