A force applicator assembly is disclosed to calibrate an in-situ force transducer (or load cell) in a force (load) applying test machine. The force applicator includes stationary member configured to be secured to fixed structure, a moving member, a load cell operably coupled to an end of the moving member, and a differential screw assembly connecting the moving member to the stationary member. A coupling assembly can be used to assure that only tension or compression loads are applied. The coupling assembly can be configured if desired such that no tension or compression loads can be applied. A method to calibrate an in-situ force transducer in a force applying test machine is also provided and uses a force generator and the coupling assembly.