Patent Application
20220154804
In published patent application no. WO2013173928A1 a device is shown which increases torque with two rows of roller-based planets all of which are contacting two other roller-based planets and at a high enough number of planets that a low camming angle is achieved. Below this angle, the camming action which occurs when the device is loaded, increases the force between the geared or rolling members and the contact pressure at the contacts between the inner and outer planets and between the inner planets and the inner race and between the outer planets and the outer race.
Achieving a coefficient of friction that is high enough to allow this camming action to happen is a challenge, because many common material combinations, such as steel on steel, have a lower Coefficient of Friction (CF) than necessary for a typical camming angle for this device. As a result materials such as nickel alloys or other material combinations must be used to achieve a high enough CF to allow the camming angle geometry to provide a tractive pressure that is proportional to the torque being transmitted.
Another challenge with a rolling contact version is to keep the planets all equally circumferentially spaced. A rolling contact does not “clock” itself relative to the other planets, and the two rows of planets are inherently unstable if the circumferential spacing of the planets is not controlled. By unstable, what is meant is that the inner race will not stay concentric with the outer race if the planets become unequally spaced.
Another challenge of embodiments of a roller-based gearbox is that bearings are required to keep the outer race axially aligned with the inner race.
Geared devices such as conventional gear reducers will commonly use a planet carrier with shafts and bearings to position the planets. A planet carrier adds rotational mass, cost and complexity.
There is provided a gearbox device having an inner race having an outer surface and defining an axis and an outer race having an inner surface and coaxial with the inner race. The gearbox device has a set of orbital planets including inner planets in geared contact with the outer surface of the inner race and outer planets in geared contact with the inner surface of the outer race, each and every inner planet being in geared contact with two outer planets, and each and every outer planet being in geared contact with two inner planets. There may be an input ring coaxial with the inner race and outer race and in geared contact with the inner planets or with the outer planets.
In one embodiment, one of A or B is the case in which A is the outer planets are longer than the inner planets and each outer planet has a respective first portion that meshes with the inner planets with which it is in contact, and the input ring has an outer surface that meshes with a respective second portion of each outer planet with which it is in contact, both the first portions and the second portions of the outer planets meshing with the outer race; and B is inner planets are longer than the outer planets and each inner planet has a respective first portion that meshes with the outer planets with which it is in contact, and the input ring has an inner surface that meshes with a respective second portion of each inner planet with which it is in contact, both the first portions and the second portions of the inner planets meshing with the inner race.
In another embodiment, the inner and outer planets have a length in geared contact, and the gears and races have respective diameters, selected to cause torque on the input ring to cause increased radial loading of the inner and outer planets sufficient to overcome a separating force between the gears caused by the torque on the input ring.
In another embodiment, at least one of the outer surface of the inner race and the inner surface of the outer race are formed of two angled gear surfaces having different helix angle. The two angled gear surfaces may be positioned on axially adjacent components. This arrangement may be used to enable the components to be moved axially into gear meshing contact with the planetary gears, easing assembly.
Various embodiments are directed to a gearbox comprising: a sun gear defining an inner race on an exterior surface thereof, wherein the sun gear defines an axis between a first end and an opposite second end of the sun gear; a ring gear defining an outer race on an interior surface thereof, wherein the ring gear is coaxial with the sun gear; an inner set of planets in geared contact with the inner race of the sun gear; an outer set of planets in geared contact with the outer race of the ring gear; wherein each of the inner set of planets is in geared contact with at least two of the outer set of planets and each of the outer set of planets is in geared contact with at least two of the inner set of planets; and an intermediate gear defining an intermediate race in geared contact with one of: (a) the inner set of planets or (b) the outer set of planets; and wherein one of the sun gear, the ring gear, and the intermediate gear is held stationary.
In certain embodiments, the inner set of planets each have a first axial length measured parallel to the axis of the sun gear; and the outer set of planets each have a second axial length measured parallel to the axis of the sun gear, wherein the second axial length is different than the first axial length; and wherein the intermediate race is in geared contact with a longer axial gear set of: (a) the inner set of planets or (b) the outer set of planets. In various embodiments, the inner set of planets and the outer set of planets having a length in geared contact, and the inner set of planets, the outer set of planets, the inner race, the outer race, and the intermediate race having respective diameters selected to enable torque provided via one of the sun gear, the ring gear, or the intermediate gear to cause increased radial loading of the inner set of planets and the outer set of planets sufficient to overcome a separating force caused by the torque. Moreover, in certain embodiments, the at least one of: (a) the inner set of planets or (b) the outer set of planets each have a length-to-diameter ratio greater than 1:1. In certain embodiments, the inner set of planets and the outer set of planets each comprise two differently tapered portions.
In various embodiments, the inner set of planets and the outer set of planets each define helical gears. In certain embodiments, the inner set of planets and the outer set of planets each define helical gears having a constant helix angle. Moreover, the inner set of planets and the outer set of planets may each define helical gears having differing helix angles along an axial length. In various embodiments, the inner set of planets and the outer set of planets each define herringbone gear patterns. Moreover, the intermediate gear may comprise two axially adjacent components each having a respective angled gear surface corresponding to a portion of the herringbone gear patterns, wherein the two axially adjacent components are fastened to one another. In certain embodiments, the ring gear comprises two axially adjacent components each having a respective angled gear surface corresponding to a portion of the herringbone gear patterns, wherein the two axially adjacent components are fastened to one another. Moreover, the sun gear may comprise two axially adjacent components each having a respective angled gear surface corresponding to a portion of the herringbone gear patterns, wherein the two axially adjacent components are fastened to one another.
In certain embodiments, the gearbox device further comprises at least one inner fence configured to axially constrain the inner set of planets. In various embodiments, the gearbox further comprising at least one outer fence configured to axially constrain the outer set of planets. In various embodiments, the inner race, the outer race, the intermediate race, and exterior surfaces of each of the inner set of planets and each of the outer set of planets all define a plurality of gear teeth separated from adjacent gear teeth by gear roots, and wherein at least a portion of the gear roots define radial slots. In certain embodiments, each of the inner set of planets and each of the outer set of planets are hollow.
Various embodiments are directed to a multi-stage gearbox device comprising a plurality of gearbox devices as discussed herein, wherein the plurality of gearbox devices are arranged in stages such that a first ring gear of a first gearbox device is connected to and drives a second intermediate gear of a second gearbox device.
Certain embodiments are directed to a gearbox device comprising: a sun gear defining an inner race on an exterior surface thereof, wherein the sun gear defines an axis between a first end and an opposite second end of the sun gear; a ring gear defining an outer race on an interior surface thereof, wherein the ring gear is coaxial with the sun gear; an inner set of planets in geared contact with the inner race of the sun gear; an outer set of planets in geared contact with the outer race of the ring gear; wherein each of the inner set of planets is in geared contact with at least two of the outer set of planets and each of the outer set of planets is in geared contact with at least two of the inner set of planets; and an intermediate gear defining an intermediate race in geared contact with one of: (a) the inner set of planets or (b) the outer set of planets; and wherein the inner race, the outer race, the intermediate race, and exterior surfaces of each of the inner set of planets and each of the outer set of planets all define a plurality of gear teeth having a continuous helix angle.
