Development of a Component Level Test Methodology to Validate the Transmission Bush of a Manual Gear Box

In the era of fierce competition, launching a defect free product on time would be the key to success. In a modern automobile, the transmission system is designed with utmost care in order to transfer the maximum power from engine to driveline smoothly and efficiently. Optimized design of all the transmission components is necessary in order to meet the power requirement with the least possible weight. This optimization may require gear designs with different internal diameters. The assembly of these gears may not be possible on a solid transmission shaft. To facilitate assembling while retaining optimum design of transmission parts, a separate bush is designed to overcome this limitation. Some bushes may require a flange to restrict any free play of the mounted gear in its axial direction. During complete system level testing of one newly developed manual transmission, bush failure was observed. Bushes are generally press fitted on the transmission shafts, on which the needle roller bearing is mounted. In some cases, the free axial movement of the gear is restricted by the hub or shaft itself. But in other cases, due to assembly constraints, the bush itself is flanged to restrict the axial free play of the gear. When the respective gear is not in engaged condition, the bush does not get any axial thrust. Once the gear is engaged, due to the axial thrust caused by the helical gear, the bush experiences a certain amount of axial thrust. This axial thrust was suspected to be the reason of failure. A component level test set up maintaining the boundary condition was made and axial load was applied on the bush using a hydraulic actuator. The failure mode was simulated consistently, and design modifications were carried out. Modified designs were also tested using the newly derived test methodology and the modified design with the required durability life was adopted for implementation. The complete system level durability test was again performed with the modified bush and no failure was reported thereafter. The component level test methodology helped to test a number of design iterations and a number of samples within a stipulated time and cost. This methodology can be used in all future projects as a part of front-loading support before performing complete system level test.