A variety of materials has been and will be used for the manufacture of special gears and thus a blanket recommendation cannot be made for the lubrication of all nonmetallic gears. While the concern here is lubrication, it is of interest that these materials tend to have less mass per unit volume than the ferrous metals and thus the resulting gearing has less inertia of rotating parts. Further, injection molded gears are about as low in cost as any gears. AGMA 250.02, Dec. 1955, makes the following statement:
“The gears should be lightly loaded if lubrication is not permissible.”
Certain small gears, particularly if made from “Teflon” or “Nylon,” are run without lubrication. However, Carlyon^9, when considering plastics for bearings, found that: “Even small amounts of oil present during typical manufacturing processes often will improve” friction and PV ratings. It was also shown that the continuous presence of oil raises the PV values of “Nylon” and “Delrin”. Graphite is sometimes used as filler in plastics and may thus aid in lubrication.
Medium sized nonmetallic gears are generally used in mesh with metal gears. Thus, “Bakelite” spur gears, which are made by impregnating fabric with “Bakelite” resin and then applying pressure and heat, are used with metal gears. Laminates may also be made of paper impregnated with phenolic resins. Heavy oil or lubricating grease is used as the lubricant on such pairs and the amount and frequency of application depends upon the severity of service.
Rawhide pinions disintegrate when in contact with mineral oil and therefore, a mixture of tallow and graphite can be used for lubrication of such gears in mesh with metal gearing. Such pinions furnish noiseless, resilient service with little vibration and prolong the life of the metal mate.
Pinions can be made from most materials which can be molded or cast, often in conjunction with other material. Thus, “Fabroil” pinions are stated to be made of pressed cotton held under compression by steel shrouds. The manufacturer of nonmetallic gears should be in position to recommend suitable lubricants for the same.
Plastic gears are sometimes used in small gear pumps, and here the material being handled, such as water, petroleum products or solvents, may serve as a lubricant.
Undoubtedly the use of gears made from plastics will increase both as to number and types. While it is not the function here to treat of their characteristics, some of the qualities should be recognized in applying lubricants. The fact that these synthetic materials have lower specific gravities and consequently less inertia than metals indicates less impact when gear teeth mesh. Also plastic gears tend to have greater elasticity than metal; therefore, they should conform to a mating gear without the necessity of a thick lubricating film. Since some of the non-metallic gears are thermoplastic they will not be used under conditions of elevated temperatures; consequently, high temperature lubricants will not be required.
The solubility characteristics of plastics will vary with the type and a lot of gears made from such materials can advise as to any restrictions in fluids to be used with specific plastics. Bueche and Flom^7 who investigated “Surface Friction and Dynamic Mechanical properties of Polymers” found that water solutions of sodium stearate were satisfactory lubricants for either “Neoprene” or branched or unbranched polyethylene when in contact with steel. While “Delrin” can be operated in a bone-dry state, pretreatment with lubricating oil has been found to reduce its erratic operations and improve the abrasion resistance. The recommended pretreatment consists of complete immersion in a low viscosity oil for one hour at 250 degree (F) after which the part is wiped dry. Gears of this materials have been operated in contact with both oil and lubricating grease with no deformation or wear. Since the variety of types of plastics used in gears will increase, the safest procedure in lubricating such parts is to consult the supplier.
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