Product Overview
Reliable custom rubber parts for industrial applications
Quick Answer
Black rubber O-rings in multiple sizes are molded circular seals for pumps, valves, fittings, cylinders, automotive parts and industrial equipment. Inner diameter, cross-section, material, Shore A hardness, tolerance, color, packaging and size assortment can be customized. NBR, EPDM, HNBR, FKM, Silicone and CR compounds are available according to application conditions.
Black rubber O-rings in multiple sizes are circular elastomer seals used in grooves to prevent leakage between compatible mating components. They can be supplied in different inner diameters, cross-sections, materials and Shore A hardness levels. Common applications include pumps, valves, hydraulic systems, pneumatic equipment, pipe fittings, automotive assemblies and general industrial machinery.
Product Overview
What Is a Rubber O-Ring?
A rubber O-ring is a solid torus — a ring with a circular cross-section — manufactured from an elastomeric compound to precise inner diameter (ID) and cross-section diameter (CS) dimensions. When installed in a machined rectangular groove (O-ring groove) and compressed between two mating surfaces by assembly or system pressure, the O-ring deforms elastically to fill the micro-gap between the mating surfaces and establishes a continuous contact seal that prevents fluid or gas passage around the perimeter of the groove.
The O-ring seals in two stages:
- Initial squeeze seal — the mechanical compression from assembly (typically 10–30% of cross-section diameter) establishes primary sealing contact before any system pressure is applied
- Pressure-energized seal — system pressure acts on the O-ring cross-section, pressing it against the low-pressure side of the groove wall and increasing contact stress proportionally with pressure; the higher the system pressure, the tighter the seal
This self-energizing pressure mechanism makes the O-ring uniquely efficient — it requires no external adjustment, no springs, and no retightening over its service life in static applications.
Product Specifications
| Attribute | Value |
|---|---|
| Product Name | Black Rubber O-Ring |
| Also Known As | O-Ring / Toric Seal / Ring Seal / Circular Cross-Section Seal |
| Cross-Section Shape | Circular torus — defined by ID × CS (inner diameter × cross-section diameter) |
| Materials Available | NBR (Nitrile), EPDM, FKM (Viton®), VMQ (Silicone), CR (Neoprene), HNBR, FFKM, Polyurethane (PU), Butyl (IIR) |
| Standard Color | Black (carbon-black filled); other colors by compound |
| Hardness | Shore A 40–90 (standard: 70A) |
| Standard Series | AS568A (Imperial / inch), ISO 3601 (Metric / mm), BS 1806, DIN 3771, JIS B2401 |
| Size Range | ID 1.0 mm to 2,000 mm; CS 0.5 mm to 12 mm |
| Temperature Range | -60°C to +250°C (compound-dependent) |
| Pressure Rating | Static: up to 400 bar (with back-up rings); Dynamic: up to 80 bar |
| Fabrication | Compression moulding (standard), injection moulding, lathe-cut (large custom) |
| Certifications | RoHS, REACH; FDA 21 CFR 177.2600 (VMQ/EPDM); WRAS (EPDM); AS9100 (aerospace) |
The O-Ring Geometry — Specifying the Correct Size
Every O-ring is defined by two dimensions:
- Inner Diameter (ID) — the diameter of the O-ring measured across the inside of the torus, in mm or inches
- Cross-Section Diameter (CS) — the diameter of the circular cross-section, in mm or inches; also called cord diameter or wire diameter
The relationship between these two dimensions determines the correct O-ring for any groove. The O-ring ID should be slightly smaller than the groove ID (typically 1–3% stretch) so the ring sits in tension in the groove and cannot rotate or fall out during assembly. The CS is compressed in the groove by 10–30% to achieve the primary mechanical seal.
