Advanced polymers let designers replace metals in large, complex gear sets that can carry heavy loads and survive harsh environments.
Senior Design Engineer
Ticona Technical Polymers
Plastic gears have had a successful run in automobiles, appliances, office equipment, and power tools. Polymer performance and processing techniques have advanced to a point where plastic gears will handle higher-torque applications, previously the domain of their metal counterparts.
Plastic gears can be good load carriers. They resist chemicals and heat, don't wear much, and keep noise to a minimum. In automotive electrical systems, they replace hydraulic and cable assemblies in interior and exterior mechanisms. These systems rely on plastic gears to quiet and smooth lift-gate operation and give stealthy movements to tracking headlights, windows, and underhood throttle body controls.
Designers at Audi, Volkswagen, Bentley, Bugatti, and Lancia have all spec'd plastic over metal in recent electronic-parkingbrake gears. The brakes employ 1.2-in. (30-mm) spur gears made from Fortron linear polyphenylene sulfide (PPS). A motor-drive belt powers the gear that drives a transmission for actuating the rear brakes. Designers chose Fortron PPS for its dimensional stability over a broad temperature range, as well as its good chemical resistance and ease of processing.
But cost alone is often a stronger motivator for designers to investigate plastic gear materials. Plastics frequently get chosen over metal because of multiple cost-cutting benefits. They provide greater design freedom, letting designers build gears too difficult or expensive if made from metal. They can consolidate parts in existing metal drives to cut fabrication and assembly costs. This includes cluster gears where advanced processing techniques let multiple gears mold as a single part.
Molded gears can take on complex shapes for split-path planetary drives and modern, high-efficiency worm and internal gears, at AGMA (American Gear Manufacturers Association) precision levels of Q6 to Q9. It is often an arduous and costly task to machine these gears from metal. And plastics are often less expensive than stamped or machined metal. Ditto for those formed with powdered metals. Powdered-metal gears, for example, are often double the cost of a comparable thermoplastic gear. And hobbing them from metal blanks can triple gear cost.
A GEAR EVOLUTION
Plastics primarily serve in involute gears, most commonly spur gears. Historically, plastic gear designs mimicked those of successful metal gears. But new design strategies for boosting power, improving function, and lowering costs are pushing the boundary on plastic gear size, structure, and geometry. A number of recent design approaches specially target plastics:
Nonlinear gears with tailored outputs involve a variety of design methods including eccentric shaft locations that can vary gear displacement profiles. Elliptical and other out-of-round gears are an option. They cycle through various speeds and torques as they rotate. Other gears feature radius step changes that adjust power output to meet application demands.
Planetary and other epicyclic gear arrangements provide split power paths for greater power and torque transfer in a relatively small package.
Herringbone gears have teeth that lie on the pitch cylinder in a V-shaped form. Half of each tooth is on a right-handed helix and the other half is on a left-handed helix. As gears rotate, the opposing forces on the teeth cancel out axial thrust loads that are typically detrimental to simple helical plastic gears.
Two-shot molded gears solve a number of common gear issues. The process can mold gears with rigid cores and lubricous surface layers as well as those with shockabsorbing materials sandwiched between gear rings and hubs molded from another polymer.
Crowned gears made from plastic can maintain transmission accuracy even if shaft alignment is less than optimum. One example involves lead crowning where the teeth can be molded taller or thicker (or both) at the center of the gear face. This lets gears overcome poor load distribution caused by misalignment.
Zero or low-backlash designs eliminate clearance issues between gear teeth. They can overcome manufacturing errors and bearing run-out. They also help limit noise in lightly or moderately loaded gears that reverse direction repeatedly.
Tooth profiles with custom pressure angles on their forward and reverse flanks improve gear efficiency and sliding, reduce heat and noise, and let designers tailor bending stress.
Designers in the 1960s were the first to deploy plastic gears on a broad scale. But they were limited to gear diameters of <2 in. and power outputs under 0.25 hp. Today, 4 to 6-in. molded gears are common. Some, including a bevel gear used in a recent washing machine transmission have grown to 15 in., or more. Power levels have reached 2 hp, but within a decade, materials advances may boost power output to 10 hp.
