In many industrial gearbox applications, thermal rating is a key factor in the practical utilization of the gearbox. The thermal capacity is affected by the efficiency (power loss) of the gearbox and heat dissipation within the environment. Methods, as found in ISO Technical Reports (ISO/TR 14179), help to estimate quasi-stationary temperature in the oil sump.

To further increase the understanding, SKF has incorporated power loss prediction and thermal equilibrium in its validated simulation tool. It goes beyond ISO methods: it includes the interaction between heat losses, thermal expansions, and (bearing) pre-loading. With this simulation tool, all these interactions can be analyzed by solving the mechanical and thermal problem simultaneously.

When changing operating conditions (e.g., during starting up), temperature differences between rotating parts (e.g., shafts) and the exterior parts (e.g., housing) may differ considerably from the steady state conditions. These effects are extremely difficult to anticipate, or complex measurements would be required. The developed method allows engineers to perform transient calculation in which the momentary temperature differences (affecting the bearing loading) are taken into account. In this way, the whole gearbox behavior can be analyzed in more detail.

In the paper, the methodology used to obtain the interaction between mechanical and thermal equilibrium will be explained. Using the simulation tool capabilities, the performance of the bearings and gears in a multi-stage gearbox will be analyzed and presented.