Get into Gear with Plastic

Plastic gears provide new design opportunities based on their ability to meet complex form factors and deliver attractive cost. With plastic gears, material selection and inspection are areas that have experienced considerable change. The key to material selection is meeting product requirements at the lowest cost. Addressing application concerns that include shrinkage, load, speed and environmental conditions and coefficient of thermal expansion are just a few of the ways plastic gear suppliers have expanded in demanding markets such as automotive. Newer material and new material combinations allow plastics to meet harsh requirements of under-the-hood automotive applications such as electric throttle control that have to withstand 185C temperatures, says Andrew Ulrich, a senior engineer at UFE.

As opposed to higher temperature requirements in some systems, the recent American Gear Mfg. Assoc. (AGMA) specification AGMA 909-A06 affects all plastic gear applications. “AGMA 909-A06 is a document developed to inform the designer of the minimum requirements needed to specify a plastic gear and to discourage additional inspection methodology that adds cost,” says Richard Wheeler, president of ABA-PGT.

Bigger, More Powerful

“What’s new in plastic gears or what continues to be new, is that our clients keep pushing for more and more metal to plastic conversions and higher horsepower,” says Andrew Ulrich, a senior engineer at UFE. One case in point is an 18-inch diameter washing machine gear. Driven by a ½ horsepower motor, the gear turns a drum full of clothes and withstands dynamic impact loading. Testing is certainly critical to this application. However, a completely different method of inspection was chosen to meet the needs of the customer. By digitizing and picking fixed points around the gear and comparing to the original file, UFE met the customer’s quality and cost requirements.

Testing Terminology

With all the recent changes in AGMA specifications, ABA-PGT has a strong push on educating plastic gear users. According to Richard Wheeler, president of ABA-PGT, ever-increasing accuracy specifications and inspection to verify that performance are key trends ABA-PGT has identified. For example, elemental gear inspection can help identify process changes and quality improvements. In spur and helical gears, this type of profile measurement can lead to gear mesh noise reduction and increased face-width load distribution. Information including a plastic gear manual and a special plastics gearing software program are available tools to help users.

Pinion Rotor

Accumold produces a micro-molded part for military and aerospace applications that requires a two-part assembly. “Gears, especially micro gears, require unique mold designs, extremely high accuracy mold building and excellent processing capabilities,” says Brent Hahn, business development manager, Accumold. The first step in the rotor pinion is a cylindrical shape that uses a magnetic material molded in a plastic compound. Then, the gear portion is molded using a partial glass-filled Polysulfone. To protect the completed assembly’s very tight tolerances from shipping damage, each unit is individually packaged. The company’s Micro-Mold parts range from about ½-inch to units with features measured in microns and frequently have complex geometry and tight tolerances.