O-Ring Hardness Guide: 70 Shore A vs 90 Shore A and Application Selection
O-ring hardness, measured on the Shore A durometer scale, is one of the most critical yet frequently overlooked material specifications. Hardness directly affects sealing force, extrusion resistance, dynamic friction, and compression set behaviour. Selecting the wrong hardness is a common cause of seal leakage, excessive wear, and premature failure. This guide explains Shore A hardness, compares 70 and 90 Shore A performance, and provides application-specific recommendations.
What Is Shore A Hardness?
Shore A is a durometer scale used to measure the indentation hardness of flexible materials such as elastomers. The scale ranges from 0 (very soft, gel-like) to 100 (very hard, approaching plastic rigidity). Most O-ring elastomers fall between 50 and 90 Shore A.
How Shore A Is Measured
A spring-loaded indenter is pressed into the material surface. The depth of indentation is converted to a hardness reading:
- 0 Shore A: No resistance (fully penetrated)
- 100 Shore A: No penetration
- 70 Shore A: Moderate flexibility — the industry standard for general-purpose O-rings
- 90 Shore A: High rigidity — used for high-pressure and dynamic applications
Common O-Ring Hardness Grades
| Hardness | Characteristics | Typical Applications |
|---|---|---|
| 50 Shore A | Very soft, low sealing force, excellent conformability | Fragile surfaces, low-pressure vacuum, large tolerances |
| 60 Shore A | Soft, good conformability, moderate sealing force | General low-pressure static seals, glass/plastic mating |
| 70 Shore A | Balanced properties, good sealing force, moderate extrusion resistance | General-purpose static and dynamic seals |
| 80 Shore A | Firmer, better extrusion resistance, higher sealing force | High-pressure hydraulics, pneumatic systems |
| 90 Shore A | Very hard, highest extrusion resistance, high sealing force | Ultra-high pressure, severe dynamic wear |
70 Shore A: The General-Purpose Standard
70 Shore A is the default hardness for most O-ring materials, including NBR, FKM, EPDM, and silicone. It offers the best balance of:
- Sealing force: Sufficient to conform to surface imperfections
- Compression set resistance: Good recovery after long-term compression
- Dynamic performance: Low enough friction for reciprocating seals
- Cost: Most widely produced and competitively priced
When to Use 70 Shore A
- General hydraulic and pneumatic systems up to ~100 bar
- Static seals in plumbing, HVAC, and automotive
- Food and pharmaceutical equipment with moderate pressure
- Applications where low friction is important
90 Shore A: High-Pressure and Severe Service
90 Shore A O-rings are significantly harder and stiffer than 70 Shore A. They offer superior extrusion resistance but require higher gland compression forces and produce more friction.
When to Use 90 Shore A
- High-pressure hydraulics above 100–200 bar
- Applications with large clearance gaps
- Severe dynamic abrasion environments
- Systems requiring backup rings (90 Shore A is standard with PTFE backups)
- Oilfield and mining equipment
Hardness vs Performance
Sealing Force
Harder O-rings require more compression to achieve the same contact width. A 90 Shore A ring needs approximately 30–50% more compressive load than a 70 Shore A ring to seal against the same surface.
Extrusion Resistance
This is where harder materials dominate. Under high pressure, softer O-rings flow into clearance gaps more easily. 90 Shore A resists extrusion roughly 2–3x better than 70 Shore A in the same material family.
Dynamic Friction
Friction increases with hardness. For reciprocating or rotary seals, 70 Shore A is preferred because it generates less heat and wear. 90 Shore A should only be used dynamically when pressure demands it.
Compression Set
Hardness and compression set are related but distinct. A harder material does not automatically have lower compression set — that depends on the compound formulation and curing system. However, high-quality 90 Shore A compounds often show better long-term recovery in high-pressure glands.
Wear Resistance
In dynamic applications with abrasive media, 90 Shore A generally outperforms 70 Shore A because the harder surface is more resistant to cutting and abrasion. HNBR 90 Shore A is particularly effective in high-wear environments.
