Plastic gears the heart of the matter

Feb. 1, 2000
Incorrect shrinkage, not material inadequacy, is the root of most plastic gear failures. Now there's something you can do about it

Microsoft may be best known for its software, but one of its most innovative products is actually a toy. It's an animated Barney doll, and it almost didn't make it out of development.

MS Barney, a mechatronic marvel, was designed to walk and talk and interact dynamically with its namesake program on PBS. Put it in front of a TV tuned to the "Barney" show, and the purple dinosaur sings, marches, and plays along, waving its hands and nodding its head on cue, in response to broadcast commands.

Early on, however, Barney had somewhat of a hitch in his giddyup. Ironically, the elaborate electronic control system worked just fine, but the plastic gear transmission, a relatively simple component, kept binding, stopping the doll in its tracks and nearly scuttling the project.

Had the engineering team blamed Barney's problem on plastic gears the story would end right here. But someone had enough insight to search for the real cause of the doll's halting gait, which turned out to be incorrect plastic shrinkage that occurred during the gear-making process. In other words, the final dimensions and profile of the gears in the transmission weren't as designed.

Rather than throw in the towel, the developers redesigned the transmission, more closely profiling the gears as well as the mold cavities to establish the true shrinkage. They not only solved the binding problem but, by switching to a softer material for the high-speed motor pinion, made the gears, and Barney's walk, smoother and quieter.

Code purple

Such misunderstandings, if not resolved, can quickly derail an engineering project; if not caught, they can sink even the most well-intentioned product, possibly dragging the manufacturer down along with it. Engineers have to learn to deal with it, however, because the trend toward plastic gears isn't going to go away. On the contrary, it's picking up steam as designers and OEMs learn lesson after painful lesson through the school of hard knocks.

Most of the confusion surrounding plastic gears centers on the shrinkage issue. When you design a plastic gear mold you have to allow for non-uniform and non-linear shrinkage, realizing that the gear that pops out of the mold, once it cools, is going to have a significantly different shape than the cavity. The differences are associated with everything from material properties to molding machines, to the habits of the equipment operators themselves.

It doesn't help that the designer or end user is usually isolated from the people who turn raw materials into finished parts. And what a gaggle of people it is. You don't just buy a molded gear, you buy into a convoluted and fragmented process.

It's not uncommon for a molder, for example, to subcontract tooling to a toolmaker who's never molded a gear. Although the molder may offer some guidance, it will typically have less to do with the function of the gear and more to do with such things as mold maintenance and faster cycle times. The bottom line is that you, the designer, will have many decisions made for you without your knowledge and perhaps without your best interests in mind.

One word – plastics

Despite the complexity of the procurement process, there are many reasons to consider plastic gears. Whether you're working on a medical system, semiconductor equipment, handheld tool, office product, or a heavy duty industrial machine, plastics have several things to offer that you're unlikely to achieve with metal.

Cost is obviously one of them. The cost of a molded plastic gear is significantly less than that of a cut steel gear.

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Design flexibility is another. Just about anything is possible with molded plastics, from orbiting compound differential gear sets to asymmetric designs. With plastic, internal gears are no harder to make than external gears and they can carry more load.

Another advantage derives from the physical properties of plastics themselves. Such attributes as chemical resistance, lubricity, low weight, and elasticity add a whole new dimension to gears. Because of plastic's elasticity, for example, a gear can even serve as a vibration damping element.

Plastic gears can also be molded in all sorts of colors. If a gear transmission is going to be visible, it might as well look good. Who knows, one day we may see a trend toward small appliances and handheld tools with translucent or even transparent housings in the image of the stylish new Apple computers.

Don't forget too that plastics are still a relatively young technology. Material scientists are continually developing new grades, materials, additives, and fillers They're also improving consistency. In addition, third-party companies that combine resins from different suppliers have begun to issue custom blends tailored specifically for gears. Some "blenders," such as LNP, even test their materials as gears, providing invaluable data to users as well as manufacturers.

Manufacturing-wise the case for plastic gears is stronger now than ever, with recent improvements in precision tooling, high-tolerance molding machines, and inspection equipment like scanning CMMs. Computers have had an impact too particularly in the area of testing. Today's automated test systems offer new insights into the function of gear transmissions, pushing them to the brink of catastrophic failure to obtain information never before possible.

