Development of a test bench to study self-loosening of bolted joints

Common methodologies to study self-loosening of bolted joint are analytical, finite element, and experimental. Analytical and finite element methods are used to find, evaluate, and quantify this phenomenon while the experimental method is mostly used for validation and verification purposes. Most of the previous experimental studies on self-loosening of bolted joints have been performed with a test machine known as Junker Machine which uses a conventional load cell and a rotation sensor which change the geometry of the joint and loading condition. Inserting a load cell or a rotation sensor changes the clamping length and joint stiffness, which both are important factors in joint integrity and self-loosening behavior. Their non-use of load cell can greatly simplify the study of bolted joints and the investigation of other parameters. It is important to not alter the clamping length and stiffness of the joint when they are the main subject of study. In the design of the current test rig, the geometry of the bolted joint is maintained exactly the same as in the real application and in the finite element analysis used for validation. The designed test bench provides the possibility of studying bolted joints with different thicknesses of clamping parts in combination with different preloads and lateral imposed displacements. A digital data gathering system is put in place to provide the possibility of measuring and recording of bolt axial load (preload), imposed lateral displacement and resulting force, relevant rotation of the bolt and nut, temperature, time and number of cycles. In the first step, a clamping device was designed to accommodate the test plate clamping joints and its instrumentation. A universal joint has been attached to the lower plate to minimize undesirable bending force and moments. A finite element analysis of the new design has been carried out to ensure the integrity of the plate attachments which are the weakest point in the mechanism. The maximum capacity of all other standard mechanical parts is defined by their respective manufacturers and are compared with results from the analysis. In the second step, the driving gears and crank system have been repaired and modified to improve their performance to adapt with the new connections. In the third step, two mounting assemblies have been designed to hold the lateral displacement sensor and the rotation sensor. Both assemblies have been designed to accommodate different test settings using simple and low cost inserts. A digital magnetic counter is added to the system to record the number of cycles. A thermocouple has also been considered to measure temperature of plates in further studies. As the last step, all sensors have been connected and synchronized to a computer via a PCB. Data gathering system is visualized and controlled by LabView software. As the first phase of the project, a full 3D FE analysis of the joint has been performed and results have been compared to experimental test. In this step, a clamping joint consisting of two steel plates (with 10 mm thickness), two steel washers, a bolt and a nut has been evaluated. The results show that the developed machine is functional and can be used for the next step of the research.