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Designing Rectangular Polymeric Sealing Rings: SAE J2310-2024 Design Guide
This article provides an overview of SAE J2310-2024, a recommended practice for designing rectangular cross section polymeric sealing rings used in automatic transmissions and hydraulic systems. The standard covers material selection, dimensioning, joint configurations, and critical design considerations such as thermal expansion and clearance calculations.
1. Standard Overview and Scope
SAE J2310-2024 is a reaffirmation of the original 1991 document, with updates to material physical properties and detailed calculation examples. The standard is intended for rotating and static grooved shaft applications and excludes aluminum contact environments. It serves as a guide for engineers to design reliable sealing rings using polymers like PTFE, polyimide, and PEEK.
2. Material Selection and Physical Properties
Polymeric sealing rings can be made from PTFE, polyimide (PI), or PEEK, often compounded with fillers to enhance wear resistance, reduce friction, and improve thermal stability. Each material exhibits distinct physical properties that influence design decisions.
Important: The physical properties listed in the standard are typical approximate values. Actual properties vary significantly with filler types and amounts. Always consult your material supplier for specific data.
| Property | Units | PTFE | Polyimide (PI) | PEEK |
|---|---|---|---|---|
| Max Use Temp (No Load) | °C | 93–316 | 200–330 | 230–315 |
| Heat Deflect Temp (455 kPa) | °C | 121 | 238–271 | NA |
| Heat Deflect Temp (1.82 MPa) | °C | 50–100 | 160–417 | 140–315 |
| Coef. of Linear Expansion | 10E-5/K | 1.4–25.0 | 0.4–6.0 | 2.2–14.7 |
| Ultimate Elongation | % | 40–650 | 1.6–90.0 | 0.9–150 |
| Specific Gravity | g/cc | 0.7–2.3 | 0.00545–2.05 | 1.23–1.53 |
| Compressive Strength | MPa | 1.5–23.4 | 77–553 | 29–183 |
| Coefficient of Friction | – | 0.008–0.28 | 0.14–0.24 | 0.11–0.4 |
PTFE offers low friction and can be used as solid or split rings. Polyimide and PEEK have higher modulus and strength but require split configurations for installation.
3. Design Parameters and Best Practices
Axial Width and Radial Wall Thickness
The axial width (W) is selected based on expected pressure and material properties. Common widths are provided in the standard. For rectangular rings, the radial wall thickness (T) should not exceed the axial width; typically, it is 90% of the axial width to balance strength and ease of assembly. Modern designs often feature chamfers or cutouts on the edges to reduce frictional torque and improve efficiency.
End Clearance and Thermal Expansion 🛠️
End clearance (or compressed gap G) is critical to accommodate thermal expansion without leakage or binding. For PTFE seals, the minimum gap should be zero at the lowest operating temperature. For polyimide and PEEK straight cut and step joints, the minimum gap is calculated to approach zero at the maximum operating temperature. The formula involves the coefficients of thermal expansion of the seal and bore materials.
⚠️ Common design pitfall: Failing to account for thermal expansion can cause leakage if the gap is too large, or seal binding if too small. Always perform the temperature-based gap calculation for your specific materials.
Solid vs. Split Rings
PTFE can be used as solid rings (must be stretched over the shaft) or split rings. Polyimide and PEEK must be split because of their rigidity. The gap dimension only applies to split rings and is essential for controlling thermal expansion.
Joint Configurations
The standard describes six common joint types: solid (no joint), straight cut, step joint, scarf cut, T-joint, and interlocking. Each has operating principles and application considerations:
- Straight Cut: Simple, leaks until gap closes with thermal expansion.
- Step Cut: Seals under pressure; check step over-engagement to avoid buckling.
- Scarf Cut: Seals under pressure but requires sufficient axial clearance to prevent ramping.
- T-Joint and Interlocking: More complex, used for specific sealing needs.
Frequently Asked Questions
- When should I use a solid ring vs. a split ring?
Solid rings are possible only with PTFE and require stretching over the shaft; split rings are easier to install and mandatory for polyimide and PEEK. Consider assembly and thermal expansion requirements. - How do I calculate end clearance for PTFE seals?
The minimum gap should be set to zero at the lowest operating temperature for all joint types. The upper limit depends on manufacturing costs and performance – consult the ring manufacturer. - What is the recommended radial wall thickness relative to axial width?
The radial wall thickness should be approximately 90% of the axial width for proper function and ease of assembly. However, non-rectangular profiles with chamfers are increasingly common to reduce friction. - Which joint type should I choose to minimize leakage?
Step cut and scarf cut joints provide better sealing under pressure compared to straight cut. Scarf cut seals need careful axial clearance to avoid ramping. PTFE scarf cuts may also risk welding at high temperatures.
For detailed design implementation, always refer to the latest SAE J2310 document and work closely with seal material manufacturers.
