Compared to their metal counterparts, plastic gears offer the advantages of less weight, lower inertia, and quieter running. They often require no lubrication or they contain their own internal lubricants. Plastic gears usually cost less, and they can easily be designed to incorporate other features, so fewer parts are needed in an assembly.
Many plastic gears are made from thermoplastic-based materials, though some use thermoset materials. Early thermoplastic gears (nylon or acetal) were limited to carrying low loads at low speeds. As new materials became available, designers used plastic gears in more demanding applications. But their strength and longevity were still no match for that of metal gears. Now manufacturers add fiber reinforcements and lubricants to thermoplastic resins to narrow the gap, further expanding the load capacity and endurance of plastics.
Reinforcements
Glass and carbon fiber reinforcements increase the load capacity of plastic parts. Compounded into a thermoplastic resin, these fibers also increase the rigidity, and in the case of crystalline resins, the temperature range. For example, nylon 6/6 with 30% glass fibers gives over twice the tensile strength, three times the stiffness (flexural modulus), and three times the heat distortion temperature, Table 1. Substituting carbon fibers in nylon 6/6 yields 25% higher tensile strength and more than twice the stiffness of its glass-reinforced counterpart.
Lubricants
Combining a lubricant with a thermoplastic resin produces a self-lubricating composite material that resists wear. PTFE (polytetrafluoroethylene) is the most widely used lubricant in thermoplastics, and it has a very low coefficient of friction (about 0.02). As mating gear teeth mesh, PTFE in the plastic gear leaves a lubricating film on the tooth surfaces. This film transfers to the mating gear, whether it be metal or plastic, thereby reducing wear of both gears.
Adding silicone to the thermoplastic also reduces wear and friction, but to a lesser degree than PTFE. When silicone is used with PTFE, it produces a synergistic reduction in wear and friction. Furthermore, new low-cost lubricants such as in Lubriloy R are being developed to bridge the performance gap between unlubricated and PTFE-lubricated nylon.
In nylon 6/6, these lubricants cut friction and wear, Table 2, which reduces heat generation and power requirements. Because the lubricants reduce heat build-up, nylon 6/6 can handle somewhat higher loads. But, it still has limited strength and temperature capabilities.
Combining these built-in lubricants with fiber reinforcements can improve the strength, wear resistance, and high temperature resistance of plastic gears.
Material selection
Many thermoplastic materials are suitable for gear applications. Some resist wear, operate at temperatures up to 500 F, and withstand corrosive chemicals. But the numerous combinations of resin, reinforcement, and lubricant makes the task of material selection seem insurmountable.
To narrow the list of candidates and avoid lots of time and frustration, compare the application requirements to the various selection factors, which include accuracy, chemical environment, mating gear material, and temperature, as well as mechanical properties (strength).
Accuracy. The shrinkage of molded plastic as it cools affects gear accuracy. With a basic thermoplastic material, this shrinkage is isotropic (same in all directions) and is easy to compensate for when designing the mold.
On the other hand, adding reinforcements makes the shrinkage anisotropic (different properties in different directions). Therefore, the manufacturer must thoroughly understand the shrinkage behavior of a material to mold an accurate gear. Amorphous resins, Table 3, have lower and more isotropic shrinkage than crystalline resins.
A secondary consideration is moisture absorption, which causes dimensional changes. Although most plastics are affected by moisture, gears molded from nylon 6 or 6/6 experience the most change.
Chemical environment. Chemicals can stress crack, or in severe cases, dissolve thermoplastic resins. Generally, crystalline resins resist chemicals better than amorphous types. Amorphous resins are susceptible to chemical attack by several aromatic hydrocarbons, and some experience stress cracking in the presence of oils and greases. In such cases, material suppliers may be able to suggest a chemically compatible grease or an alternate chemically resistant resin for the plastic part.
High temperatures accelerate chemical attack on thermoplastics. In severe cases, a chemical that is harmless at normal temperatures can stress crack or even dissolve a material at high temperature. Within each group (crystalline or amorphous), resins with higher melt temperatures tend to be more chemically resistant.
