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.

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:

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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