A guide to o-ring seals
We’ve put together this introduction to o-rings to help you understand them better, what they are and why you need them. We’ll cover:
What is an o-ring?
Sometimes written as o ring and o’ring seals – at other times referred to as ring seals – these are sealing rings used across many industries for demanding applications. O-rings are essentially a gasket shaped like a ring with a circular cross section, used on components such as pumps and valves. The ring design forms a seal to prevent fluid or air from leaking.
Considerations for choosing o-rings have to include these factors:
Durometer
Also called Shore A hardness. O-Ring durometer is the hardness of the o-ring material. Typically, o-rings are made of rubber or thermoplastic elastomer. Durometer scales provide numbers 1-100 to define how soft or hard the material is, with 70 being standard. Any Shore A hardness less than 70 is considered a softer material, while greater than 70 is harder. You can learn more in our guide, What’s shore hardness and why should you care?
Chemical compatibility
Your material will need resistance to its environment and the chemicals it will be containing. The performance of your o-ring is only as good as your material choice. Consideration here needs to be thoughtful.
Pressure compatibility
If the pressure that o-ring seals are subjected to is too high or low, the o’ring will fail to seal adequately. Anything under 100psi is considered too low. The pressure has to be high enough to activate the seal. Here again, material plays a critical role. The most reliable seal is achieved by the component design increasing the o-ring’s seal cross-section and reducing the seal durometer.
How do o-ring seals work?
The o-ring seal is placed within a groove – also called a gland – which is designed to handle compression between two surfaces intended to be mated. The gap between these two parts is the diametrical clearance. Compression causes the o-ring to block the diametrical clearance, which in turn creates the seal.
When fluid puts pressure on the o-ring, the ring is pushed against the low-pressure side, forcing it against the walls of the gland, increasing the strength of the seal. When the pressure is freed, the properties of the o-ring material enable it to regain its shape.
O-ring applications fall into one of two categories, depending on the type of motion that happens between the two mating surfaces.
Static applications: sealing between the two surfaces that do not move. The pressure is on the top and bottom of the o-ring’s cross section.
Typically used: flange applications
Dynamic applications: the two surfaces, or parts, move against the o-ring seal interface. High resistance – or good resistance – to fluids must be part of your product’s design considerations than is necessary in static seal applications. This is due to the volumetric increase in the o-ring. Any increase that exceeds around 20% can result in friction and excessive wear. Another consideration for dynamic applications is that at most, only 4% shrinkage can be tolerated.
Typically used: hydraulic applications
O-rings are the most common oil seals available. If you’re working with a hydraulic system that uses oil, pay close attention to the dynamic application during sealing.
How are o-rings made?
The manufacturing process for producing ring seals varies, but the most common methods include:
- Extrusion
Raw materials for o-rings are forced through a ring-shaped die. Sometimes extrusion involves a hopper, where the material is heated. Afterwards, the material is put through the die. Different forms of extrusion include friction extrusion, cold extrusion and microextrusion.
- Injection Moulding
Injection moulding is similar to extrusion in that it involves raw materials and a mould. Metal can be used, but usually it’s plastic polymers in the form of pellets. Thermoplastic elastomer is commonly used for injection moulding to make o-rings and seals. The pellets are heated and the liquified material is then injected into a mould, where it cools and hardens.
- Pressure Moulding
Pressure moulding is also known as compression moulding. This manufacturing process uses heat and pressure to create o-ring seals. Let’s say you’re making rubber o-rings. The rubber is first preheated before being injected into a mould cavity. A plug is then fitted on top, putting pressure on the mould cavity. As the pressure intensifies inside of the cavity, the preheated rubber takes its o-ring shape.
- Transfer Moulding
Transfer moulding is similar to pressure moulding. Both involve preheated materials and a mould cavity. While compression moulding requires an open mould cavity, transfer moulding uses an enclosed mould cavity. As the cavity is closed, higher pressure builds. The result is thicker and more consistent o-rings.
How to use o-rings
O’rings have specific applications. They’re most commonly used for sealing in valves, pumps, cylinders and connectors. Specifically, they are indispensable for these and other applications and industries:
- Hydraulic seals
- Pipe joints
- Oil seals
- Equipment and machinery with fluid seal points, such as rotating shafts
- Fuel systems
- HVAC systems
- Food processing, o-rings must be 3A sanitary, specialty 3A sanitary, NSF-61 Medical industry, such as respiratory equipment and devices with valves, pumps, fluid transfer and connectors
- Oil & Gas industry, for valves, gas pumps, fittings, dispensers and storage tanks
- Electronics, for semiconductor processing and dust protection
As we already noted, to use o’rings so that you get the best performance for your component, you need to choose your material carefully to ensure chemical compatibility and pressure compatibility.