In a typical application of chemically resistant material, a self-propelled lawnmower uses two front drive wheels made of injection molded polypropylene with long glass fiber reinforcement, Figure 1. An internal gear is molded into each drive wheel. The long fibers provide higher fatigue strength than short fibers.
The polypropylene resists chemical attack by fertilizers and pesticides on the lawn, and it doesn’t rust. Its low moisture absorption helps maintain dimensional accuracy even in damp grass. Also, its lubricity lets the wheel run without grease that could otherwise collect dirt and other abrasive debris that may cause premature failure.
Mating gear material. Where a plastic gear is intended to operate with another gear, the other gear’s material is a crucial factor in selecting a suitable thermoplastic material. To narrow the list of candidates, consult appropriate thrust washer test data, such as in Table 2. Materials with higher wear factors than nylon 6/6 (see table) may wear excessively and so should be excluded.
Thrust washer data is available for a variety of thermoplastic composites running against steel and other thermoplastics. Composites with reinforcements and lubrication wear less than unfilled (no reinforcement or lubricant) resins, with a few exceptions (acetal).
Wear characteristics of one plastic running against another vary widely, even among materials with good lubricity. For this reason, general conclusions about what makes a good wearing combination are hard to find. The best advice is to consult the thrust washer wear data for plastic-on-plastic combinations, then conduct gear prototype tests with the chosen materials.
An example of a plastic-on-plastic application is a gear drive for a van door, Figure 2. The powered sliding doors in these passenger vans originally used high-cost, PTFE-lubricated nylon gears with low moisture absorption. Later, engineers switched to lower-cost 40% glassreinforced, self-lubricated nylon 6/6. This material provides acceptably low wear rates in a temperature range from 240 to 185 F, plus resistance to corrosion or chemical attack from road salt.
Service temperature. High temperatures cause most thermoplastics to wear faster, severely limiting material selection. Mechanical properties also degrade as temperature increases to the material’s melt point or glass transition temperature. Composites such as nylon and PBT have been used intermittently at temperatures up to 300 F. But their strength drops severely, particularly when exposed to high temperature for a long time. Base materials for most high temperature applications are limited to crystalline resins PEEK, PPS, and PPA, or amorphous resins PEI or PES.
An example of a high-temperature application is the gearing used in a cruise control system by a major U.S. automotive manufacturer, Figure 3. Designers chose a glass-fiber-reinforced, PTFE-lubricated PPS material for the 1/4-in. diameter worm because of its ability to flow into and fill the small mold. For the mating spur gear, they chose glass-reinforced, PTFE-lubricated PEI due to the need for accuracy. These gears operate under the hood where temperatures exceed 300 F.
Strength. The use of plastic materials for gears is hampered by a lack of sufficient strength or load capacity data. Also, the mechanical and thermal behavior of thermoplastics varies more with temperature than those for metals. This variation has frustrated attempts to interpolate needed values from available data.
Two major concerns are the tooth bending stress and the tooth contact stress generated by loads between mating gear teeth. Designers can roughly estimate the maximum bending stress using the Lewis equation (see Dudley’s Gear Handbook or Mechanical Engineering Design). This calculated value must be multiplied by a safety factor based on performance requirements (temperature and cycles), then compared to the tensile strength of the material. Most of the time, a value of 2 or more is used. The appropriateness of this factor can only be confirmed through prototype testing.
Likewise, contact stress can be estimated using the Hertz theory of contact stress between two cylinders (see previous references for bending stress). For satisfactory gear life, the contact stress must be less than the endurance limit of the material. Endurance limits are also seldom available for plastic materials, although they are usually about the same as the compressive strength.
