Abstract: A testing apparatus for determining dynamic strength of sample paper cores includes a rotatably driven pneumatic chuck for engaging the inner surface of a sample core to rotatably drive the sample core, and a variable-tension belt assembly having a flexible belt which is looped over the sample core and tensioned to simulate the load applied to a paper core in actual use. The belt is tensioned by a pneumatic cylinder which is controlled under feedback control by a servo valve and a computer controller so that the load applied to the sample core accurately tracks a predetermined desired load schedule versus time.