Retrotechtacular: Planetary Gears Are Out of This World

If you’ve ever been to a Maker Faire, perused Thingiverse, or been within 100 feet of a 3D printer, you’ve probably seen a plastic set of planetary gears with a little crank or maybe a set of hyperbolic planetary gears. Those don’t serve much of a purpose aside from looking cool and starting conversations, but the ones you’ll find if you open a pencil sharpener or an automobile gearbox certainly do. ‘But Hackaday,’ you ask, ‘how do they work?’. Once again, the United States Army Information Corps is here to tell you in their inimitably no-nonsense, mid-century, black-and-white style with a training film from 1953.

We can already hear you crying foul: ‘Part II?! Surely you jest, Hackaday.’ No, and don’t call us Shirley. Either Part I is forever lost to the annals of time, or it’s so utterly spectacular that it can’t be released to the public. Not even [Jeff Quitney] has found it yet. But Part I is important and probably explains what planetary gears are in addition to outlining the five governing laws of planetary gear systems that Part II recalls.

Planetary or epicyclic gears are named after their original purpose of modelling the five known planets of the solar system as observed by the ancient Greeks. In its most basic form, a planetary gear system is made up of a sun gear in the center, planet gears that mesh with and revolve around the sun gear, and and outer ring gear or annulus that meshes with the planet gears. The interactions between these components are easy to see in this animation. Changing the radio button will fix that part in place, and you can watch the way that this affects the other components.

This week’s film explains that multiple sets of planetary gear systems in differing sizes are used to gear automobiles for different forward speeds as well as reverse operation. This gearing is accomplished by putting one or more sets in reduction mode.

The case of reverse is an interesting problem that is explained once in overview and then again more carefully. In the interest of saving both space and weight, a compound gear system is used. If you can appreciate that holding part of the system firmly in place will change the behavior of the other parts, you’re well on your way to understanding the method by which a compound planetary gearing system sends reverse motion to the wheels.

Essentially, the ring gear (that’s the big one around the outside) of the intermediate gear system is connected with a hollow shaft to the sun gear of the reverse unit. A second shaft inserted into the hollow shaft connects the planet carrier of the intermediate system to that of the reverse system. The output shaft becomes the reactionary in this arrangement. When the planet gears of the intermediate system are held reactionary, the ring gear of the its system moves in a direction opposite that of its sun gear. This is where reverse motion begins.

If the ring gear of the reverse system is held in place with clutch, then it becomes the reactionary. Thus, the planetary gears of the reverse system must follow the motion of their sun gear. This reverse motion from the intermediate system is transferred through the reverse gear system and out to the wheels. When the vehicle is shifted out of reverse, the reverse gear system is taken out of the picture. Once a forward gear is engaged, the reverse system will act as a pass-through, transferring forward motion in whatever gear it is fed from the intermediate gear system.

The rest of the film deals with the effects of dual input to the system. That is, what occurs on the output when the ring gear is given a constant input speed and the sun gear a variety of input speeds. The output of the system will always be between the fixed speed of on the ring gear and whatever input is given to the sun gear. It will never exceed the speed of the ring gear. If the sun gear is run in the reverse direction of the ring gear, the planet gears and thus the output shaft will follow the direction of the component with the most teeth, which is always the ring gear.

However, if the rotation of the sun gear is increased to match the speed of the ring gear, the output shaft will stop completely. If the sun gear’s speed cranked up past this tipping point, the output shaft will move again in the opposite direction. This principle is crucial to the differential system that allows a tank to do donuts in place.

[Thanks Hans for the tip!]

Retrotechtacular is a weekly column featuring hacks, technology, and kitsch from ages of yore. Help keep it fresh by sending in your ideas for future installments.

15 thoughts on “Retrotechtacular: Planetary Gears Are Out of This World

  1. Having worked in a factory which manufactured Automatic Transmissions, it’s just amazing how all these fit together with a series of hydraulically controlled clutches, etc to control all these planetarys.

    I specifically was responsible for the machining of the Valve Bodies (Think a 3D PCB for hydraulic fluid where the transistors are valves attached to solenoids and diodes are little plastic balls)

  2. Software for designing gears

    Sizing calculations are available for all common gear types (cylindrical gears, bevel gears, worm gears, helical gears, hypoid gears, and crown gears). For cylindrical gears, the software also works with planetary sets and gear racks. In addition to a feature for reviewing calculations with respect to standards (ISO, DIN, AGMA, VDI, Klingelnberg), the program also offers a number of different design and optimization functions.
    Also includes full transmission design/analysis

    Gearotic <- This guy is in it for the fun and hobby, but is also knowledgeable.
    Spur, Helical. Bevel, and Crown; Involute, Cycloidal, Random, Geometry, CNC

    Spline, Spur, Helical, Spiral Bevel, Gleason; Involute, Spirol?; Geometry, Inspection, Stress, CNC

  3. YouTube up Toyota’s Synergy Drive. Spend some time looking at models, teardowns, etc. One vid I saw uses actual parts on a demo bench with clear illustration into the messy differential of engine, motor, and drivetrain.

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