Engineering
- Why is an O-ring gland groove so much wider than the O-ring?
- How do I know which of the five standard O-ring cross sections to select?
- Will O-rings with a smaller cross section seal higher pressures than O-rings with a larger cross section?
- When should backup rings be used?
- What is the maximum stretch an O-ring should have?
Why is an O-ring gland groove so much wider than the O-ring?
The O-ring gland groove is so much wider than the O-ring because most O-rings expand. Most O-ring gland grooves are 35 percent wider than the cross section of the related O-rings. A typical static O-ring seal with 25 percent compression will expand about 20 percent in width. As a result of this expansion, there will be only 15 percent side clearance after the O-ring is installed. O-rings will take a compression set early if restrained on all four sides.
How do I know which of the five standard O-ring cross sections to select?
You should consider the following when selecting the appropriate cross section:
- Tolerances of gland. More precisely made grooves and surfaces can use O-rings with a smaller cross section.
- Space and wall thickness of assembly. Thin assemblies can use only O-rings with a small cross section.
- Warpage of gland surfaces. Plastic parts that can warp or deform under pressure require O-rings with a larger cross section.
- Surface finish of gland. Surface finishes that are rougher than standard will seal better when using O-rings with a larger cross section.
- Price. O-rings with smaller cross sections cost less.
Will O-rings with a smaller cross section seal higher pressures than O-rings with a larger cross section?
No. A harder (higher durometer) O-ring and a smaller extrusion gap will resist extrusion at higher pressure. An industry standard graph shows the maximum pressure rating for each combination of hardness and extrusion gap.
When should backup rings be used?
You may use contoured backup rings (Back-O's) with O-rings to give greater pressure ratings (higher extrusion resistance) when you cannot reduce the extrusion gap of the gland. Generally, a 70-durometer O-ring with a backup ring will have the pressure rating of a 90-durometer O-ring without a backup ring.
What is the maximum stretch an O-ring should have?
Usually, the maximum stretch an O-ring should have after installation is 5 percent to avoid reducing the life of the O-ring. However, in cases where the O-ring has a very small cross section compared to its diameter, 5 percent stretch will not keep the O-ring from rolling in the groove, making installation and sealing difficult. Using 10 to 15 percent stretch is usually a good compromise.
