At first sight, the shaft seal may appear to be an article of utmost simplicity. However, it conceals a complex technology and encompasses a series of extremely delicate functional parameters. It must at this point be understood how these elements operate separately and jointly in order to provide a seal of top reliability.

This term indicates the arithmetic difference between the shaft diameter and the inner diameter of the free sealing lip. Its function is to maintain the lip in contact with the shaft, while exerting such a force on it as to guarantee a sealing action with a minimum of wear and wasted energy.

This terms indicates the force exerted by the sealing lip on the shaft. It is normally represented by the joint action of the forces generated by the Garter spring and by the interference. The measure of this parameter precisely defines the sealing capacity of a shaft seal. During its operation, the radial force actually decreases depending on the operating parameters (speed, temperature, pressure, quality of the fluid to be retained) which cause wear on the lip and physical-chemical changes in the compound.

Another essential parameter from a sealing viewpoint is the shape of the sealing lip. The main elements to be taken into account in this case are:
- the position of the fulcrum
- the angles of the lip (on the oil side and air side)
- the length of the diaphragm.

3.3.1 - Position of the fulcrum

The fulcrum is the point around which the lip may perform a flexing motion. Changing the shape of the lip allows one to vary the position of its fulcrum and consequently its flexibility as well as its range of action. These changes are fixed at the design stage and are based on the operating requirements.

3.3.2 - Lip angles

The angles on the oil side and air side differ substantially, due to their practically opposite functions:
- angle on the oil side: from 40° to 60°
- angle on the air side: from 12° to 30°
The angle on the oil side serves two purposes:
A) To cut off the flow of oil, by easing its centrifugation
B) To facilitate the installation of the shaft fitted with an acceptance chamfer, according to the DIN 3760 standard. The angle on the air side serves the purpose of a capillary sealing against the permeated oil that would tend to drip off when shutting down the machine. This detail also helps to provide a good seal during operation.

3.3.3 - Length of the lip

The absorption capacity for the vibrations of a rotating shaft also and above all depends, in addition to the parameters hereto examined, on the length of the lip. Within certain limits, for certain speeds and in the absence of pressure, its absorption capacity for radial vibrations is directly proportional to its length. On the contrary, the presence of pressure reduces its minimum value so as to offer less available surface to the pressure itself (also refer to the par. 5.3.2).

For standard applications the distance between the sealing edge and the horizontal axis of the spring may vary between a minimum of 0.1 mm and a maximum of 1.2 mm in the direction of the base face. The exact position is defined at the design stage, depending on the technical requirements, the material employed, the dimensions of the shaft seal and its configuration.