This article explains the engineering design concepts behind injection-molded gears that make drives lighter, quieter, and less costly. As the experience base with plastic gears has grown, computer-aided-design (CAD) tools have advanced. The most powerful advantages of plastic gears may be the design opportunities they afford. Gear geometries overlooked by designers used to metal are often easy to mold in plastic, and they can reduce drive size, weight, and cost. The operating speed of plastic gears obviously impacts operating temperature. However, rapid-loading rates can also affect material properties. For some materials, the faster a tooth is loaded, the higher the effective modulus and strength. It is highlighted that designers of plastic gears should also pay special attention to shaft attachment. Bore tolerances naturally impact true center distances, sometimes resulting in loss of proper gear action. Problems with prototypes can also tempt gear designers to change resins—a costly mistake given the different shrinkage characteristics of various plastics. It is better to rework the tooth profile than switch the material, unless it is clear that the wrong material was chosen.
For mechanical engineers, plastic gears are a powerful means of cutting drive cost, weight, noise, and wear. Plastic gears also open new opportunities for smaller, more efficient transmissions in many products. So, what are the payoffs when using plastic gears in place of metal? Where do they make the most sense? How are they specified, and which resins are best? These questions are timely, as more engineers turn to plastic gears in higher-power, high-precision applications. Some current examples illustrate the possibilities.
When Maytag engineers designed their new washer transmission around plastic gears, they effectively eliminated the noise of steel gears. They also saved 13 lbs. and did away with 42 parts compared with a previous metal gearbox. Gears that are injection-molded from unfilled and fiberglass-reinforced Celcon acetal copolymer keep their strength and tight tolerances, even in an oil-bath transmission. They also demonstrate the long-term durability essential in an appliance expected to have a long service life.
Hewlett-Packard and molder UFE took plastic gears to new standards of manufacturing quality in the DeskJet 660 color printer. Acetal copolymer cluster gears were specified to comply with the high quality standards of the American Gear Manufacturers Association (AGMA) Quality Class Q9. The accuracy was necessary for precise paper movement to prevent “banding”—obvious skipped lines or overprinting. For 48-pitch gears, 1.25 inches in diameter, AGMA Class Q9 denotes total cumulative error (TCE) of just 0.0015-inch, and tooth- to-tooth (TTT) error of 0.00071-inch.
To improve the reliability of the “world washer” that’s manufactured in several countries, Whirlpool Corp. introduced a splined clutch, or “splutch,” containing a spline and gears molded in acetal copolymer. The low-wear epicyclic gear assembly lasts four times the projected life of the washing machine. It also reduces the number of moving parts by 20 percent when compared with earlier designs using metal gears.
Injection-molded plastic gears have come a long way. Historically, they were limited to very-low-power transmissions, such as clocks, printers, and lawn sprinklers. Today’s stronger, more consistent engineering polymers, and better control of the molding process, now make it possible to produce larger, more precise gears that are compatible with higher horsepower. For example, Whirlpool enhanced a 3/4-horsepower drive for another washing machine with a spin gear molded in fiberglass- reinforced acetal copolymer. The molded plastic gear cost about a fifth of what the original machined metal gear cost, and made the drive lighter.
As the experience base with plastic gears has grown, computer-aided-design (CAD) tools have advanced. For instance, CAD software can now optimize plastic gear designs based on temperature, moisture pickup, and other environmental factors. Ticona developed P-GEAR, the Plastic Gear Evaluation and Research tester, specifically to characterize gear resins in different service environments. The new data will allow design engineers to predict gear performance more accurately. And better predictions mean faster, shorter design cycles, since the development phase may be approached with greater confidence.
Payoffs in Plastic
Typically, gears are a means of positively transmitting uniform motion with constant drive ratios. Thermoplastic and thermosetting polymers have long provided alternatives to metals in low-powered, unlubricated geartrains. Gears machined from phenolics and other thermosets can be used at higher operating temperatures, and they are more resistant to lubricants that are generally required. However, injection-molded thermoplastic gears have better fatigue performance and, unlike those manufactured from thermosets, can cut manufacturing costs significantly compared with metal gears. Thermoplastics are now finding their way into applications demanding lubricated drives, higher horsepower, and higher AGMA quality standards.
For drive designers, thermoplastic gears offer multiple advantages over metal and thermosets. They enabled the maker of a gear motor drive for a convalescent bed to eliminate powder metal gears and reduce parts count from three to two. The acetal gears reduced noise, improved durability, and cut total drive costs by one-third when compared with the original design.