In various embodiments, the gearbox further comprising at least one inner fence attached to the sun gear and configured to constrain axial movement of the inner set of planets. In certain embodiments, the at least one inner fence comprises two inner fences each secured on opposing axial ends of the sun gear. In various embodiments, at least one outer fence attached to the ring gear and configured to constrain axial movement of the outer set of planets. In certain embodiments, the at least one outer fence comprises two outer fences each secured on opposing axial ends of the ring gear. Moreover, each of the axial ends of the inner set of planets may have a hemispherical shape and the at least one inner fence has a curved shape corresponding to the hemispherical shape of the axial ends of the inner set of planets. In various embodiments, the intermediate gear is an output ring and one of the sun gear or the ring gear is driven by an input motor. In certain embodiments, the inner set of planets each have a first axial length measured parallel to the axis of the sun gear; and the outer set of planets each have a second axial length measured parallel to the axis of the sun gear, wherein the second axial length is different than the first axial length; and wherein the intermediate race is in geared contact with a longer axial gear set of: (a) the inner set of planets or (b) the outer set of planets. Moreover, the at least one of: (a) the inner set of planets or (b) the outer set of planets each have a length-to-diameter ratio greater than 1:1. In certain embodiments, each of the inner set of planets and each of the outer set of planets are hollow.
Various embodiments are directed to a gearbox device comprising: a sun gear defining an inner race on an exterior surface thereof, wherein the sun gear defines an axis between a first end and an opposite second end of the sun gear; a ring gear defining an outer race on an interior surface thereof, wherein the ring gear is coaxial with the sun gear; an inner set of planets in geared contact with the inner race of the sun gear; an outer set of planets in geared contact with the outer race of the ring gear; wherein each of the inner set of planets is in geared contact with at least two of the outer set of planets and each of the outer set of planets is in geared contact with at least two of the inner set of planets; and an intermediate gear defining an intermediate race in geared contact with one of: (a) the inner set of planets or (b) the outer set of planets; at least one inner fence attached to the sun gear and configured to axially constrain the inner set of planets; and at least one outer fence attached to the ring gear and configured to axially constrain the outer set of planets.
In various embodiments, each of the axial ends of the inner set of planets has a hemispherical shape and the at least one inner fence has a curved shape corresponding to the hemispherical shape of the axial ends of the inner set of planets. Moreover, each of the axial ends of the outer set of planets has a hemispherical shape and the at least one outer fence has a curved shape corresponding to the hemispherical shape of the axial ends of the outer set of planets. In certain embodiments, the inner set of planets each have a first axial length measured parallel to the axis of the sun gear; and the outer set of planets each have a second axial length measured parallel to the axis of the sun gear, wherein the second axial length is different than the first axial length; and wherein the intermediate race is in geared contact with a longer axial gear set of: (a) the inner set of planets or (b) the outer set of planets.
In certain embodiments, the intermediate gear is an output gear, and one of the sun gear or the ring gear is driven by an input motor. In various embodiments, the at least one of: (a) the inner set of planets or (b) the outer set of planets each have a length-to-diameter ratio greater than 1:1. Moreover, the inner set of planets and the outer set of planets each define helical gears. In certain embodiments, the inner race, the outer race, the intermediate race, and exterior surfaces of each of the inner set of planets and each of the outer set of planets all define a plurality of gear teeth separated from adjacent gear teeth by gear roots, and wherein at least a portion of the gear roots define radial slots. In certain embodiments, each of the inner set of planets and each of the outer set of planets are hollow.
Certain embodiments are directed to a gearbox device comprising: a sun gear defining an inner race on an exterior surface thereof, wherein the sun gear defines an axis between a first end and an opposite second end of the sun gear; a ring gear defining an outer race on an interior surface thereof, wherein the ring gear is coaxial with the sun gear; an inner set of planets in geared contact with the inner race of the sun gear; an outer set of planets in geared contact with the outer race of the ring gear; wherein each of the inner set of planets is in geared contact with at least two of the outer set of planets and each of the outer set of planets is in geared contact with at least two of the inner set of planets; and an intermediate gear defining an intermediate race in geared contact with one of: (a) the inner set of planets or (b) the outer set of planets; and wherein each of the inner set of planets and each of the outer set of planets have a stiffness greater than a stiffness of each of the sun gear, the ring gear, and the intermediate gear, such that one or more of the sun gear, the ring gear, or the intermediate gear deforms to balance radial loads on the inner set of planets and the outer set of planets.
In certain embodiments, each of the inner set of planets and each of the outer set of planets comprise a metal material. Moreover, one or more of the sun gear, the ring gear, and the intermediate gear comprise a plastic material.
In various embodiments, the inner set of planets each have a first axial length measured parallel to the axis of the sun gear; and the outer set of planets each have a second axial length measured parallel to the axis of the sun gear, wherein the second axial length is different than the first axial length; and wherein the intermediate race is in geared contact with a longer axial gear set of: (a) the inner set of planets or (b) the outer set of planets. In certain embodiments, the at least one of: (a) the inner set of planets or (b) the outer set of planets each have a length-to-diameter ratio greater than 1:1. In various embodiments, the inner set of planets and the outer set of planets each comprise two differently tapered portions. In certain embodiments, the inner set of planets and the outer set of planets each define helical gears. In certain embodiments, the inner set of planets and the outer set of planets each define helical gears having a constant helix angle. In various embodiments, the inner set of planets and the outer set of planets each define helical gears having differing helix angles along an axial length. Moreover, the inner set of planets and the outer set of planets each define herringbone gear patterns. In certain embodiments, the intermediate gear comprises two axially adjacent components each having a respective angled gear surface corresponding to a portion of the herringbone gear patterns, wherein the two axially adjacent components are fastened to one another.
In various embodiments, the ring gear comprises two axially adjacent components each having a respective angled gear surface corresponding to a portion of the herringbone gear patterns, wherein the two axially adjacent components are fastened to one another. In various embodiments, the sun gear comprises two axially adjacent components each having a respective angled gear surface corresponding to a portion of the herringbone gear patterns, wherein the two axially adjacent components are fastened to one another. Moreover, the gearbox may further comprise at least one inner fence configured to axially constrain the inner set of planets. In various embodiments, the gearbox device further comprising at least one outer fence configured to axially constrain the outer set of planets. In certain embodiments, the inner race, the outer race, the intermediate race, and exterior surfaces of each of the inner set of planets and each of the outer set of planets all define a plurality of gear teeth separated from adjacent gear teeth by gear roots, and wherein at least a portion of the gear roots define radial slots. Moreover, each of the inner set of planets and each of the outer set of planets may be hollow.
Various embodiments are directed to a gearbox device comprising: a sun gear defining an inner race on an exterior surface thereof, wherein the sun gear defines an axis between a first end and an opposite second end of the sun gear; a ring gear defining an outer race on an interior surface thereof, wherein the ring gear is coaxial with the sun gear; an inner set of planets in geared contact with the inner race of the sun gear; an outer set of planets in geared contact with the outer race of the ring gear; wherein each of the inner set of planets is in geared contact with at least two of the outer set of planets and each of the outer set of planets is in geared contact with at least two of the inner set of planets; and an intermediate gear defining an intermediate race in geared contact with one of: (a) the inner set of planets or (b) the outer set of planets; and wherein each of the inner set of planets and each of the outer set of planets have a stiffness less than a stiffness of each of the sun gear, the ring gear, and the intermediate gear, such that one or more of the sun gear, the ring gear, or the intermediate gear deforms to balance radial loads on the inner set of planets and the outer set of planets.
