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Nylon has good wear resistance, even when operating without lubricant. A major disadvantage is instability in the presence of moisture and humidity. Delrin* is similar to nylon in many respects, but is super or with regard to rigidity, dimensional stability, and resistance to moisture. Properties are listed in Table 1.28.
These comments and data apply in particular to gears machined from plastic stock. Alternately, a greater volume of plastic gears are produced by molding. This subject is covered in detail in Par. 20.2.

14.6 Application, and General Comments 
For large gears and power applications, the ferrous materials are used. The greater the load and durability requirements, the more essential are the high-alloy steels. Plain carbon steels are in common use for low-quality commercial gears.
An exception in the ferrous group are the stainless steels. These are predominantly used in the small-gear, fine-pitch instrument fields because of their corrosion resistance. For fine-pitch precision applications, stainless steels are excellent. Although the 400 series is easier to machine and can have superior properties as a result of heat treatment, the 303 type of stainless steel has reasonable machinability and offers superior corrosion resistance. In addition, when used in conjunction with aluminum housings, its coefficient of thermal expansion matches that of aluminum much better than the 400 series.
The aluminum alloys, particularly 2024-T4, are excellent instrument gear materials when used within their strength ratings. Aluminums have no value as a power gear material and should not be used beyond low-load instrument-type applications.
Bronze is excellent for worm gears through the full range from light loads to power applications. It is also appropriate far use in spur and helical meshes that have high velocity and/or significant loading.

Plastic materials are best suited for small gears of the instrument and light commercial product variety. Their poorer machining characteristics and greater instability make them undesirable for precision applications. Their quiet operation and minimal lubrication requirements render them particularly attractive far consumer products.
A summary of material features is presented in Table 1.29.

15.0 FINISH COATINGS

Thin finish coatings are often applied to metal gears for protection against the environment or for decorative purposes. The type of finish chosen is related to the material, corrosive conditions, and level of gear quality and precision.

Finish coatings on the active surfaces of gear teeth must accomplish their objectives without altering dimensions, profile, or surface finish. This limits coatings to thin coverings of oxides or a substance  that permanently adheres to the base, and not all are suited to extend over the active tooth surfaces.

15.1 Anodize

An excellent finish for aluminum gears is anodize. This is an artificially induced thin, but even and 
hard coating of oxide. The thickness of the coating can be varied by process control, and can be 
troublesome in the maintenance of close tolerances. Consequently, anodizing of precision aluminum
gears is usually limited to the gear blank prior to tooth cutting.

* Registered trade name of E.I. duPont de Nemours and Co.

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