Injection molding is fast and economical compared with hobbing teeth in metal blanks. Plastic gears usually can be used as molded and require no finishing. Thus, they cost less in production quantities. The cost of plastic alternatives can be one-half to one-tenth that of stamped, machined, or powder metal gears, depending on the manufacturing technique. For example, the manufacturer of a damper actuator for heating, ventilating, and air-conditioning systems calculated that 14 acetal copolymer gears in the gear train cost half as much as comparable metal gearing.
Plastic gears are also inherently lighter than metal. The specific gravity of steel is 7.85, while the specific gravities of glass-reinforced nylon 6/6 and low-wear acetal copolymer are close to 1.4. However, differences in specific gravity alone are not direct indicators of weight saving. For example, to transmit the same power, plastic gears must usually be larger than metal gears. However, once tradeoffs in size and power are made, plastics can lend themselves to smaller, lighter drivetrains as well as innovative gear designs that may not be possible in metal. One case is split-path planetary drives that are rarely considered by designers because they demand greater numbers of expensive metal gears. With inexpensive plastic gears, compact, split-path transmissions can actually be less costly than multistage, single-branch spur drives.
Quiet and Smooth
Low coefficients of friction associated with acetal copolymer and other engineering plastics help minimize gear wear. Lower friction also means less horsepower wasted in heat. Maytag estimates that its cooler-running plastic transmission reduced heat rise by 10 to 15 percent when compared with previous metal drivetrains. Greater efficiency can be important in light of anticipated future U.S. Department of Energy standards for appliances.
Oil-bath or grease lubrication enables drive designers to exploit the added strength of glass-reinforced plastic gears without excessive wear. A major automotive supplier, for instance, eliminated squeaks and wear in motorized car seats by replacing metal seat-adjuster gears with those molded in acetal copolymer compatible with lubricants.
Self-lubricating plastic gears also lend themselves to gear trains where the use of grease must be avoided, such as those in Hewlett-Packard printers or K’Nex motorized toys, where oil or grease leaks cannot be tolerated.
Plastic gears provide the opportunity to cut drive noise by reducing dynamic loading. Gear misalignment and small tooth errors create tiny impacts, resulting in running noise. However, lower-modulus plastic gear teeth deform to compensate for the inaccuracies, and their softer material absorbs impacts, often making plastic gears quieter than more costly metal gears that are one or two AGMA classes higher in quality. In the convalescent bed mentioned earlier, acetal gears reduced operating noise significantly.
The most powerful advantages of plastic gears may be the design opportunities they afford. Gear geometries overlooked by designers used to metal are often easy to mold in plastic, and they can reduce drive size, weight, and cost. For example, a common arrangement of two external spur gears with a large ratio demands a wide center distance. However, the same ratio can be achieved in a smaller space by replacing an external gear with an internal gear that, while tough to machine in metal, is easy to mold in plastic. Low-cost, low-wear plastic gears may also let designers reconsider the axiom of the fewer parts, the better. Split power paths in parallel or nonparallel axis drives can indeed have more parts, but they afford advantages in space, weight, efficiency, and cost. Plastic gears impose no special restrictions on gear ratios, and the required accuracy can be achieved with today’s molding machines and materials.
As performance requirements for a drive increase, thé upfront design effort required to make plastic gears work becomes more complicated. The state-of-the-gear-art has advanced to where plastic gears are now in drives ranging from ¼ to ¾ hp. Future applications may take them between 1 and 10 hp in the near term and up to 30 hp in the long term. Horsepower limits for plastic gears vary with the polymer, depending upon the modulus, strength, wear, and creep characteristics that change with temperature. Nevertheless, plastic gear limits can be defined in terms of contact stress, velocity, and temperature for dry running gears. For lubricated gears, fatigue strength and temperature are the critical issues.
Plastic gear trains are generally built around involute gear technology. This system is very forgiving of the center distance shifts inherent in plastic gears. Conversely, plastic gears are not satisfactory in noninvolute systems that are center-distance sensitive. In particular, many nonparallel axis systems are not based on involute technology and are difficult to manufacture with plastic gears.
