Part 4 of Concept realization of an 4x4 Vehicle's Transfer case
- June 30, 2015
- Marcel Romijn
The last phase in the 4X4 Transfer Case project was to complete the housing design. In the third stage, as seen in the previous blog of the Transfer Case project (https://brace-automotive.com/en/blog/part-3-of-concept-realization-of-an-4x4-vehicles-t/), the concept model was shown and the ways of construction were illustrated. The main goal in this phase of the project was to prepare the final model for 3D printing and to make it in principle series production ready. Therefore, several design changes had to be made and improvements were implemented along with the making of the preliminary production and assembly drawings.
The concept phase was closed by a design review. In this review the pros and cons of the final concept model were discussed and the changes to still be made were specified.
The biggest issue with the concept design was that the ribs were unable to cast. For casting products, draft angles of 1.5 degrees are needed to prevent casting failures. In the length of the ribs, perpendicular to the split line, no draft angle was applied. That meant that the ribs were as thick on the flange as they were on the end of the housing. When removing the casting molds it will be nearly impossible to take them off without damaging the product. Therefore a draft angle of 1.5 degrees on the ribs was added to solve this problem.
Change in wall thickness
While the concept model had a general wall thickness of 5 millimeter, the final model does it with 4 millimeters. The requirements listed 4 millimeters as a minimum so a research was done to see in how far it was possible to realize this number. After several studies and inspections it was concluded that it was safe to use 4 millimeters for the wall thickness. Because the ribs serve as the main attribute to distribute the forces, the housing itself will be nearly free of forces. Other benefits of reducing the wall thickness are the decrease in weight and the reduction of costs due to material volume.
Addition of space for the shim rings
Because of casting and machining tolerances it never is possible to make the gear train fit to the housing perfectly. This is partially solved by the implementation of the shim rings. It basically is a washer to fill the ‘gap’ between the bearing and the housing in the horizontal direction. The space required to place them was added in this model.
All of the points were implemented in the new model, which was build in the same way the concept model was. For more information about this method, it is recommended to take a look at the previous blog about the Transfer Case project.
The next step in this phase was the making of the preliminary assembly and production drawings for the housing. An assembly drawing shows all the parts needed to put the Transfer Case including the gear train together. Production drawings are required for the tooling to be done correctly. These drawings contain the dimensions and geometry of the machining that is applied. For example; boring the holes for the bearings to match the dimensions desired. An important feature on these production drawings are tolerances and surface roughness requirements.
The performance and properties of any casted product are affected by the degree of roughness on the various surfaces of the product in question. With a higher surface smoothness the higher the corrosion resistance and fatigue strength will be. Also the friction between various components is reduced with a smoother surface finish. During the designing process, it is important to keep in mind that the model that is to be made must also be able to be produced. In order to achieve this, various tolerance studies were done on this model which eventually led to the production and assembly drawings delivered. An example of them is shown below.
With the final concept model completed along with the preliminary production and assembly drawings the designing phase is finished. The next step in this project is to have the whole product printed in 3D.