28.11.2007 | Autor / Editor: Dr. Manfred Achenbach / Dr. Jörg Kempf
Users increasingly look at the foreseeable lifetime costs of a planned investment when making their investment decisions. Total Cost of Ownership (TCO) is, in that regard, always also a question of how good the seal engineering is. How long do seals in fact last? The Parker Seals group has developed processes which not only investigate this question, but provide the answers. They make forecasting of lifetimes possible where previously they were impossible in this way.
Total cost of ownership (TCO) plays an increasingly important part in capital expenditure decisions. Producers of machinery, vehicles, plant and equipment are therefore not only expected to consider manufacturing cost efficiencies when developing new systems, but also the maintenance and service requirements that arise throughout the equipment’s lifetime. Sealing technology is an essential element of such considerations. The service life of seals is influenced, for example, by the chemical impact of ambient media, heat, (UV) light and pressure. Due to the numerous influencing factors and diverse operating conditions in the field seal life may vary greatly and is very difficult to assess. Nevertheless, the Parker Seal Group has developed viable methods to investigate and answer these questions. The use of non-linear finite elements programs and physical-chemical model concepts allows predictions regarding the ageing behavior of rubber seals to be made.
The advantage of a reliable investigation and prediction of a material’s behavior is that it provides an overview of the long-term properties of the material in the specific application conditions as early as in the development stage of components. This would also allow service life assessments to be made. Traditional methods for evaluating an elastomer’s potential as a static or dynamic seal use ASTM or other standard immersion tests. These tests involve immersing the material in a test fluid for a specific time. One then compare physical properties such as hardness and ultimate tensile strength, before and after immersion, and make a judgement as to the material’s suitability as a seal. Immersion testing certainly plays a role in screening potential materials. One, obviously, would not choose a material that is severely deteriorated in the immersion test. However, some physical degradation and volume swell can generally be tolerated. The problem is, elastomers are not tested in their final form — squeezed and with little surface exposed to the fluid or environment. Thus, immersion tests do not predict how long elastomers will seal in specific environments. Yet many industrial seals must remain serviceable for a long time in severe environments. That makes physical tests impractical. Fortunately, another method is emerging for evaluating the long term of elastomeric seals, namely numerical simulation using appropriate aging models.
Today, commercially available finite element (FE) programs can be used to investigate the “virgin” properties of elastomeric seals. However, ageing effects that would enable a predictive analysis to be made cannot be in extensio simulated numerically yet, as the influencing factors vary for every application and/or material. The elasticity of sealing components consistently degrades as a result of a chain cleavage of the polymers and the subsequent cross-linking caused by the influence of heat and oxygen. Ruptures in the network chains reduce the number of effective chain segments, which are the “pillars” of strength and elasticity. For these internal processes, it is necessary to find equations that are able to describe the processes, some of which are interlinked, depending on external variables such as temperature and pressure. The reason is that network density and structure change within the rubber and thus limit the usable thermodynamic properties of the elastomer seals throughout their service life.
Whereas the influence of low temperatures on the elasticity of rubber is reversible, the process of aging permanently reduces rubber elasticity. This means that aging increases the risk of leakage, particularly whenever factors that enhance sealing action, such as high temperature and system pressure, are absent, for example after a system shut-down. But not only age can impair the effectiveness of a seal. High-frequency pressure loads such as those typically occurring in hydraulic applications will ultimately reduce the elasticity of rubber as well.
The aging model described by Parker surpasses the conventional structural-mechanical concept and enables correlations with thermal and physico-chemical effects in elastomers. Diffusion and swelling of ambient media as well as the resulting chemical reactions that lead to changes in elastomeric behavior can be considered in the aging model via FEM.
The advantage of modelling the long-term performance of seals by means of numerical simulation is that it enables virtual “testing“ of concepts (material, geometry, service conditions) in the initial design stage by means of numerical models. This eliminates the need to fabricate prototypes at such an early stage and therefore reduces development cycles and costs. The resulting aging model largely corresponds to the properties of the system to be modelled, enabling product properties, which often escape identification in physical trials, or where such identification can only be achieved with a major effort, to be identified. The question, “What happens if .... ?“ can be answered quickly, reliably and cost-effectively by a binding answer regarding service life.
The design of suitable seals can therefore be supported by this type of modelling and numerical simulation. Based on calculations of forces and deformations the computer-aided simulation of operating performance helps to optimise the design prior to the fabrication of a prototype and to ensure reliable service up to the end of the seal’s specified usable life. The following example serves to illustrate this.
The O-ring in this example serves to provide sealing in a water-glycole mixture and uses an HNBR sealing compound. The seal is supposed to provide reliable sealing performance for 20,000 hours at 95 °C, as premature leakage would result in expensive downtime and high labor costs for the repairs. FEA predicts not only changes in sealing force over time but also the resulting compression set. Each image represents stress distribution in the O-ring cross section at various stages in its life. They show, clearly, a decay in stress over time. After 20,000 hr, the test ends and conditions approach initial values: 0-bar pressure and ambient temperature. Any remaining strain energy stored in the O-ring is too small to compensate for thermal shrinkage. A gap on the outer sealing surface leads to leakage. This example of computer simulation using aging data shows the advantages of computer-simulation versus traditional compression-set testing.
For this application an optimized fluoroelastomer (FKM) has been developed. It resists oils containing aggressive additive packages, water-glycol, as well as acids and lye. Using once again finite element modeling simulated behavior of the new compound is possible. Results indicated good sealing performance at 95°C and 2 bar, even after 20,000 hr of service. Field tests performed by the customer confirmed the predictions.
Conclusion: Nevertheless, elastomer seals continue to be subjected to development by means of experimental investigations. These are often time-consuming and costly. The simulation of service conditions can simplify this optimisation process and achieve higher cost efficiencies. Even though the prediction of seal life using numerical methods such as the Finite Elements Method (FEM) is not state-of-the-art across the board it offers invaluable advantages for the development of seals, both in terms of development cycles and the subsequent costs of ownership of systems equipped with such seals. Aging is predictable, at least for seals.
The author is the Head of Engineering and Analytical Services/FEA at Parker Hannifin GmbH & Co. KG, Prädifa – Packing Division, Bietigheim-Bissingen/Germany.
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