Nitrile Sealing Rings & Cord

What are nitrile sealing rings and cord?

Nitrile sealing rings, better known as O-rings, are round rubber rings with a circular cross section, used to seal the gap between two parts. Nitrile cord, also called O-ring cord, is the same rubber made as a continuous length, which can be cut and joined to make an O-ring of any size. Both are made from nitrile rubber (NBR), which resists oils and fuels, so they are widely used in automotive, hydraulic, and general engineering applications.

A standard nitrile sealing ring or cord has these main features:

• A solid, round cross section, measured as its section diameter
• Nitrile rubber (NBR), which resists oil, grease, and petroleum fuels
• A hardness rating on the Shore A scale, usually 70 or 90
• For O-rings, an inside diameter that sets the size of the ring.

O-ring cord is supplied as a continuous extruded length.

• It is cut to the length needed and the ends are joined to make a custom O-ring, which is ideal where a standard size does not fit.
• Cord can also be used on its own as a seal, gasket, or packing strip in a groove.

Nitrile sealing rings and cord work by being squeezed slightly in a groove between two parts, so the rubber presses against both surfaces and blocks the leak path.

Why are nitrile sealing rings and cord essential?

O-rings are one of the simplest and most widely used seals in engineering. They are small, low in cost, and easy to fit, and they seal reliably against oil, fuel, water, and air in both static and moving applications. Nitrile is the most common O-ring material because it gives a good balance of oil resistance, wear resistance, and price.

O-ring cord adds flexibility. Instead of stocking many sizes of O-ring, a length of cord can be cut and joined to make a seal of exactly the right size, which is especially useful for large or non-standard diameters, repairs, and one-off jobs.

Typical applications include:

• Hydraulic and pneumatic equipment
• Engines, gearboxes, and fuel systems
• Pumps and valves
• Plumbing and water fittings
• Garden hoses and irrigation equipment
• Covers, lids, and inspection hatches
• Agricultural and construction machinery
• Custom seals for non-standard grooves

The main types of nitrile sealing rings and cord

• Metric O-ring cord - Supplied in metric section sizes, from small sections of around 1mm up to large sections of 20mm or more, sold by the metre.
• Imperial O-ring cord - Supplied in inch section sizes, such as 1/8 inch and 3/8 inch, for imperial grooves and equipment.
• 70 Shore nitrile - The standard hardness, flexible enough to seal well in most applications and easy to fit.
• 90 Shore nitrile - A harder grade, which resists being squeezed out of its groove under higher pressure, and resists wear in moving applications.
• Standard size O-rings - Ready-made rings in standard metric and imperial sizes, including the British Standard (BS) imperial range.

How to choose the right nitrile sealing ring or cord?

• Section size - Choose a section diameter that suits the groove depth and width, so the ring is squeezed by the right amount when fitted.
• Inside diameter or length - For an O-ring, choose the inside diameter to suit the groove or shaft. For cord, work out the length needed to make the ring.
• Hardness - Choose 70 Shore for most applications, or 90 Shore for higher pressures or where the seal could be pushed out of its groove.
• Temperature - Nitrile suits temperatures from around -30°C up to +120°C. For higher temperatures, a different material such as FKM may be needed.
• Fluid compatibility - Nitrile resists mineral oils, greases, petrol, diesel, and LPG. Check it is compatible with the fluid being sealed, as some chemicals and brake fluids attack nitrile.

Essential references and specialised ranges

How the numbering works:

O-ring cord is referenced by its section diameter, material, and hardness:

3mm NBR 70 = 3mm section nitrile cord, 70 Shore hardness

1/8 inch NBR 70 = 1/8 inch section nitrile cord, 70 Shore hardness

O-rings are referenced by their inside diameter and section diameter:

20 x 3 = 20mm inside diameter, 3mm section

Imperial O-rings are often referenced by a BS number, such as BS050, which identifies a standard size in the British Standard range.

To work out the length of cord needed to make an O-ring, add the inside diameter to the section diameter, then multiply by 3.14. For example, for a 100mm inside diameter ring in 5mm cord: (100 + 5) x 3.14 = about 330mm of cord.

Suffixes:

NBR = nitrile rubber

70, 90 = hardness on the Shore A scale

Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.

FAQ - All you need to know about nitrile sealing rings and cord

What is the difference between O-rings and O-ring cord?

An O-ring is a ready-made ring in a set size. O-ring cord is a continuous length of the same rubber, which can be cut and joined to make an O-ring of any size.

How do I join O-ring cord?

Cut both ends squarely with a sharp blade, then join them with a suitable instant adhesive or a splicing kit. Hold the ends together firmly until the joint has set, and check the joint is smooth before fitting.

What does Shore hardness mean?

Shore hardness is a measure of how hard the rubber is. 70 Shore is the standard grade, which is flexible and seals well. 90 Shore is harder, and better at resisting pressure and wear.

Can nitrile be used with all oils and fuels?

Nitrile resists most mineral oils, greases, and petroleum fuels, including petrol, diesel, and LPG. It is not suitable for some fluids, such as certain brake fluids, strong acids, or ozone-rich outdoor exposure, so check compatibility before use.

How do I fit an O-ring?

Clean the groove and the mating surface, and remove any burrs or sharp edges that could cut the ring. Lightly lubricate the O-ring with a compatible oil or grease, then fit it into the groove without twisting or overstretching it.

How do I identify the size I need?

For an O-ring, measure the inside diameter and the section diameter of the old ring, or measure the groove it fits in. For cord, measure the section diameter with callipers, and work out the length from the size of the groove.

How can I tell if an O-ring needs replacing?

Warning signs include:

• Leaks around the seal
• A flattened ring that has lost its round shape
• Cracks, cuts, or nicks in the rubber
• Hardened, brittle, or swollen rubber
• Damage where the ring has been squeezed out of its groove
• A failed or split joint on a cord ring