Form the design basis for screw joints

This master thesis was performed at Scania CV AB in Södertälje at the engine development department, NMKA and at Scania Latin America in Sao Paolo, Brazil. Scania is a global company with operations in Europe, Latin America, Asia, Africa and Australia. Scania is the world’s third largest manufacturer for heavy trucks and heavy bus segment. The constant development of new design solutions demands more stable mounting in Scanias production. Assemblies with new screw joints with different parameters affect the value of the clamping force for the joint and consequently the final product. It is critical that the screw joint has the right clamping force to assure that it fulfils its purpose. Large deviations can result in total loss of clamping force and engine breakdown. The purpose with this master thesis was to find the most important factors that affect the clamping force and form the basis of how different screw joints should be handled. Knowledge about Scania screw joints were collected by working in the production, observing drawings, a thorough literature study and by interviewing representatives from different departments. The problems regarding screw joints were found to be rather different at the departments and the problems had been an issue for a longer period of time. The mapping of screw joints resulted in a number of interesting screw joints with different kinds of problems. A discussion about the interesting screw joints was held in order to select joints to take a closer look at. By focusing on area of interest, area of responsibility and resources, five screw joints were selected, rocker arm shaft, main bearing cap, connecting rod and the oil sump in both aluminum and steel. The chosen screw joints represented specific types of screw joints on which the chosen parameters were tested. The important parameters that were tested are driver speed, friction and settlement. The equipment that was used throughout the work was the Ultrasonic, which measures the length of a screw with ultrasound. The authors feel that the equipment has given reliable results and that it has been easy and convenient to use during the tests since time consuming strain gages were avoided. The test results shows that the speed of the nut runner has a great influence on the clamping force and torque. Depending on the nut runner and which kind of screw joint that is examined the clamping force and torque varies. The friction affects the result of the tightening substantially, especially when tightening by torque. If torque and angle tightening is used it is an advantage to keep the initial torque as low as possible but high enough to keep the surfaces tight together before applying the angle. The test results regarding settlement show that the settlement is extremely dependent on the ingoing parts in the screw joint. Parameters such as screw and clamped parts affect both the tightening procedure and degree of settlement. The authors recommendations are that a decision has to be made about what information is important for each department. Furthermore, as the tightening procedure is really important for the final clamping force it is advisable to decide which department that is responsible of making the correct tightening parameters of a specific screw joint in the production. Hopefully this master thesis has woken an interest which in the future can make a difference for an even more stable production and in the end an even better quality at Scania CV AB.Validerat; 20101217 (root