What compression set is

Compression set is the permanent deformation an O-ring keeps after being held compressed for a long time. When you install a ring it is squeezed into a rectangular gap; ideally it would push back forever. In reality the elastomer slowly “forgets” its round shape and stays partly flattened, so it pushes back with less force. When that residual force drops below what the joint needs, the seal leaks — often first as a weep during a cold start or a pressure drop.

It is one of the most common causes of gradual, age-related seal failure, distinct from sudden damage like extrusion or installation cuts.

How it is measured

Compression set is tested to standards such as ASTM D395: a sample is compressed by a fixed amount, held hot for a set time, released, and its recovery measured.

Compression set % = (t0 − t2) / (t0 − ts) × 100

Here t0 is the original thickness, ts is the compressed (spacer) thickness, and t2 is the thickness after recovery. 0 % means the ring sprang all the way back; 100 % means it kept the full flattened shape and lost all recovery. A good high-temperature seal material might show 15–25 % after 70 hours at its rated temperature; a poor combination can exceed 50 %.

What causes it

  • Heat. The dominant driver. Every material has a temperature above which its set climbs steeply. Running NBR near or above 120 °C is a classic cause.
  • Time. Set accumulates with hours under load, quickly at first and then more slowly.
  • Over-squeeze. Excess squeeze raises internal stress and speeds up set. More squeeze is not “more sealing” — past the ideal band it shortens life.
  • Chemical attack and swell. A fluid that swells or degrades the polymer accelerates set, which is why material compatibility matters as much as temperature.

Why it makes seals leak

A static O-ring seals because it is held in compression and keeps pushing outward against both sealing faces. As set accumulates, that stored energy bleeds away. The seal can hold at steady temperature but weep the moment conditions change — a cold start shrinks the metal faster than the hardened rubber can follow, opening a momentary gap. Thermal cycling therefore exposes compression-set problems long before a constant-temperature test would.

How to prevent it

Pick the right material for the temperature. This is the biggest lever. FKM holds far lower set than NBR at high temperature, and FFKM lower still for extreme heat and chemicals. Match the material’s rated band to your real duty, including short excursions.

Material Compression-set resistance Best use
NBR (nitrile) Moderate; poor above ~120 °C Ambient oil and hydraulic
FKM (fluoroelastomer) Good to 200 °C Hot oil, fuel, chemical
FFKM (perfluoroelastomer) Excellent, extreme heat Aggressive chemical, high heat
VMQ (silicone) Excellent recovery, low strength Wide temperature, static

Design the squeeze into the ideal band. Enough to seal, not so much that stress and set climb. The radial static calculator reports squeeze across the tolerance stack so you can keep it in range.

Control temperature and dwell. Where possible, keep continuous temperature below the material limit and avoid long static soaks at peak heat. If the duty is unavoidably hot, move up a material grade rather than push a cheaper polymer past its limit.

Frequently asked questions

Is compression set the same as creep? They are related but not identical. Creep is continued deformation under constant load; compression set is the deformation that remains after the load is removed. Both erode sealing force over time.

What is an acceptable compression set value? It depends on the application, but for a hot static seal, keeping tested set below about 25 % at the rated temperature is a common target. Lower is better for long service life.

Can more squeeze compensate for a material that takes a set? No — extra squeeze raises stress and usually makes set worse. The right fix is a material with better set resistance, sized to the ideal squeeze band. Compare the FKM and FFKM material pages when heat is the problem.