Abstract: The impact energy of gears in meshing is produced by different excitation sources such as machining or mounting errors, time-varying stiffness, and damages in critical components. In order to find the relationship between contact interfaces and the vibration energy delivered to the gear-shaft-bearing-housing-casing multi-interfaces, the definitions of multi-interface, through which the vibration energy is delivered, are provided in this paper. The energy peak value and the energy root mean square (RMS) are investigated. The term energy-maintaining factor is proposed to illustrate the relationships between the contact interfaces and the vibration energy. Finally, a table of factors depicting the relationships between energy on multi-interface and energy-maintaining factor is obtained based on experimental studies. The results show that energy-maintaining factors and the energy relationships can be used to explain the decaying of shock vibration energy in mechanical transmissions.