5.1 Influence of the elastomer on the fluid to be retained

It is first of all appropriate to consider the influence that the material constituting the shaft seal may exercise on the fluid to be retained. This is the case, for example, where the shaft seal comes in contact with foodstuffs or pharmaceutical products for which, also according to the prevailing legal regulation, a careful protection against any product alteration is mandatory. If we consider foodstuffs, even the faintest odor communicated to the food by the rubber may exclude it from the specific application. The choice of the most suitable material for retaining the fluid is therefore crucial.

5.2 Influence on the elastomer of the fluid to be retained

The material of the sealing lip is exposed to chemical and physical effects due to the nature of the fluid to be retained and the operating temperature. These effects may act in a combined fashion. An excessive temperature may determine a premature degradation of the characteristics of the material constituting the shaft seal, and an occurrence of chemical reactions between the additives contained in the oil and the elastomer. The consequences of these chemical reactions may be:
- a hardening of the sealing lip as a consequence of the losses of one or more components of the compound.
- a softening of the elastomer caused by the reaction with substances from the fluid to be retained, or by chemical breakdowns.
It is important to analyze the elastomer's behaviour in the presence of hypoidal oils (par. 5.2.1) and greases (par. 5.2.2).

5.2.1 - Hypoidal oils

The hypoidal oils contain additives (sulphur, peroxides) that get free as a result of a temperature increase, which can start chemical reactions within the materials constituting the sealing lip. This occurs in particular with the elastomers containing the same substances as the additives in the oils. The chemical reactions occurring in such cases determine a further reticulation of the elastomers with a consequent hardening and a resultant premature destruction from wear of the sealing lip. An example of this reaction is shown by nitrile rubber, which despite a maximum temperature limit of 120°C cannot operate above 80°C in the presence of additive-containing oils. If the operating temperatures while using hypoidal oils exceed 80°C, the use of a more heat resistant compounds, such as a polyacrylic (ACM) or fluorinated elastomer (FPM), must be considered.

5.2.2 - Grease

In a grease seal, the heat generated by friction during operation is dissipated with greater difficulty. This may cause some overheating at the sealing point, making it necessary to impose a limitation on the maximum allowable surface speed. For this reason, when the peripheral speeds reach 50% of the limit given for the particular type of compound, a verification of their effect by practical tests is normally in order before authorising the use of the seal.

Particular cases may demand the use of gaskets, lubricated by a grease contained in an internal recess of the seal, in contact with various fluids (of an aqueous nature). In these cases the overheating is limited by the cooling action of the fluid itself, so that the peripheral velocities may be higher. It is important to observe that the level of the liquid to be retained determines the heat-dissipating conditions and thus the temperature at the sealing point.

The temperature of the fluid to be retained exerts a fundamental influence on the lifetime and therefore on the efficiency of a shaft seal. However, it must be considered that the fluid's own temperature must be added to the over temperature caused by friction between the sealing edge and the shaft surface. The value of the over temperature depends on a number of factors, the foremost of which are:
- roughness of the shaft (see par. 6.2)
- surface velocity (see par. 5.4)
- working pressure (see par. 5.5)
- type of fluid to be retained (see par. 5.2.2)
- lubrication (see par. 5.6)
- shape of the sealing lip (see descriptions in par. 4).

V=  [p x d [mm] x n [turns/min]] / 60 x 1000

Because of the friction between the shaft surface and the sealing edge, this velocity causes a temperature increase of the fluid in the sealing area. This parameter must be kept in mind when choosing the suitable type of polymer constituting the shaft seal. Experience suggests that the maximum peripheral velocity limits should not be exceeded.

The maximum allowable peripheral velocities are a function of the shaft diameter, the revolutions per minute and the type of material of the sealing lip. For a more complete discussion of the mentioned parameters, please refer to the pertinent chapters.

The shaft seals of a standard form are not recommended for applications at fluid pressures exceeding 0.5 bar. If used at higher pressures, they are in fact subjected to a radial force acting in addition to that of the spring, leading to an excessive compression of the sealing lip on the shaft. This results in increased friction, production of heat and rapid wear. In extreme cases the lip may be even flipped over. Pressures from 0.5 to 6 bar demand the application of a "supporting ring", whose shape must precisely "marry" the configuration of the sealing lip (on request, the ROLF Technical Service will supply details on the size).

In addition to eliminating the use of a "supporting ring", these models are of a more compact size and easier to install. They may also be used for shaft seals working under pressurizing and depressurizing cycles. In the latter case, two opposing or RD-type shaft seals may also be used. We recall that it is always appropriate to fill the free space between the shaft seals at least partially with grease. All shaft seals used in the presence of pressures over 1 bar must be secured in the seating by a retaining system (seger rings, etc.) to avoid being expelled. For pressures over 6 bar we recommend consulting the ROLF Technical Service. In special cases and where the pressures do not attain the maximum level, a saw-tooth of appropriate size can be carved out in the housing.

The duration and correct functioning of a shaft seal depend on the quality of its lubrication. During operation it is essential to guarantee that the shaft seal is continuously in contact with the lubricating liquid most suitable for the expected operating conditions. If there is a possibility of running dry, even if only for brief periods, it is advisable to use a shaft seal fitted with a dust cover, while filling at least 70% of the cavity between the main lip and the dust cover with an adequate type of grease. A greater volume of grease may keep the lips in a raised position and generate leaks. The same procedure is also advisable in the presence of poorly lubricating fluids (such as water, lyes, etc.) and for crankshafts of two-stroke engine.

Important notes:

1) The lubricating fluid must circulate freely within the sealing zone. Bearings, bushings or miscellaneous supports must allow a free passage from the sealing zone to the discharge zone.

2) It is essential that any stagnating points or other hindrances to the flow of lubricant be avoided. Avoid flanges or ledges exceeding the thickness of the supporting steel sheet on the spring side. If this is not possible, arrange for appropriate discharges.

3) Carefully avoid overpressures, especially of a pulsating type. Always provide a vent.

4) Wherever possible or if necessary, screen off the shaft seals to avoid the turbulences induced by roller bearings.

5.7 Friction and power absorption

The friction generated by the sliding action of the shaft surface against the sealing edge leads to a dissipation of energy. To contain the loss of energy within proper limits, the following parameters must be correctly set:
- The geometry and the type of material constituting the sealing lip
- The tension of the Garter spring
- The speed of rotation and the degree of finishing of the shaft
- The viscosity of the lubricant
- The operating pressure.
The first two factors determine the radial loading of the shaft seal, meaning the force by which the sealing lip impinges on the surface of the shaft. The radial loading is a factor of fundamental importance for the shaft seal. Any excessive radial loading causes heavy friction and a resulting high absorption of power and rapid wear, while an insufficient radial loading will not provide a properly assured sealing action. Its value derives from optimising the requirements of seal safety and maximum reduction of power absorption. The absorption of power is not constant with time.

The values of power absorption can be gauged experimentally. 
For example, the power absorption of a shaft seal for a shaft of 100 mm. diameter, rotating at a peripheral speed of 10 m/sec, amounts to 0.19 kW or 0.26 HP.