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]
|Gland clearance in relation to hardness and 0-ring cross section|
|Cross section||Max. clearance 70 ° Shore A|
|.070||1,0-2,0||.002 - .004||0,05 - 0,1|
|.103||2,0-3,0||.002 - .005||0,05 - 0,13|
|.139||3,0-4,0||.002 - .006||0,05 - 0,15|
|.210||4,0-6,0||.003 - .007||0,07 - 0,18|
|>.275||>6,0||.004 - .010||0,1 - 0,25|
|Gland Clearance Table|
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