Because of their versatility, as well as their ability to speed-up and automate the complex motion sequences, today’s automated computer-controlled machineries, intelligent robots, and majority of the modern automated equipment’s use highly dynamic servo-motors.


Precision planetary gearheads are frequently used in conjunction with such servo motors in order to: balance inertial loading conditions seen during frequent speed cycling sequences, decrease the motor speed, boost the torque, and at the same time, provide a robust mechanical interface for pulleys, cams, drums, and other mechanical transmission components. Actually the servo-motor and gearbox combination became one of the basic “mechatronic” assemblies.


The Planetary (Epicyclical) Gear System as the “System of Choice” for Servo Gearheads

Frequent misconceptions regarding planetary gears systems involve backlash: Planetary systems are used for servo gearheads because of their inherent low backlash; low backlash is the main characteristic requirement for a servo gearboxes; backlash is a measure of the precision of the planetary gearbox.


The fact is, fixed-axis, standard, “spur” gear arrangement systems can be designed and built just as easily for low backlash requirements. Furthermore, low backlash is not an absolute requirement for servo-based automation applications. A moderately low backlash is advisable (in applications with very high start/stop, forward/reverse cycles) to avoid internal shock loads in the gear mesh. That said, with today’s high-resolution motor—feedback devices and associated motion controllers it is easy to compensate for backlash anytime there is a change in the rotation or torque-load direction.


If, for the moment, we discount backlash, then what are the reasons for selecting a more expensive, seemingly more complex planetary systems for servo gearheads? What advantages do planetary gears offer?


High Torque Density: Compact Design

An important requirement for automation applications is high torque capability in a compact and light package. This high torque density requirement (a high torque/volume or torque/weight ratio) is important for automation applications with changing high dynamic loads in order to avoid additional system inertia.


Depending upon the number of planets, planetary systems distribute the transferred torque through multiple gear mesh points. This means a planetary gear with say three planets can transfer three times the torque of a similar sized fixed axis “standard” spur gear system.


Rotational Stiffness/Elasticity

High rotational (torsional) stiffness, or minimized elastic windup, is important for applications with elevated positioning accuracy and repeatability requirements; especially under fluctuating loading conditions. The load distribution unto multiple gear mesh points means that the load is supported by N contacts (where N = number of planet gears) hence increasing the torsional stiffness of the gearbox by factor N. This means it considerably lowers the lost motion compared to a similar size standard gearbox; and this is what is desired.


Low Inertia

Added inertia results in an additional torque/energy requirement for both acceleration and deceleration. The smaller gears in planetary system result in lower inertia. Compared to a same torque rating standard gearbox, it is a fair approximation to say that the planetary gearbox inertia is smaller by the square of the number of planets. Again, this advantage is rooted in the distribution or “branching” of the load into multiple gears meshes locations.


High Speeds

Modern servomotors run at high rpm’s, hence a servo gearbox must be able to operate in a reliable manner at high input speeds. For servomotors, 3,000 rpm is practically the standard, and in fact speeds are constantly increasing in order to optimize, increasingly complex application requirements. Servomotors running at speeds in excess of 10,000 rpm are not unusual. From a rating point of view, with increased speed the power density of the motor increases proportionally without any real size increase of the motor or electronic drive. Thus, the amp rating stays about the same while only the voltage must be increased. An important factor is in regards to the lubrication at high operating speeds. Fixed axis spur gears will exhibit lubrication “starvation” and quickly fail if running at high speeds because the lubricant is slung away. Only special means such as expensive pressurized forced lubrication systems can solve this problem. Grease lubrication is impractical because of its “tunneling effect,” in which the grease, over time, is pushed away and cannot flow back into the mesh.


In planetary systems the lubricant cannot escape. It is continuously redistributed, “pushed and pulled” or “mixed” into the gear contacts, ensuring safe lubrication practically in any mounting position and at any speed. Furthermore, planetary gearboxes can be grease lubricated. This feature is inherent in planetary gearing because of the relative motion between the different gears making up the arrangement.


If you want to learn further details about servo gearheads, you can continue to visit Or you can visit the well-known manufacturer for servo gearhead – Jia Cheng Precision Machinery Co., Ltd. for more information.


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