Prototypes. Although expensive and time-consuming, the best way to ensure satisfactory life is to make prototype gears from the chosen material and test them under service conditions. Though prototypes can be machined from plastic rod and slab stock, the results may differ from a molded gear due to differences in surface finish, molded-in stresses, and accuracy. The only way to accurately predict how a plastic gear will perform is to test a molded prototype. For accurate results, tests should simulate service conditions as close as possible.
For example, accelerated testing at speeds higher than required for the application may cause higher temperatures that lead to rapid failure, whereas the gear may have had sufficient life at normal temperatures.
Static tooth load tests may be useful if the gear is not intended to run at stresses close to its endurance limit. In this case, a gear that withstands a static test load of 8 to 10 times the working loads can be expected to have a long service life.
Gear failures
If a gear fails despite all your efforts to select a good material, determining the type of failure is crucial to preventing future failures.
Failure modes for plastic gear teeth include adhesive and abrasive wear, pitting, plastic flow, fracture, and thermal cyclic fatigue.
Adhesive wear. Intermittent welding and tearing of small areas of the tooth contact surfaces is called adhesive wear. If the welding is microscopic, the wear is normal.
Keeping a gear lubricated, either externally or internally, reduces friction and wear. In plastic-on-plastic gear pairs, it usually helps if at least one gear contains PTFE. Using an external lubricant with PTFE-lubricated gears may not give as good a result because the grease may prevent formation of the PTFE film. However, PTFE-lubricated gears have a break-in period during which the wear rate is higher while the film forms. Therefore, a light external lubricant may slow this initial wear if it does not inhibit formation of the PTFE film.
In unlubricated gears, failure at the tooth contact surface usually occurs due to nonuniform or excessive wear. This type of wear causes friction and heat build-up that softens the material; and it reduces the tooth cross-section, which increases stresses due to tooth loading, Figure 4. These conditions usually cause smearing or breakage at the contact surface.
Abrasive wear. When hard particles get between the tooth contact surfaces, abrasive wear occurs. These particles may be wear debris from the gears, or dirt. This type of wear may also occur if one of the gears (usually metal) has a rougher surface than the other. The particles first penetrate the plastic and then “plow” off pieces from the surface. If a plastic gear is to be run against a metal gear, the metal gear tooth face should have a 12-16 min. finish for good wear resistance.
There is no way to design a plastic gear for increased abrasion resistance. Therefore, try to avoid abrasive wear conditions (for example, by designing the gearbox to let wear particles fall away from the gears).
Pitting. A fatigue failure, called pitting, occurs in the tooth contact area when stresses exceed the endurance limit of the material. If the loads that cause these stresses are repeated often enough, portions of the surface will fatigue and fall off. Pitting is generally independent of lubrication. It is rare in plastics, but can occur, especially if the gears are well lubricated and have low wear.
Plastic flow. High contact stresses, combined with the rolling and sliding action of meshing gears, causes plastic flow or deformation of the tooth surface. Because plastics are insulators and have low melting temperatures (compared to metals), they tend to melt and flow in situations where metal gears would score, Figure 5. The initial plastic flow is in the radial direction and may relieve itself. In more severe cases, flow occurs in the axial direction, which indicates that operating conditions are too severe and tooth failure will soon occur. Lubrication (internal and external) helps prevent plastic flow by reducing the amount of heat generated by friction.
Fracture. Two conditions cause fracture of a tooth: overloading (stall, impact), and cyclic stressing (fatigue) beyond the endurance limit of the material. These fractures generally occur at the root fillet and propagate along the base of the tooth, Figure 6. Fractures in lubricated gears are usually caused by overload. Fractures located higher on the tooth are usually wear related.
Thermal cycle fatigue. Both unlubricated and lubricated gears may fail due to thermal cyclic fatigue. Tooth bending stresses always cause some heat and, because plastics are good thermal insulators, this increases the temperature of the material. Such a temperature rise can reduce the strength of the material and cause pitch line deformation (tooth fold over).
Kevin R. Quinn is senior applications engineer, and Ed Williams is advanced applications engineer, LNP Engineering Plastics, Exton, Pa.