The design of octagonal ring dynamometers

The use of thin rings is considered as a means of measuring two orthogonal forces and a moment applied in the plane of the ring, with appropriately mounted strain gauge bridges. The fundamental formulae for the moment and strain distributions in circular, octagonal and extended octagonal rings are presented. Expressions are also given for calculating ring deflections or stiffnesses. A design equation for determining ring thickness is derived, based on maximum strain criteria for the ring material and on data from measurements of the strain gauge bridge sensitivities of six orthogonal ring dynamometers which have been used by other workers. A procedure is given for the design of extended octagonal rings in terms of geometrical parameters. Design curves are presented for the determination of an appropriate mean ring radius (r) and the calculation of ring thickness (t). Formulae are given for the calculation of the strain gauge bridge sensitivity to the applied orthogonal forces (F and P) and moment (M). The stiffness of the dynamometer and its natural frequencies are discussed. Dynamometers in which the ring cannot be considered to have a thin section (r/t < 3) are also considered. (C) 1995 Silsoe Research Institute.