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Comparing Cycloidal and Planetary Gearboxes

Posted by: Willie Costa on Feb. 13, 2011

Gearboxes are, on the surface, relatively simple mechanisms that adjust the operating speed and torque of an application’s prime mover. However, understanding the two major types of gearboxes is essential for lowering the cost and increasing the performance of an application.

Precision gearboxes have a long and illustrious history in the robotic and industrial automation histories. Mated with servomotors, gearboxes allowed engineers to control the heavy loads and high cycle rates found in these applications with a great deal of precision. Today, there are two primary types of gearboxes that dominate these applications: the cycloidal and the planetary. These are discussed in detail below.

Cycloidal gearboxes
A cycloidal gearbox (or cycloidal reducer) is comprised of four components: the input shaft, a cycloidal cam (single or compound), cam followers, and an output shaft. The input shaft attaches to a drive member that induces an eccentric rotation of the cycloidal cam; in compound reducers, the first cycloidal cam engages a second cycloidal cam (double reduction), which may then engage a third cycloidal cam (triple reduction). The cam followers act as gear teeth, and will exceed the number of cam lobes. Cycloidal gearboxes offer ratios from as low as 10:1 to over 300:1 without stacking stages in the manner of a planetary gearbox, and thus cycloidal gearboxes may have a more compact footprint. A typical cycloidal gear is seen here:

Planetary gearboxes
A planetary gearbox is comprised of three members: a sun gear, multiple satellite or planet gears (hence the gearbox’s name), and an internal ring gear. The input shaft attaches to the sun gear, which transmits rotational motion to the planet gears, which in turn rotate the internal ring gear, which is part of the gearbox housing. Planet gears rotate on rigid shafts attached to a plate called a planet carrier; this rotation of the planet carrier is what causes the output shaft to rotate. As with all mechanical speed reduction, this gives the output shaft a lower rotational speed and higher torque than the input shaft. Planetary gearboxes may also be single- or double-reduction, with reduction ratios ranging from 3:1 to over 100:1. Additional stages can be added for even higher reduction, or to change output shaft orientation (i.e. via bevel gearing, miter gearing, etc.). A typical planetary gearbox is seen below:

Comparing the two
To select the right type of gearbox, engineers and application specialists should consider more than just cost. Of primary concern is the level of precision required in the application. If accurate positioning with minimal backlash are key design drivers, the application would be best served through a cycloidal gearbox. Minimising backlash can also allow the servomotor to handle high-cycle, high-frequency applications. Cycloidal reducers also have immense shock load tolerance – up to 500% in many cases – which planetary gearboxes can’t hope to match.

The application’s reduction ratio must also be considered, especially with regard to equipment footprint. For reduction ratios ranging from 3:1 to 100:1, planetary gearboxes offer the best “torque density” (amount of torque available per cubic foot of space taken up by the unit), unit weight, and precision. Despite the shock load capacity of cycloidal units, few cycloidals are available in gear ratios much less than 30:1. Conversely, for ratios above 100:1, few planetary gearboxes can be found that match a cycloidal’s torque density, due to the fact that stage stacking is rarely needed with a cycloidal, so the gearbox can be shorter and less expensive.

How a gearbox will physically mate to the application should also be a consideration. Planetary gearboxes are usually offered with square frames that mate precisely with servomotors; however, planetary gearboxes grow in length as reduction ratio increases. Conversely, cycloidal reducers increase in diametre as reduction increases, but do not generally increase in length. Because of this, compound-reduction cycloidals can be shorter – albeit wider – than their planetary equivalents with the same reduction ratio.

These guidelines provide advice for a preliminary gearbox selection. Choosing the right gearbox also involves examining bearing capacity, torsional stiffness, shock load presence and duration, environmental considerations, duty cycle, and expected life of the gearbox. Mechanically, gearboxes have become something of an accessory to servomotors; however, care must be taken to select the appropriate gearbox for the application, in order to ensure that the drive unit will return the best value per dollar spent. Either a cycloidal or a planetary gearbox can be used in any application that employes servomotors or stepper motors, provided that the correct research and selection processes have been followed.

Summary

Benefits of planetary gearboxes

· High torque density

· Load distributed between planet gears

· High efficiency

· Low input inertia

· Low backlash

· Low cost

· Smaller diametre compared to cycloidal gearbox of equal ratio

Benefits of cycloidal gearboxes

· Zero- or very-low backlash throughout the life of the application

· Rolling contact, rather than sliding contact

· Low wear

· High shock load capacity

· High torsional stiffness

· Shorter overall length, compared to planetary gearboxes of equal ratio