Abstract
The sealing system of a radial shaft seal consists of three components: the shaft surface, the radial shaft seal and the fluid. These kind of sealing systems have been used since more than 70 years. During the last decades the fluids have changed more and more. An increasing usage of synthetic base oils, which contain an increasing number of additives, can be recognized. The influence of these fluids as part of the sealing systems on the functional behavior of the systems is not known.
One goal of this thesis was to determine the influence of synthetic fluids and their additives onto the functional properties of a sealing system. These functional properties are the friction torque, the pumping rate and the wear of the radial shaft seal. All investigations have been done with radial shaft seals made from NBR and FPM. Another goal of this thesis was to quantify the wetting behavior of the fluids. Therefore two wetting properties have been examined: the contact angle and the wetting factor. The contact angle was measured with the fluids on the surface of the radial shaft seals made from both elastomers. Therefore wetting interactions between the fluid and the elastomeric surfaces were detected. The wetting factor was measured with the fluids on the flat surface of a sample made from steel. The wetting factor describes the change of the wetting area after a period of some hours. In this investigations the time was at least two to a maximum of 20 hours. There was also the possibility to do these investigations at different temperatures between 20 and 90°C. The wetting factor was used to represent the wetting interactions between the fluids and the shaft surface, made from steel.
Synthetic fluids have been used in all investigations. Their base oil is polyglycole or polyaphaolefine. Each fluid contains of a base oil and one single additive. Additionally the base oils without any additive have been used. In sum 19 different fluids were part of the investigations. The results of the investigations of the named functional und wetting properties show an influence of the base oils. Higher friction torques, pumping rates and wear of the radial shaft seal was measured with fluids based on polyglycole. The contact angles were also higher and the wetting factors were lower with this kind of base oil. Apart from the influence of the base oils, the single additives have shown an influence on the properties. Some additives have always an increasing influence on one property, others always have a decreasing influence.
The main goal was to describe all connections between the five functional and wetting properties. All results have been evaluated regarding to the different fluids. This final step was based on the results of the previous investigations, which had been done separately for every property. The first result is that the characteristic of every connection between two properties is similar, despite of the different elastomers.
In sum there are five different properties. Looking at three arbitrary properties, there are also three connections between them. Assumed that the characteristics of two connections are known, the characteristic of the unknown third connection must follow a constraint. All determined results have led to an internally consistent overall context. Because of this the results confirm each other.
The characteristics of the connections between the friction torque, the pumping rate, the wear of the radial shaft seal and the contact angle are equal. The characteristic of the connection of the wetting factor is opposed to all the named properties. While the friction torque, the pumping rate, the wear and the contact angle increases, the wetting factor decreases at the same rate. The characteristic of the connection of the contact angle and the wetting factor is opposed. Both properties are describing the endeavor of a fluid of wetting a surface. The higher the endeavor of a fluid is, the lower must be the contact angle and the higher must be the wetting factor. The opposed characteristic of their connection is conclusive. This result is remarkable because of the different materials of the surfaces. Different fluids have different endeavors of wetting a surface, but they respectively show the same endeavor on both surfaces which are made from different materials.
The friction torque and the wear are dependent on the thickness of the fluid film in the sealing contact. A high pumping rate should reduce the thickness of the fluid film. Finally the assumption can be formulated, that the endeavor of a fluid of wetting a surface is the opponent of the pumping rate. Then the thickness of the fluid film and at the end the friction torque and the wear of the radial shaft seal, are dependent on the wetting properties of the fluid.
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