Topics within the text below: Molded Gear & Direct Gear Design Schematic Comparison Print Specification         Molded Gear Design     The very first thing to understand is: molded plastic gears are not like metal gears. They are not made the same way, the materials differ significantly, their failure modes are quite often different, and their print specification should be completely different. A number of articles in our download area deal specifically with the technical details of better molded gear design. Here we will present a capsulated overview of this subject. Direct Gear Design     Characteristics influencing metal gear design include: Metal gears are almost always cut with hobs. Their design is based on the hob that cuts the gear. Any modification to improve the strength or performance of the gear usually translates to necessary changes to the cutting setup, such as 'addendum modification'.     In contrast, characterists influencing plastic gear design include: Plastic gears are primarily molded, and their shape is not dependent on traditional hob designs. Plastic gear mold cavities are almost always cut with wire EDM or specialized machine tools. Because only one cavity has to be constructed for a specific gear shape, much more attention and cost can be applied to its development. Even more to the point, plastic gears need to be optimized to the greatest extent possible--any tool in the designer's arsenal to improve performance will greatly aid suitability to the application for this weaker material       The method we employ at Kleiss Gears for optimal molded gears is what we term direct gear design. A gear set is designed as a whole. The tip of the teeth of one gear mate at maximum material condition with the root of the mating gear, and vice versa. In effect, we design the gears at their maximum material condition, with the teeth built as long as possible for maximum contact and strength. The figure below shows the comparison between a standard AGMA specified gear design and a comparable Shape Optimized gear design. Comparison of Standard Gear Mesh to Custom Shape Formed Gears Standard Gear set Shape Formed Gear set Shared Attributes: Center Distance: .75 inches minimum Gear Ratio: 2 to 1 (24 to 12 teeth) Differences: 24 Diametral Pitch 20 Degree pressure angle .06545 Tooth Thickness (both gears) Max contact ratio: 1.15 Max center distance variation: +.022 inches .11865 Base Pitch 25 Degree operating pressure angle .070 pinion tooth thk’ns/.061 gear tooth thkn’s Max contact ratio: 1.59 Max center distance variation: +.032 inches Standard Gear set Shape Formed Gear set Print Specification for Molded Plastic Gears     Designing plastic gears is the first step, but assuring that they are produced correctly and measured accurately is just as important. Plastic gears are not cut, they are molded. All features on a plastic molded gear shrink from the cavity that forms the gear. In exaggerated terms, the cavity may measure 47.5 DP while the molded gear measures 48 DP. The Diametral Pitch (or more accurately, the base pitch) shrinks from the cavity to the molded gear. Therefore, these attributes must be treated as variables, not as unchangeable Basic Datums. Furthermore, the root geometry (of trochoid) of a directly designed gear must be generated as the design requires. At Kleiss, we have developed a gear data field that we believe accurately defines and controls the dimensions and quality of the molded gear. Our suggested data field follows:     The method of direct designed roots has been described in AGMA Standard 1006-A97 Appendix F. Hopefully, in the next year the above print specification for molded gears will be published in a new AGMA Standard for Specification of Molded Plastic Gears.

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