1. The hardness of the O-ring
O-ring material hardness is the most important indicator for evaluating sealing performance. The hardness of the O-ring determines the amount of compression of the O-ring and the maximum allowable extrusion gap of the groove. Since the nitrile seal of Shore A70 can meet most of the conditions of use, if there is no special description for the sealing material, the nitrile rubber of Shore A70 is generally provided.
2. Extrusion gap
The maximum allowable extrusion gap gmax is related to system pressure, O-ring section diameter and material hardness. Generally, the higher the working pressure, the smaller the maximum allowable extrusion gap gmax. If the gap g exceeds the allowable range, the O-ring will be squeezed out or even damaged.
3. Compression permanent deformation
Another indicator for evaluating the sealing performance of O-rings is the compression set of the selected material. Under the action of pressure, the O-ring as an elastic element produces elastic deformation, and as the pressure increases, permanent plastic deformation also occurs.
4. Pre-compression amount
The O-ring is installed in the groove, in order to ensure its sealing performance, an initial compression amount should be reserved. For different applications, the amount of pre-compression relative to the section diameter W is also different. Typically, it is about 15%-30% in static seals and about 9%-25% in dynamic seals.
5. Tension and compression
When the O-ring is installed in the groove, it is subject to tension or compression. Excessive values of tension and compression will result in an excessive increase or decrease in the cross-section of the O-ring, since 1% stretching correspondingly reduces the cross-sectional diameter W by about 0.5%. For hole seals, the O-ring is preferably in a stretched state, and the maximum allowable stretch is 6%; for shaft seals, the O-ring is preferably compressed along its circumference, and the maximum allowable circumference compression is 3 %.
6. O-rings are used as rotary shaft seals
O-rings can also be used as rotary shaft seals for low-speed rotary motion and short operating cycles. When the peripheral speed is lower than 0.5m/s, the O-ring can be selected according to the normal design standard; when the peripheral speed is greater than 0.5m/s, the phenomenon that the elongated rubber ring will shrink after being heated must be considered, so the sealing ring is selected. The above phenomenon can be avoided by making its inner diameter about 2% larger than the diameter of the shaft sealed by it. After the sealing ring is installed in the groove, the sealing ring is compressed radially, and the O-ring forms a small amount of corrugation in the groove, thereby improving the lubrication conditions.