Things you should know about sizing and applying
PT design ●  ionsystemdesign ●  July 2000●  27
Size wise
S
ome say don’t call on plastic to do a metal’s job, but the lines between the two materials’ capabilities are blurring.Plastic gears can be used successfully without lubrication in 60 to 70% of open gear applications. They’ll even work lu-brication free in such demanding applica-
tions as a 21-in. diameter main drive for a packaging machine or a 300-rpm drive for a diaper-making machine.
Even further closing the gap between metal and plastic are hybrid gears, incor-
porating nylon 12 castings around knurled metal hubs. Nevertheless, when a gear design incorporates plastic, a num-ber of variables come into play.
Sizing protocol
The key to any gear’s performance is proper sizing. A metal gear is generally rated by evaluating load data, but plastics have different properties than metals and are sen-sitive to changing operating conditions. A plastic gear, therefore, must be selected, ac-
tually, calculated, with load data, environ-mental conditions, and material properties in mind. Things to consider include:
•D RIVE GEOMETRY — center dis-tance; available space (face width).
• L OAD DATA — torque; rpm; trans-mission ratio.
•E NVIRONMENT AND OPERATING CONDITIONS — operating temperature;shock loading; exposure to chemicals;exposure to water or humidity; clean room, etc.
• P LASTIC MATERIAL PROPERTIES —moisture absorption; swelling and back-lash requirements; impact strength at low and wear resistance at high temperatures.You’ll likely know the gear ratio, the center distance, and possibly the motor’s horsepower. If you’re lucky, you’ll have the freedom to determine the space needed for the gears. It’s usually better to start working with a plastic gear supplier early in the design process, rather than reaching for a cookie cutter catalog solu-tion at the last minute.
Selecting the right plastic
There are many engineered nylon gear materials on the market, choosing the right one depends on your specific appli-cation needs. Some materials, such as ny-lon 6, have limitations such as moisture absorption and swelling (sometimes more than 3%) resulting in dimensional change and loss of tensile strength. Other polyamide (nylon) gears are not as lim-ited, for example, with cast nylon 12, you get high torque transmission, self lubrica-tion, quiet operation, light weight, shock and vibration absorption, long wear, low maintenance, and no corrosion.
Plastic gears
The knurled metal core around which nylon 12 is cast provides ample torque
transmission and even heat dissipation from the gear teeth through to the shaft. The metal also reduces thermal expansion by about 50%, increasing operating stability.Once the nylon 12 material is cast around the core, the finished bar, second from left,is “sliced” into blanks. Then the blanks are hobbed to create teeth, while the hub center is machined out, usually with a keyway for secure attachment to the shaft.
Assignment: metal gear replacement
With so many variables involved,each plastic gear application is likely to be different, as evidenced in the follow-ing example. The problem was quite common, oil leaks in a machine using lubricated metal gears. To complicate matters, the machine must operate for 12 months without failure, as wear com-ponents are to be changed only during the annual shut-down or after 2,000hours of operation.
Plastic gear selection for the applica-tion begins with review of the existing gear set parameters:Center distance:  5 in.Output speed:300 rpm Input speed:1,200 rpm Start-up torque:1,200 in.-lb Continuous torque:700 in.-lb The original metal gears have the fol-lowing specs:
Pinion: spur; 20 teeth (T), 10 diame-tral pitch (DP); 20°pressure angle (PA);attachment to shaft via keyway; 1.25-in.face width; 2.25-in. length trough hub (LTH); mild steel material.
Gear:spur; 80 T; 10 DP; 20°PA; key-way; 1.25-in. face; 2.25-in. LTH; mate-rial cast iron.
Rated torque is 1,307 in.-lb.
If these gears were to run without lu-brication, they would last anywhere from
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●  PT design ●  ionsystemdesign ●  July 2000
Cast nylon 12 (PA 12G) gears grow less than 0.2% even when permanently immersed in fluid.
How polyamids handle moisture
cast nylon 12 gear, are suggested. The potential of each is calculated using a
modified Lewis formula for tooth root stress, including an algorithm that fac-tors in stresses and wear b
ehavior when gears are run without lubrication.
The application calls for a flank pres-sure safety (wear) factor between 1 and 1.3; this is an especially important issue with unlubricated gears. It’s determined an initial grease application is acceptable. With this information, engineers reverse the calculation, starting with the target of 2,000 hours of gear life. By working with flexible parameters, such as face width, number of teeth, metal core size,
At low
temperatures
stresses cause
nylon 6 to
become brittle,
while cast
nylon 12 is
stress-free and
retains its
reaction in the shaft“toughness”
throughout the
temperature
range.
PT design●  ionsystemdesign●  July 2000●  29
lon 12 with a 5-in. metal core. About 10% of expected life is offered for ma-chine start-up.
Because the gears are hobbed, the the-oretical calculation can be verified eco-nomically in physical tests.
How plastic gears got a bad rap
At one time, metal was the only gear material choice. But metal means main-tenance. You have to keep the gears lu-bricated and hold the oil or grease away from everything else by putting it in a housing or a gearbox with seals. When oil is changed, seals sometimes leak after the box is reassembled, ruining products or components. Metal gears can be noisy too. And, because of inertia at higher speeds, large, heavy metal gears  can cre-ate vibrations strong enough to literally tear the machine apart.
In theory, plastic gears looked promis-ing with no lubrication, no housing,longer gear life, and less required mainte-nance. But when first offered, some de-signers attempted to buy plastic gears the way they did metal gears — out of a cata-log. Many of these injection-molded plastic gears worked fine in non-demand-ing applications, such as small household appliances. However, when designers tried substituting plastic for metal gears in tougher applications, like large pro-cessing equipment, they often failed.Perhaps no one thought to consider that plastics are affected by temperature,humidity, torque, and speed, and that some plastics might therefore be better for some applications than others. This turned many designers off to plastic as the gears they put into their machines melted,cracked, or absorbed moisture compro-mising shape and tensile strength.  ●
Material provided by Georg Bartosch, president, Tody Mihov, engineering manager, and Ruth Emblin,
marketing manager of Intech Corp., Closter, N.J.
Next step
• Did you find this article interesting? • For more information on gears go ionsystemdesign.
• For more information from Intech Corp.,
will wear out in a relatively short time. Gear alternatives 2 and 3, on the other hand, meet the operating life require-ments. Both alternatives show a comfort-able tooth root safety, but alternative 3wins out for its higher flank pressure safety, while gear dimensions remain the same. So, by choosing tooth modifica-tion, we can make the gear 17% nar-rower, or 2.1 in. wide, and still achieve the flank pressure safety of the unmodi-fied, wider gear. The tooth root stress safety would still be adequate in this case,and the face width reduction would let the plastic gear fit into the 2.25-in. avail-able on the shaft. A further face width re-duction of up to 20% is possible with a plus-plus modification
Alternative 3 will give 2,500 hours of gear life, and has a modified tooth for ad-ditional wear safety. The pinion is made of steel, and the gear is made of cast ny-
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●  PT design ●  ionsystemdesign ●  July 2000
See ad on pages A62,299 in the 2000 PTD Handbook.

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