The ability to craft far more precise plastic gears is also growing. Gear molders commonly attain an AGMA quality level of Q7. Q10-level gears are in production with molders who commit to the equipment and systems necessary to produce this result.
The ongoing quest for quieter drives has helped advance plastic gear design, especially in cars, homes, and offices. The trend has pushed designers to use more lubricious or flexible polymers, manufacturing processes that improve molding precision, and radical tooth profiles. Plastics readily create such special profiles as those with teeth having varying pressure angles. This tooth profile has proven successful in minimizing transmission noise and effort, especially at low power.
One strategy to reduce gear noise involves two-shot molding. Here teeth made of a softer polymer go on a stiffer gear center or stiffer teeth can be overmolded on to a softer, vibrationabsorbing core. An elastomer molded between the hub and teeth absorbs gear-shock load, limits noise, and prevents tooth damage during hard stops. In large gears, cored teeth lower noise and improve accuracy. Coring may make gears easier to mold. It eliminates large plastic masses that make gear geometry hard to control.
Designers must not ignore gear-housing design. Gear housings may need as much attention as the gears themselves. That's because they must maintain precise gear alignment during service. Housings, therefore, must be stiff, strong, and dimensionally stable. Long-fiber-reinforced plastics offer one option for gaining such properties.
Designers often choose plastic gear and housing combinations that sport equivalent or nearequivalent coefficients of thermal expansion. This is especially important for smaller gearboxes with tight tolerances. Given their smaller volumes, they often work at higher temperatures because it's harder to dissipate heat.
Thermally conductive polymer blends and engineering thermoplastics that tolerate elevated temperatures are less likely to see dimensional changes caused by frictional heating from gears in tight quarters. However, it's sometimes necessary to choose gear and housing materials with greatly different thermal expansions. This requires considerably more attention to dimensional change over the operating-temperature range of the gear set to produce satisfactory results.
Internally lubricated polymer blends are changing how gearboxes are designed. Smaller, lower-power gears often run dry or need low-wear, internally lubricated plastics. The trend is to push dry-running gears to higher power levels. Doing so lets designers employ simpler gearboxes as well as eliminate the expense and nuisance of handling grease and oil.
But running gears dry at higher power will only be practical with further refinement of such current lubricious polymers as acetal copolymers. Blending the acetals with polytetrafluoroethylene (PTFE) or silicone is one option for lowering coefficients of friction, wear, and noise.
PLASTICS GEARED FOR GEARS
Many factors come into play when selecting gear plastics. A polymer's modulus must withstand tangential forces and impacts, but at the same time could allow some gear-tooth deflection so teeth run quieter. Polymers also need enough fatigue strength to handle cyclical loading and unloading of the gear teeth. They must also have sufficient tensile strength to withstand repeated shock loads. And good creep resistance coupled with dimensional stability will help the polymer maintain geartooth contact ratio, tip clearance, and geometry.
Dimensional stability is, however, a complex factor. Polymers that absorb water or chemicals will be less dimensionally sound. Temperature changes can also play havoc on part dimensions. If molded badly, geometry can change as the gear cools after molding and be deformed by the working environment and friction.
It's also important to note how resins function with other plastics or metals in the assembly. Mating dissimilar polymers in a gear set often creates drives that run smoother, quieter, cooler, and with less wear. For tight-tolerance gear sets, it's often advantageous to spec the same plastic for adjacent gears if moisture and temperature will likely change during service. In these cases, designers should also spec the same material for the gear housing.
The most common thermoplastic gear materials are acetal, polyester, and nylon. They create strong and precise gears with good fatigue and wear resistance. The range of plastics available for gears helps designers cope with the temperature fluctuations gear sets will see. Acetal copolymer works well to 212°F (100°C), polybutylene terephthalate (PBT) can serve at 302°F (150°C), nylon 6/6's limit is 347°F (175°C), and PPS stands up well at 392°F (200°C). High-temperature nylons and polyphthalamide have somewhat lower thermal limits than PPS, while liquid-crystal polymers (LCPs), imides, and polyetheretherketones have higher limits.
