Shore A hardness of O-rings: why 70 is the standard and when to choose another

On a seal drawing, two digits usually follow the material code: NBR 70, FKM 80, VMQ 50. That number is the Shore A hardness — and after the medium and temperature, it is the next figure that decides whether the ring survives in your application. A soft compound conforms to a rough surface and seals at low clamping force; a hard one withstands high pressure without being squeezed into the gap. In this article we explain what Shore A hardness expresses, why 70 ShA is the industry standard, when to go softer or harder, and why a ring's hardness changes in service.

What Shore A hardness expresses

Shore A hardness states the compound's resistance to indentation by a durometer tip — the higher the number, the harder the material. For elastomer seals, Shore A is the standard scale; hard plastics are measured on the Shore D scale, which you will rarely meet with O-rings. Common sealing compounds span roughly from 30 ShA, where the softest silicones sit, up to 95 ShA, where hard polyurethanes and carboxylated nitriles top out. A 30 ShA compound squeezes between your fingers like a soft rubber band; you can barely press a fingernail into a 90 ShA compound.

Important: hardness says nothing about chemical or temperature resistance. NBR 70 and FKM 70 share the same hardness but behave completely differently in media. Choose hardness only as the next step after medium and temperature — verify the medium in the chemical compatibility chart; the full medium → temperature → pressure → motion sequence is covered in our material selection guide.

Why 70 Shore A is the standard

If the drawing omits hardness, you will almost always be supplied a 70 ShA compound — our configurator makes the same assumption when you leave hardness blank. Around 70 ShA is the compromise that works best in static and ordinary dynamic applications: the compound is compliant enough to fill surface micro-roughness at standard squeeze — roughly 22 percent of the cross-section in a static groove and about 10 percent in a dynamic one — yet firm enough not to extrude into the gap at ordinary pressures.

What is actually stocked and produced matches this. NBR and HNBR typically come in 70, 80 and 90 ShA, FKM in 70, 75 and 90 ShA, EPDM in 60, 70 and 80 ShA, silicone in 50, 60 and 70 ShA, chloroprene in 60 and 70 ShA, and polyurethane in 85, 90 and 95 ShA. When you enter a size and material in the configurator without a hardness, it offers the most common values: 70, 75, 80 and 90 ShA.

Softer compound: when 50 to 60 ShA helps

Reach for a softer compound in specific cases: sealing rough, cast or slightly damaged surfaces; assemblies with low clamping force — thin-walled plastic parts, hand-tightened joints — and wherever the ring must follow larger manufacturing tolerances. A soft compound fills irregularities that a hard ring would only bridge with substantially more squeeze. The price is lower resistance to extrusion and abrasion — a soft ring does not belong in high-pressure hydraulics.

Mind that not every chemistry is available soft. Silicone (VMQ) is made from 30 ShA, NBR, EPDM and chloroprene from 40 ShA, fluorosilicone likewise from 40 ShA. FKM, however, starts at 60 ShA, FFKM at 65 ShA and polyurethane only at 70 ShA. If the application demands an extremely soft seal and fluorocarbon chemistry at the same time, you are hitting a material limit — the answer is usually a design change, not a hunt for a compound nobody makes.

Harder compound: high pressure and extrusion risk

As pressure rises, so does the force pushing the ring into the gap between parts. Too soft a material is a typical cause of extrusion: the rubber gets squeezed into the gap and the ring's edge looks nibbled. With a nibbled edge there are three proven fixes: raise the hardness to 80–90 ShA, choose a more abrasion-resistant material such as polyurethane, or support the ring with a PTFE back-up ring. You will find the same recommendations in our failure analysis tool.

A hard compound has its costs, though: it needs more force for the same squeeze, follows rough surfaces worse, and in a dynamic application can wear the counter-surface faster. If a back-up ring reliably solves the extrusion, staying at 70 ShA with support often beats jumping to 90 ShA without it. The groove matters too — a tighter gap and a correctly designed groove cut extrusion risk about as effectively as a harder material; run the ratios through the groove calculator.

