Polyurethane Sealing Rings & Cord

What are polyurethane sealing rings and cord?

Polyurethane sealing rings are O-rings made from polyurethane (PU), and polyurethane cord is the same material supplied as a continuous length. Polyurethane is one of the toughest sealing materials available. It has outstanding resistance to wear, tearing, and being squeezed out of its groove under pressure, which makes it a popular choice for hydraulic systems and seals on moving parts.

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

• A solid, round cross section, measured as its section diameter
• Polyurethane, which is very tough and resists wear, tearing, and abrasion
• A hardness on the Shore A scale, commonly 70 or 90
• Good resistance to mineral oils, greases, and hydraulic oils.

Polyurethane is chosen for toughness under pressure and movement.

• Its high strength lets it resist being pushed out of the groove at high pressure, a common cause of failure in softer O-rings.
• Its wear and tear resistance make it last longer than most rubbers in seals on moving rods, pistons, and shafts.

Polyurethane cord is also widely used as round drive belting, as it can be joined by heat welding to form a strong, continuous belt.

Why are polyurethane sealing rings and cord essential?

In hydraulic cylinders, valves, and pumps, seals face high pressure, constant movement, and contamination. Softer O-rings can wear, tear, or be squeezed out of their groove, leading to leaks and downtime. Polyurethane stands up to these conditions, giving a longer-lasting seal in demanding hydraulic and pneumatic equipment.

It also performs well in dirty and abrasive environments, such as construction and agricultural machinery, where grit and dust would quickly damage softer seals.

Typical applications include:

• Hydraulic cylinders, rams, and pistons
• Hydraulic and pneumatic valves
• Pumps and compressors
• Construction and agricultural machinery
• Mining and quarrying equipment
• Seals exposed to dirt, grit, and abrasion
• Round drive belts for conveyors and light machinery

The main types of polyurethane sealing rings and cord

• Polyurethane O-rings, 70 Shore - A softer grade, which seals well at moderate pressures and is easier to fit, while still offering polyurethane’s toughness.
• Polyurethane O-rings, 90 Shore - A harder grade, which gives the best resistance to being squeezed out of the groove, ideal for high-pressure hydraulic applications.
• Polyurethane O-ring cord - Supplied as a continuous length, for cutting and joining into custom-sized seals, or for heat welding into round drive belts.
• BS size polyurethane O-rings - Ready-made rings in standard British Standard imperial sizes, such as BS104 and BS614.

How to choose the right polyurethane sealing ring or cord?

• Pressure - Choose 90 Shore polyurethane for high-pressure hydraulics, or 70 Shore for lower pressures where easier fitting and softer sealing are useful.
• Movement and wear - Choose polyurethane for seals on moving parts, or where dirt and abrasion would wear out softer materials.
• Fluid compatibility - Polyurethane resists mineral oils, greases, and hydraulic oils. It is not suited to hot water, steam, strong acids or alkalis, or brake fluids, which can break it down.
• Temperature - Polyurethane suits moderate temperatures. It is not designed for high-heat applications, where FKM or silicone may be better.
• Size - Choose a standard BS size O-ring to suit the groove, or the right cord section and length for custom seals.

Essential references and specialised ranges

How the numbering works:

Polyurethane O-rings are often referenced by a BS number, which identifies a standard imperial size, followed by the material and hardness:

BS104 PU90 = BS104 size O-ring in 90 Shore polyurethane, with a 2.85mm inside diameter and 2.62mm section

BS614 PU70 = BS614 size O-ring in 70 Shore polyurethane, with an 11.91mm inside diameter and 2.62mm section

Polyurethane cord is referenced by its section diameter, for example 5mm PU cord.

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

Suffixes:

PU = polyurethane

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 polyurethane sealing rings and cord

What is the difference between polyurethane and nitrile?

Both resist mineral oils and greases. Polyurethane is much tougher and more resistant to wear, tearing, and high pressure, so it suits demanding hydraulic and moving seals. Nitrile is more flexible, has a wider temperature range, and suits general sealing.

What is the difference between 70 Shore and 90 Shore?

70 Shore is softer, easier to fit, and seals well at moderate pressures. 90 Shore is harder, and better at resisting high pressure without being squeezed out of its groove.

Can polyurethane be used with water?

Polyurethane is not suited to hot water or steam, which can slowly break it down. For water and steam, EPDM is usually a better choice.

How do I join polyurethane cord?

For seals, join the cut ends with a suitable adhesive. For drive belts, polyurethane cord is usually joined by heat welding, which melts the ends together to form a strong, continuous joint.

How do I fit a polyurethane O-ring?

Clean the groove and remove any burrs. Lightly lubricate the ring with a compatible oil, then fit it without twisting. Harder 90 Shore rings are less stretchy, so take extra care when fitting them over threads or sharp edges.

How do I identify the size I need?

Check the BS number if the ring is a standard size. Otherwise, measure the inside diameter and section diameter of the old ring with callipers, or measure the groove.

How can I tell if a polyurethane seal needs replacing?

Warning signs include:

• Leaks from hydraulic cylinders, valves, or fittings
• A flattened ring that has lost its shape
• Nibbling or chipping on the edge where it has been squeezed into the gap
• Cracks or a sticky, softened surface from heat or water
• Wear or scoring on moving seals