a torque applier for use in endurance testing a planetary differential aircraft propulsion gear box of the type having a pair of coaxial output shafts and a colinear input shaft wherein the test system also includes an identical slave gear box arranged nose - to - nose with the test gear box with output shafts of the test and the slave gear boxes connected for unitary rotation . the torque applier is disposed between the test gear box and a prime mover and permits straight - through connection to the test gear box input shaft . a pair of rotary actuators are mounted on the torque applier housing and are connected through combining gears to each of the test and slave gear box input shafts and apply opposite torsional moments to the input shafts while the shafts rotate thereby to simulate operational torque transfer through the test gear box .