1. What is the function of the part?
2. What is the expected lifetime of the part?
3. What agency approvals are required? (UL, FDA, USDA, NSF, USP, SAE, MIL spec)
4. What electrical characteristics are required and at what temperatures?
5. What temperature will the part see? And, for how long?
6. What chemicals will the part be exposed to?
7. Is moisture resistance necessary?
8. How will the part be assembled? Can parts be combined into one plastic part?
9. Is the assembly going to be permanent or one time only?
10. Will adhesives be used? Some resins require special adhesives.
11. Will fasteners be used? Will threads be molded in?
12. Does the part have a snap fit? Glass filled materials will require more force to close the snap fit, but will deflect less.
13. Will the part be subjected to impact? If so, radius the corners.
14. Is surface appearance important? If so, beware of weld lines, parting line, ejector location, and gate vestige.
15. What color is required for the part? Is a specific match required or will the part be color coded? Some glass or mineral filled materials do not color as well as unfilled materials.
16. Will the part be painted? Is a primer required? Will the part go through a high
temperature paint oven?
17. Is weathering or UV exposure a factor?
18. What are the required tolerances? Can they be relaxed to make molding more
19. What is the expected weight of the part? Will it be too light (or too heavy)?
20. Is wear resistance required?
21. Does the part need to be sterilized? With what methods (chemical, steam, radiation)?
22. Will the part be insert molded or have a metal piece press fit in the plastic part? Both methods result in continuous stress in the part.
23. Is there a living hinge designed in the part? Be careful with living hinges designed for
crystalline materials such as acetal.
24. What loading and resulting stress will the part see? And, at what temperature and environment?
25. Will the part be loaded continuously or intermittently? Will permanent deformation or creep be an issue?
26. What deflections are acceptable?
27. Is the part moldable? Are there undercuts? Are there sections that are too thick or thin?
28. Will the part be machined?
29. What is the worst possible situation the part will be in? (For example, the part may be
outside for an extended period of time and intermittently put in water, or the part may
see a constant high load while submerged in gasoline at 150°F.) Parts should be tested
in the worst case environment.
checklist was developed by TICONA
Exploiting the Potential of Plastics Gears
Written by Zan Smith, Staff Engineer, Ticona LLC and Andy Ulrich, Senior Product Engineer, UFE Inc.
Plastic gears are now being used in drives of higher power and higher precision than in the past.
They afford appliance designers dramatic opportunities to reduce drive cost, noise and weight.
However, due to the properties of engineering resins, plastic gears require a greater engineering
effort than metal gears.
This paper examines current applications of plastic gears and explains the payoffs in reduced cost,
weight and noise. It also provides insight into the design process for plastic gears and discusses the
importance of a gear design team.
The presentation covers:
Current plastic gear applications
Accuracy of plastic gears
Weight and cost savings possible with plastic gears
Drive design considerations when using plastic gears
Plastic gear design fundamentals
The gear design team
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. What are the payoffs when using plastic gears in place of metal? Where do they
make 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 (Fig. 1). They also saved 13 pounds and did
away with 42 parts compared with a previous metal gearbox. Gears injection-molded from
unfilled and fiberglass-reinforced Celcon® acetal copolymer maintain 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 (Fig. 2). Acetal copolymer cluster gears were
specified to comply with the high-quality standards of AGMA (American Gear Manufacturers
Association) 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” manufactured in several countries,
Whirlpool Corporation introduced a splined clutch or “splutch,” containing a spline and gears
molded in acetal copolymer (Fig. 3). 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%
when compared with earlier designs using metal gears.
Gears are critical, complex drive components that directly affect function and reliability.
Engineers must therefore understand both the potential and the pitfalls of plastics, to get the most
from them in gearing applications.
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 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 tools have advanced.
For instance, CAD software can now optimize plastic gear designs based on temperature, moisture
pickup and other environmental factors.
The unrealized potential of plastic gearing is becoming more apparent to the industry. Testing of
plastic gears specifically to characterize gear resins in different service environments has begun.
The new data will allow design engineers to more accurately predict gear performance. 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 gear trains. 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. Consequently, they have a
significant cost advantage 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 (Fig. 4).