In certain embodiments, each of the inner set of planets and each of the outer set of planets comprise a metal material. Moreover, each of the inner set of planets and each of the outer set of planets are hollow. In various embodiments, each of the inner set of planets and each of the outer set of planets comprise a plastic material. Moreover, in certain embodiments, the inner set of planets each have a first axial length measured parallel to the axis of the sun gear; and the outer set of planets each have a second axial length measured parallel to the axis of the sun gear, wherein the second axial length is different than the first axial length; and wherein the intermediate race is in geared contact with a longer axial gear set of: (a) the inner set of planets or (b) the outer set of planets. In various embodiments, the at least one of: (a) the inner set of planets or (b) the outer set of planets each have a length-to-diameter ratio greater than 1:1. In certain embodiments, the inner set of planets and the outer set of planets each comprise two differently tapered portions. In certain embodiments, the inner set of planets and the outer set of planets each define helical gears. In various embodiments, the inner set of planets and the outer set of planets each define helical gears having a constant helix angle. In certain embodiments, the inner set of planets and the outer set of planets each define helical gears having differing helix angles along an axial length. In certain embodiments, the inner set of planets and the outer set of planets each define herringbone gear patterns. Moreover, the intermediate gear may comprise two axially adjacent components each having a respective angled gear surface corresponding to a portion of the herringbone gear patterns, wherein the two axially adjacent components are fastened to one another. In certain embodiments, the ring gear comprises two axially adjacent components each having a respective angled gear surface corresponding to a portion of the herringbone gear patterns, wherein the two axially adjacent components are fastened to one another. Moreover, the sun gear may comprise two axially adjacent components each having a respective angled gear surface corresponding to a portion of the herringbone gear patterns, wherein the two axially adjacent components are fastened to one another.
In certain embodiments, the gearbox device further comprising at least one inner fence configured to axially constrain the inner set of planets. In certain embodiments, the gearbox device further comprising at least one outer fence configured to axially constrain the outer set of planets. In various embodiments, the inner race, the outer race, the intermediate race, and exterior surfaces of each of the inner set of planets and each of the outer set of planets all define a plurality of gear teeth separated from adjacent gear teeth by gear roots, and wherein at least a portion of the gear roots define radial slots. In various embodiments, each of the inner set of planets and each of the outer set of planets are hollow.
Various embodiments are directed to a method of assembling a gearbox device, the method comprising: placing a set of outer planets in geared contact with an inner surface of an outer race; placing a set of inner planets in geared contact with the outer set of planets, each and every inner planet being in geared contact with two outer planets, and each and every outer planet being in geared contact with two inner planets; placing a first component of an inner race in geared contact with the inner planets and coaxial with the outer race, the first component having a first angled gear surface; placing a second component of an inner race in geared contact with the inner planets and coaxial with the outer race, the second component having a second angled gear surface, the first angled gear surface and the second angled gear surface having different helix angle; and placing an input gear in geared contact with the outer planets and coaxial with the outer race. In certain embodiments, the first angled gear surface and the second angled gear surface have opposite helix angles that collectively form a herringbone gear surface. In various embodiments, the input gear comprises a first input gear component having a first angled input gear surface and a second input gear component having a second angled input gear surface, and the step of placing an input gear in geared contact with the outer set of planets and coaxial with the outer race comprises placing the a first input gear component coaxial with the outer planets and with the first angled input gear surface in geared contact with the outer set of planets, and placing the a second input gear component coaxial with the outer set of planets and with the second angled input gear surface in geared contact with the outer set of planets, the first angled input gear surface and the second angled input gear surface having different helix angle.
In certain embodiments, the first angled input gear surface and the second angled input gear surface have opposite helix angle to together form a herringbone input gear surface. Moreover, the first angled input gear surface may be placed into geared contact with the outer planets before the step of placing the set of inner planets in geared contact with the outer set of planets, and the second angled input gear surface is placed into geared contact with the outer set of planets after the steps of placing the first input gear component and the second input gear component of the inner race in geared contact with the inner planets.
Moreover, certain embodiments are directed to a method of assembling a gearbox device, the method comprising the steps of: placing a set of inner planets in geared contact with an outer surface of an inner race; placing a set of outer planets in geared contact with the inner set of planets, each and every outer planet being in geared contact with two inner planets, and each and every inner planet being in geared contact with two outer planets; placing a first component of an outer race in geared contact with the inner planets and coaxial with the inner race, the first component having a first angled gear surface; placing a second component of an outer race in geared contact with the outer set of planets and coaxial with the inner race, the second component having a second angled gear surface, the first angled gear surface and the second angled gear surface having different helix angle; and placing an input gear in geared contact with the inner set of planets and coaxial with the inner race.
In certain embodiments, the first angled gear surface and the second angled gear surface have opposite helix angles collectively forming a herringbone gear surface. Moreover, according to certain embodiments, the input gear comprises a first input gear component having a first angled input gear surface and a second input gear component having a second angled input gear surface, and the step of placing an input gear in geared contact with the inner planets and coaxial with the inner race comprises placing the a first input gear component coaxial with the inner set of planets and with the first angled input gear surface in geared contact with the inner planets, and placing the a second input gear component coaxial with the inner set of planets and with the second angled input gear surface in geared contact with the inner planets, the first angled input gear surface and the second angled input gear surface having different helix angle. In various embodiments, the first angled input gear surface and the second angled input gear surface have opposite helix angle to together form a herringbone input gear surface. Moreover, in certain embodiments, the first angled input gear surface is placed into geared contact with the inner set of planets before the step of placing the set of outer planets in geared contact with the inner planets, and the second angled input gear surface is placed into geared contact with the inner set of planets after the steps of placing the first input gear component and the second input gear component of the outer race in geared contact with the outer planets.
Reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
The present disclosure more fully describes various embodiments with reference to the accompanying drawings. It should be understood that some, but not all embodiments are shown and described herein. Indeed, the embodiments may take many different forms, and accordingly this disclosure should not be construed as limited to the embodiments set forth herein. Immaterial modifications may be made to the embodiments described here without departing from what is covered by the claims. Rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.
Embodiments of the present device eliminate the need for a planet carrier by transmitting torque from an inner fixed ring to an outer output ring directly through two rows of planets. The gear reduction ratio is determined by the difference between the OD of the inner ring and the ID of the outer ring with the inner and outer planets acting as a torque transfer load path between them. As the planets are caused to orbit, the outer ring will rotate at a ratio such as approximately 3:1 or possibly lower, or up to approximately 6:1 or possibly higher. The closer the OD of the inner ring is to the ID of the outer output ring, the greater the reduction ratio.
Embodiments of the device disclosed here use a combination of features to provide equal circumferential spacing as well as axial alignment of the planets and races as well as eliminating the need for additional bearings in some applications or reducing the strength (and therefore the cost and weight) of the additional bearings by virtue of the interaction of the planets and races providing axial alignment from the inner race to the outer race. Furthermore, embodiments of the device disclosed here provide a structure that applies a magnetic force directly to the planets to eliminate the need for a separate motor rotor where the planets themselves act as the rotor with a reduction ratio because they are orbiting at a higher speed than the output ring. This eliminates the need for a sun ring input which simplifies the manufacturing and assembly of the motor-gearbox combination. The fact that the planets (and therefore the contained magnets) are spinning is not believed to be a significant detriment because they are still providing magnetic flux to the airgap and stator.
