Rubber O-ring is one of the most used seal components,which is commonly used in mechanism equipments for its nice structural and performance goodness [1], the performance of O-ring would affect the normal working of equipments,sometimes would cause magnitude accidents. The design of　　rubber seals involve solid mechanics, tribology, macromolecule material, fluid erudition, and manufacture technic of mechanism. So it is difficult to study the deformation and contact stress on the seal surface of the rubber seals while been installing and working. However with the development of computers, computation methods, material, and commercial finite element software, it is possible to study the significant nonlinear problems of rubber seals while been installing and working using finite element software, and some achievements have been acquired.

　　Domestic researchers have taken variety of methods to analyze the mechanical performance of O-rings, and gotten the stress distribution of O-ring on different conditions. Guoding Chen have analyzed the mechanical performance of O-ring when the O-ring is installing and the fluid pressure is 6.8 Mpa,using commercial software MARC / Mentat320 [3]. Zhengxing Zuo has studied O-ring using ANSYS, and obtained the deform maps of the O-ring on the condition that the oil pressure is 5 Mp and there is no oil [4-5]. Zhihong Zhou have analyzed the deform and stress of O-ring under different pressure ratio and oil pressure, and obtained the relationship between von mises stress, contact stress and the maximum contact stress.

　　The main role of O-ring is to prevent leakage of fluid, so whether there is leakage or the O-ring breaks, the O-ring would fail. As for the failure criteria of the O-ring, the most common failure criterion is that: the contact stress on the seal surface must be beyond the work pressure of the fluid, and the shear stress of the O-ring must be lower than the strength of the rubber material [5]. In the design and selection of O-rings,these two criterions must be satisfied. The paper mainly talks the fist criterion.

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