Material-Specific Hardness Availability
| Material | Common Hardnesses | Maximum Typical |
|---|---|---|
| NBR | 50, 60, 70, 80, 90 Shore A | 90 Shore A |
| HNBR | 70, 80, 90 Shore A | 90 Shore A |
| FKM | 50, 60, 70, 75, 80, 90 Shore A | 90 Shore A |
| EPDM | 50, 60, 70, 80, 90 Shore A | 90 Shore A |
| VMQ (Silicone) | 30, 40, 50, 60, 70, 80 Shore A | 80 Shore A |
| FFKM | 65, 70, 75, 80, 90 Shore A | 90 Shore A |
| PTFE | N/A | Rigid plastic |
Note: Silicone is difficult to formulate above 80 Shore A and still maintain good physical properties. PTFE is a rigid thermoplastic and has no meaningful Shore A hardness.
Application Selection Matrix
| Application | Recommended Hardness | Rationale |
|---|---|---|
| General hydraulics (<100 bar) | 70 Shore A | Balanced sealing and friction |
| High-pressure hydraulics (100–400 bar) | 80–90 Shore A | Better extrusion resistance |
| Ultra-high pressure (>400 bar) | 90 Shore A + backup rings | Maximum extrusion resistance |
| Reciprocating dynamic seals | 70–80 Shore A | Lower friction, reduced heat |
| Rotary shaft seals | 70–80 Shore A | Minimise spiral failure risk |
| Vacuum seals | 60–70 Shore A | Better conformability |
| Food/pharma static seals | 70 Shore A | Standard, FDA compliant |
| Pneumatic cylinders | 70 Shore A | Low friction, fast response |
| Oilfield downhole | 90 Shore A | Resists H₂S, high pressure, abrasion |
| Plastic/glass mating surfaces | 50–60 Shore A | Lower force prevents cracking |
Hardness and Tolerance
Harder O-rings are generally easier to manufacture to tight tolerances because they are less prone to deformation during molding and trimming. For aerospace and precision instrumentation, 80–90 Shore A materials often meet tighter AS568 or ISO 3601-1 tolerance classes more consistently.
Common Hardness Mistakes
Using 70 Shore A for 500 Bar
A 70 Shore A O-ring in a 500 bar application will extrude rapidly unless the clearance gap is essentially zero. For pressures above 200 bar, 90 Shore A with backup rings is the norm.
Using 90 Shore A for Delicate Plastic Seals
The high sealing force of 90 Shore A can crack brittle plastics like acrylic or polycarbonate. Use 50–60 Shore A for plastic or glass mating surfaces.
Ignoring Material When Comparing Hardness
A 70 Shore A FKM and a 70 Shore A NBR have the same hardness but very different chemical and thermal resistance. Always specify material and hardness together.
Summary
For most applications, 70 Shore A is the correct starting point. It provides the best all-round balance of sealing, friction, and cost. Move to 80 or 90 Shore A when pressure, clearance gaps, or abrasive wear demand better extrusion resistance and durability. Use 50–60 Shore A for delicate surfaces and low-pressure conformability.
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Frequently Asked Questions
Q1: What does 70 Shore A mean for an O-ring? 70 Shore A indicates a moderately flexible elastomer with balanced sealing force and compression set resistance. It is the industry standard hardness for general-purpose O-rings.
Q2: Is a 90 Shore A O-ring better than 70 Shore A? Not universally. 90 Shore A is better for high pressure and abrasion resistance but produces higher friction and requires more compression force. For general sealing below 100 bar, 70 Shore A is usually preferable.
Q3: Can I use a 70 Shore A O-ring for high pressure? 70 Shore A is generally acceptable up to approximately 100 bar. Above this, extrusion becomes a significant risk, and 80–90 Shore A with backup rings is recommended.
Q4: What is the softest O-ring material? Silicone (VMQ) is available in very soft grades down to 30–40 Shore A. These are used in medical devices, food processing, and delicate sealing applications.
Q5: Does hardness affect temperature resistance? Hardness and temperature resistance are independent properties controlled by the polymer base and compounding. However, harder compounds sometimes retain mechanical strength better at elevated temperatures.
Q6: Which hardness is best for dynamic seals? 70–80 Shore A is best for reciprocating and rotary dynamic seals because it minimises friction, heat generation, and wear. 90 Shore A should only be used dynamically when pressure dictates it.
Q7: How do I measure O-ring hardness? O-ring hardness is measured with a Shore A durometer. For accurate readings, test on a flat surface or stack multiple rings. Curved surfaces can give falsely low readings.