High Power Density Speed Reducers: A TRIZ Based Classification of Mechanical Solutions

Abstract

This paper aims to create a conceptual map of problems and solutions concerning High Power Density Speed Reducers (HPDSRs), i.e. planetary gearboxes, cycloidal gears and harmonic drives. The existing designs of HPDSRs are explored and classified through the Problem Solution Network (PSN), i.e. a method based on the Network of Problems from the TRIZ base of knowledge that considers different levels of abstraction. Through the PSN, it was possible to highlight conceptual design differences and communalities among the various HPDSRs in order to clarify the working principles of existing solutions. HPDSRs carry out the speed reduction through components that perform planetary motions. Therefore, a first distinction has been made based on input and output motions. Cycloidal and harmonic solutions have as output the rotation motion of the planet while planetary gear trains have as output the revolution motion of the planetary pinion. A second classification has been made on the strategy for avoiding the secondary path of contact, i.e. the unwanted contact between two components outside of the expected contact area. Cycloidal solutions modify the tooth profile while harmonic solutions deform the planetary pinion. Further considerations have been made on multi-stage solutions that take into account differential principles to multiply the useful function.