How Are the Standard Sizes of O-Rings Determined?

The standard size of an O-ring is defined by the dual parameters of inner diameter d1 and cross-sectional diameter d2 (wire diameter). All mainstream standards specify sizes based on these core dimensions, along with supporting tolerance requirements and serialization rules for specifications. The specific determination rules and mainstream standards are set out as follows:

1. Core size benchmark

All standards first establish a standard series for the cross-sectional diameter d2 (wire diameter) (e.g., 1.8, 2.4, 3.1, 5.7, 7.0 mm, etc.), and then match the standardized values of the inner diameter d1 to the wire diameter. In addition, d1 increases by a fixed step to avoid disorganized specifications.

2. Size marking rule

The general marking format is d1×d2 (inner diameter × wire diameter). The outer diameter (d1+2×d2) may be marked in some scenarios, yet the core of the standard still takes d1 and d2 as the reference.

3. Tolerance specification

Different standards stipulate the upper and lower deviations of d1 and d2 according to size ranges (e.g., small inner diameter/large inner diameter, thin wire diameter/thick wire diameter) to ensure interchangeability.

Mainstream O-Ring Size Standards

Chinese National Standard GB/T 3452.1: It is the mainstream standard in China. The wire diameter is divided into a narrow series (1.0~4.0 mm) and a wide series (5.7~12.0 mm). The inner diameter is adapted to the wire diameter, covering conventional industrial working conditions.

American Standard AS568: It is the most globally universal standard (predominant in the hydraulic and pneumatic fields). Numeric codes correspond to a unique combination of d1×d2 (e.g., AS568-010 corresponds to 1.78×1.78 mm). The core wire diameters include 1.78, 2.62, 3.53, 5.33 mm, etc., suitable for European and American equipment.

Japanese Standard JIS B2401: It is divided into Type P (general purpose) and Type G (precision). Its wire diameter and inner diameter series have slight differences from the Chinese and American standards, suitable for Japanese-made equipment.

International Standard ISO 3601: It is highly aligned with GB/T 3452.1, serving as a unified global basic specification with consistent core size series.

Key Supplementary Note

Standards clearly specify the matching rules for the minimum groove size. The d1 and d2 of an O-ring must be adapted to the inner diameter and width of the installation groove (the compression ratio is generally 10%~20%), which is also a supporting design requirement for size standardization.