Standard cross-section sizes (ISO 3601 metric series):
| CS Designation | CS Diameter | Typical ID Range | Application |
|---|---|---|---|
| N1 | 1.8 mm | 1.8–6.0 mm | Miniature instruments, medical |
| N2 | 2.65 mm | 4.5–50 mm | Light duty hydraulic, pneumatic |
| N3 | 3.55 mm | 5.5–200 mm | General purpose — most common |
| N4 | 5.3 mm | 12.5–400 mm | Heavy duty hydraulic, large bore |
| N5 | 7.0 mm | 22.5–600 mm | Large pressure vessels, large bore |
Complete Size Reference Tables
ISO 3601 Metric O-Ring Size Chart (Selected Common Sizes)
| ISO Size | ID (mm) | CS (mm) | Typical Application |
|---|---|---|---|
| 3 × 1.5 | 3.0 | 1.5 | Miniature instrument seals |
| 5 × 1.5 | 5.0 | 1.5 | Small bore connectors |
| 10 × 2 | 10.0 | 2.0 | Hydraulic fittings |
| 15 × 2.5 | 15.0 | 2.5 | Pipe fittings, valves |
| 20 × 2.5 | 20.0 | 2.5 | Hydraulic cylinder ports |
| 25 × 3 | 25.0 | 3.0 | General hydraulic |
| 30 × 3 | 30.0 | 3.0 | Pump covers, end caps |
| 40 × 3.5 | 40.0 | 3.5 | Cylinder end covers |
| 50 × 4 | 50.0 | 4.0 | Large bore hydraulic |
| 75 × 5 | 75.0 | 5.0 | Pressure vessel ports |
| 100 × 5 | 100.0 | 5.0 | Large cylinder end covers |
| 150 × 6 | 150.0 | 6.0 | Large vessel flanges |
| 200 × 7 | 200.0 | 7.0 | Pressure vessel covers |
| 300 × 8 | 300.0 | 8.0 | Large equipment covers |
Material Selection Guide
| Rubber Material | Key Characteristics | Recommended Application Conditions |
|---|---|---|
| NBR | Oil resistance and practical cost-to-performance balance | Petroleum oils, hydraulic fluids, lubricants and general industrial machinery |
| EPDM | Water, steam, ozone, weather and coolant resistance | Water systems, outdoor equipment, HVAC systems and coolant-contact applications without oil exposure |
| HNBR | Improved heat resistance, mechanical strength and oil compatibility | Automotive, hydraulic and higher-temperature oil-contact sealing applications |
| FKM | Heat resistance and compatibility with demanding oils, fuels and chemicals | Fuel systems, chemical processing, high-temperature oil service and demanding industrial applications |
| Silicone | Wide temperature flexibility and soft elastic behavior | High-temperature, low-temperature, food-contact or medical-related applications where suitable grades are specified |
| CR | Balanced weather resistance, moderate oil resistance and mechanical durability | Outdoor equipment, refrigeration, marine and general-purpose sealing applications |
Compound Selection Matrix
| Media / Condition | Temp | Best Compound |
|---|---|---|
| Mineral oil, hydraulic oil, diesel | ≤ +100°C | NBR 70A |
| Hot oil, oilfield, CNG | ≤ +150°C | HNBR |
| Aggressive chemicals, solvents | ≤ +200°C | FKM |
| Extreme chemical + ultra-high temp | ≤ +320°C | FFKM |
| Water, steam, HVAC, outdoor | ≤ +150°C | EPDM |
| Food, beverage, pharma (FDA) | -60°C to +250°C | FDA VMQ |
| Gas, LPG, vacuum | ≤ +130°C | Butyl (IIR) |
| Marine, moderate oil + weather | ≤ +120°C | CR |
| Economy water / general | Ambient | EPDM / NBR |
O-Ring Application Types
Static Sealing (Face Seal & Radial Static)
In a static application, the two O-ring groove walls do not move relative to each other once assembled. The O-ring maintains a constant compression set that does not change during service. Static applications tolerate higher compression percentages (up to 30%) and are the most common O-ring application — pipe unions, valve bodies, equipment covers, boss seals, face seals.