Embodiments of the device use gears or lobes that are small enough and numerous enough to provide what acts and feels more like a rolling contact than a gear. In the claims, the term “lobes” also encompasses the term “gears”. Lobes have the advantage of providing a high surface area in the radial direction (as opposed to a gear that has gear teeth which act like wedges). In an example, the pressure angle of the lobes or gears may be greater than 20, 30 or 40 degrees. In an alternate configuration, high angled gears can be used instead of lobes. By configuring the gears or lobes in a herringbone configuration, a number of characteristics can be achieved, including: circumferential planet spacing as a result of the gear-specified circumferential positioning of the planets; axial alignment of planets to races and inner planets to outer planets as a result of the herringbone helical gears; and the ability to eliminate or reduce the need for a bearing between the inner and outer races because the herringbone gears on the planets provide multi-axis (i.e., radial and axial location) constraints. The use of permanent magnets (PMs) in the planets allows one or more (e.g., two) electromagnetic stators positioned on axial ends of the device to be commutated in such a way as to impart rotational torque and motion to the planets, and by doing so to generate torque on the outer ring (using the inner ring as a fixed reference in these non-limiting examples, although it is understood that the outer ring can be used as the fixed reference and the inner ring can be the output ring. It is also understood that the stator(s) can be attached to the inner or outer ring regardless of which one is fixed and which one is the output).
A typical conventional differential gear with a planet carrier cannot include PMs in the planets because the differential gearbox requires bearings and shafts in the planets. Furthermore, if a conventional planetary gear (with a single circular array of planets) uses PMs in the planets together with a fixed sun gear it will act as a speed increaser rather than as a reducer.
In
The axially locating member 24 need not separate the magnets. The member 24 merely prevents the magnets from moving together. If separated, such as with two simple cylindrical PMs that are separated by a ring of plastic (if plastic gears are used) to form axially locating member 24, then there needs to be a soft magnetic material disk 112 (e.g., steel) between them.
The axially locating member 24 is preferably molded or fabricated as one piece with at least an inner portion 114 (inner diameter) of the planets. The entire planet can be formed as a single piece, or the gear faces of the planet may be one or more separate pieces into which the inner portion 114 is inserted. A soft magnetic member, such as a steel disk 112, is preferably used as a flux linkage path between the two magnets. In certain embodiments, the PMs may have a smaller diameter cylindrical end section instead of the soft magnetic material disk. Simple cylindrical magnets are considered to be less expensive to build, and the use of a steel disk spacer for flux linkage between them allows this disk to be easily adjusted to the ideal thickness (whereas PMs are more difficult to machine to the same tolerance).
The embodiment shown in
This single row of magnets configuration has alternating polarities of the magnets in a single array of PM planets.
The stator may have a plurality of poles. Each pole may be embodied as, for example, an electromagnet having a post, or an air coil. For a conventional three phase motor, the stator has a number of poles divisible by 3 (the term “poles” or “posts” when referring to the stator, refers to each individual post and coil, or coil, if air coils are used). It can also be useful to have the number of poles divisible by 4, so if the number of poles is both divisible by 3 and divisible by 4, the number of poles is divisible by 12.
The number of rotor posts (rotor posts, here, refers to the number of planets with permanent magnets of alternating polarity relative to adjacent planets with magnets) is then based on the number of stator posts and, for a concentrated winding, the number of rotor posts is greater than or lesser than the number of stator posts. For example −2 or +2, but −4 or +4 is preferred, because this distributes the magnetic force around the air gap to reduce the bending load on the stator. Other differences will work also.
Here, the number of rotor posts is the number of planets with magnets in them, which is typically either the number of total planets or the number of planets in one of the rows of planets.
An example of a suitable number of planets in a row, in an embodiment with magnets in one row of planets, is 16, as shown in
The embodiments shown in
Although this device could possibly be configured to work with traction surfaces, the use of lobes as for example shown in
The pressure angle of certain device 10 embodiments can significantly affect the loading of the gears/lobes 30, 32. Because there is a self-camming action (as discussed herein) when the device 10 (gearbox) is loaded and designed in accordance with certain embodiments places a radial load on the planets 16, 18. With a pressure angle below a lower threshold, there is risk that the gear teeth bind, such that the device 10 does not rotate or produces high friction. With a pressure angle above an upper threshold, the gears/lobes 30, 32 become too shallow to take significant load, which may result in skipping of the gears/lobes 30, 32 under load. The lower threshold and upper threshold are affected by the camming angle of the inner and outer planets 16, 18.
Adjusting the pressure angle may increase or decrease radial forces in order to improve load sharing among the plurality of planets 16, 18 or to increase the longevity of the planets 16, 18 or other gears in a device 10 (e.g., by decreasing the loads experienced by the gears). For example, in a device 10 incorporating planets having a low stiffness (e.g., comprising a low-stiffness material or a low-stiffness configuration), the material stiffness alone provides a significant amount of deflection to allow for load sharing between the planets 16, 18 or the device 10. Providing gears/lobes 30, 32 having a high pressure angle would increase the radial load on the planets 16, 18, but decrease the bending load on the planets 16, 18. Such a configuration may increase the longevity of the planets 16, 18 in implementations in which tooth root bending is identified as a critical failure mode. More typically, one would want to decrease the radial load on the less-stiff planets 16, 18 and make use of a lower pressure angle of the gears/lobes 30, 32 to maximize the overall stiffness of the device 10. In a device 10 having high stiffness planets 16, 18, additional radial load may be utilized to ensure there is enough deflection of the planets 16, 18 (or other gears/rings as discussed herein) to ensure load sharing between the planets 16, 18. As an example, utilizing a higher pressure angle may provide sufficient deflection of the planets 16, 18 to increase load sharing among the planets 16, 18.
It may be beneficial to have a higher pressure angle for the planets 16, 18 due to the decreased stress concentration at the tooth root. This would translate to a lower stress in the tooth root, which may increase the longevity of the planets 16, 18.
A simplified example of a high effective pressure angle lobe profile is shown in
High effective pressure angle—In a conventional gearbox, a high pressure angle would result in a high separating force between the gears during torque transfer. In embodiments of the device, the lobe pressure angle is low enough to increase the effective friction coefficient of the contact areas so a camming angle is established. Once this critical effective friction coefficient (EFC) is established, the self-energizing effect will cause the planets to increase the traction pressure rather than to slide or skip.
As described in WO2013173928A1 (the content of which is incorporated by reference herein), each of the inner race and outer race may be circular and centered on an axis. A traction angle øi may be defined as follows: for each pair of a first inner planet 16 that contacts a first outer planet 18, the traction angle øi is defined as the angle between a first line extending outward from the axis through a center of the first inner planet 16 and a second line extending from the contact point of the first outer planet 18 with the outer race 14 and a contact point of the first inner planet 16 with the inner race 12. Orbital motion the planets 16, 18 leads to differential motion between the inner race 12 and outer race 14, and thus torque forces are transmitted between the inner race 12 and outer race 14 via the planets 16, 18. The torque forces are transmitted between the contact points of adjacent planets 16, 18 and thus are transmitted at the traction angle having a ratio of a circumferential component to a radial component equal to the tangent of the traction angle øi. Thus, as described in WO2013173928A1, for traction surfaces if a coefficient of friction between the inner race 12 and inner planet 16 is greater than the tangent of the angle, the torque will generate a radial component sufficient to maintain traction between the inner planet 16 and the outer planet 18 as the torque increases, without requiring a large preload or any additional mechanism to increase radial loading. This is referred to herein as the “camming effect”; a device 10 exhibiting this camming effect may also be referred to herein as “self energizing” (e.g., a self-energizing gearbox).
With gears or lobes 30, 32 on the planets 16, 18, the coefficient of friction between the surfaces of the planets 16, 18 is not relied on to create a self-energizing effect to keep the planets 16, 18 from rotationally sliding on each other. Instead, the gears or lobes 30, 32 serve to time the planets 16, 18 to each other and to their respective races.
