Compression ratio and the relationship between the amount of stretch and permanent deformation

Update:13-10-2018
Summary:

The rubber of various formulations used in the producti […]

The rubber of various formulations used in the production of O-rings produces a compressive stress relaxation phenomenon in a compressed state, at which time the compressive stress decreases with time. The longer the use time, the greater the compression ratio and the amount of stretching, the greater the stress drop caused by the relaxation of the rubber stress, so that the O-ring is insufficiently elastic and loses the sealing ability. Therefore, it is desirable to try to reduce the compression ratio under the allowed use conditions. Increasing the cross-sectional dimension of the O-ring is the easiest way to reduce the compression ratio, but this will result in an increase in the size of the structure.

It should be noted that when calculating the compression ratio, it is often overlooked that the height of the section caused by the O-ring being stretched during assembly is reduced. The change in the cross-sectional area of the O-ring is inversely proportional to the change in its circumference. At the same time, due to the tensile force, the cross-sectional shape of the O-ring will also change, which is represented by its height reduction. In addition, under the surface tension, the outer surface of the O-ring becomes flatter, that is, the height of the section is slightly reduced. This is also a manifestation of the compression stress relaxation of the O-ring.

The degree of deformation of the O-ring section also depends on the hardness of the O-ring material. In the case where the amount of stretching is the same, the height of the section of the O-ring having a large hardness is also greatly reduced. From this point of view, a material having a low hardness should be selected as much as possible according to the use conditions. Under the action of liquid pressure and tension, the O-ring of the rubber material will gradually plastically deform, and the height of the section will be correspondingly reduced, so that the sealing ability is finally lost.

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