Gears for demanding applications. A comparison between cast iron, wrought steels and P/M steels

摘要

The most important standards covering gear design are reviewed. Some small differences, mainly formal, are analyzed. The calculation procedures require the introduction of different parameters, which are determined by material, working conditions, stress level and type, geometrical features, surface finish and lubrication requirements. As a basic comparison, the material-dependent parameters are reviewed, for fully dense steels, cast iron, P/M steels. The most important properties, to be considered for a reliable and sound design of mechanical parts, are presented, with their typical ranges. For sintered - and heat-treated - steels the influences exerted on significant properties by porosity, uneven diffusion of alloying additions and production sequence, are discussed. A comparison with the most traditional ferrous materials systematically shows the advantages of a controlled lack of homogeneity, in cases of fatigue resistance requirements, provided that sintering bonds are properly developed and carbon content and distribution is achieved. Strength improvements linked to density increases are the ground for development of new processing methods, such as overall or selected densification. This is especially valuable when a controlled gradient of local properties is a desired result. Different porosity and microstructure patterns are presented and their corresponding influences on strength and performance features are analyzed. The effects of local changes in elastic parameters are examined for torque- transmitting teeth. The requirements of various gear applications enable us to classify some specific material choices and corresponding processing sequences, aimed at reaching the optimum balance between suitability for manufacturing, functional reliability and cost. The forming possibilities inherent of P/M processing when applied properly to gear processing allow for the overcoming of some geometric constraints that are required when machining steels. Some examples of improved design are presented. Finally, the prospects of future enhancements of P/M processing and materials for demanding gear applications are outlined.