Zusammenfassung
Hydraulic cylinders are an important part of modern machinery. Especially where high forces are needed, with little space available, they are a good choice. Though hydraulic cylinders have been used for many years and a lot of research was done to improve them, it is still a challenge to reduce and avoid leakage.
To proceed on this subject, this thesis examined the tightness of polyurethane hydraulic rod seals with special respect to geometric changes of the seals and compared different methods to describe the tightness of the seals.
Long term tests were carried out to analyse changes of leakage and changes of the seal shape of PU lip seals. Seal profiles with different radii at the sealing edge and a second, flat edge on the air side of the sealing lip were examined using FEM. Some of these profiles were applied to real lip seals and tested in long term tests. During this long term tests the tightness was analysed with the determination of the leakage and of the pumping effect. Additionally, the absolute thickness of the oil film was analysed using Ellipsometry.
During the long term test the sharp sealing edge changed to a radius, going along with improving tightness. Also Seal profiles with rounded sealing edges showed better tightness in FEM simulations compared to seal profiles with sharp edges. Contrary to this, seals with sealing edges rounded before starting the long term test, leaked from the beginning.
Seals with a second, flat edge on the sealing lip showed a tilting effect of the sealing lip. With this, the pressure contribution between seal and rod can be influenced by the pressure lasting on the seal. Especially for cylinders driven in differential mode this should enable good possibilities to reach a thin oil film dragged out during outstroke and a high ability to pump oil back in the cylinder during instroke. That should result in good tightness. Tests with turned seals couldn’t reach that potential. In contrast, injection moulded seals where the lip was cut-off to reach a second, flat edge on the lip, showed a significant improvement of differential mode tightness.
The methods used to characterise seal tightness are all helpful and eligible for a comprehensive examination. Nevertheless the results of the different methods differ. Finally, the quality of a sealing system has to be proofed with a concrete test installation using guides, seals and wipers.
The leakage measurements of injection moulded seals showed temporal and local different values. After a phase with tight seals, leakage occurred, followed by a phase without leakage. Selective measurements of the pumping effect did not show leakage at all. Besides, the film thickness measured with the ellipsometer showed big circumferential differences.
Lip seals with a second, flat edge on the lip offer a good possibility to improve tightness for differential mode seals. Nevertheless, further work is still necessary to put into practise a seal for daily use. The ellipsometer measurements with big circumferential differences require a new understanding of what happens in the sealing gap. Till then tests and experiments will be essential to design seals
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