Fatigue Life Investigation in the Design Process of Metacone Rubber Springs

A case study is reported on the design of a Metacone mounting used as a primary spring fitted at the wheel axle box on a railway vehicle. In use the rubber layers are loaded in shear and compression and the Metacone mounting provides controlled complicance along three axes. Requirements for increased service life and dimensional limitations (implying high strain levels) have led to an ever closer focus on suspension spring design. (In the current example the general space envelope was 250 mm in diameter by 250 mm high.) Furthermore, as was the case here, end of life is often defined in terms of clearly noticeable cracks or changes in stiffness rather than final failure. The location, formation and early stage development of fatigue cracks are therefore key issues in such cases. Design methods involving finite element (FE) and fatigue life analysis (FLA) were used to redesign the Metacone. As a result the life of the Metacone undergoing customer-specified (single axis) fatigue testing was increased more than ten-fold.