In brief
A planetary gearbox is an epicyclic gear unit with sun gear, planet gears, ring gear and carrier that transmits high torques in a small space.
Contents
What is Planetary gearbox?
In a planetary gearbox, several planet gears mesh simultaneously with the central sun gear and the internally toothed ring gear. The planets are mounted on a planet carrier. Because the load is distributed over several tooth meshes, the gearbox is very compact and power-dense and has coaxial input and output shafts. Depending on which element is held stationary, different ratios result.
Two groups are common in industry. Precision planetary gearboxes are used on servo motors; what matters there is low backlash in arc minutes, torsional stiffness and smooth running. Heavy-duty planetary gearboxes drive mills, conveyors, agitators, cranes and wind turbines. Key parameters are ratio, number of stages, nominal and peak torque, backlash, permissible radial and axial forces at the output, and connection dimensions for motor and machine.
In maintenance, the focus is on bearing damage, gear tooth wear and leaks. In servo gearboxes, wear often shows up as increasing backlash, which causes positioning errors or oscillation in the control loop. In heavy-duty gearboxes, oil condition and bearing temperature are often monitored in order to detect damage to planet bearings at an early stage.
In practice: maintenance and procurement
For servo planetary gearboxes, besides type and ratio, the motor adapter, shaft version and backlash class in particular must be stated so that mounting fits without modification. For heavy-duty gearboxes, the nameplate, serial number and mounting position are the basis for spare parts.
Synonyms and abbreviations: planetary gear · epicyclic gearbox · planetary gear reducer · servo gearbox · planetary gearhead
Frequently asked questions
How does a planetary gearbox work?
A sun gear in the centre drives several planet gears, which also mesh with an outer ring gear. If the ring gear is fixed, the planets travel around the sun gear and take the planet carrier with them, which forms the output. This produces a reduction with coaxial input and output. Several stages in series give higher ratios.
What does backlash mean in planetary gearboxes?
Backlash is the small angle through which the output can be moved back and forth without load while the input is held. It is usually stated in arc minutes. For positioning tasks with servo motors, low backlash is important because it directly affects accuracy. Backlash increases as wear progresses.
What information is needed to replace a planetary gearbox?
Important are type designation, ratio, frame size, number of stages and backlash class, as well as output type such as keyed shaft, smooth shaft or flange, and the motor adapter with shaft diameter and flange dimensions. For heavy-duty gearboxes, mounting position, oil grade and serial number are added. A photo of the nameplate makes identification easier.
What is the difference between a planetary gearbox and a helical gearbox?
In a helical gearbox, one pair of gears meshes at a time, and the shafts are parallel offset or coaxial over several stages. In a planetary gearbox, several planets share the load, which allows smaller dimensions for the same torque. Helical gearboxes are simpler in design and usually less expensive; planetary gearboxes are more compact and stiffer.