In an embodiment shown in
Embodiments of the present device 10 incorporate a geared contact between the two rows of planets 16, 18 and between planets and races. This geared contact allows a larger camming angle and potentially higher torque transmission. One challenge to be solved with a geared contact is that the radial compression between geared components can result in non-conjugate motion, and high friction and cogging as a result of the wedging effect of teeth of one planet acting as wedges that are being forced between the receiving teeth of the meshing planet. This wedging effect results in a high mechanical advantage of the radial force between the planets planar to the gear contact faces resulting in high friction and wear. Forcing gears together radially will also result in a variable friction force as the mechanical advantage changes throughout different phases of the gear tooth contact during planet rotation. This variable friction force can result in cogging and irregular wear.
A new gear tooth profile for the device provides a combination of rolling contact at a coefficient of traction, combined with an involute gear tooth profile that provides the rest of the torque transfer not provided by the rolling contact.
The use of a cylindrical rolling contact surface between the gear teeth and if used with spur gears, will reduce the amount of geared contact (i.e., it will reduce the contact ratio). At a high enough percentage of cylindrical rolling contact, a geared contact ratio of less than 1 will occur. Up to this ratio, it is difficult or impossible to achieve a rolling contact ratio of greater than 1. The use of a helical tooth pattern as described here, can provide a continuous rolling contact between gears as well as a continuous geared contact for smooth rolling contact and uninterrupted geared torque transmission. Helical teeth having helical direction at different axial portions of planets can form herringbone teeth.
It should be noted that embodiments may use a camming angle and coefficient of friction that allows the rolling surfaces to transmit a high percentage of the torque. In other applications, it may be preferable to use a camming angle and CF which does not result in a self-energizing effect. In this case the gear teeth may provide a greater percentage of the total torque.
Reasonable performance has been shown with a relatively simple gear tooth profile that uses a sine wave shape gear form. This shape can be a pure sine wave or an approximate sine wave such as a series of linked arcs which form lobes. With a high enough number of lobes, the height of the teeth is short enough to reduce the sliding motion between the gear teeth while providing enough surface area at the tips and roots of the lobes in the radial direction for smooth rolling contact. For example, the lobe height may be less than 1/20, 1/30 or 1/40 of a radius of a gear, for example an inner planet gear 16 or outer planet gear 18. The use of a high helix angle provides a consistent radial contact and consistent torque transmission surface area in the tangential direction. When this lobed shape is used with the self camming geometry of the present device 10, the traction angle will determine how much of the torque transmission is provided by the tangential contact and how much is provided via traction of the tooth roots in semi-rolling contact with the tooth tips.
Embodiment of the device 10 provide rigid torque transmission, even when constructed from plastic. The rotational stiffness potential of embodiments of the device 10 are believed to be much higher than is possible from a conventional planetary gear train. This is because the torque is transferred from the inner gear 16 to the outer gear 18 along a nearly straight line though the inner planets 16 and outer planets 18. This straight line torque transfer is shown in a simplified FEA analysis in
Increased radial preload may increase stiffness, but also increase rolling friction. Increased rolling friction is not always beneficial, but there are cases where increased rolling friction may be helpful. In machining, for example, it is desirable to prevent backdriving of the gearbox as a result of tool load or vibration. In other uses, like applications where a safety brake is needed, high preload can be used to make the gearbox non-backdriveable below a certain backdrive torque. This reduces the cost and complexity and power consumption of a brake which must be disengaged with an electric current, for example.
In one example, a self-energizing portion of a device comprises a stationary inner sun gear meshing with a plurality of spaced inner planets 16 (e.g., 17 equally spaced inner planets 16), which in turn mesh with a corresponding number of spaced outer planets 18 (e.g., 17 equally spaced outer planets 18). The outer planets 18 then mesh with a race of the outer ring. The input of this stage is the orbit of the planets 16, 18, while the output is the motion of the outer ring. The input stage drives the planets 16,18 in the self-energizing stage by using a planetary gear. This stage uses the sun gear as an input, the planet rotation as the output, and an idler outer ring. In an example embodiment, a 45° helix angle is used in a herringbone configuration for each of the gears, however other helix angles (whether provided in a herringbone configuration, a continuous helix configuration, a changing helix-angle configuration, a spur-gear configuration, and/or the like).
The diameters and number of gear teeth used in the embodiment having a 45° helix angle is embodiment are shown in Table 1.
Traction and geared configs of embodiments of the device are described in published patent application no. WO2013173928A1. Various embodiments as discussed herein include configurations using geared input and gear tooth profiles and configurations to provides benefits which include effective ways to keep planets equally spaced (circumferentially and axially), ways of minimizing part count through a non-symmetric input, and a simplified way of increasing reduction ratio though a non-symmetric sun ring input to the inner or outer planet arrays.
The use of geared contact between the planets and ring gears keeps them equally spaced circumferentially. Moreover, in accordance with certain embodiments, the use of herringbone gear or lobe teeth prevents movement of the gears in the axial direction. This allows the gears to be used as a bearing for relative location of the inner fixed gear and the outer output gear in both the radial direction and the axial (thrust bearing) direction.
Moreover, this combination of herringbone gears or lobes provides the ability to drive the inner or outer planets from only one side of the gearbox without significant twisting of the planets about a radial axis of the device 10. By using a gear 90 in
In an example of how a non-limiting exemplary embodiment of the device can be assembled, the following describes one way the device can be assembled if the geometry is created according to the principles described here.
Order of assembly is as follows, and indicated by boxes with step numbers in
To operate this non-limiting example embodiment, turning the sun gear and holding inner ring will cause outer ring to spin at a reduced ratio of approximately 7:1.
If the outer planets are driven by the sun gear, as shown here, input by a larger gear than the outer planet diameter as shown here, it is preferable to have the smallest dimension of the larger sun input ring gear larger than the OD of the fixed ring gears. In this way, assembly of the gearbox is enabled because the two halves of the inner fixed ring (4, 5) can be “threaded” together from either side of the inner row of planets after the inner sun gear ring member (2) is threaded onto the larger sun input planet gears from the inner plane outward as described above. Furthermore, if the OD of the inner fixed ring is smaller than one half of the sun input ring, the sun input ring gear assembly can be a herringbone profile so it requires no bearing. The inner half of the sun input ring can be “threaded” into engagement with the sun gear input planet gears from the inside of the assembly before the yellow inner planets are inserted, and then the other half of the sun gear herringbone can be threaded on from the outside bolted to the first half of the sun gear after the inner (yellow) row of planets has been inserted and the two halves of the inner fixed gear herringbone has been assembled from both axial ends.
This configuration would make the assembly fully constrained in the axial direction, however such a configuration does not necessarily balance the axial loads as seen on the planets since it not a symmetric herringbone arrangement.
In order to minimize the axial loads on the planets, one of three design constraints may be implemented:
In embodiments in which the helix angle of the gears is kept constant along the axial length of each gear, the gear mesh between the inner planets and stationary sun gear has a different length relative to the gear mesh between the inner planets and the stationary sun ring; and the gear mesh between the outer planet and the input sun gear has a different length relative to the gear mesh between the outer plant and the input sun ring. These lengths may be selected to reduce axial forces.
In embodiments in which the axial length is maintained as constant, the gear mesh between the inner planets and stationary sun gear has a different helix angle relative to the gear mesh between the inner planets and the stationary sun ring; and the gear mesh between the outer planet and the input sun gear has a different helix angle relative to the gear mesh between the outer planet and the input sun ring. These lengths may be selected to reduce axial forces.