Acetal is widely used in gears because of its dimensional stability, fatigue resistance, and ability to withstand many chemicals over a wide thermal range. It's highly lubricious and moves smoothly over both metals and plastics. But even with 40 service years as a gear material, acetal continues to evolve. For example, recently introduced grades of Celcon acetal copolymer include nonsqueak grades that combine high toughness and fatigue resistance; those with elevated PTFE levels; higher lubricity grades that cost less than those containing PTFE; and special glass-filled grades containing silicone.
PBT, a polyester, works well in mixed gear sets having gears made from other plastics and metals. It molds with extremely smooth surfaces and is often used in housings.
Nylons offer exceptional toughness and wear little against other plastics and metals. Their tendency to absorb water and many lubricants can lead to dimensional change, so designers usually turn to other polymers for precision gears. Nylons often serve in worm gears and housings.
PPS is stiff and dimensionally stable. It sports good fatigue life, resists chemicals, and is often top choice for use in drives running in hot, corrosive environments. In automobiles, PPS gears go into electronic throttle controls, turbo actuators and their housings, and lubricant circulating pumps for transaxles and transmissions.
LCPs work well in watch gears and other small, precision and lightly loaded gears. LCPs are dimensionally stable to 428°F (220°C) and resist most chemicals. They flow exceptionally well to fill intricate, thin-walled parts with great accuracy. And they have low shrinkage during molding and yield little flash.
Polycarbonate, along with other plastics have had limited success in gears. It is not commonly spec'd for gears because of its low chemical and fatigue resistance and poor lubricity. Commodity resins, such as acrylonitrile butadiene styrene (ABS) and low-density polyethylene (LDPE) lack chemical, thermal, and creep resistance and aren't dimensionally stable. Their use is generally restricted to basic, low-load or low-speed gears.
Copolyester elastomers with 10 to 250-kpsi moduli serve in slow and low-load gears. Grades with hardness values of 25 to 30 Shore D work well in two-shot molding and give gears good noise and vibration dampening. In one case, polyester elastomer created flexible teeth in planetary gears that were virtually unaffected by misalignments and manufacturing errors. This made them quieter at high speed and low load.
Fibers and fillers dramatically alter gear-resin properties. They can serve as a way to fine-tune mechanical properties. Acetal copolymer reinforced with 25% short 0.08-in. (2-mm) glass fibers can double base resin tensile strength and triple its flexural modulus. Adding glass fibers longer than 0.4 in. (10 mm) has an even more profound effect on polymer strength, stiffness, creep and impact resistance, dimensional stability, and toughness.
Long-fiber-reinforced plastics can have fiber loadings of up to 60%. This gives housings and large gears greater strength and better dimensional stability. And substituting carbon fibers or aramid fibers for glass improves gear wear. Long-fiber-reinforced plastics also yield better surface quality because fewer fiber ends break the surface, as is often the case with short-fiber-reinforced plastics. The boost in performance gleaned from long-fiber reinforcement can let designers spec lesscostly commodity resins such as polypropylene (PP). Long-fiberreinforced PP housings are often stronger and more dimensionally sound than those made from short-fiber-reinforced nylon.
The inherent lubricity of many gear plastics lets them serve in computer printers and toys where lubricants are unwelcome. Designers continue to take lubricated and unlubricated plastic gears to new levels. Not only are designers creating ever larger, more-powerful and more-precise gears, they are finding ways to shrink gearboxes without losing power.
Designers may choose plastics for one benefit and find gains from many others. For example, a manufacturer switched from a cast-iron transmission to one using glass-reinforced Celcon acetal copolymer to reduce noise in a five-gear, dual-drive washing machine. The design employs a split power path in which gear arrangement is symmetrical and balanced across the axis.
Polymer gears in the design also avoided the counterweight typically found in other washing-machine-transmissions and eliminatedthe need for many secondary-finishing operations. The new transmission has significantly fewer parts and is much lighter, making it easier to assemble the unit manually.
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