The new Barney doll from software giant Microsoft is chock full of electronic wizardry that's essentially useless if three plastic gear transmissions don't work right. Versatile and inexpensive, plastic gears make all sorts of machines possible today unlocking design creativity like never before.

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.

State-of-the-art plastic gear design and a little ingenuity have come together in the development of a planocentric or orbiting drive. This particular gear set achieves a 40:1 reduction, and even though it has no glass filling, it can run continuously, delivering 30 lb-in. of output torque. The output is taken off the wobbling external gear.

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.

Scanning coordinate measurement machines are ideal for profiling precision gears. The CMM shown, from Electronic Measurement Devices, Flanders, NJ., has a 40-in. volumetric accuracy and is capable of both video and analog contact scanning. In the inset, a plastic gear and the cavity insert from which it was molded are set up for inspection.

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.

Continue on Page 2