Planetary gear reducer corrects small misalignments

Speed with low torque characterizes fractional horsepower motors. In applications such as checkout belts in supermarkets, however, no one wants their groceries to go whizzing by without stopping-so such applications get stopping power from gear reducers that apply the necessary torque. Either parallel or planetary gear reducers can be used, but although planetary reducers transmit more torque, typically they cost significantly more than parallel reducers. Now Groschopp engineers have developed a planetary gear reducer with a floating sun gear that they say is cost-competitive with parallel gear reducers, while providing a quieter answer for fractional horsepower motors-all within a 3.5-inch-square housing.

The gear reducer consists of a ring gear around three planetary gears and a floating sun gear. The sintered metal components are two to five times less costly than cut and machined parts and have greater repeatability, says Groschopp Design Engineer Scott Hulstein, who was responsible for the conceptual design. Sintered parts, however, have slightly lower tolerances. For example, tolerances for tooth thickness in a cut gear may be as tight as 0.0002 in., while the sintered part's tolerance can be as loose as 0.002 in. To compensate, Groschopp engineers chose to have mating splines on the motor shaft and sun-gear center. Unlike the planetary gears, which are fixed in place, the sun gear has a toothed central spline loosely fit onto the shaft spline-floating the sun gear. The mated splines correct any slight misalignments-up to 0.001 inch maximum-that result due to the differences in tolerance between the shaft and the sintered gear.

"Compared to cut and machined planetary gear reducers, this reducer provides less tight positioning of the motor shaft. Because sintered gears are less precise than machined parts, positioning of the motor shaft is less precise than it is in other gear reducers," says Matt Van Schouwen, design engineer. Backlash up to 0.75 degrees can occur. "This gear reducer is best suited for applications that do not require precise positioning," he says. For that reason, it is marketed to compete against parallel shaft reducers.

The floating sun gear also cuts down on noise. "When the sun gear is clamped to the motor shaft it's fixed solidly, lead-ing to 'rattle' if tolerances vary at all," says Hulstein. "When it is allowed to roll completely into the planet gears, which happens when the sun gear floats, the operation is quieter."

The company has not published the decibel levels experienced with the gear reducer, because they vary by specific application and test methodology, but claims that the high contact ratio, gear design, and use of powder metal result in sound levels lower than the 70 dBA claimed by competing reducers, given the same ratio and input speed.

Floating the sun gear also helps to keep it centered and reduce gear tooth damage. "Because it's in constant contact with the planets, it needs to be perfectly centered to provide equal load sharing," he says. Hulstein says that the floating design provides true involute action, which he defines as complete rolling motion be-tween the mating gears-a key to fewer broken gear teeth and long life. "A fixed sun gear can achieve this only if every tolerance is perfect."