In embodiments in which neither the helix angle or the axial length is maintained as constant, the gear mesh between the inner planets and stationary sun gear has a different length and helix angle relative to the gear mesh between the inner planets and the stationary sun ring; and the gear mesh between the outer planet and the input sun gear has a different length and helix angle relative to the gear mesh between the outer plant and the input sun ring. These lengths may be selected to reduce axial forces.
While there are many potential benefits of this device 10, at this point it has been shown by the inventors that there are no known gear combinations that provide a perfect gear mesh. Each solution has some amount of error in one or more parameters such as the gear diameter, module, meshing contact, and/or the like.
Some gearing solutions have errors that would be less than the manufacturing tolerances of the individual gear parts. The number of solutions that have such a low error is limited though, and it is desirable to have additional solutions.
So far, over 100 million combinations of planet numbers and gear tooth numbers on planets and gear rings has been tested with no perfect solutions found. This has required that the possibilities be narrowed down to the least imperfect possibilities.
The constraints for selecting a usable combination include the following:
The diameter differential of the sun and the outer ring is large enough to provide a reduction ratio between the inner fixed ring and outer output ring of greater than 2:1 (2 orbits of the planet results in 1 or more rotations of the output ring). Planet numbers range from min of 5 to max of 30, although there are additional solutions beyond this range of planets.
A gear tooth pitch of greater than 0.7 mm (this is to allow manufacturing by common gear production methods including injection molding).
An outer ring OD of approximately 89.25 mm was set as constant, knowing that the gear diameters can be scaled to larger or smaller diameters as required. By the application. This diameter was selected as one that is of useful size for the robotics market.
Only non-perfect solutions have been found. The imperfection in the gear combinations shows up as either an imperfect alignment of the gear teeth or a mismatch in the module of the meshing gears. Typically, the inner row of planets will mesh well with the inner fixed gear, and the outer row of planets will mesh well with the outer output ring gear, but the inner planet teeth will be misaligned to the outer planet row gears. Some misalignment can be tolerated due to the compliance/flexibility of the materials (and the resulting compliance/flexibility of the constructed components) but the greater the misalignment, the lower the torque transmission capacity of the gearbox and the greater the friction due to interference between the gears.
The use of more, smaller teeth increases the number of potential options, but small gear teeth make manufacturing and assembly more difficult and small teeth may also reduce torque transmission in some cases.
The use of fewer planets increases the manufacturability of the planets, but more planets allows for a larger maximum torque assuming the load is shared between planets and provides additional solutions.
With all of these considerations taken into account, the number of usable combinations is surprisingly low. An inaccuracy index was used to compare the different options with the index indicating how misaligned the planet-to-planet mesh is for a given option.
The potentially usable configurations have been limited to those solutions with an RMS error factor of less than 0.0004 and are shown in the following table. Error factors higher than shown will be appropriate for certain applications. In addition, the shown configurations may be scaled geometrically while keeping the number of teeth constant.
The error factor reflected within the data shown in Table 2 below accounts for both angular error and diameter errors. The ratio given assumes that the input is the rotation of the input sun, with the inner ring held stationary and the outer ring as the output.
Selecting for gear/lobe teeth of at least 1 mm (which allows for manufacturing such as by 3D printing) and minimizing the error factor, a discovery analysis according to the algorithm produced only hundreds of positive options from several hundred million possibilities that were examined
Other less ideal options are shown.
Included in the gear combinations are gears that are both in phase and out of phase. Including out of phase gears significantly increases the number of solutions when compared to only in phase solutions. Additionally, the error factor tends to be lower in out of phase solutions. Out of phase refers to a configuration where a pinion meshes with a ring gear at a different phase of the gear tooth mesh as compared to another pinion meshing with the same ring gear, for a given rotational position of the output.
In certain embodiments, higher error factors can be accommodated to provide a functional gearbox having components that appropriately move relative to one another by providing gear teeth on various gear surfaces that are subject to a profile shift. Using concepts of profile shift, gears may be designed having an adjusted center distance between two meshing gears, having adjusted designs encompassing undercuts in a gear with too few teeth, and/or having an adjusted amount of sliding in a gear mesh. In such a case, the same tool can be used to manufacture the gear, but its placement with respect to the center axis of the gear is changed. This results in a slightly different tooth form as shown in
By making use of profile shift, a higher error threshold can be used for generating gearbox solutions for a device as discussed herein, while maintaining functionality of the device, even with gearing combinations that may otherwise render the device nonfunctional.
Profile shift is also used to optimize efficiency in a gear mesh by adjusting the amount of sliding that occurs in the gear mesh. By minimizing specific sliding, each gear tooth will have the smallest amount of sliding in the mesh, increasing efficiency and minimizing potential issues with wear failures. Commonly, this will shift the profile of one gear in a pair inward (−x) while shifting the mating gear profile outward (+x). The sum of these shifts in most applications tends to be very near zero in order for the system as a whole to work well.
In devices of self-energizing gearboxes provided in accordance with embodiments as discussed herein, the sum of these shifts is related to the amount of error in the gearbox solution. However, the magnitude of the sum of the profile shifts of gears within designs may increase to accommodate increased error factors while providing a functional self-energizing gearbox. Accounting for included shifts, the gear mesh between adjacent gears maintains an optimum point for its specific sliding as in a typical gearbox. In embodiments as discussed herein, there are significantly more gear meshes as compared to a typical planetary gearbox having analogous dimensions, meaning that once the error is accounted for in profile shift, any additional shift in one gear will correspond to a complimentary shift in every other gear. As such, the gear meshes are not optimized for specific sliding individually, but as a whole. This has the added benefit of allowing the profile shift due to the error in the gear mesh be spread between multiple gear meshes.
In certain example embodiments, the planet gears may be provided with a positive profile shift to effectively increase the pressure angle of each included gear tooth and to reduce the stress concentration at the root of each tooth in order to increase the life of the planet gears.
Separate components were designed and 3D printed to be fixed to the stationary and input components of the gearbox in order to test output torque capabilities.
As shown in
To prevent bending of the planets, the self-energizing gears can be positioned on either side of the input as shown in
In certain embodiments, a gearbox may have a single sided input, as well as a single helical tooth pattern (no herringbone pattern is necessary). The input drives the outer planets in this embodiment, and the planets cause a differential movement of the inner and outer rings, one of which is fixed and the other is the output.
Helical gears typically require thrust bearings or an opposing helix angle (herringbone pattern) to maintain axial positioning of the gears. However, these require space and weight. Embodiments as discussed herein encompassing helical gears that float axially have been found to demonstrate axial positioning stability during use. The inventors found that self-energizing gearbox configurations having the double row of planet gears with no planet shafts creates a situation where the axial forces on one gear mesh on a planet are canceled out (opposed) by the opposite helical interface on the other gear mesh on each planet. That is, the axial forces on the outer planet from the inner planet cancel with the axial forces on the outer planet from the outer ring, and the axial forces on the inner planet from the outer planet cancel with the axial forces on the inner planet from the inner ring. The result is much lower axial forces on the planet than would be the case with a planetary gearbox having a single row of planets.
While the axial forces on the planets cancel, the axial forces on the input, inner, and outer rings do not cancel and other elements, such as for example thrust bearings, can be used to bear the axial load on these elements.
A significant advantage of a single angle helix is that the gearbox can be assembled. A herringbone pattern is very difficult to assemble if not impossible. The single helical angle allows the gears to be slid together axially and the cancellation of the axial forces allows them to operate without shafts or bearings.
