# Nonsymmetric U-cup Seal

A u-cup seal consists of two sealing lips which are forced towards walls as the pressure builds up. The major advantage of u-cup seals is that as the fluid pressure increases so does the force on sealing lips, enhancing the sealing capability of the seal.

The model consists of three axisymmetric parts: piston, housing and u-cup. The u-cup sits inside  the gland, cut into the piston. This is a challenging problems as it involves multiple nonlinearities: material nonlinearity, geometric nonlinearity and contact between multiple components. As the geometries and loads are axisymmetric, the problem is analyzed using an axisymmetric model.

The analysis of u-cup seal is performed in three steps. In the first step, interference is resolved and in the second step housing is moved downwards. In the third step, a fluid pressure is applied.

Note: This exercise is part of the Solving Non-linear Problems with Abaqus course.

(Note: Click on the figure for closer view)

As the geometries and loads are axisymmetric, the problem is analyzed using an axisymmetric model.

At the beginning of the analysis, an interference exists between seal and piston. During the first step, interference is resolved which results in stresses in the seal.

(Note: Click on the figure for closer view)

During second step, as the housing moves downward, it also pulls the seal downward.

A certain amount of contact pressure is required to maintain contact with the sealing surfaces. This is necessary to prevent fluid leakage.

In the third step, a fluid pressure is applied. As the fluid pressure increases on the seal, it is pushed upwards.

Extrusion occurs when fluid pressure forces the seal material into the gap between sealing surfaces. Extruded material can break away, creating leak paths. Furthermore, heat generated from friction can cause a seal to be compression set, shortening its life. Optimizing clearance gaps and selecting a proper material based on the temperature and pressure involved are necessary to reduce the risk of extrusion.

During the second step, as the housing moves downwards, the contact pressure builds on the lips of the seal.

As the fluid pressure acts on the seal, the seal is pushed against the sealing surfaces. As a result, the contact pressure increases significantly.

The following animation shows the evolution of von Mises stresses and contact pressure.