engineering fundamentals Introduction to O-Rings
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Find O-Ring cross-section diameter from gland design input.
Introduction
O-Rings are torus-shaped (i.e. doughnut-shaped) objects made from elastomeric compounds such as natural or synthetic rubber, and are used to seal mechanical parts against fluid movement (air or liquid). O-Rings perform their sealing action by deforming to take the shape of their cavity, after being oversized to guarantee an predetermined interference fit.

O-Rings are inserted into cavities defined as glands, and are typically used in one of two seal designs, axial or radial. These seal designs along with their gland geometries are shown in the following schematic:

 

An O-Ring is specified by its inner diameter, its cross-section diameter, its material hardness/durometer (typically defined by the Shore A hardness), and its material composition.

In order for an O-Ring to seal against the movement of fluid, it must be compressed when seated inside the gland. A standard set of design guidelines exist to determine the proper O-Ring dimensions for radial and axial seals of a given dimension.

O-Ring Symbol Definitions
Parameters used in the discussion of O-Rings are defined in the following table:
O-Ring Parameters
Symbol Parameter Description
ID Inner Diameter Diameter of the inside edge of the cross-section.
CSmax Maximum Cross-Section Diameter Upper bound on the cross-section diameter for a given set of input requirements.
CSmin Minimum Cross-Section Diameter Lower bound on the cross-section diameter for a given set of input requirements.
CStol Cross-Section Tolerance Manufacturing tolerance on the O-Ring cross-section diameter.
Cmax Maximum Compression Upper bound for the cross-section compression (in %) when the O-Ring is seated in the gland; used as a design input.
Cmin Minimum Compression Lower bound for the cross-section compression (in %) when the O-Ring is seated in the gland; used as a design input.
Radial Gland Symbol Definitions
Parameters used in the discussion of glands for radial seals are defined in the following table:
Gland Parameters for Radial Seals
Symbol Parameter Description
Bd Bore Diameter Inner diameter of the bore which confines the outer diameter of the O-Ring.
Btol Bore Diameter Tolerance Manufacturing tolerance on the bore diameter.
Gd Groove Diameter Minimum diameter of the gland which confines the inner diameter of the O-Ring.
Gtol Groove Diameter Tolerance Manufacturing tolerance on the groove diameter.
GW Groove Width Groove length in the axial direction; must be large enough to accommodate O-Ring axial expansion.

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