Bevel gears are an exception, as they are noninvolute but often made of plastics. The low modulus of plastics makes them relatively forgiving of the alignment and manufacturing errors that are inherent in mass-produced bevel gears. Crossed-axis helical worm gears that make point contact when new are good candidates for plastic at low loads. Their capacity is increased by initial wear that produces a line contact. Involute face gears have a line contact and are preferred to worm drives at higher power levels.
Many of today's engineering resins provide the necessary performance for working gear trains.
In the past, plastic gear applications were typically air-cooled, either unlubricated or greased. As engineering resins now move into drives with higher horsepower and greater precision, the drive designer faces the choice of oil- or grease-lubricated, or unlubricated, gearboxes. The decision to lubricate or not, and the choice of a lubricant, are key factors for the drive designer to consider.
For plastic gears running in an oil bath, the oil facilitates removal of frictional heat and allows higher load capacity. Unlubricated and greased gears are aerodynamically cooled and, therefore, run hotter with lower load capacity. Unlubricated gear sets are often molded in different materials for a reduced coefficient of friction (COF). Acetal copolymer is often mated with nylon 6/6 or polybutylene terephthalate (PBT). The combinations have much lower COFs than any of these materials working against themselves. Unlubricated plastic gears often have lubricants such as PTFE, silicone, or graphite compounded into the polymer. While these additives reduce friction, the COF is still higher than that of greased gears.
Generally, the load capacity and life of lubricated plastic gears is governed by bending fatigue at the tooth root. Unlubricated gears, which run hottest with the lowest load capacity, often fail due to wear or overheating on the tooth flanks. Greased gears will occasionally fail due to wear if the grease does not stay in the mesh.
While engineering resins can resist oils and greases, lubricants must be carefully chosen because some can cause dramatic changes in gear properties and dimensions. For example, extreme pressure oils are unnecessary with the low contact pressures found in plastic gearing, and some can attack plastics chemically. Likewise, the choice of resin for the application is important. PTFE and other low-friction additives compounded in the material of plastic gears may have little or negative value if the gears are oiled or greased.
Plastics are naturally more prone to dimensional creep than metal, and creep in plastic gears depends on duty cycle and temperature. Consequently, molded gears are best used in applications without static loads. If static loads cannot be avoided, plastic gears must be designed to operate properly after teeth have deflected due to creep.
The operating speed of plastic gears obviously impacts operating temperature. However, rapid-loading rates can also affect material properties. For some materials, the faster a tooth is loaded, the higher the effective modulus and strength. Higher temperature reduces the modulus and strength of most plastics, and accelerates creep. These effects must be considered in the design process, and studies to quantify them are just beginning.
Gear load analysis is complicated, regardless of gear material, and gear design remains an area of special expertise. Gears also usually demand more precision than commonly molded parts, so their tooling can be expensive. A good plastic gear design, however, saves money in reducing trial-and-error mold iterations. For the project engineer, building a drive with plastic gears ideally should start with a team that includes a gear designer, molder, tool builder, and resin supplier, all experienced with gears.
The team needs the most complete application information available to create as detailed a gear specification as possible. Ambient temperature, lubrication, and duty cycle impact gear life and drive performance. A housing material that matches the thermal and moisture expansion of plastic gears can help maintain precise center distances. However, plastic housings cannot dissipate heat as well as metal. Gear swelling due to moisture absorption in some resins can also stall tight-meshed gears. CAD tools can help designers allow for worst-case tolerances. Universal Technical Systems of Rockford, Ill., is one supplier of such CAD tools.
Plastics also change the rules of gear and drive design. The designer of a metal pinion gear would usually limit the aspect ratio to one or less. With plastics, an aspect ratio of two or three may be acceptable, since full tooth contact may be achieved. Plastic gears can require tip relief that’s not needed in metal gears. The lower mesh stiffness of plastic teeth requires more backlash than is found in metal gears. A Hunting Ratio that is desirable in many metal gear trains to equalize wear actually may accelerate wear with plastic gears. The guidelines for metal gear design must be examined carefully before applying them to plastic gears.
Rack tooth form remains a useful way to define and generate gear teeth in metal or plastic. Standard metal gear profiles can provide a starting point for plastic gears, though some plastic profiles are preferable. The most common profile systems are described in ANSI/AGMA 1006- A97, “Tooth Proportions For Plastic Gears.” Most profiles are based on a 20-degree pressure angle and a working depth of two over the diametral pitch, or two times the module.