When torque is applied to the input ring 138, there is a torsional twisting load transferred to the outer planets 136, in addition to a rotational torque transfer to each of the planets around their individual axes. As a result of the self-energizing (or camming) effect between the inner ring 130 and outer ring 134 through the two rows of planets, the gears on two rows of planets and the inner ring 130 and outer ring 134 are forced into engagement proportionally more as the torque output of the device increases. At a certain length of outer geared planet and a certain reduction ratio, the self-energizing effect which causes the gears to mesh together will have a greater straightening effect on the outer planets than the twisting effect of the input from the input sun gear 138. The length of the longest planets may correspond to an overall width of the device in the axial direction. This combination of width and reduction ratio can be calculated by someone skilled in the art to ensure that the meshing of the outer planets 136 with the outer ring 134 straightens the outer planets 136 when torque is applied to the input gear 138 as a result of the output torque that is transferred from the inner ring 130 to the outer ring 134 which causes the camming effect to push the gears into mesh rather than the separating force of the gears causing them to unmesh which would allow them to twist. Because of the gear ratio of the gearbox, the input torque on the input gear 138 will be significantly lower than the torque transferred through the planets 136 and 132. At a ratio of 7:1, the input torque would be roughly 1/7 of the output torque. As a result, the dominant force in the outer planet 136 will be the load due to the torque transfer from the inner ring 130 to the outer ring 134. The radial load component from the camming effect ensures that the contacting gear tooth of the outer planet 136 is forced radially into the corresponding gear tooth in the outer ring 134. This radial load causes the straightening effect that counteracts the twisting effect due to the input torque from the input gear 138. This effect is stronger with a higher pressure angle in the gear teeth or a higher camming angle due to the resulting increase of radial load in the gears.
The greater the aspect ratio of the pinon length to planet diameter, the less likely the planets are to twist as a result of the twisting force from the sun ring input. This relationship exists for two reasons. Generally speaking, the greater the aspect ratio for a given gearbox OD and width, the smaller the pinon diameter and therefore the higher the reduction ratio. As a general trend, the higher the reduction ratio, the greater the radial forces on the pinons which can be used to generate a deeper mesh between the pinons and the rings as compared to the decreased twisting force that is generated by the input of the sun, because of the increased reduction ratio which requires lower torque at the sun ring input, and therefore the greater the aligning effect. For this reason, it is believed that a planet length-to-diameter ratio of greater than 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1 is suitable for causing the pinons to self-align when the gearbox is transmitting torque from the sun input to the output ring.
Moreover, a high aspect ratio between planet length and planet diameter allows a low helical angle of gear teeth on the planets to still achieve a high contact ratio (gear mesh overlap). Lower helical angles further reduce axial forces.
Aspect ratios can be greater than 2:1 and can be 3:1 or higher, 4:1 or higher, 5:1 or higher, 6:1 or higher, 7:1 or higher, 8:1 or higher, 9:1 or higher, or 10:1 or higher. In an embodiment, the planets include an aspect ratio of at least 4:1.
In a typical planetary gearbox having a single row of planets, a high aspect ratio will be decreasingly beneficial because the gears will twist and lose torque transmitting ability. Longer gears in a typical gearbox are subject to “knockdown factors” because the planet carrier, for example, will twist and biases torque transfer to one end of the planets. By contrast, self-energizing gearboxes provided in accordance with embodiments herein transmit the torque purely radially, thereby rendering the length of the gears relatively moot, and enabling longer planet lengths without the knockdown issue.
The fact that there is no twisting or tweaking of the gears in this gearbox enables the usage of extra long planets without a substantial decrease in expected torque output.
The embodiment shown in
A gearbox with included motor as shown above can also be formed in a single sided configuration, for example with one or more motors located inside the ID of the small inner ring gear.
Other embodiments comprise only one sun ring at one end. In this configuration, there are two sun rings which are driven by two individual motors. The motor stators are fixed to the inner ring which is fixed in this configuration. The motor rotors are fixed to the input sun rings.
In another embodiment as shown in
Both the inside and outside motor configurations can also encompass an inner rotor at both axial ends of the device instead of an outer rotor surrounding and forming an outside diameter of the device.
In the illustrated embodiments of
In a further embodiment shown specifically in
Any one of the inner ring 324, the outer ring 320, and an intermediate ring (such as the input ring 321 shown in
In an embodiment with many small planets, the relative motion of the inner ring 324 and outer ring 320 is much smaller that the relative motion of the intermediate ring (e.g., input ring 321) with respect to the inner ring 324 and outer ring 320. Thus, while the intermediate ring can be fixed, with one of the inner ring and outer ring the input and the other the output, this configuration will lead to a small (near 1) speed change ratio with faster internal moving parts than the input and output, and is generally not preferred. With the intermediate ring as the input or output, the device will be a speed increaser if the intermediate ring is the output and a speed decreaser if the intermediate ring is the input.
With multiple intermediate rings, it is also possible to have one intermediate ring be the input and one be the output. Of the inner and outer rings, one can be fixed and the other free spinning. With this arrangement, a gearing ratio different than 1 can be obtained if one of the intermediate rings connects to the inner planets and the other to the outer planets. This gearing ratio can be varied by varying the sizes of these planets. Of note, more variations can be obtained by allowing planets to change in size along their axial length so that the intermediate ring contacts the planets at a different diameter.
Also, both the outer ring and inner ring could be movable and the gearbox will provide a differential between these as output, with a gearing ratio that depends on the movement of the inner and outer rings.
Another possibility is using the self-energizing gearbox as a tool output device. Specifically, if a motor is attached to the sun gear input and if the inner ring is attached to a shaft that turns clockwise in the outer output ring is attached to a shaft that must turn counterclockwise, a reversing differential joint can be created.
It is understood that if the input ring meshes with the outside of the inner planets in an embodiment, for example having first and second portions that mesh with the outer planets and input ring respectively, both portions meshing with the inner ring, then the same principle would apply.
This design may make use of straight cut gear teeth, helical gear teeth, lobes, friction surfaces, or other profiles.
A straight cut gear tooth design like that described above may be advantageous for assembly, with a significantly lower part count when compared to a herringbone design, and a design which allows the gears to be inserted into the assembly from one side.
The straight cut gear tooth design does not have an axial constraint on the planets like the herringbone design, and thus needs some mechanism to constrain the planets axially. This design makes use of fences (not shown in
Bearings and shafts can be used in some configurations of the device to locate the planet gears axially. For some configurations, especially smaller devices, it is preferred to eliminate any bearings or shafts in planets.
In this case, whether the gears are helical or straight cut, an axial location strategy is required. Shown in
The embodiment shown here uses straight cut gears, however the fences 337-340 are operable with helical cut gears in alternative embodiments.
The illustrated embodiments of
Moreover, as shown the device has planet gears arranged in two rows, in this embodiment the outer planets 332 being axially longer than the inner planets 333. The inner fence 338 which contacts the inner planets 333 can be fixed to the inner ring 334, and the outer fence 340 which contacts the outer planets 332 can be fixed to the outer ring 330. In the illustrated embodiment, fences are provided at both axial ends of the device. In an embodiment as illustrated, where the planets 332, 333 extend to different axial positions on the input axial end, the inner fence 337 and outer fence 339 on the input axial side can be at different axial positions to contact the planets of the respective rows of planets.
