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issue: August 2006 APPLIANCE Magazine

Motor Technology
Switching Gears


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A traditional harmonic-drive gear mechanism finds a new application in power tool motors.

Johnson Electric engineers have found a new application for harmonic gear drive motors that gives power tool engineers a high reduction gear ratio with a reduced number of components in a small package.

Best known for its lack of backlash and simple construction, the harmonic-drive gearmotor has been used for years in applications requiring great precision, such as robotics and automation. But as supplier Johnson Electric discovered, even a 50-year-old motor technology can have new uses.
According to Roger Baines, a consultant for the Hong Kong motor maker, it is the gear reduction characteristic of harmonic drives that tempted the company to experiment with the use of harmonic-drive gearing in power tool motors. “It has been common to use a long train of gears to reduce the high speed of the motor to a speed more useful for a screwdriver or drill,” Baines explains. “However, because of their length, these trains of gears, even epicyclical systems, tend to defeat the objective of keeping these hand tools small enough for comfortable handling.”
While Baines admits that the gear design most power tools have used for decades is efficient and long lasting, it requires more than one stage of gearing to create the speed reduction needed. The mechanism becomes large in size due to the number of gears and components required.
This is what pointed Johnson Electric engineers to the simplicity of a harmonic gear drive, which only has three main components—a wave generator or harmonic drive, a flexible gear ring and a rigid circular gear ring. Baines says that by using the relative motion between components, the harmonic gear mechanism can achieve large gear ratios with fewer components. “To create an eccentric force, the motor shaft that runs through the center of the mechanism can drive a single cam, or two wheels set apart by 180 degrees,” Baines explains. “This creates an ellipsoid in the flexible gear ring so that the gear teeth in the flexible ring engage with the teeth in the rigid circular gear ring.”
As an example, Baines says that if there are 48 teeth in the flexible gear ring and 50 teeth in the rigid circular gear ring, then, after one revolution of the motor shaft, there will be a two-tooth differential in the angular position of the flexible gear ring and the rigid circular gear ring. “This creates a relative angular motion of one twenty-fifth of a revolution that equates to a 25:1 reduction in motion, which is a substantial gear reduction in a single stage,” he says. “This converts a 15,000-rpm motor into a 600-rpm output drive.”
The gear’s smaller number of components saves costs by using less material and requiring a smaller number of assembly operations. It also provides design freedom. According to Baines, the harmonic-drive motor, whose housing can be made from steel or plastic, is 67 percent shorter in the gearbox section than the typical gear train used in power tools. “This represents an important savings that gives power tool designers many more design options in creating ergonomically designed tools with good hand-holding balance,” he says.

Suppliers mentioned in this article:
Johnson Electric N.A.
 

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