January 8, 2014

Part II: All Plastic Gears Are Not The Same

Engineers sometimes lump all plastic gears together, with injection molded nylon gears as the point of reference. When we talk about plastic gears here at Intech, we mean something very different. Before we get any further into the intricacies of how to design with our gears, let’s go over the key differences:

  • Fabrication. With traditional plastic gears, an injection molding process typically produces the entire gear, including the teeth. Intech’s plastic gears have a very different construction. They consist of a polymer gear surface that is gravity cast over a metal hub. The polymer part of the gear starts out as a round blank, and we machine the teeth. In this regard, our gears have more in common with cut metal gears than molded plastics.

  • Material Properties. Just as all plastic gears aren’t created equal, neither are the polymers themselves. Traditional plastic gears typically made from injection molding grades of nylon 6, acetal and a handful of other engineering polymers. We typically craft our gears from a casting grade of nylon 12, though we sometimes use other engineering polymers if the application requires it. Cast nylon 12, in particular, has a very clear advantage over other engineering polymers when it comes to striking a balance between tensile strength, wear resistance, dimensional stability, moisture resistance and vibration damping capabilities—all of which are extremely desirable properties in gear applications. Whether we use nylon 12 or another engineering polymer, the fact that the gear blanks are cast and not molded also changes the material properties. In particular, our cast and machined gear teeth have lower internal stresses than molded teeth. And casting directly over a metal gear hub also eliminates the keyway stresses that occur with molded plastic gears and metal shafts.

  • Design Takeaway. None of this is to suggest that molded plastics gears don’t have a place. They can be a great fit for lightly loaded applications that can take advantage of useful plastic properties—such as vibration damping and corrosion resistance. Injection molding also fits well, from a cost standpoint, in applications with high production volumes. Our advanced polymer gears target really are different. Their strength, dimensional stability and mechanical damping actually make them an alternative to metal gears, particularly in applications where metal gears are failing prematurely or having noise and vibration issues. Because our plastic gears have cut teeth, we achieve similar AGMA quality levels to the metal gears that we often replace. And the design freedom afforded by plastics has allowed us to develop some unique gear geometries that reduce backlash and increase torque transmission.

In upcoming installments of our gear series, we’ll delve into the design considerations and tips you’ll need to know when using cast-and-cut plastic gears.

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