O-Ring Thermal Expansion and Extrusion Clearance Calculator
A seal that is perfect at the assembly bench can extrude or over-compress at operating temperature because the bore, piston and rubber expand at different rates. This calculator shows the change in extrusion clearance and squeeze across a temperature range.
Your numbers
Linear CTE in um per m per deg C.
Linear CTE in um per m per deg C.
AS568 2xx series is 3.53 mm.
Typical solid elastomers fall near 100 to 250. Use your compound datasheet.
Your results
Uses linear thermal expansion for uniform temperatures. Fluid swell, hardware temperature gradients, rubber modulus change and tolerance stack-up are not modelled.
Squeeze at temperature vs operating temperature
Sweeps Operating temperature from -40 to 300, holding your other inputs fixed.
Automate seal checks from drawing data
An optional session on reading drawing and item data into a private assistant for design rule checks.
Optional. The tool and every export work without this. No spam.
How to use it
Set the assembly and operating temperatures, bore and piston diameters, and pick the material of each part. Enter the O-ring cross-section and groove depth, and the elastomer linear expansion from the compound datasheet. The primary outputs are hot diametral clearance and hot squeeze. The sweep chart plots hot squeeze from cold to 300 deg C so you can see where the design leaves its window.
- Aluminum housings with steel pistons open up fastest as temperature rises.
- Use the same CTE for both parts to confirm the clearance stays constant.
- Run the chart at your minimum temperature too; cold shrinkage can drop squeeze.
The formulas
Linear expansion is L(T) = L0 x (1 + alpha x deltaT), with alpha in parts per million per degree C. Diametral clearance at temperature is bore(T) minus piston(T). Elastomer cross-section and groove depth grow with their own coefficients, and squeeze is recomputed as (CS hot minus depth hot) divided by CS hot. Rubber expands roughly ten times more than steel, so squeeze rises with temperature even when the hardware is stiff.
What the numbers mean
With the defaults, a 50 mm aluminum bore, 49.9 mm steel piston and FKM-like rubber at 150 deg C, clearance grows noticeably as the bore outpaces the piston, while squeeze climbs as the rubber outgrows the groove. Extrusion risk scales with clearance, pressure and low modulus, so a hotter, softer material needs less clearance or a backup ring. Rising squeeze adds friction in dynamic glands and accelerates compression set.
Common mistakes
Do not mix imperial and metric inputs. Do not assume steel hardware everywhere, since many fluid power bodies are aluminum. And do not read the output as a replacement for tolerance analysis: the machined clearance at the extremes of tolerance, not the nominal, sets the extrusion gap you must survive.
Frequently Asked Questions
What is the coefficient of thermal expansion for O-rings?
Solid elastomers generally expand about 100 to 250 millionths per degree C linear, roughly ten times steel. The exact value depends on the compound, so use the supplier datasheet. Volume expansion is about three times the linear value.
How does temperature affect extrusion?
Extrusion risk rises with pressure, diametral clearance and falling material modulus. Heat usually lowers the modulus and, with aluminum housings, can increase the clearance too, so a gland that holds at room temperature may need a backup ring when hot.
Why does squeeze increase with temperature?
The elastomer expands several times faster than the metal groove, so its cross-section grows relative to the groove depth. That raises squeeze and gland fill, which increases friction and compression set unless the groove is sized with hot conditions in mind.
Which CTE values should I use?
Common handbook values are about 23.6 for 6061 aluminum, 17.3 for 304 stainless, 19 for brass and 11.7 for carbon steel, in millionths per degree C. Use your material certificate or supplier data for anything critical.
If your quote engineers keep reference limits in spreadsheets, we can explain how to expose material and temperature limits to a quoting assistant that checks them against the ERP item and drawing.
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