When you’re ready to use your o-ring – that is, when you apply it to your component – take care not to overstretch it, which can cause it to break or tear during assembly. Proper installation is just as important to ensuring high performance.
Simply slide it onto your component – don’t roll it. That will result in spiraling, which causes leaks and damage to your assembly. Use a lubricant to make the sliding process easier – but do not use a lubricant made of the same material as your seal. Also, if you’re mating your o-ring with a threaded surface, take extra caution to avoid tears that might not be visible, but will leak later on.
Finally, make sure you use the correct o-ring size.
How to measure o-rings
For your seals to work properly and prevent leakage, you need to install them correctly. Your first step is to determine the right size. To get an accurate measurement, you really should use a calliper, although a ruler can be useful to help give you an estimate.
Step 1: Step 2: Measure the I.D. (inside diameter).
Step 2: Measure the O.D. (outside diameter).
Step 3: Measure the cross section. The cross section is the width of the o-ring. When measuring, be careful not to clamp the o’ring too tightly. This will cause displacement, which will give you an inaccurate measurement.
Another way to determine your cross section measurement is with this simple formula:
(OD - ID) ÷ 2.
What tolerances do o-rings need?
So that your o-rings can seal effectively, you’ll need to leave tolerances. Examples below are for the cross section and I.D. in accordance with ISO 3601-1B.
| Cross section (mm) | Tolerances (mm) |
|---|---|
| ≤ 2.62 | +0.08 |
| 2.63 – 3.00 | +0.09 |
| 3.01 – 4.50 | +0.10 |
| 4.51 – 5.50 | +0.13 |
| 5.51 – 7.50 | +0.15 |
| 7.51 – 8.50 | +0.18 |
| 8.51 – 10.0 | +0.20 |
| I.D. (mm) | Tolerances (mm) |
|---|---|
| 1.80 – 2.79 | +13 |
| 2.80 – 4.86 | +14 |
| 4.87 – 6.69 | +15 |
| 6.70 – 8.75 | +16 |
| 8.76 – 10.59 | +17 |
| 10.5 – 11.79 | +18 |
| 11.8 – 14.99 | +19 |
| 15.0 – 16.99 | +20 |
| 17.0 – 18.99 | +21 |
| 19.0 – 21.19 | +22 |
| 21.2 – 22.39 | +23 |
| 22.4 – 24.99 | +24 |
| 25.0 – 25.79 | +25 |
| 25.8 – 27.99 | +26 |
Rubber o-rings
Nowadays, rubber seals refer to o-rings made from a synthetic-rubber material, or an elastomer. Natural rubber o-rings do exist but synthetic rubbers, or elastomers, have more desirable properties for o-rings.
Natural rubber is harvested from rubber trees in the form of latex. The latex is a fluid which is refined into rubber for commercial processing. Natural rubber, like other materials, can be enhanced through manufacturing processes to take on the qualities desired.
Natural-rubber materials cannot stand up to the demands of a hydraulic application. They’re mostly used in automotive brakes and food & beverage applications. Natural rubber also can’t be used in applications with a continuous temperature above 85°C. For many high-temperature applications, that won’t do.
On the positive side, resistance to fatigue from wear is a natural-rubber characteristic. But it only has moderate resistance to ozone damage, nor can it be used with petroleum oils.
Elastomers have many advantages, on the other hand. Nitrile rubber, also called buna n, is an example of an elastomer. It’s an excellent, cost-effective o’ring material. Nitrile o-rings withstand most hydraulic fluids, petroleum oils and crude oil. We’ll look at other o’ring materials made of elastomers, such as silicone, in the next section.
Right now, it’s worth returning to the subject of static and dynamic applications. Static rubber o-rings create a seal between two surfaces that don’t move. Rubber o-rings for dynamic applications involve forming a seal between two surfaces that do move.
If your application is static, you don’t need an especially durable or hard-wearing synthetic rubber. In this instance, your considerations should include silicone.
If your application is dynamic, you’ll want an elastomer that can withstand abrasion, such as fluoroelastomer. This material is also known under the trade name VITONTM.
O-ring materials
O-rings are available in a wide range of materials. The most important considerations for choosing your o-ring material include chemical compatibility, operating temperature, sealing pressure and durometer. Your application might require other considerations, such as tear resistance.
A note about durometer shore hardness: each material varies. O-rings made of silicone are available in a Shore A 50, but it’s also available in Shore A 90. Always pay attention to the durometer shore hardness when investigating materials.
Common o-ring materials include:
Silicone rubber
Silicone o-rings offer high performance in high-temperature applications. Silicone rubber also stands up to cold, ozone exposure, water, steam and petroleum fluids.
