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Orbitless drive

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From top to bottom, a Parallel-Axis, Planetary, and Orbitless drive. Each has a 3:1 ratio although the Parallel-Axis drive ratio is negative so the output spins in reverse.

An orbitless drive is a patented [1][2] epicyclic drive which scales the speed and torque of an input shaft by a ratio. It is sufficiently efficient[3] to be back-driveable, and may be used as speed reducers or overdrive mechanisms.

Like a planetary drive, the input shaft drives a central sun gear which in turn, drives a group of planet gears that ride on a planet carrier. Unlike a planetary drive, the planets do not engage a ring (orbit) gear. Instead, a second carrier which is misaligned with respect to the first carrier prevent the planets from rotating. They circulate around the sun at a fixed orientation while transferring mechanical energy between the sun and carrier.

Theory of operation[edit]

The reduction ratio (i) of the orbitless drive with a pinion sun is obtained from the following formula, where Zs is the number of sun gear teeth, Zp is the number of planet gear teeth, the sun is the input, and the carrier is the output[4]. The resulting ratio is half the ratio obtained from a planetary gear using similar pinions.

When the sun is a ring gear, the reduction ratio is as follows[4]. Since Zs must be larger than Zp, the ratio is always positive and less than unity so it acts as an overdrive. Like any overdrive, it may be used as a reducer by interchanging the roles of the input and output shafts. The same ratio results if the sun is a pinion and is engaged with the planets through a non-direction-reversing transmission path such as a belt, a chain, or an idler gear. In this case, a negative or very high overdrive ratio is possible.

Properties[edit]

Gearing systems may be categorized as either low-ratio (close to 1:1) or high ratio (usually above 10:1). There are a large number of high-ratio configurations, but only three low-ratio configurations. In fact, many high-ratio configurations are modified versions of a low-ratio configuration, such as a compound Planetary drive or a worm drive. Low-ratio configurations are typically used when efficiency and back-drive-ability are desired. Higher ratios are often achieved by connecting multiple low-ratio drives in series.

Each of the three low-ratio gearing configurations have distinguishing properties which motivate how they are typically used.

Parallel-Axis[edit]

  • Axial offset
  • Single gear mesh
  • Negative ratio

Planetary[edit]

  • Co-axial
  • Two gear meshes per parallel transmission path
  • Positive ratio

Orbitless[edit]

  • Co-axial or small axial offset (optional)
  • Single gear mesh per parallel transmission path
  • Positive ratio

Compared to a planetary drive, the absence of a ring gear and the inclusion of a second carrier offset one another in terms of cost and complexity. Load capacity is generally between that of a parallel axis and planetary drive. Similar to a planetary drive, compound planets may be used to achieve high reduction ratios[5]. Similar to a parallel-axis drive, the absence of internal teeth enables the use of belts and chains.

Any gear geometry, such as spur, helical, or herringbone may be used, but axial loads must be supported by thrust bearings, similar to a parallel-axis drive.

A unique property of orbitless geometry is that none of the planet axes are required to be in the centre of the planet. Both may be offset to accommodate larger planet bearings.

Compound Planets[5][edit]

The addition of an idler gear between the sun and planet gears reverses the direction of meshing between the sun and planets. This inverts the ratio whereby, for speed reduction, the carrier is the input and the sun is the output. A high ratios is possible when the sun and planet have a similar pitch diameter (number of teeth). This technique for converting a low-ratio drive into a high ratio drive by adding an idler gear is applicable to epicyclic gearing in general and is used in certain planetary configurations such as the Wolfram drive.

The sun may be configured to be a ring or a pinion, but higher ratios are obtained when the sun is a pinion so it may be similar in size to the planets. The reduction ratio (i) with a pinion sun is obtained from the following formula, where Zs is the number of sun gear teeth, Zp is the number of planet gear teeth, the carrier is the input, and the sun is the output.

When the sun and planet differ by a single tooth, the ratio is equal to the number of teeth in the sun gear. When the sun is larger than the planet, a positive ratio is obtained (the input and output shafts rotate in the same direction). When the sun is smaller than the planet, a negative ratio is obtained (the input and output shafts rotate in opposite directions). If the sun and planet have an equal number of teeth, an infinite ratio is obtained and the output shaft (sun) does not rotate at all.

The reduction ratio (i) with a stepped idler gear is obtained from the following formula, where Zs' is the number of idler gear teeth in the step that engages the sun gear and Zp' is the number of idler gear teeth in the step that engages the planet gear. An identical ratio is obtained if Zs' and Zp are interchanged.

Many high ratio drives such as harmonic or cycloidal drives involve a pinion engaging a ring. In order for the associated gears to differ by one tooth, the tooth geometry must be modified to avoid mechanical interference. Since all the gears in a compound orbitless drive are pinions with external teeth, high ratios may be obtained without tooth modifications.

Unlike a conventional orbitless drive where the planet axes may be placed anywhere on the planet, a compound orbitless drive requires at least one centrally located planet axis because the idler gears must be supported by a central carrier. Any gear geometry, such as spur, helical, or herringbone may be used, but axial loads must be supported by thrust bearings, similar to a parallel-axis drive, although the idler gears will experience some cancellation of thrust forces.

See also[edit]

References[edit]

  1. Stocco, Leo. "US Patent 9,970,509 B2".
  2. Stocco, Leo. "Orbitless Gearbox", Chinese Patent ZL 2015800376718.
  3. Stocco, Leo (2017). "A New Standard of Epicyclic Efficiency - A Practical Comparison of Planetary and Orbitless Gear-Heads". Forschung im Ingenieurwesen. Vol 81, Issue 2-3: 153–161.
  4. 4.0 4.1 Stocco, Leo (2016). "The Orbitless Drive" (PDF). ASME International Mechanical Engineering Congress & Exposition.
  5. 5.0 5.1 Stocco, Leo (2016). "The Coupled Orbitless Drive" (PDF). ASME International Mechanical Engineering Congress & Exposition.


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