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.