O-Rings - Application Details

'O' RING GROOVE SIZE

All Seals CHEM-RINGS are designed to be used in all standard 'O' Ring grooves with the same housing dimensions and clearances as for standard elastomeric seals. Our recommendations for BS 1806/AS 568 Imperial Standard 'O' Ring grooves are tabulated below for the standard groove design show below.

SEAL EXTRUSION RADIAL CLEARANCE GAPS (D on above diagram)

If the radial clearance gap (D) between the sealing surface and the groove corners are too large and the pressure exceeds the deformation limit of the 'O' Ring, extrusion of the ring material can occur.

When this happens, the extruded material wears or frays with cycling and can cause seal failure.

There are two possible options to minimise the potential for 'O' Ring Extrusion;

Preferably, close the clearance gap (D) by modifying the shaft or housing dimensions, or use backup washers or other anti-extrusion devices.

Alternatively, use the harder, higher modulus Viton™ core, at the possible expense of higher friction and greater tendency to leak at lower pressure ranges.

INSTRUCTIONS FOR INSTALLING CHEM-RINGS

All surfaces that the 'O' Ring will contact in operation should be smooth and free from sharp edges, burrs and deep scratches. A 30 micro-inch finish is best recommended on the bore, 15 micro-inch finish on the shaft.
Lubricate the 'O' Ring and all contact surfaces with a clean light oil or grease.
3. If it is difficult to install the 'O' Ring over a shaft, it may be 'stretched' by first immersing the 'O' Ring in boiling water for a few minutes. This softens and enlarges the 'O' Ring and allows it to be fitted more easily. Install the 'O' Ring when it is still hot and it will return to its original size when cooled. Elongation at Break is 108 to 150% but it is strongly preferable not to stretch the 'O' Ring more than 20% for Silicone core and 7% for Viton™ core 'O' Rings.
4. Do not bend the 'O' Ring too sharply as the Teflon will tend to kink under the strain. If it does become kinked however, it will not effect the operation in most applications.

The degree of squeeze best recommended is;

15/22% for static applications
10/15% for dynamic applications
8/10% for pneumatic applications

The amount of squeeze required varies with many factors, most critically, the pressure to be sealed.

COMPRESSION SET

All Seals CHEM-RINGS resist hardening and embrittlement due to the TEFLON® FEP encapsulation. The 'O' Seals retained elasticity and recovery properties at temperatures up to 205°C., is due to the Silicone or Viton™ core material.

The results of compression set tests, which compare the retained sealing force of perfluoroelastomers, fluoroelastomers and FEP Encapsulated 'O' Rings can be observed in the following diagram.

The results of these tests demonstrate that, by combining the mechanical properties of Viton™ or Silicone elastomers and the chemical resistance of TEFLON® FEP/PFA resins, the CHEM-RING possesses the ability to resist compression set and retain its mechanical properties.