Investigation of different joint types for a multi-component calibration device based on a hexapod structure

The project for a new calibration device for multi-component forces and moments [1] is based on a hexapod structure. One of the main problems is the accurate transformation of the external loads into the single-component forces acting on the force transducers in the links. This transformation can be realized using joints at the ends of these links. The paper deals with two types of such joints: ball joints and elastic joints. The advantage of an ideal ball joint is that there is no mechanical response to the deformation. On the other hand, real ball joints always suffer friction, resulting from the pressure between the balls and the socket introduced by the transmitted force. The advantage of elastic joints is that they are frictionless. The disadvantage is that they have a mechanical reaction to the deformation: they generate bending moments which affect the force/moment transformation and the forces in the links. The paper shows experimental results for ball joints. For elastic joints, finite element results are presented which allow the estimate of the bending moment, which increases with the deformation.