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Failed Plastic Worm Gear
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LRT is a Gearbox Specialist, Mechanical Engineer, Product Design Engineer, Gears and Power Transmissions Design and Development Consultant, Gearbox Technology Consultant with world-class expertise in gearbox technology, design and development of gears and power transmissions.
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The task at hand was to design a new cost-reduced worm gear drive for an automotive application.
The worm was made out of steel and mated with a worm gear which was over-molded onto a drive spindle. The chosen material was an unfilled acetal.
During the 5 endurance tests the worm gears failed between 30% to 50% of the required cycles. The gearboxes were torn down and the parts were inspected.
The gears showed excessive wear and had eventually sheared off. On all 5 samples about half of the worm gear tooth was worn away on one side, but on only 4 to 6 teeth. This meant that the damage happened in the acceleration phase when the torque was the highest.
The old design had a flexible shaft between the motor and the worm which dampened the impact. On the new design this flexible shaft was removed for cost reduction and the worm and motor shaft were one piece.
The amperage draw was then measured against the acceleration time on the new and old gear drives. The production gearbox had an acceleration time of 230 msec and the new design only 75 msec. This meant that the acceleration torque had increased by 35%.
The Hertzian stress or surface contact stress was calculated and the results showed that the surface endurance limit of the gear material was at the limit. Based on these results the pressure angle was changed from 10° to 15°, which lowered the Herztian pressure by about 22%.
This type of wear is also present if one of the gears has a rougher surface than the other. In this case it was the metal worm. The worm surface finish was measured at about 30 to 35 ģin. A good surface finish range for steel gears wearing against plastic gears is 10 to 15 ģin.
The worm was produced by a cutting process. Roll-forming the worm would create a much better surface finish. Therefore the process was changed.
The surface finish of the roll-formed worms was measured at 8 to 10 ģin.
With the implementation of these two changes, lifecycle testing was performed again. All 5 endurance tests finished without incidents. The gearboxes were taken apart and it was noticed that there was no wear on the gears.
The problem was solved and the production tooling for the worm and gear were released.
Read other articles by this KKAI Associate:
Design of New Cost Reduced Gearbox
Development of Plastic Servo Cover
| Gearbox Specialist, Mechanical Engineer, Product Design Engineer, Gears and Power Transmissions Design and Development Consultant, Gearbox Technology Consultant, gearbox technology, design and development of gears and power transmissions. | |
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Kevin Kennedy & Associates, Inc.
Rapid Response Engineering® Solutions
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