Thermal Expansion of a Gearbox

The Problem

Problems with bearing wear were reported after exposure of a gearbox to temperatures of -40° C. It was thought that the problem may lie in the different thermal expansion coefficients present in the design, combined with an incorrect tolerance. It was decided that a thermal stress analysis of the component should be carried out in order to highlight any inherent problems.

The Approach

A 3D model of the component was generated and material properties, thermal boundary conditions and appropriate constraints were applied. In order to verify the solution, an assembled gearbox was taken down to the local meat factory and left in a deep freeze freezer with a temperature of -40° C for 24 hours. A thermal sensor and monitoring equipment was also fitted to the unit.

The Result

Following the analysis and after inspection of the frozen gearbox, the problem was located. Due to the position and location of bearings and the fact that they ‘shrunk’ much less at these temperatures than the gearbox, the area between the bearings became highly stressed and deformed. After reheating to room temperature, the ‘shrink fit’ was relaxed resulting in a loose bearing housing allowing the bearings to vibrate in their housing leading to bearing wear. A solution to the problem was found by increasing the tolerance of the bearing housing to close to its maximum limits. Upon cooling, the stresses experienced in the problem area were found to be significantly reduced and deformation was minimised.