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.
Differences in specific gravity alone, however, 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 drive trains as well as innovative gear designs that may not be feasible 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 with multi-stage, 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
its cooler-running plastic transmission reduced heat rise by 10 to 15% when compared with
previous metal drivetrains. Greater efficiency can be important in light of anticipated future US
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 the Hewlett-Packard printer or K’Nex motorized toy 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 home healthcare 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 accustomed 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, which, while tough
to machine in metal, is easy to mold in plastic.
Low-cost, low-wear plastic gears may also allow designers to reconsider the axiom: The Fewer
Parts, The Better. Split power paths in parallel or non-parallel 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.
The higher the performance requirements for the drive, the more complicated the up-front design
effort required to make plastic gears work. 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 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 to plastic gears. Conversely, plastic gears are not
satisfactory in non-involute systems that are center-distance sensitive. In particular, many non-parallel
axis systems are not based on involute technology and are difficult to manufacture with
Bevel gears are an exception as they are non-involute 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.
To Lubricate Or Not To Lubricate
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 lubricated, grease lubricated or unlubricated gearboxes. The
decision to lubricate or not lubricate, and the choice of a lubricant, are essential 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. Therefore,
they run hotter with lower load capacity. Unlubricated gear sets are often molded in different
materials for 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 by
wear or overheating on the tooth flanks. Greased gears will occasionally fail by 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.
In The Know
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 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 design of plastic gears, 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 including a gear designer, molder, tool builder and resin supplier; all
experienced with gears.
The team needs the most complete application information available to create the most detailed
gear specification 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 in Rockford, IL, 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
normally limit the aspect ratio to one or less. With plastics, an aspect ratio of two or three may be
acceptable as full tooth contact may be achieved. Plastic gears can require tip relief unnecessary
in metal gears. The lower mesh stiffness of plastic teeth requires more backlash than found in
metal gears. A hunting ratio considered desirable in many metal gear trains to equalize wear
might accelerate wear with plastic gears. The guidelines for metal gear design must be examined
carefully before applying them to plastic gears.
Tooth forms defined in terms of a “basic rack” remain a convenient way to define and generate
gear teeth in metal or plastic. Standard metal gear profiles can provide a starting point for plastic
gears, although there are some plastic profiles that are preferred. The most common profile
systems is 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. However, standard tooth profiles are a starting point for plastic
gears. The profile needs to 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% 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 to enhance resin flow into the teeth during injection
molding. This reduces molded-in stresses and more uniformly removes heat from the plastic
during solidification. A more stable geometry results. A full root radius will also reduce stresses
at the root.
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 mold precision and attention to processing for a secure mount without
over-stressing 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 preferred.
Molded In What?
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,
It is generally easier to mold high-quality gears with resin containing minimal additives than with
highly filled blends. The specifier should call for only as much glass or 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 shrink
out of the mold, but its fatigue characteristics make it suitable for only lightly loaded gears and
short service life. Liquid Crystal Polymer (LCP) has exceptional dimensional stability and fills
the most intricate molds. To date, LCP has been used for only 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 die-cast 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.
Specify and Mold
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, increase the creep rates 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 error of plastic gear
designers is starting with insufficient application specifications. The specifics of the application
must be factored into detailed analysis before prototyping. The detailed drawings must contain
sufficient information to manufacture the gear. The accompanying table is an example of the
minimum specifications for a plastic spur gear.
Problems with prototypes can also tempt gear designers to change resins—a costly mistake given
the different shrink 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.
Ticona manufactures and markets a broad line of engineering and high-performance polymers
used in applications including automotive, electrical/electronic, appliance, healthcare, industrial
and consumer products.
Celcon is a registered trademark of Ticona.
UFE Incorporated is an engineering and manufacturing services company providing Product
Engineering, Mold Manufacturing, Injection Molding, and Contract Manufacturing services to
diversified customers worldwide.
Figure 1. The pioneering dual-drive washer transmission from Maytag uses spur gears molded in
Celcon® acetal copolymer. It saves 13 pounds and eliminates 42 parts compared with a
conventional metal gearbox, and dramatically reduces gear noise while enhancing long term