Hardness by material: what is actually made

Every chemistry has its manufacturable hardness band. The general-purpose rubbers cover the widest range: NBR, EPDM and chloroprene (CR) from 40 to 90 ShA, HNBR and epichlorohydrin (ECO) from 50 to 90 ShA. Silicone (VMQ) is the softest — 30 to 80 ShA; fluorosilicone (FVMQ) covers 40 to 80 ShA. The fluoroelastomers start higher: FKM 60 to 90 ShA, FFKM 65 to 90 ShA and FEPM (Aflas) 70 to 90 ShA. The automotive specialities ACM (60 to 80 ShA) and AEM (50 to 75 ShA) sit in the middle. The hardest are polyurethane (PU) and carboxylated nitrile (XNBR) — both 70 to 95 ShA, built for abrasion and high pressures.

PTFE stands apart: it is not an elastomer but a rigid polymer with a hardness around 55 to 65, measured by a different characteristic. Compressed PTFE does not spring back, which is why it serves as a back-up ring or in special designs — not as a drop-in replacement for a rubber O-ring in an ordinary groove.

Hardness changes in service

The hardness on the drawing is the hardness of a fresh compound; in service it shifts, and the hardness change of a removed ring is a valuable diagnostic signal. An overheated ring hardens, embrittles and cracks radially; the surface tends to be glossy or baked. A chemically attacked ring, depending on the incompatibility, swells and softens — or hardens instead — along with changes in volume, mass or colour. In vacuum, volatile components, plasticisers above all, evaporate out of the elastomer: the ring hardens, loses mass and shrinks.

The practical takeaway for maintenance: if the removed ring is noticeably harder than a new piece from the bag, do not just replace it — find the cause. Hardening almost always points to exceeded temperature or an incompatible medium, and a new ring of the same compound will end the same way. Compare your find with the typical damage patterns in our failure analysis.

How accurate the stated hardness is

Hardness is measured with a durometer on a flat specimen of prescribed thickness, not on the ring itself — measuring on the curved, thin cross-section of an O-ring gives indicative values only. Allow for natural spread between production batches too. That is why our configurator accepts a hardness match within ±3 ShA when looking for an exact replacement and ±5 ShA when searching wider alternatives. The difference between 70 and 72 ShA does not matter in practice; the difference between 70 and 90 ShA does.

Frequently asked questions

What hardness does a standard O-ring have?

70 Shore A. Order without specifying hardness and you will get a 70 ShA compound — the right choice for most static and ordinary dynamic applications. Our configurator also uses 70 ShA as the default.

When should I choose 90 Shore A?

At high pressures, with larger gaps between parts, and wherever extrusion threatens — typically in hydraulics. The alternatives are a PTFE back-up ring next to a softer O-ring, or polyurethane at 85 to 95 ShA, which has the best abrasion resistance among elastomers.

Does higher hardness mean better chemical or temperature resistance?

No. Hardness is a mechanical property of the compound and has no direct link to chemical resistance or temperature range. NBR 90 fails in brake fluid just like NBR 70. Solve medium and temperature by choosing the material chemistry; pick hardness afterwards, by pressure and motion.

Can hardness be measured on a finished ring?

Only approximately — a durometer needs a flat specimen, and a curved cross-section skews the reading. When identifying an unknown ring, start from the dimensions via the size converter, the colour and the application; compare hardness at least by feel against a new piece of a known compound.

Pick the hardness in seconds

For a routine replacement, the configurator on the homepage is enough: enter the dimensions, the material and optionally the hardness — the default 70 ShA suits most applications, and variants in other hardnesses appear right next to it. Check your medium in the chemical compatibility chart, the groove dimensions in the groove calculator, and compare a damaged ring in the failure analysis. If you are still unsure — say, an unusual combination of high pressure and a soft compound — write to Filip; he advises from hands-on practice.