Axial seals

Axial seals are arranged with one sliding friction partner seated axially against the other. The contact surface in this case is a flat annular ring that separates the inner and outer spaces. In this case, the partners can shift radially in relation to each other - within limits - without impairing the seal\'s effectiveness. The seal is taken up to compensate for wear by shifting one or both of the rings axially. Because this does not effect the annular geometry, hard materials can be used. Generally, axial seals are referred to as mechanical seals.

A pair of seal rings consists of one stationary and one rotating ring, both with extremely smooth contact surfaces (<1µm). This makes the seal gap around the entire periphery small enough to prevent leakage regardless of the relative positions of the rings. The rings themselves are sealed against suitably designed housing parts with static secondary sealing elements.

Because one ring is stationary and the other turns, friction is created. The amount of heat generated depends on the friction conditions in the seal gap and the shaft speed. Additional thermal loading can be applied if the product being sealed is very hot. Frequently, therefore, it is necessary to augment the tribological properties of the friction partners with appropriate lubrication.

Creation of a lubricant film
In the case of lubricated seals, it is necessary to provide mechanical relief by building up hydrostatic or hydrodynamic pressure. Depending on the pressure difference to be withstood and the shaft speed, the design can adapted and the right fluid selected to produce an unchanging hydrodynamic pressure level in the seal gap. In an initial approximation, the pressure pattern can be assumed to be linear, see Fig. 2.

The hydraulic balance ratio K is the ratio between the surface area A2 exposed to the pressure to be withstood and the common sealing surface A1 of the two seal rings, see Fig. 3. It is an important criterion for assessing the hydrostatic closing forces that oppose the opening forces in the seal gap and prevent the seal rings from lifting off.

A distinction is made between mechanical seals where K>1 (in actual practice between 1.1 and 1.2) and balanced mechanical seals where K<1 (usually between 0.6 and 0.9). The lower the ratio K, the higher the security against thermal overloading, but measures still have to be taken to prevent the sealing surfaces from lifting off.

The pressures in the spaces to be sealed act on the seal rings, producing a certain pressure pattern at the sealing points. Additional forces are created by friction at the O-ring, on the sealing surface, and at the antirotation stop. If the pressure differences acting on the sealing surface are low, the closing forces are also supported by spring action that may or ay not be dependent on the direction of rotation. Dependable operation of the seal is ensured if the closing forces are always slightly greater than the opening forces.

The above forces and the temperature gradients in the components deform the seal rings. These deformations affect the geometry, which alters the pressure pattern in the seal gap. To ensure unchanging conditions at the seal face, very rigid seal rings made of hard material are therefore essential - especially where the sealing duty is highly demanding.

Parameters affecting seal selection
the selection of seal configuration, materials, and peripheral devices depends on a number of parameters. the pressure and temperature of the medium being sealed and the diameter and rotary speed of the shaft must be taken into consideration if one wishes to arrive at a reliable seal design. A figure frequently used for evaluation purposes is the product of pressure and velocity at the seal face, or "p x v".

Other factors to be considered are the seal\'s mounting position and product properties such as chemical attack, tendency to agglomerate, and solids content. Where slurries or products with high solids concentrations are involved, these product properties often have a fundamental influence on the seal design.