Wheels: Plastics replace metals
Second-gen power lift gate uses high-precision plastic shafts and gears
An innovative transmission from Hi-Lex America continues to reliably and quietly open and close automotive lift gates thanks to design and development assistance from Ticona Engineering Polymers, which helped the automotive tier supplier create new injection-molded high-precision plastic shafts and gears.
"For several years, Hi-Lex customers have used a two-stage reduction transmission that relies on precision steel ball bearings mounted on plastic shafts and accompanying 2.5-in.-diameter plastic gears to achieve the desired reduction between the electric motor and a flexible torsional cable assembly to lift and close big, heavy automotive rear lift gates," says Fred Eberle, technical engineer at the Hi-Lex Automotive Center in Rochester Hills, MI.
Hi-Lex is a leading supplier of automotive electromechanical, power closure devices, and control cables.
"A first-generation model used a powder metal shaft system, which was subsequently replaced with a plastic shaft system that provides an immense improvement in NVH (noise, vibration, and harshness) and makes our customers very happy," says Eberle.
With input from plastic gear experts at Ticona, the American Gear Manufacturers Association, Ohio State University Gear and Power Transmission Research Laboratory, and gear software developer Universal Technical Systems, Hi-Lex optimized and refined the second-generation lift gate transmission via extensive research and development and testing. It relies on first- and second-stage plastic splined shafts attached to plastic gear wheels in a gearbox with a rated torque output capacity of about 12 N-m (106 in.-lb).
The second-generation lift gate transmission assembly.
"From the outset, we knew we wanted to design the transmission with plastic gears to reduce cost, weight, and especially noise," Eberle says. "We knew Ticona had the gearing and material expertise to help Hi-Lex achieve its business goals."
To avoid any timely and costly setbacks during development of the two-stage transmission, Hi-Lex established a plastic gear team early on that included Eberle and David Sheridan, senior design engineer, Ticona. The team defined, analyzed, and highly optimized the design to take advantage of many considerations unique to plastic gearing. These considerations can be significantly more complex than those for standard metal gears, and they pose challenging implications for gear and drive designers.
"Although Eberle has extensive experience and skill in regards to gearing and drive development, the Ticona / Hi-Lex gear team took the design and optimization process to the highest level of geometry analysis, material application, and plastic gear experience," Sheridan says. "Ticona used its 50-plus years of plastic gear experience to help Hi-Lex develop this high-precision plastic gear transmission."
Once the requirements were defined, the team identified materials for the gears and shafts that would maintain the required mechanical properties and dimensional stability, even at elevated temperatures.
The gear team selected Ticona grades of Celcon acetal copolymer (POM) and Celstran long-fiber-reinforced plastics (LFRT):
- Celcon M90, a general-purpose, unreinforced POM that provides a good balance of toughness and fatigue strength for the first-stage gear and shaft;
- Celcon GC25T, a 25% glass-coupled POM that provides the necessary stiffness and fatigue strength for the second-stage output gear; and
- Celstran PA 66-GF50-02, a 50% long-glass-fiber polyamide 66 with high strength and stiffness combined with high heat deflection for the second-stage output shaft.
"For several years, original equipment manufacturers have used our power lift gate transmissions in a variety of automotive platforms," Eberle says. "Not once has a gear broken, worn out, or failed in any of our testing, even with multiple design lives, or in service.
"The gears are nice and quiet. In fact, they get even quieter at elevated temperature, exactly like they were designed to do."
"It's nice when things work out the way you expect them to," adds Sheridan. "You know you have it right when the design behaves the way the analytical models predicted it would."
Want more information about plastic gear design? Designfax suggests downloading the "Ask The Ticona Gearheads" plastic gear design guide (.pdf) pictured here.
Published February 2012
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