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20.14 Lubrication

Depending on the application, plaslic gears can operate with continuous lubrication, initial lubrication  or no lubrication. According to LD. Martin lnjection Molded Plastic Gears, Plastics Design and Processing, 1968; Pert 1, August,pp 38-45; Part 2. September,pp.33-35);

(a) all gears function more effectively with lubrication and will have a longer service life; 
(b) a light spindle oil (SAE 10) is generally recommended as are the usual lubricants; these include silicone and hydrocarbon oils, and in some cases cold water is acceptable as well ; and 
(c) under certain conditions dry lubricants, such as molybdenum disulfide, can be used to reduce tooth friction.

Ample experience and evidence exists that substantiates plastic gears can operate with a metal mate without the need of a lubricant so long as the stress levels are not exceeded. It is also true that in the case of a moderate stress level, relative to the materials rating plastic gears can be meshed together without a lubricant However, as the stress level is increased there is a tendency for localized plastic to plastic welding which increases friction and wear. The level of this problem varies with the particular plastic type.
A key advantage of plastic gearing is that for many applications running dry is adequate. When a
situation of stress and shock level is uncertain, using the proper lubricant will provide a safety margin and certainly will cause no harm. The chief consideration in choosing a lubricants chemical compatability with the particular plastic. Least likely to encounter problems with typical gear oils and greases are: nylons, delrins, phenolics, polyethylene and polypropylene. Materials requiring caution are: polystyrene, polycarbonates, polyvinyl chloride and ABS resins.
An alternate to external lubrication is to use plastics fortified with a solid state lubricant. Molybdenum disulfide in nylon and Delrin are commonly used. Also, graphite, coloidal carbon and silicone are used as fillers.
In no event should there be need of an elaborate sophisticated lubrication system such as for metal gearing. If such a system is contemplated then the choice of plastic gearing is in question. Simplicity is the plastic gears inherent feature.

20.15 Inspection

In view of the compliance of injection-molded gears, the dimensional accuracy of such gears is determined by a variable-center-distance fixture (see Section 18.1). This type of gear testing is both functional and utilizes a much more consistent measuring contact force than an over-wires measurement. In view of the hygroscopic nature of plastic gears, care should be taken to minimize dimensional changes between inspection and use of the gear.

20.15 Molded vs. Cut Plastic Gears

Although not nearly as common as the injection-molding process, both thermosetting and thermoplastic plastic gears can be readily machined. The machining of plastic gears can be considered for high-precision parts with close tolerances and for the development of prototypes for which the investment in a mold may not be justified.
Standard stock gears of reasonable precision are produced by molding blanks with brass inserts, which are subsequently hobbed to close tolerances.
When to use molded gears vs. cut plastic gears is usually determined on the basis at production quantity, body features that may favor molding, quality level and unit cost Often the initial prototype quantity will be machine cut, and investment in molding tools is deferred until the product and market is assured. However, with some plastics this approach can encounter problems.