Planet Reducer

Planetary gear reducers belong to the epicyclic family of reducers, since the planet gears trace epicycloidal curves in space—very similar to how the popular kid’s toy, the spirograph, works[i]. Simple planetary gears are arranged with a centrally located, externally toothed sun gear that meshes with one or more externally toothed planet gears. As the name implies, the “planet gears” rotate around the “sun gear.” The planet gears also mesh with an internally-toothed ring gear—which encompasses the entire gear set.

CHARACTERISTICS

High torque capacity in a compact package – Due to the load sharing of several planet gears, planetary gear reducers can handle the same torque that a larger parallel axis gear set can take. Although planetary gear sets will likely have more components than a parallel axis gear set, the smaller-size-for-equivalent-torque planetary reducer will be less expensive.

High efficiency – A single stage of planetary gears will likely be over 95 percent efficient.

High reduction ratios – Ratios as high as 11:1 can be achieved in a single planetary gear stage whereas it is difficult to achieve much higher than 5:1 in a single parallel axis stage.  However, it is impossible for planetary sets to achieve a ratio much under 3:1 due to geometric interferences.  A parallel shaft set is not limited on low ratios as they can be as far down as 1:1 (idler gear).

High torsional stiffness – Since the sun gear will likely contact multiple planet gears, the torsional stiffness, or resistance to elastic deformation, is higher in a planetary gear set than parallel axis gear set[ii].

Lubrication flexibility – Planetary gear sets may be lubricated with either oil or grease.  The gears within a planetary set prevent grease from escaping, unlike standard spur gear sets, and constantly redistribute the lubrication allowing for many different mounting orientations[iii].

Integrated housing option – The ring gear in a planetary set may be designed such that it is also the outer housing for the entire gearbox.  Reducing the number of components reduces assembly time and part cost adding to the bottom line.

Coaxial alignment & Direction of Rotation – The output shaft on a planetary gear set is “coaxially-aligned” with the motor shaft meaning they share the same axis.  The planetary output shaft also rotates in the same direction as the motor whereas the output shaft on a single-stage parallel axis gear set rotates opposite of the motor direction.  If an offset of the output shaft is desired, a parallel axis gear set may be added to the planetary set, or a full parallel axis gear set may be used.

Thermal capacity – since the size of a planetary gear reducer is smaller than that of an equal torque capacity parallel axis unit, the thermal capacity will likely not be as high on the planetary gear reducer—making this type of reducer a good fit in intermittent duty applications.  Planetary gear reducers can still run continuously, but measures may have to be taken to prevent the gearbox from overheating.

APPLICATIONS

Construction equipment

Door & gate openers

Food & beverage mixers

Material handling equipment

Medical devices

Packaging equipment

Power tools

Vehicle transmissions

Wind generation

Planetary gear reducers have many positive features and advantages over other types of gear reducers.