Development and Evaluation of a Sealing Concept for Actuators of Extraterrestrial Rovers

Abstract

Robotic spacecraft on other celestial bodies are under the influence of extreme environmental conditions. Especially the aggressive dust is a thread for mechanical systems like actuators. Scope of this thesis is the rotational seal, which protects the interior from the environment. For the ILM38-actuators, which were developed at the German Aerospace Centre’s Robotics and Mechatronics Institute for deployment on Moon and Mars, industrial Teflon-seals were used, but with disappointing results. My first task was to research on the environmental conditions and which other sealing concepts were already developed for space rovers, as well as to find companies, which offer suitable seals. It turned out, that spring-loaded Teflon-seals are indeed the most promising approach. I selected two companies, ElringKlinger and GFD, which offered suggestions for the specified problem. Prior my thesis, a test bed was constructed to measure friction torque, temperature and the sealing quality of different seals with various shaft-surfaces. The trials should be conducted in vacuum, so my second task was to modify the testbed. One problem was that the included torque measurement device is not vacuum-proof. The whole test bed was separated in two parts, with the actuator and seal inside the chamber and the torque measurement device outside. Before the actual tests could begin, the measurement error induced by the transmission shaft needed to be determined. This data was not yet available, so I measured in experiments and wrote a correction program. After the phase of preparation was completed, three different seal types (the old seal and two new ones) with four surface coatings for the shaft (which provided different surface roughnesses for the aluminium material) were ready for testing: To enhance the precision of the results, three independent test runs were performed for each seal/surface-combination, utilising unused seals. It turned out that except two events, the leakproofness was always provided. However, after analysis of the results, the model from ElringKlinger performed best in comparison. This seal consists of a spring-loaded Teflon-seal with one sealing element and small dimensions. The second seal, the model from GFD, was similar in appearance, but possessed two sealing elements, which lead to a much higher friction torque. At two surface coatings, the friction was so high, that the tests were not possible at all. It also turned out, that a certain degree of surface roughness is necessary for operation, while to smooth or too rough surfaces showed worse results.