Fretting test rig with variable normal force

Fretting is small amplitude reciprocating sliding between surfaces, and it may quickly causes surface cracks, which can continue growing under cyclic loads, until the structure breaks entirely as a result of the fretting fatigue. Fretting can also produce hardened wear particles as a result of adhesive wear, which then accelerates abrasive wear. In this case, the community uses the term fretting wear. The design of heavily loaded contacts, susceptible to fretting, is a difficult task because there is no generally accepted design guide. More extensive fretting research is needed to create them. This paper introduces detailed design phases for a equipment (rig) for a variable normal force fretting test. Supporting high radial and normal forces such that there is minimal run-out between the specimens was the most significant design challenge. The combination of a hydrostatic radial bearing and elastic torque shaft was selected for the detail design phase based on FE-analyses, calculations, and overall evaluation. The frame of the test rig consists of the main frame, which supports mainly the normal force and two torque frames, which support torque cylinders. Many solutions, which were found to be working in the current "ring-ring" apparatus of Tampere University, could be utilized in the new test rig like the tapered connections of the specimens, the elastic rod of the torque lever, axial displacement plate, and contact pressure adjustment system. The designed test rig enables fretting tests with 0 Hz to 20 Hz cycle frequency so that normal and tangential force or displacement can be controlled independently of each other. The normal force cannot change from compression to tension dynamically, but the adhesive force of the contact can be measured by slowly increasing the tension force. The designed fretting test rig fulfills all essential requirements, which were set.

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