In the manufacture of a sheet material, such as cardboard, the measuring of the flexural stiffness of the cardboard material is of critical importance to both end users and manufacturers. The flexural stiffness of the cardboard correlates directly to its strength and its strength determines the quality of the end product in which the cardboard material is used. Additionally, in the manufacturing process of this type of sheet material, it is advantageous to measure the sheet material's flexural stiffness during its manufacture in order to monitor and maintain a predetermined strength of the material and to intervene directly into the manufacturing process "on-line" at the first indication of a deviation from the desired strength. Present cardboard manufacturing methods employ a high-speed manufacturing process that manufactures cardboard in a continuous web at speeds of 500 meters/minute. Therefore, deviations from a quality standard must be recognized early and the manufacturing process adjusted or large amounts of undesirable material will be manufactured.
In a laboratory setting, a sample of cardboard material is tested as a transverse beam. The German Standard DIN53121 includes a two-point measuring method with a unilateral biasing and a load at the free end and a three-point measuring method that utilizes two supports that support the specimen to be measured from below with a force applied from above. This force is applied and increased until a selected bending is achieved. The force is measured and its value inserted into a formula to calculate the specific flexural stiffness of the material under test.
In a continuous manufacturing process the material web under test is moved through a similar apparatus. The difference with respect to the laboratory method is that the material under test is supported by two rollers. The force acting from above also is applied to the material under test by means of a roller. This force is measured by a load cell, implemented by an arrangement of inductive or capacitive strain gauges, by piezoceramic elements, or by accordingly-shaped semiconductor elements. The load cell is mounted at a stationary support, and the force measured by the load cell is converted into an analog electrical signal, which is evaluated by associated evaluation electronics. The deflection of the material web is done by means of an actuator which may be positioned between the stationary support and the load cell or between the load cell and the roller, respectively.
However, during the continuous manufacturing process the material web is also under an additional force imparted by the tension of the material web. When the signal of the load cell alone is evaluated, it cannot be recognized whether the measured force is due to the web tension, the flexural stiffness, or to both values.
FIG. 6, of European Patent 0 541 518 B1, shows an apparatus for determining the strength of sheet materials. The material web is guided over a circular-shaped supporting device, and a deflection device arranged in the middle of the support device deflects the material web. A load cell measures the force exerted by the material web onto the support device. However, in order to correctly calculate the flexural stiffness, other values, including web tension, must be inputted into a computer to correct the value read by the load cell.