Machinery's Handbook, 31st Edition
2666
O-RINGS O-RINGS Overview
An O-ring is a one-piece molded seal that distorts when squeezed between two parts to fill a gap. Most O-rings are toroidal (a round donut shape with a hole in the middle) and made from highly resilient materials, such as thermoplastic elastomers. Some O-rings are made from materials that permanently deform in use, such as PTFE (polytetrafluoroethylene). To create the sealing geometry, an O-ring is contained in a cylindrical or planar groove cut into one part. An O-ring gland is composed of the groove in one part and a sealing surface in the other part. If the two parts fit together with no gap, the groove and gland depths are the same. From a sealing standpoint, it usually does not matter which part in the cylindrical or planar joint carries the gland or groove. Ease of manufacture and/or assembly normally drives that decision. When properly installed in a gland, an O-ring is slightly deformed, so that the naturally round cross section is squeezed dia- metrically out of round prior to application of pressure. This compression ensures that under static conditions, the O-ring is in forceful contact (or “interferes with”) the seal - ing surfaces of both parts enclosing it. The resiliency of the material ensures adequate sealing force at zero applied pressure and allows some dimensional variation within the gland. When pressure is applied, it tends to force the O-ring across the gland, causing the O-ring to further deform and increase the sealing force, as in Fig. 1(a). As additional pressure is applied, the O-ring deforms into a D shape and can flow into the clear - ance gap between the two parts, as in Fig. 1(b). When the applied pressure changes direction (sometimes referred to as cycling ), the O-ring can be forced back and forth within the gland. In such cases, a stiffer or abrasion-resistant O-ring, backup rings, or a second O-ring seal (one for each direction) may be needed. (Other seal types also are available that are better suited to pressure-reversal applications.) In gaseous ap- plications, the permeability of the O-ring material must be considered in estimating leakage rates and avoiding explosive decompression failure of the O-ring, particularly at high pressures.
Fig. 1. Effect of Fluid Pressure on the Cross Section of an O-Ring in a Gland O-Ring Standards and Designations.— O-ring size is based on two parameters: inner diameter and cross-sectional diameter. However, to completely characterize an O-ring, both the size and the material must be specified. Manufacturers often use different desig - nations to identify O-ring compounds. Table 1 provides some prevailing standards for general-purpose inch and metric O-rings and gland designs. Note that previous O-ring metric standards BS 1806, DIN 3771, and JIS B 2401 have been superseded by the ISO standards shown in this table.
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