Development of Fretting Test Rig with Dynamic Normal Force

Abstract

Fretting is short amplitude reciprocating sliding between surfaces and it easily causes surface cracks, which may continue growing under cyclic loads, until the structure breaks completely as a result of the fretting fatigue. Often the damage happens unexpectedly, because the initiation and the growth of the cracks are difficult to observe. Fretting can also produce hardened wear particles as a result of adhesive wear, which then accelerates abrasive wear. In this case, term fretting wear can be used. The designing 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.

The current fretting test apparatus of Tribology and Machine Elements Research Group of Tampere University is based on annular flat-on-flat contact, constant normal load and reciprocating tangential slip created with an electric shaker. However, there is need to study larger contacts with dynamic normal loading. Based on these needs, it was decided to develop a new fretting test rig whose requirements were defined in collaboration with industrial partner. The designing of the new fretting test rig was carried out by adapting three phases of the generic development process model. The planning phase consisted of literature research and definition of requirements. The Quality Function Deployment method was used at the beginning of the concept development phase to evaluate the requirements and to create metrics for the performance evaluation of the test rig. Supporting high radial and normal forces so that there is minimal run-out between the specimens was the greatest design challenge. It was estimated that specimens with 38 mm outer diameter would be suitable for 400 MPa contact pressure test requiring 250 kN normal force. A function structure, six concepts and one combined concept were obtained as a result of the concept development phase. The greatest radial force caused by uneven distribution of friction was estimated so that concepts could be evaluated. The combination of hydrostatic radial bearing and elastic torque shaft was selected for the detail design phase based on FE-analyses, calculations and overall evaluation.

During the detail design phase, the examined range of the contact pressure was 24 MPa - 400 MPa and the outer diameter of specimens was 38 mm - 150 mm. The maximum required force was 250 kN and torque 23200 Nm. 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 apparatus, 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 thesis contains design procedures for three different types of hydrostatic bearings, one of which was used to design a hydrostatic journal bearing with an orifice restrictor. The maximum radial force causes under 18 μm eccentricity on the shaft. The designed test rig enables fretting tests with 0 - 20 Hz cycle frequency so that normal and tangential force or displacement can be controlled independently of each other. 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.