Don't be fooled

To the unsuspecting designer, the advantages of plastic, unfortunately, can quickly become disadvantages – starting with its versatility and the fact that there are almost no bounds to what you can do with it. Too often the person who gets a brilliant idea has no idea how to design it, and neither do the molders or toolmakers. Once in a while someone will crack open a book, but standard gear design as defined in most texts doesn't work for the typical plastic application.

Then there's the overload associated with choice. Today there's such a vast number of polymers and blends, each with its own unique properties and shrinkages, that the material selection process usually turns into a wild goose chase. At best – and this is where a lot of designers get into trouble – someone will look at a spec sheet and attempt to make assessments and selections based on properties taken out of context. Material properties in gears manifest themselves in a complex manner, however, requiring a more thoughtful approach.

Another pitfall has to do with tooling. Although there's no shortage of opinions about gear tooling, very little is documented. In fact, what's presumably known about tooling and shrinkage is kept secret. There's also a shortage of toolmakers with sophisticated inspection equipment. When you're "cooking" with plastic, the ingredients, the temperature, and even the samples you choose for inspection have to be just right.

Although everyone claims to inspect gears for size and shrinkage, what they really mean is that they employ gear-roll testers which are great for proving that the gear shape stays constant, but nearly useless for telling you what the shape actually is. Molders can and do inspect simple dimensions, but there's usually nothing simple about the dimensions of a molded gear.

The best advice, since there are no standards in the plastics industry for gear design, inspection, shrinkage, and tolerances, is to proceed with caution. If a supplier seems to be maintaining secrets about shrinkage or tooling or molding, you're probably not in control of your destiny. Even something as far removed as the work ethic of the person operating the molding machine can play a major role in determining the outcome of your design.

Last but not least is the trap set by production costs. Though the material cost may be insignificant, the cost of plastic gear tooling can take your breath away. Huge volumes of gears are often required to amortize the tooling investment. Furthermore, because cut plastic gears are often poor approximations of the molded version, it's usually necessary to bite the bullet and mold prototype gears to test their viability. Once in a while you can cheat the system, but you're living dangerously if you try to do it often.

Plastic makes perfect

The good news is that a growing number of designers and OEMs are learning how to get what they want out of plastic gears. Most are finding success by keying in on shrinkage and by taking the time to perform thorough inspection and test for both the part and the mold cavity. Those who once made the mistake of trying to apply standard gear design methods have learned their lesson as well.

What's potentially even more valuable are the lessons and knowledge accumulated through false starts and failures. Manufacturers are learning that if a plastic gear doesn't work right, rather than fight the symptoms, they can more easily and more effectively solve the problem at the source. Take the experience of one manufacturer that did just that.

Engineers at Varitronics, a manufacturer of label makers, were struggling with an indexing transmission that failed to provide consistent precision. After expending significant effort to treat the symptoms, they redirected their attention to shrinkage errors and the possibility that standard gear design wasn't working.

The engineers decided to redesign the gears, with molded plastic construction in mind, and they also had the mold cavities recut for proper shrink. During initial productions, the shrinkage estimates were verified by inspecting both the cavities and gears. Product variations due to plastic gears are now history thanks to an indexing transmission that's also more precise.

Fueled by its success, Varitronics recently embarked on a new project. It did everything by the book. The new gears were designed for accuracy and tolerance relief, and tooled in a single-frame mold with multiple inserts to reduce tooling cost. The tool was carefully inspected as were the first molded gears, revealing that the initial shrinkage estimates were close, but not close enough.

The tool was subsequently re-cut for nominal shrinkage and the gears inspected again to verify size. Although it passed inspection, the engineers didn't stop there. They also performed in-process roll testing to verify process stability and tool wear. The final product, like Microsoft's new Barney doll, is off and running without a hitch. And with what you now know about plastic gearing, yours can be too.

Rod Kleiss is President and Chief Technologist of Kleiss Gears Inc., Shoreview, Minn. Jack Kleiss is President of Kleiss Engineering, Indianapolis, Ind.

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