Some 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 lubrication free in such demanding applications 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, incorporating nylon 12 castings around knurled metal hubs. Nevertheless, when a gear design incorporates plastic, a number 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 sensitive to changing operating conditions. A plastic gear, therefore, must be selected, actually, calculated, with load data, environmental conditions, and material properties in mind. Things to consider include:
• DRIVE GEOMETRY — center distance; available space (face width).
• LOAD DATA — torque; rpm; transmission ratio.
• ENVIRONMENT AND OPERATING CONDITIONS — operating temperature; shock loading; exposure to chemicals; exposure to water or humidity; clean room, etc.
• PLASTIC MATERIAL PROPERTIES — moisture absorption; swelling and backlash 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 solution 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 application needs. Some materials, such as nylon 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 limited, for example, with cast nylon 12, you get high torque transmission, self lubrication, quiet operation, light weight, shock and vibration absorption, long wear, low maintenance, and no corrosion.

Assignment: metal gear replacement

With so many variables involved, each plastic gear application is likely to be different, as evidenced in the following 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 components are to be changed only during the annual shut-down or after 2,000 hours of operation.

Plastic gear selection for the application 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 following specs:

Pinion: spur; 20 teeth (T), 10 diametral 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; keyway; 1.25-in. face; 2.25-in. LTH; material cast iron.

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Rated torque is 1,307 in.-lb.

If these gears were to run without lubrication, they would last anywhere from three days to three weeks. An alternative is to use plastic gears.

Three gear sets with different specifications, each pairing a steel pinion with a 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 factors in stresses and wear behavior when gears are run without lubrication.

The application calls for a flank pressure 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, three alternatives are speced.

Alternative 1 (see chart on p. 28) is a cast nylon 12 gear with the same face width as the cast iron gear it replaces. The life expectancy does not meet the 2,000-hr target. The gear has sufficient tooth root safety to carry the load, but because of the low flank pressure safety it will wear out in a relatively short time.

Gear alternatives 2 and 3, on the other hand, meet the operating life requirements. Both alternatives show a comfortable tooth root safety, but alternative 3 wins out for its higher flank pressure safety, while gear dimensions remain the same. So, by choosing tooth modification, we can make the gear 17% narrower, or 2.1 in. wide, and still achieve the flank pressure safety of the unmodified, 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. available on the shaft. A further face width reduction 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 additional wear safety. The pinion is made of steel, and the gear is made of cast nylon 12 with a 5-in. metal core. About 10% of expected life is offered for machine start-up.

Because the gears are hobbed, the theoretical calculation can be verified economically in physical tests.

How plastic gears got a bad rap

At one time, metal was the only gear material choice. But metal means maintenance. You have to keep the gears lubricated 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 create vibrations strong enough to literally tear the machine apart.

In theory, plastic gears looked promising with no lubrication, no housing, longer gear life, and less required maintenance. But when first offered, some designers attempted to buy plastic gears the way they did metal gears — out of a catalog. Many of these injection-molded plastic gears worked fine in non-demanding applications, such as small household appliances. However, when designers tried substituting plastic for metal gears in tougher applications, like large processing 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 compromising 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.

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