DESIGN
CHECKLIST
These
are the questions the product development team needs to answer before the right
plastic material can be selected:
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
economical?
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.
The above
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.
Abstract
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
Introduction
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.
Gearing Up
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.
Powerful Potential
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
plastic gears.
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.
Driving Design
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,
accurate molding.
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.
Authors’ Note:
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
performance.
