Optimal Polymer Gear Design: Metal-to-Plastic Conversion
Recent achievements in polymer development have inspired a tendency to replace metal gears with plastic ones in many moderately loaded mass-produced gear drives. This metal-to-plastic conversion takes advantage of the benefits of plastic gears, such as low production cost, reduced weight and inertia, low noise and vibration, zero corrosion and electric current conductivity, and the advantages of the injection molding process in producing complicated multifunctional parts. However, a simple material replacement is insufficient for a successful metal-to-plastic conversion. Some polymer material limitations — low strength and wear resistance, low thermal conductivity that reduces maximum operating temperature, sensitivity to operating conditions (temperature and humidity), limited injection molding process accuracy — must be compensated for by innovative gear design. Unlike machined metal gears, which are typically constrained by standard tooth proportions and hobbing rack generation technology, a polymer gear injection molding process allows for a deep optimization of gear tooth geometry. Such optimization of plastic gears for a particular custom application, essential for a metal-to-plastic conversion, is comprehensively covered by the Direct Gear Design method.
The article describes the optimal selection of operating pressure angle and contact ratio to maximize load sharing between contacting tooth pairs, and root fillet optimization to minimize root stress concentration.
The article presents a numerical example of a metal-to-plastic conversion, comparing a standard steel gear pair to its replacement polymer gears, whose optimal design utilizes all the advantages of polymer materials and compensates for their limitations. It outlines basic guidelines for optimal polymer gear design.
Author: Dr. Alexander Kapelevich
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