Non-metallic squeeze gaskets

Description

Characteristics of non-metallic squeeze gaskets

Non-metallic type squeeze packing (rubber gasket) requires material selection at high and low temperatures, but because it is softer than other materials, it can be assembled with relatively little force and has good sealing properties. Types of non-metallic squeeze packings include seat gaskets, liquid adhesive gaskets, and O-rings. Similar to O-rings, there are D-rings and square rings for twist prevention and proper seating on the groove. The O-ring, which is the simplest representative of the squeeze packings, is introduced here to explain how to select and use it. There are many applications and types of fixed O-rings, and they have long been used in all fields, from household products to automobiles, industrial equipment and aircraft.

Properties required for O-ring materials

Since the sealing function of an O-ring is achieved by the stress generated by the crushing of the O-ring, it is basically required to have appropriate stress within the range that does not cause abnormal deformation. In addition, this property must not be lost during use. Synthetic rubber is the best material with such characteristics. However, it is difficult to cover all of these properties with one material, so it is necessary to use suitable materials for different applications. NOK offers a variety of materials to suit all conditions of use.

Types of NOK’s O-ring materials

The sizes of NOK-O-Rings (standardized products) are based on JIS, ISO, JASO, AS and NOK standards.

Types and characteristics of NOK’s O-ring materials

NOK prepares materials equivalent to JIS and JASO standards as standard materials, and a variety of special materials are available to meet a wide range of usage conditions. Typical examples are shown below. For other materials prepared for special applications, please contact the nearest sales office.

*Class 4E and Class 5 are standard materials for JASOF 404. * Material 'H' is NOK’s material symbol.

The squeeze and groove dimensions of O-rings

The squeeze is set at a minimum value of approximately 8% for sealing purposes, and a maximum value of approximately 30% for the compression set limit of the rubber material. For the dimensions of the grooves, please refer to the catalog.

Surface roughness of grooves

The table below shows the surface roughness of the parts of the O-rings that come into contact.

Use at high pressure (clearance and protrusion)

The O-ring exerts a sealing function due to the compressive stress of the material, and also exerts a self-sealing effect due to the pressure of the sealed fluid. Therefore, in principle, the limit of pressure may not be considered; however, in practice, there is always a gap between the O-ring and the fluid to be sealed, which may cause the O-ring to become incapable of functioning due to the pressure of the sealed fluid. When the groove clearance is expected to exceed a limit, it is recommended to use a backup ring. For the groove design and use of a backup ring, contact our sales office nearest to you. Back-up rings should be installed on both sides of the O-ring when pressure is applied from both directions, and one ring should be installed on the opposite side of the O-ring when pressure is applied from one direction. There are three types of backup rings: endless, bias cut and spiral. Endless is the most effective; however, bias cut and spiral are more convenient in terms of installation.

Chamfering holes and shafts

To prevent damage to the O-ring during installation, make sure to taper the corners of the cylinder and piston rod as shown in the table below. In addition, it is recommended to set the L dimension to (G + b) or more to prevent the O-ring from galling due to bent shafts at the time of installation.