Abstract: An improved testing device for testing the tensile strength of sheet material, such as paper, including a first pair of vertically moving jaws mounted on a support and a second pair of vertically moving jaws hanging from the first support in a pendulum manner and adapted to be at a dead-center position with the second pair of jaws juxtaposed to the first pair of jaws and in contact therewith for gripping a sheet of material to be tested therebetween. Low pressure, large diameter, pneumatic cylinders are provided for moving the lower jaws of each pair upwardly towards the fixed jaws against a self-aligning cylindrical spring.