The misalignment is undoubtedly a very serious problem as to different kinds of gears. The old problem is the common condition of misaligned mating parallel-axis (spur and helical) gear tooth surfaces. There is a solution which is new to molded plastic gears but old in machined gear practice. It is the modification of gear flanks by making them full thickness at mid-face-width and tapering them to each edge.
The above mentioned method, i.e. modification, extending over the full height of the gear tooth, is referred to as crowning. Misalignment of gear teeth is very important. Instead of the ideally parallel gear rotation axes, the axes may be intersecting in the same plane, or skew in different planes, or a combination of both.
Crown is commonly specified by the height of the circular arc spanning the width of the gear tooth in a direction perpendicular to the tooth surface. The full required crown is generally applied to only one of the two mating gears, preferably to the gear which will provide the greater shrinkage clearance to assist in the ejection of the molded part.
Let us see the crowning of many gears. First, let us see something about the crowning of machined gears. There is a long standing practice of crowning machined steel gears. The modification of metal tooth surfaces is generally accomplished by secondary operations. Crowned gears have been used in a great variety of gear transmissions. There is a need even when the transmission housing is of rigid metal construction with accurately machined features for mounting ball bearings with negligible clearances. In applications associated with molded plastic gears, the need is greater.
The second is the crowning of molded plastics gears. The recent introduction of crowned molded plastic gears has required significant development in new tooling and processing methods. The new tooling includes the construction of the mold cavity with varying cross-section, smallest at the ends and largest at the center of the face width. And the new processing covers the ejection of the molded gear while preserving the modified tooth surfaces. This requires the optimum control of the ejection timing so as to take advantage of the initial shrinkage and the limited elasticity of the still hot plastic material. This researched process is readily available for spur gears and some helical gears.