Recommended groove compression: 15–30% of CS
Recommended compounds: All compounds suitable; select based on media and temperature
Dynamic Reciprocating (Piston & Rod Seals)
The O-ring slides axially back and forth in its groove as the piston or rod strokes. Friction, heat generation, and wear replace compression set as the primary failure mechanisms. Groove compression is lower (10–20%) to reduce friction and heat; surface finish of the dynamic bore is critical (Ra 0.2–0.4 μm recommended).
Recommended groove compression: 10–20% of CS
Recommended compounds: NBR 70A (oil), EPDM (water), FKM (chemical/high temp), PU (high pressure)
Speed limit: Up to 0.5 m/s standard; up to 1.5 m/s with low-friction compounds
Dynamic Rotary
The O-ring seals against a rotating shaft or bore. Friction and heat generation are more severe than reciprocating service; O-ring speed limits are lower. For high-speed or continuous rotation, lip seals are generally preferred over O-rings; O-rings in rotary service are limited to low-speed intermittent rotation, oscillating motion, or slow continuous rotation (< 0.5 m/s surface speed).
Recommended groove compression: 10–15% of CS (minimize friction)
Recommended compounds: NBR, HNBR, FKM with low friction formulation
Speed limit: ≤ 0.5 m/s surface speed for continuous rotation
O-Ring Groove Design Reference
| Application | Groove Width | Groove Depth | Surface Finish (Ra) | Compression |
|---|---|---|---|---|
| Static — face seal | 1.4 × CS | 0.70–0.74 × CS | ≤ 3.2 μm | 26–30% |
| Static — radial | 1.4 × CS | 0.72–0.76 × CS | ≤ 3.2 μm | 24–28% |
| Dynamic — reciprocating | 1.4 × CS | 0.78–0.82 × CS | 0.2–0.4 μm | 18–22% |
| Dynamic — rotary | 1.4 × CS | 0.82–0.86 × CS | 0.2–0.4 μm | 14–18% |
Back-Up Rings — When to Use
At pressures above 70 bar in dynamic applications and above 100 bar in static applications, the pressure differential across the O-ring begins to extrude the rubber into the clearance gap between the groove and the mating bore or shaft. Extrusion causes spiral failure or nibbling of the O-ring leading edge — the primary high-pressure O-ring failure mode. PTFE or nylon back-up rings installed on the low-pressure side of the O-ring in the groove close the clearance gap and prevent extrusion, allowing O-rings to seal reliably up to 400 bar static.
| System Pressure | Back-Up Ring Required? |
|---|---|
| < 70 bar dynamic / < 100 bar static | Not required |
| 70–200 bar dynamic | Single PTFE back-up ring (low-pressure side) |
| > 200 bar dynamic | Double PTFE back-up ring (both sides) |
| > 100 bar static | Single back-up ring recommended |
Quality Standards & Testing
| Test | Method | Specification |
|---|---|---|
| Hardness | ISO 48-4 / ASTM D2240 | Shore A ± 5 of nominal |
| Tensile Strength | ISO 37 / ASTM D412 | ≥ 10 MPa (NBR 70A) |
| Elongation at Break | ISO 37 | ≥ 300% (NBR 70A) |
| Compression Set | ISO 815 (22h @ 70°C) | ≤ 15% (NBR 70A) |
| Volume Change in Oil | ISO 1817 (IRM 903, 70h @ 100°C) | ≤ +20% (NBR) |
| Ozone Resistance | ISO 1431-1 | No cracking (EPDM, CR) |
| Heat Aging | ISO 188 (70h @ 100°C) | ≤ ±10% property change |
| Dimensional | AS568A / ISO 3601 | ID ± 0.25%, CS ± 0.13 mm |
| FDA Compliance | 21 CFR 177.2600 | VMQ and EPDM available |
| WRAS Approval | WRAS scheme | EPDM compound available |
| RoHS / REACH | EU Directives | All standard compounds compliant |
| AS9100 / NADCAP | Aerospace quality | Available for aerospace-grade supply |
Installation Best Practices
Pressurize gradually at first use — bring system to operating pressure slowly at first pressurization; sudden pressure surges can displace an un-bedded O-ring from its groove
Verify O-ring size — confirm ID and CS against groove dimensions; an oversized or undersized O-ring is the most common installation error
Clean the groove thoroughly — remove all metal chips, old O-ring fragments, and contamination; a single chip under the O-ring causes immediate leakage
