The phenomenon that the medium flows from the interior of the limited space to the outside, or the inside of the limited space does not want to occur from the outside. The medium flow is leaked through the interface of the inner and outer space. The root cause of the leak is due to the existence of the contact surface, and the pressure difference and concentration difference between the two sides of the contact surface are the driving force for leakage. Due to the influence of factors such as the cover form and processing accuracy, there is an inevitable gap on the cover surface, which will cause incomplete coincidence of the cover and leakage. To reduce leakage, it is necessary to maximize the contact surface, that is, reduce the cross -sectional area of the leakage channel, increase the leak resistance, and make it greater than the leakage driving force.

Pressing loads to the seal to generate compression correspondence, which can increase the degree of contact with the cover surface. When the stress increases to suffering from obvious plastic deformation of the surface, it can fill the clearance of the cover and block the leakage channel. The purpose of using the pads is to use the pad material with the characteristics of the compressive load, which is more likely to produce the characteristics of plastic deformation, so that it fills the tiny unevenness of the French cover, thereby achieving sealing.