Tooth root strength of worm gear shafts with concave and convex tooth profiles

Abstract The load-bearing capacity of industrial worm gearboxes is primarily limited by failure of the worm wheel due to the soft bronze material. While worm gear boxes with higher strength wheel materials like cast iron or steel are most used in low-speed applications, the usage of these materials is generally increasing for a variety of applications. Therefore, the load limiting part of the gearbox is shifting towards the worm shaft, which is usually only designed against deflection. Fractures of worm teeth have been observed occasionally in the past, but only simplified approaches for the stress calculation—if any—are provided in worm gear standards. This paper therefore aims to further develop existing methods from the authors by investigating the influence of the tooth profile on the stress in the worm tooth root. A detailed analysis of the stress in the tooth root using the finite element method is provided. To determine the fracture behaviour due to static overload, two geometrically different worm gear shafts with concave and convex flank profiles taken from an industrial worm gearbox are investigated. A single tooth of the worm shaft is loaded by the means of a tensile test machine until a fracture occurs. The theoretical stresses are compared with the observed fractures.