Lubricate the O-ring before installation — apply a thin film of system-compatible lubricant (petroleum jelly for NBR/oil service; silicone grease for EPDM/VMQ/water service); lubrication prevents installation damage and reduces roll-in friction
Do not use sharp tools — fit O-rings by hand or with smooth-edged fitting tools; screwdrivers and picks cut O-rings and cause immediate or delayed leakage
Protect O-ring from sharp edges during installation — use installation sleeves or chamfered lead-ins when pressing O-rings past threaded sections, keyways, or sharp bore edges
Do not overstretch — avoid stretching O-rings more than 50% above nominal ID during installation; overstretching thins the cross-section and reduces sealing contact stress
Check for twisting — after installation, confirm the O-ring sits evenly in the groove with no twist visible; a twisted O-ring has reduced effective sealing contact area
Storage & Shelf Life
Per ISO 2230 and AS1646 elastomeric sealing element storage guidelines:
| Compound | Recommended Shelf Life | Storage Temperature | Key Requirements |
|---|---|---|---|
| NBR | 7 years | +15°C to +25°C | Away from ozone, UV, oils |
| EPDM | 10 years | +15°C to +25°C | Away from ozone (good natural resistance) |
| FKM | 10+ years | +15°C to +25°C | Excellent long-term stability |
| VMQ | 10+ years | +15°C to +25°C | Avoid compression in storage |
| CR | 5 years | +15°C to +25°C | Most sensitive to ozone |
| NR | 3–5 years | +15°C to +25°C | Most sensitive compound |
| Butyl | 10 years | +15°C to +25°C | Excellent stability |
Store all O-rings in original sealed packaging, in cool dark conditions, away from electric motors, welding equipment, and fluorescent lighting (ozone sources), and away from petroleum products and solvents.
FAQ
What are black rubber O-rings used for?
Black rubber O-rings are used to seal between compatible mating components in grooves. They help reduce leakage of liquid or gas in pumps, valves, cylinders, pipe fittings, automotive parts and industrial equipment when the material, size and compression are correctly selected.
Can O-rings be supplied in multiple sizes?
Yes. Black rubber O-rings can be supplied in multiple inner diameters and cross-section sizes according to standard references, drawings, size lists or physical samples. Buyers should provide the required size list and quantity for each size before quotation.
How should buyers select the rubber material?
Material selection depends on the contact media, temperature, pressure and environment. NBR can be reviewed for oil exposure. EPDM is suitable for water and weather resistance. FKM can be evaluated for fuel, heat and chemicals. Silicone can be considered for broad temperature requirements.
What dimensions are needed to specify an O-ring?
The two most important dimensions are inner diameter and cross-section. Outer diameter can be calculated from these values or confirmed from the drawing. For critical sealing, buyers should also provide groove dimensions, compression requirements and dimensional tolerance.
Are O-rings suitable for both static and dynamic sealing?
O-rings can be used in static sealing and selected dynamic sealing applications. Dynamic applications require additional review of movement type, speed, lubrication, friction, extrusion gap, pressure and material wear resistance. The final design should be checked against the actual operating conditions.
Are samples available before bulk production?
Samples are available upon request. Sample planning depends on size, material, hardness, tolerance, tooling requirements and quantity. Please provide a drawing, standard reference, size list or physical sample so the black rubber O-rings can be reviewed before batch production.
Request a Custom Quote
Send your O-ring size list, drawing, standard reference or physical sample for technical review. Please include the ID, cross-section, material, Shore A hardness, contact media, temperature range, pressure, static or dynamic sealing condition, quantity by size and packaging requirements.
