Engineering Design Guide for Gland Clearance

The gland clearance diagram gives a guide to the relation between hardness, pressure, clearance, and extrusion. This figure is based on NBR O-rings with a cross section of .139 inch (3,53 mm) without back up rings. When there is risk for extrusion use contoured hard rubber or plastic back-up rings. The results are based on tests at temperatures up to 70°C.

Note: for silicone and fluorosilicone O-rings reduce all the clearances shown by 50%.

The most effective and reliable sealing is generally provided with the diametrical clearance as shown in Table 3.B1a. The maximum allowable gaps are indicated for 70°hardness O-rings with different cross sections without back-ups for reciprocating and static seals. These values correspond to a pressure of ca. 1200 PSI (80 bar) (8 Mpa) at 70°F (21°C). When greater clearances occur, the diagram indicates conditions where O-ring seals may be used - depending on the fluid pressure and O-ring hardness. [See Table]