Machine Design

Planetary reducers eliminate machining step to keep costs down

A face-mount (MM7300SA) model in the NEMA-34 configuration (threaded holes in the reducer face and a 0.75-in.diameter, 1.5-in.-long shaft with a standard 0.1875-in. keyway). Other options include an offset shaft, mounting from the reducer side via through holes in the reducer flange, base mounting, and custom shaft and flanges. In the design and prototype stage is an industrial-grade, right-angle planetary reducer for use in tight spaces. It's expected sometime this year.

A maker of hand-operated movers for boats and airplanes needed a motor drive able to provide a high power density. A quarterhorsepower parallel-shaft gearmotor proved too large and a servodrive was considered overkill.

The solution came through use of a new planetary gear reducer from Groschopp, Sioux Center, Iowa. It operates at input speeds to 4,000 rpm for servolike performance at about the same cost as a parallel-shaft gearmotor. In another application involving an RV slide out, the 377:1 ratio planetary gearmotor routinely operates (intermittently) at output torques to 1,200 lb-in.

The new planetary units target uses where less precision is acceptable but cost is important. The design meshes a sun gear with three planet gears. Such configurations tend to last longer than parallel-shaft spur geartrains and better handle larger continuous and peak torque. Tough involute gears are pressed net shape from a sinter-hardened, powder metal which eliminates secondary machining and the associated cost.

Another important design consideration is heat management because the amount of heat generated in a gearbox is proportional to transmitted power, gear ratio, and mechanical efficiency. The Motion Master 7300SA planetary unit is lubricated for life with an advanced synthetic-blend lubricant that remains stable at extreme temperatures. The lube lets the gearbox run cooler than those filled with grease but still permits operation at temperatures to 150F (60°C). A low internalfriction and high oil-viscosity index combine to reduce frictional power loss by 5% in some cases. No breather vent is required provided the reducer operates within specified thermal limits.

Groschopp engineers also specified a relatively higher gear-contact ratio as a means of lowering noise and boost torque capacity. Continuous-duty torque ranges from 90 to 700 lb-in. with corresponding peak torques three to five times these levels. This makes the reducer ideal for intermittent or slower applications at high torque levels.

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