While standard tooth profiles are a starting point for plastic gears, the profile must be optimized for a material with a lower modulus, greater temperature sensitivity, and different coefficients of friction and wear than metal. Plastic gears commonly have greater working depths than metal gears, sometimes up to 35 percent greater. This allows for variations in effective center distance due to thermal, chemical, and moisture expansion. The designer of plastic gears should strive for a full root radius. Not only will this reduce stresses at the root, but it will also enhance resin flow into the teeth during injection molding, which reduces molded-in stresses and removes heat more uniformly from the plastic during solidification. A more stable geometry results.
Designers of plastic gears should also pay special attention to shaft attachment. Bore tolerances naturally impact true center distances, sometimes resulting in loss of proper gear action. A simple press fit demands extra molding precision for a secure mount without overstressing the plastic. A press-fit knurled or splined shaft can transfer more torque, but also puts more stress on the gear hub. Insert-molded hubs grip better but, during molding, as the plastic shrinks onto the shaft, they can induce residual stresses. Ultrasonic insertion of a knurled shaft produces the lowest residual stresses. In some cases, a single- or double-D keyed shaft prevents slippage and minimizes distortion with assembly. However, if torque is high, these can become loose. For high-torque applications, splined assemblies are preferable.
A fundamental misconception in plastic gear design is that, whatever the resin, “It’s just plastic.” The choice of a gear resin demands careful study. Inexpensive commodity resins generally lack the fatigue life, temperature resistance, lubricant resistance, and dimensional stability required for quality plastic gears in all but the most primitive applications. However, many of today’s engineering resins provide the necessary performance for working gear trains. They also have the consistent melt viscosity, additive concentrations, and other qualities essential to consistent, accurate molding.
It is generally easier to mold high-quality gears with resin containing minimal additives than those with highly filled blends. The specifier should call for only as much glass, mineral filler, or lubricant additives as are actually needed. If external lubrication is required, the drive designer, resin supplier, and lubricant supplier should work together to select an appropriate lubrication system.
Crystalline resins generally have better fatigue resistance than amorphous plastics, and most gear applications have utilized the crystalline resins, nylon, and acetal. Nylon 6/6 was used successfully, for example, in a lawn mower cam gear. Nylon, both with and without glass reinforcement, continues to serve in many gear and housing applications. Acetal copolymer provides long-term dimensional stability, and exceptional fatigue and chemical resistance over a broad temperature range.
Other resins have found limited gear success. ABS has good dimensional stability and low shrinkage out of the mold, but its fatigue characteristics make it suitable only for lightly loaded gears and short service lives. Liquid crystal polymer (LCP) has exceptional dimensional stability and fills the most intricate molds. To date, LCP has been used only in small precision gears under light loads, such as tiny wristwatch gears.
Linear polyphenylene sulfide (PPS) has exceptional temperature and chemical resistance and good fatigue life. It has been effective in other highly loaded parts molded with fine details, and should prove to be a high-performance gear material. Spur gears molded in PTFE-lubricated linear PPS were incorporated in an automotive steering column, where their coefficient of thermal expansion matches that of surrounding diecast parts. As plastic gears move into higher loads with larger gears in lubricated environments, the improved fatigue resistance, dimensional stability, and high-impact strength of long-fiber reinforced thermoplastics (LFRT) should make these materials leading gear candidates.
Gear resin selection requires the designer to focus on resin performance at the high end of the operating temperature range planned for the drive. Heat-deflection temperatures for engineering resins range from 170°F for unfilled nylon and 230°F for acetal copolymer to 500°F for reinforced linear PPS at 264 psi. However, higher temperatures can lower the modulus and strength of gear resins. They increase creep rate and introduce thermal expansion into precision parts. Fortunately, the temperature response of engineering resins is well understood, allowing designers to predict the effect on their gears.
The initial engineering effort to design plastic gears is greater than that required with metals, if only to cope with changing properties and dimensions. The most common errors of plastic gear designers start with insufficient application specifications. The specifics of the application must be factored into detailed drawings before prototyping.
Problems with prototypes can also tempt gear designers to change resins—a costly mistake given the different shrinkage characteristics of various plastics. It is better to rework the tooth profile than switch the material, unless it is clear that the wrong material was chosen.
To avoid the pitfalls of plastic gears and to realize their potential, expertise is available from software, gear consultants, and resin suppliers. With careful design and material selection, the power transmitted by plastic gears can be significant and the potential savings enormous.