Aircraft engines can be made more efficient by integrating planetary gears. In such an application, the planetary gears experience very high load cycles under fully reversed bending loads. Pulsator test rigs, which nowadays offer the possibility to perform UHCF investigations, can only be used for purely pulsating loading of gears. Therefore, for the investigation of the UHCF tooth root load carrying capacity under fully reversed bending load, a back-to-back test rig is required. Back-to-back test rigs usually have speeds of n = 3,000 rpm, which makes investigations in the UHCF range take a very long time. Therefore, a high-speed back-to-back test rig was developed.