Machine Design

What To Put On Part Prints For Plastic Gears

Dan Fritzinger
Mechanical Engineer
Grabill, Ind.

When creating a part print for a gear, the geometric drawing often becomes top priority. If you want a new plastic gear to perform to expectations, however, its print also needs some critical textual details. In fact, a well-drawn gear print often includes as much text as geometry. Novice designers frequently provide incomplete information on part prints, which hinders design, manufacturing, inspection, and assembly of the gearset. Complete part prints are important before building tooling, when engineers change the gear design to cut costs or improve performance, and when tool builders design and construct molds. Part prints also help when inspecting the gear for quality assurance or when troubleshooting on the shop floor.

Don’t limit details to the gear part. Information about the mating gear is also necessary but commonly excluded. Though each gear has its own part print, data about the mating gear may be required when checking the design prior to tooling building or when solving design problems. A good print leaves no room for interpretation during an inspection. Gear prints are no exception. It’s best to include as much inspection information as possible. Along with standard part-print information such as material, nominal draft and wall thickness, and standard tolerances, the following items should be included on a gear print.

Drawing of basic rack - A graphical representation of the tooth is invaluable for describing any tooth modifications.
Part number - for part and mating gears
Number of teeth - for part and mating gears
Diametral pitch - teeth/in. or mm/tooth
Standard pitch diameter - This is a theoretical reference value for the part gear.
Pressure angle - This is a nominal value when gear size and center distance are nominal.
Tooth form - Some tool builders have tooth forms that differ from AGMA standards. When this is the case, note it on the drawing.
Minimum and maximum circular tooth thickness - for part and mating gears
Minimum and maximum outer diameter - for part and mating gears
Minimum and maximum root diameter - for part and mating gears
Minimum and maximum operating center distance - This is the allowable mounting centerline for the mating gears in the actual application
AGMA quality rating
Tooth-to-tooth error - for the part gear
Total composite tolerance - for the part gear
Serial number - for the master gear
Test diameter - for the master gear
Number of teeth - for the master gear
Circular tooth thickness - for the master gear
Bore diameter - for the master gear
Diametral pitch - for the master gear
Pressure angle - for the master gear
Outer diameter - for the master gear
Minimum and maximum testing radius
Testing pressure
Measurement over wires - Minimum and maximum values
Diameter of measuring wires and number of wires
Shrink factor - Some engineers feel this is necessary although others feel a blanket value is not useful because gears shrink unequally in all directions.
Gear materia

2010 Penton Media, Inc.

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