Proper Squeeze
•Compression expressed as a percentage of the free-state
cross-sectional thickness:
(O-Ring C/S) - Gland Depth
(O-Ring C/S)
•Face Seal: 20-30%
•Static Male/Female: 18-25%
•Reciprocating: 10-20%
•Rotary: 0-10% |
Pressure vs Clearance
•Excessive clearance can result in seal extrusion
Eccentricity & Side Loading
• Too much squeeze on one side and not enough on the other or none at all
•Can open too wide a clearance gap and result in extrusion of one portion of seal
Pressure vs Clearance Gap Chart |
Gland Fill
•Gland volume vs. O-Ring volume
(O-Ring Volume)
(Gland Volume)
•About 25% void space or 75% nominal fill
•Need space in groove to allow for volume swell, thermal
expansion, and increasing width due to squeeze
•Narrower groove for sealing vacuum or gas
•O-Ring can get squeezed out into clearance gap or get squeezed in two directions |
Stretch
(Groove Diameter) - (O-Ring ID)
(O-Ring ID)
•General rule is 0-5%
•Excessive stretch can overstress material
•Thins cross section and reduces % squeeze
•% cross section reduction due to stretch about half of the % ID stretch |
Tolerances
•Have to be considered for O-Ring and the gland
•Affects squeeze and affects pressure rating
•Less a factor on larger cross sections |
Surface Finish
•Seal material must fill in voids in surface.
•Static surfaces
–32 rms to seal liquid
–16 rms to seal gas
•Dynamic Surfaces
–8 to 16 rms
•Too rough of a surface can result in abrasion or spiralling, even with a static seal |
Sharp Corners
•Can cause damage during seal installation |
Standard Sizes
The standard sizes used by allorings.com and
other major O-ring manufacturers in the United States are defined by Aerospace Standard
AS568A, Aerospace Size Standard for O-rings. That document, published by
the Society of Automotive Engineers, Inc. (SAE), groups most standard sizes under one of
five cross-sectional diameters (or series). Those standard sizes are also used for
most military specifications.
allorings.com offers O-rings in all 369
standard sizes, including 20 sizes for boss
seals listed under the 900 series.
Metric Sizes
Metric Sizes are available in most millimeter increments as
well as many fractions. Please contact us for pricing
on Metric sizes.
Non-Standard Sizes
Non-standard sizes 26"ID or
less in most materials can be produced with no tooling / set-up
charges. Please contact us with for pricing.
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