Plastic gears can bring many advantages to power transmission designs, says UFE Senior Product Engineer Andrew Ulrich. First among them is cost-effective manufacturing by molding rather than hobbing, the way in which metal gears are traditionally made. Molded gears can use special custom geometries that simply aren't available from the standard rack of hobbed tooth forms.
Engineers considering plastic gears as lighter, quieter, cheaper alternatives to metal need to be aware of several points concerning plastics, according to UFE, which recently created a downloadable brochure listing nine MAQs, or "must ask questions."
One point is the way in which temperature affects gear growth. "A pair of plastic gears on a 2-inch center distance could vary by as much as 0.015 inch over 100F of temperature change," Ulrich says.
Point number eight advises: "To take advantage of long addendum gears, it is normal for plastic gears to operate on extended center distances . . . your final design should minimize the undercut and target a specific sliding ratio of 3.0 or less."
In other words, the teeth can be designed to use different properties of the involute to let them expand and shrink without creating too much or too little backlash, Ulrich explains.
Another point to consider is loads, Ulrich says. Be they intermittent, continuous or standing, loads on plastic gears demand calculation of the flexure rate for the material and tooth design, point number five advises.
Standing loads, such as the kind a motorist might apply to a power window by cinching up on it, are best avoided in plastic gears, especially where heat's a factor, Ulrich says. When a standing load can't be sidestepped, a reinforced resin can be used to support the continuous force better over a wider range and temperature, the document states.
And speaking of temperatures, point number four tells us that the relationship between speed and load should be evaluated carefully. Unlubricated plastic gears at high speeds can undergo changes in material properties due to heat from friction. That's not to say high speeds are out of the question, Ulrich adds. Speeds of 10,000 and even 20,000 rpm are certainly within reach of plastic gears, he says.
UFE Marketing Manager Steven Druley says about 80 percent of the company's customers are thinking plastic for their initial designs, while the rest are looking to plastics as metal gear replacements.
The company created its Parts Smarts document to help engineers with infrequent plastic gear experience sift through the technical considerations on their way to realizing plastic's full potential, Druley adds.