Seeing seal failures or unsure which ring profile to use? The shape of the sealing ring can directly impact its performance, especially under pressure or in motion.
The main difference is in the cross-sectional shape: flat O-rings have a rectangular or square profile, while round O-rings have a circular cross-section. Each offers different sealing behavior and load distribution.
Understanding the differences can help you pick the right seal for static, dynamic, or custom groove applications.
What Is a Round O-Ring?
Need a general-purpose seal for standard grooves?
Round O-rings are the most common type, featuring a circular cross-section and designed to compress evenly in radial or axial sealing grooves.
✅ Features:
- Circular cross-section
- Excellent sealing under pressure
- Ideal for both static and dynamic applications
- Wide availability in ISO, AS568, BS standards
- Easily installed and cost-effective
Round O-rings1 are best for hydraulic cylinders2, pumps, valves, and general-purpose static sealing3.
What Is a Flat O-Ring?
Facing extrusion or groove misalignment issues?
Flat O-rings, also called square-cut or flat gaskets, have a rectangular cross-section that offers more surface contact and less rolling in dynamic applications.
✅ Features:
- Square or rectangular profile
- Better stability under side loads
- Ideal for flanges or wider groove designs
- Reduces chances of twisting or rolling during installation
- Offers broader sealing surface in static sealing
Flat O-rings4 are useful in vacuum systems5, flange joints6, and machined grooves where standard round profiles don’t seat properly.
Flat vs. Round O-Ring Comparison
| Feature | Round O-Ring | Flat O-Ring (Square) |
|---|---|---|
| Cross-section shape | Circular | Square/rectangular |
| Sealing surface contact | Line contact | Broad surface contact |
| Pressure resistance7 | ✅ Excellent | ✅ Good |
| Dynamic motion suitability | ✅ Good | ❌ Limited |
| Rolling/Twisting risk | ⚠️ Moderate | ✅ Very Low |
| Cost | 💲 Lower | 💲💲 Slightly Higher |
| Availability | ✅ Widely stocked | ⚠️ Less common |
When to Use Each Type?
Choose round O-rings for most general applications. Use flat O-rings when you need extra sealing surface, stability, or specific groove geometry.
- Use round O-rings for pistons, rotating shafts, water fittings
- Use flat O-rings for glass seals, flanges, and static joints
Custom grooves or uneven mating surfaces often benefit from a flat ring to reduce leakage risk.
Conclusion
Flat and round O-rings serve different purposes. Round ones offer versatility and dynamic sealing ability, while flat O-rings provide more surface contact and better alignment in static applications.
💡 Need help selecting flat or round O-rings for your design?
Contact Julong Rubber or Request a Quote to get expert advice and customized sealing solutions.
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Explore this link to understand the diverse applications and advantages of Round O-rings in various industries. ↩
Learn about hydraulic cylinders to see how they function and their critical role in machinery and equipment. ↩
Discover the principles of static sealing and its importance in preventing leaks in various applications. ↩
Explore this link to understand how Flat O-rings enhance sealing in various applications, ensuring better performance and reliability. ↩
Learn about vacuum systems to see how Flat O-rings play a crucial role in maintaining effective seals and performance. ↩
Discover the importance of flange joints in engineering and how Flat O-rings improve their sealing capabilities. ↩
Learn about the critical differences in pressure resistance between O-Ring types to make informed choices for your sealing needs. ↩
