Plastic Gear Design Considerations

Plastic gear is a powerful means of reducing drive-cost, weight, noise, and wear in many mechanical applications. It’s lightweight, non-rusting, quiet, injection molding enables low cost and large production, and it can operate without lubrication when meshed with metal gears (within a limited range of stress levels). On the other hand, plastic gears have some drawbacks including poor strength, tendency to hold heat, dimensional change – including backlash – and lower load sharing than metal gears.

There are two ways to produce plastic gears: CNC machining or injection molding. Injection molding is cheaper for larger volumes and has the added benefit of creating a gear from a blend of multiple polymers. This allows for a wide array of specialized properties in the final product, such as resistance to tension and pressure, heat, and corrosion.

It is important for designers to recognize that the specific crystalline resin selected for a gear application will greatly impact its ability to meet a particular requirement. While amorphous nylons have been used successfully in some applications, the best gears are made from a blend of crystalline and amorphous polymers like Nylon 6/6 and Acetal copolymer. Nylon, with and without glass reinforcement, continues to serve in many gear applications and acetal has excellent fatigue and chemical resistance across a wide temperature range.

The most critical design consideration for a plastic gear is the quality of its shaft connection. Bore tolerances naturally impact true center distances – and thus gear tooth contact – and are more significant for a molded gear than those of a machined one. A simple press-fit can induce tensile stresses in the plastic, while a knurled or splined shaft can transfer more torque but requires special attention to mold design and processing to achieve a secure fit without over-stressing the plastic. Injection-molded gears typically have a splined or knurled shaft to promote proper assembly and minimize the risk of distortion during operation.

Plastics are also more prone to dimensional change than metals, and the degree of creep depends on duty cycle, temperature, and moisture absorption rates. Injection-molded plastic gears can be produced to high accuracy if the right blend of materials, mold design and tooling, and quality control are utilized. Ideally, Total Composite Error and Tooth-to-Tooth Composite Error should be kept to an absolute minimum in order to minimize gear noise, vibration, or other problems.

The fact that plastic gears can be meshed without lubrication is a great advantage, but the lack of self-lubrication can also lead to localized “plastic-to-plastic welding” that increases friction and wear. The addition of dry lubricants can alleviate this problem, but care must be taken to ensure that the lubricant will not adversely affect the structural integrity of the gear or its performance. Some oil-based lubricants may also attack plastics chemically. For these reasons, it is usually preferable to use a lubricant that contains a low molecular weight additive such as molybdenum disulfide or PTFE. This type of lubricant can be infused in the plastic at the manufacturing stage, or compounded into the finished gear.

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