What Is an O-Ring? Answers From a Trelleborg Engineer Who Learned the Hard Way

Every week, our application team at Trelleborg gets the same questions — from plant engineers and from people who just inherited a machine with a greasy binder of part numbers. I'm one of the people answering them now, but I didn't start here. For six years I maintained molding machines and hydraulic systems, and I made nine documented mistakes in that period. Roughly $38,000 in wasted parts and downtime. I joined Trelleborg in late 2022, and in Q1 2024 I finally turned those scars into a checklist. This FAQ is the part worth sharing.

What is an O-ring, actually?

One of our most-searched phrases is “what is a o-ring” — missing the n. Trust me, when you're holding a caliper in one hand and a phone in the other, grammar loses. An O-ring is a circular elastomeric seal with a round cross-section. It sits in a machined groove, and when the mating parts are tightened, the ring gets squeezed. That squeeze, usually around 10% to 25% for static seals, is what blocks fluid or gas from getting past.

According to ISO 3601-1, an O-ring is defined by its inside diameter and its cross-section. North American drawings often use the AS568A dash-number system instead, where the series number tells you the cross-section range. And here's the trap: a size written as “18 x 2.5 mm” means inside diameter 18 mm and cross-section 2.5 mm only if someone actually labeled it. I've seen “standard size” mean two different things in the same conversation.

Why do people search for “EPDM Trelleborg” instead of just EPDM?

Because EPDM is a family, not a single recipe. EPDM stands for ethylene propylene diene monomer, and ASTM D1418 classifies it in the M family because of its saturated backbone. Trelleborg formulates and compounds EPDM in-house, for sealing, damping, hoses, and custom molded parts, rather than buying generic rubber and cutting it into circles. That's why “EPDM Trelleborg” tends to show up when engineers want a compound with a documented formulation, not a mystery puck.

The material has excellent resistance to ozone, UV, water, steam, and weathering. What it doesn't handle is petroleum oil. That's the 2017 mistake. I compared operating temperatures, ignored the fluid, and sent EPDM seals into a hydraulic oil system. Within hours they swelled, softened, and extruded out of the gland. Peroxide-cured EPDM compounds are now common and generally beat older sulfur-cured grades on heat aging and compression set. What was best practice in 2020 isn't always the best choice in 2025, but the fundamentals haven't changed: check the fluid before you check the temperature.

What counts as a Trelleborg industrial hose?

Trelleborg makes a lot more than O-rings. The Trelleborg industrial hose range includes water suction and discharge, air, chemical transfer, food and beverage, steam, and abrasive materials. These hoses are built around the media they carry and the environment they live in — the tube has to resist the product, and the cover has to resist the world.

Hydraulic hose is a more specific animal. It transmits fluid power under high pressure, which is why hydraulic hoses are built and tested to standards such as SAE J517 and its 100R-series classifications. A general-purpose industrial hose can look like a hydraulic hose from three feet away, but swapping one for the other is a gamble. I lost that gamble in September 2022. I specified a 150 psi industrial hose for what I believed was a low-pressure slurry line. The pump's startup spike hit 200 psi, and a $3,200 order turned into a floor-washing exercise plus a one-week delay. The hose manufacturer wasn't the problem; my pressure calculation was.

So when someone asks about a Trelleborg industrial hose, my first question is always: what media, what pressure, what temperature, and what abuse will it see? That list has saved us far more than the September mistake cost.

What does the hydraulic hose symbol on a schematic mean?

If you expect a special icon for hoses, I understand. But the hydraulic hose symbol is more of a drawing convention than a complicated graphic. In circuits drawn to ISO 1219-1:2012, a flexible hose is usually shown as a continuous line with a bend or loop — the visual way of saying “this is a hose, not rigid tubing.” Solid straight lines are the main working lines. Dashed lines generally indicate pilot or drain lines, not hoses.

That last point causes real confusion. I've seen technicians mark every dashed line as a hose, and I made the same lazy assumption myself on an old drawing. The line type tells you the signal or function; the loop or curve is what tells you the connection is flexible. Also remember: the schematic symbol will not tell you the hose grade. A line labeled 100R2 or EN 853 2SN carries information that the graphic itself can't show. The symbol is a starting point, not a shopping list.

What's the most common O-ring mistake you see?

Two mistakes tie for first place. One is choosing the material before checking the fluid, which is how EPDM ends up in oil and NBR ends up in outdoor ozone service. The other is ordering a “standard size” without saying which standard, or without labeling the dimensions. I've written “18 x 2.5 mm” meaning an 18 mm inside diameter. The supplier read it as an 18 mm outside diameter with a 2.5 mm cross-section. Same words, different part. That mismatch cost about $890 in redo plus a week of delay.

Measuring an old O-ring with calipers is risky if the ring is worn, distorted, or swollen — it will lie to you. Measure the gland if you can, or use a proper cone-style gauge if you can't. And don't assume 70 Shore A is right for every job. For high pressure or large extrusion gaps, a harder compound such as 90 Shore A may be needed. When I compared a 70 Shore A seal and a 90 Shore A seal side by side in the same high-pressure groove, the softer one failed within hours. That contrast made something click: hardness is a performance specification, not a preference.

Does Trelleborg make plastic goblets?

I'll answer this directly, because enough people type “plastic goblets” and end up on our site: no, we don't. Trelleborg is a B2B business. We supply engineered polymer solutions — seals, hoses, and custom components — not finished consumer drinkware.

The reason a goblet search can point toward us is probably the polymer connection. Plastic goblets are typically molded from materials like polypropylene, polystyrene, or PET, and the injection molding machines that make them are full of our products: O-rings in the hydraulic clamp cylinders, hoses carrying hydraulic fluid, and sealing systems in the hot runner. We're behind the scenes, not on the shelf. So if you're buying goblets for a party, we can't help. If you're building or maintaining the machines that mold them, now we're talking.

What's on your checklist before you order seals or hose?

After the third rejected order in Q1 2024, I wrote the checklist that's now pinned to our team's board. Here it is, more or less:

1. Fluid compatibility first. Temperature and pressure second. Check both, not just the one that's easy to find.
2. Write the size in full: inside diameter x cross-section, in mm, or give the AS568A / ISO 3601 designation. Never write a bare number.
3. Confirm which standard your supplier uses. A metric “standard” and an inch-series “standard” are different animals.
4. Pick hardness based on pressure and extrusion gap, not habit. 70 Shore A is common; it's not universal.
5. Note whether the application is static or dynamic. Dynamic seals need different gland designs and often need lubrication.
6. When in doubt, measure the groove or use a cone gauge. A worn O-ring is the worst measuring tool you own.

Since Q1 2024, this checklist has caught 47 potential errors before they became purchase orders. That's the satisfying part of this job: past mistakes finally start working for you instead of against you.