The fences 337-340 and planets 332, 333 may have curved faces in contact with one another. The curvature on the axial ends of the planets 332, 333 contacts the curvature on the fences 337-340 such that the contact circle on the end of the planets 332, 333 is outside of the planet through hole and inside of the roots of the planet teeth. Such embodiments provide contact between the planets 332, 333 and fences 337-340 close to the pitch diameter of the planets 332, 333, near where the planets 332, 333 contact an element with respect to which the fence 337-340 is fixed or at rest, so the sliding velocity is minimized. Here, the fences 337-340 are fixed to, and the planets 332, 333 contact, the inner ring 334 and outer ring 330.
In a typical planetary gearbox, it is expected that a number of planets greater than 3 would not share the load evenly without very precise tolerances. The self-energizing gearbox has more than 3 planet pairs and must have some mechanism to ensure that load sharing exists to best make use of the additional planets' strength. There are several mechanisms that this gearbox could take advantage of, with several non-limiting mechanisms described here which take advantage of the unusual load distribution of this gearbox.
One non-limiting mechanism of load sharing in the self-energizing gearbox is radial flexibility of the planets, the inner ring, or the outer ring, or any combination of these. Because of the camming effect of the planets described above, there is a strong radial load component within the gearbox, transmitted between the outer ring, planets, and inner ring. If any of these gears has radial flexibility, the gear will be able to compress under the radial load of the camming effect. Because of this flexibility, the tolerance band of the large number of planets can be taken up, allowing the planets to share load. This radial flexibility can come from a number of features or parameters, including, but not limited to, a thin wall, lower material stiffness, or gear tooth root extension such as a radial slot between the teeth.
In order to ensure that there is load sharing between the large number of planets, the previous documentation referred to the need to allow for some sort of deflection in the planets of the gearbox. This deflection could be due to geometry such as thin walled gears or undercuts in the gear teeth, or it could be due to material stiffness in one or more of the gears.
The overall size of the planets of various embodiments, as well as the capability to provide such planets with relatively thin walls (and a hollow interior) to enable radial flexibility of the planets provides adequate flexibility of the planets to enable load sharing in certain embodiments. The planets may be manufactured from a stiff material such as steel, but have thin walls (and a hollow interior) and deflect under load such that the remaining planets could make contact with the associated gears and carry load. Alternatively or additionally, the planets are manufactured from a less stiff material and have a solid construction, but still deflect sufficiently to allow the other planets to begin to share the load. It was found that relatively small differences in the design of the planets could make a significant difference in the amount of load sharing seen in a gearbox.
It should be understood that the design and manufacture of the planets may be provided to withstand high stress and/or high cycle counts. In certain device designs, the planets will sustain the highest concentration of stress of all components of the device. Accordingly, high-strength and/or long-life materials may be utilized for planets in certain implementations.
To maintain high strength within the planets, load sharing can be accomplished instead by reducing the stiffness of the other gears (outer ring, inner ring, and/or sun gears), where there is some ability to reduce strength without affecting the gearbox's critical margins of safety. It has been shown that by manufacturing the planet gears from a stiff material such as steel, and the remaining gears from a less stiff material such as carbon fibre filled PEEK, load sharing can be achieved.
Interestingly, the reduction of stiffness required by this approach is significantly higher than the reduction of stiffness required by changing the materials of the planets alone. In one simulation, the planets had ½ the stiffness of the remaining gears and was shown to be able to sufficiently load share. In order to achieve the same amount of load sharing in a configuration with full-stiffness planets, the remaining gears must have on the order of 1/7 of the stiffness of the planets.
No matter the load sharing mechanism, the higher the radial (camming) load, the more similar the planet load due to a greater load sharing effect. A higher radial load is present with a higher pressure angle of the gear tooth geometry as well as a higher camming angle of the planet contacts.
Another load sharing mechanism results from the 2 level planet construction of the gearbox. As the planets cam onto one another, the non-loaded planet-planet mesh between inner and outer planets acts to stabilize the loaded planet-planet mesh. As a result, it is believed that there is a small amount of shifting in the planet position prior to developing a high enough radial load to “lock” into place. This effect is expected to increase load sharing between the planets and be a stronger effect with a lower pressure angle.
The stress distribution on the self-energized gearbox under load induces a radial load on the planets and geared components. This radial load may further deform one or more of these components and cause the planets to load share effectively, by making the self-energized components more susceptible to deform. This can be achieved by reducing the overall stiffness of the self-energized components (i.e. outer ring, planets and the inner ring). Three different methods could be implemented to achieve this type of change in stiffness (
The disclosed design may eliminate the need for a planet carrier and bearings as the input is supplied by the input ring, circumferential location is supplied by the gears, and axial location may be supplied by, for example, fences, tapered planets, or by portions with different angled gears.
By eliminating the need for a planet carrier and bearings, the tolerance stack-up of these locating elements is eliminated. This allows for much more consistent meshing of greater than three planet gears with the ring gears.
Tolerance stack up elements which are eliminated include the location of the planet carrier pins. The concentricity of the planet carrier, the runout of the bearings, and the eccentricity of the bearing bores in each of the planets with the pitch circle of the gears.
In addition to eliminating these tolerance stack up factors, radial flexibility can be introduced into the design in a number of different ways. Introducing radial flexibility has the effect of reducing the load variation from planet to planet that would result from variations in planet sizes.
Also as a result of eliminating the planet carrier, for example, the planets can be hollow and therefore radially flexible.
A gearbox as described above can be made a two stage gearbox as shown in
If the outer ring gear of stage one is the same pitch diameter and tooth number and one piece with the other outer ring gear of state two, then the inner ring gear from the first stage is connected to the input gear of the second stage and the inner ring gear of the second stage becomes the output of the second stage.
If the inner ring gear is shared by both stages, then the outer ring gear of the first stage is linked to the input gear of the second stage, and the outer ring gear of the second stage becomes the output of the device. More than two stages can be connected in this way.
Another exemplary embodiment of the single sided self-energizing gearbox is the tapered design shown in
By tapering the gears, the planets become axially constrained and backlash can be reduced or removed by adjusting shims in the locations shown in
The tapered gear profile is currently difficult to manufacture by traditional gear manufacturing methods such as hobbing or skiving. As such, another method such as but not limited to injection molding, surface milling, powdered metallurgy, or gear rolling, will likely be used. There is also a potential increase in part count due to manufacturing limitations with these tapers.
Either the tapered or non-tapered tooth profiles may make use of straight, or helical gears or lobes. It may be beneficial to use a helix angle on the tapered gears due to the manufacturing method or to optimize strength or noise.
The longer (outer) gears may also have a split, not shown, at their necks 222 in order to ease manufacturing using injection molding, if injection molding is chosen as the manufacturing method.
Tapered gears may be used with straight or helical, including herringbone, gears. The taper, in addition to providing some axial location, allows backlash adjustment with shims. Herringbone teeth allow more precise positive axial positioning of the planets and ring gears. Used together, all of the benefits are realized but some applications will benefit from one or the other.
As shown for example in
In the claims, the word “comprising” is used in its inclusive sense and does not exclude other elements being present. The indefinite articles “a” and “an” before a claim feature do not exclude more than one of the feature being present. Each one of the individual features described here may be used in one or more embodiments and is not, by virtue only of being described here, to be construed as essential to all embodiments as defined by the claims.
Many modifications and other embodiments will come to mind to one skilled in the art to which this disclosure pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the disclosure is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
This patent application claims priority from U.S. Provisional Patent Appl. Ser. No. 62/827,786, filed Apr. 1, 2019 and U.S. Provisional Patent Appl. Ser. No. 62/828,320, filed Apr. 2, 2019, each of which are incorporated herein by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2020/052939 | 3/27/2020 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62827786 | Apr 2019 | US | |
| 62828320 | Apr 2019 | US |
