Patent US 4,073,007

A method and apparatus for measuring a mechanical characteristic of a band subjected to tension. A disturbance is produced, for example by a hammer, transversely of the band and there is measured the time that the disturbance takes to travel through a predetermined distance along the band. This measurement is related to the value of the tensile stress in the band. In this way planarity defects in the band can be detected by comparison of the times the disturbance takes to travel in different regions of the band.