Operating temperature range: -60°C to 232°C
O-rings typically used: Fuel injection ports, medical devices, food storage and electronics
You need to know: Poor tear resistance, abrasion and tensile strength
Nitrile
Also called Buna-Nitrile, or NBR, o-rings. An all-around, general purpose seal with excellent oil resistance. Not recommended for automotive brake fluids. Suitable for use with water, making these good plumbing o-rings. Also works with petroleum oils and some hydraulic fluids. Good abrasion resistance.
Operating temperature range: -54°C to 149°C
O-rings typically used: Fuel tanks, automotive and aerospace applications
You need to know: Resists tears; chemical resistance ranges from fair to good.
Polyurethane
Excellent abrasion, oil, weather and extrusion resistance. Narrow temperature range and can be used with mineral oil and grease, silicone oil and grease.
Operating temperature range: -54°C to 100°C
O-rings typically used: Hydraulic fittings, cylinders and valves and pneumatic tools
You need to know: Not suitable for applications needing good compression and heat resistance
Neoprene
Exceptional resistance to refrigerant gases, ammonia, silicone oils and greases. Good abrasion resistance and relatively low compression set. Outstanding weather resistance.
Operating temperature range: -43° to 107°C
O-rings typically used: air conditioning and refrigeration
You need to know: Good for outdoor applications
Fluoroelastomer (FKM)
These o-rings provide excellent chemical, thermal and oxidation resistance. Excellent for high-temperature and explosive environments. Suitable for explosive decompression applications, handling and transporting chemicals and fuels. Not resistant to superheated steam, low temperatures and brake fluids containing glycol.
Operating temperature range: -45°C to 204°C
Typically used for: Oil and gas, chemical, automotive, aerospace industries and general industrial applications
You need to know: Premium material
Fluorinated Propylene Monomer, known as FPM, is the same material as fluoroelastomer (FKM). FPM is the international name according to DIN/ISO, whereas FKM is for the fluoroelastomer category according to the American standard ASTM.
Metal o-rings
You can also find metal o-rings, but these have more specialised uses. Metals are good for when your application requirements exceed the capabilities of elastomers. For example, this might be for an extremely low- or high-temperature application, or if extreme pressure is involved. stock, bonded washers,
Types of ring seals in our range
We manufacture our comprehensive range to industry-leading standards. Typical applications include hydraulic and pneumatic sealing.
Available in two materials:
Nitrile rubber, Shore A hardness: 70, 75 & 90
I.D. range: 3.0 mm to 56.7 mm
Cross-section range: 1.6 mm to 5.33 mm
Fluoroelastomer, Shore A hardness: 75
I.D. range: 2.90 mm to 45.0 mm
Cross-section range: 1.6 mm to 5.33 mm
Available in two materials in different standards:
Fluoroelastomer, Shore A hardness: 75
Imperial kit: 382 assorted o-rings
Metric kit: 510 assorted o-rings
SAE flange ring seal kit: 125 o-rings
Nitrile rubber, Shore A hardness: 70 & 90
Imperial kit: 382 assorted o-rings
Metric kit: 510 assorted o-rings
SAE flange ring seal kit: 415 o-rings
Euro metric kit: 404 assorted o-rings
ORFS kit: 230 assorted o-rings
Ring seal kits – SAE flange seals
Assorted, popular sizes in easy-to-access box
Polyurethane, Shore A hardness: 90
Number of o-rings: 60
Prevents fluid loss and gas leaks
Polyurethane, Shore A hardness: 90
Kit containing 100 o-rings in one size
I.D. range: 12.7 mm to 50.8 mm
Cross-section range: Available in six standard SAE J518 sizes from 0.500" to 2.000"
Ring seal cord splicing kits and cords
Eliminates the need for extensive o-ring inventories
Cord can be cut to desired length to make fast, on-the-spot o-rings of any size
Nitrile Rubber, Shore A hardness: 70
Diameter range: 1.7 mm to 4 mm
Also available as a kit in easy-to-access box containing knife, tape, adhesive & cutting block
Also think about:
Sometimes called bonded seals or sealing washers, these are a type of washer that provide sealing properties for screws and bolts.
Usually used in applications that need a stronger seal than typical washers can offer. Range consists of nitrile washers; made from steel and with a nitrile rubber, Shore A hardness: 70.
Ideal for: applications involving hydraulic fluids or engine lubricating oils
Stock available for BSP and metric threads
Download free CADs and try before you buy
Free CADs are available for most solutions, which you can download. You can also request free samples to ensure the o-ring you’ve chosen is exactly what you need.
If you’re not quite sure which o-ring will work best for your application, our experts are always happy to help. Whatever your requirements, you can depend on fast despatch. Request your free samples or download free CADs now.
Questions?
Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application 0345 528 0474.