Planetary gear mechanism

Abstract

The invention discloses a planetary gear mechanism (1) comprising a first and a second ring gear (2, 4) and at least four double planets (6, 8, 10, 12), wherein each double planet (6, 8, 10, 12) has a first and a second gear wheel (14, 16; 18, 20; 22, 24; 26, 28) which are arranged rotatably about a common shaft, wherein the toothing of the first gear wheel (14; 18; 22; 26) meshes with the toothing of the first ring gear (2) and wherein the toothing of the second gear wheel (16; 20; 24, 28) meshes with the toothing of the second ring gear (4), wherein the number and size of the double planets (6, 8, 10, 12) is adjusted such that a predetermined size of an interior (30), a predetermined torsional rigidity and / or a predetermined load-bearing capacity of the planetary gear mechanism (1) are given.

Description

[0001] Description

[0002] planetary gear

[0003] The present invention relates to a planetary gear with a first and second ring gear and at least four double planets according to the preamble of patent claim 1.

[0004] Planetary gears can be used in various technical fields, for example, in industrial robotics. Various functions, such as a gripper arm, require cables, wires, pipes, or similar components to supply power or hydraulic fluid, or to transmit signals. However, routing cables, etc., along planetary gears is currently a complex matter.

[0005] It is therefore an object of the present invention to provide a planetary gear that enables simple and cost-effective passage of cables and the like.

[0006] This object is achieved by a planetary gear according to patent claim 1.

[0007] Such a planetary gear set has a first and a second ring gear and at least four double planets. Each of the double planets has a first and a second gear which are arranged to rotate about a common shaft. The teeth of the first gear of each double planet engage with the teeth of the first ring gear and the teeth of the second gear of each double planet engage with the teeth of the second ring gear. The drive provided in a planetary gear set, i.e. an element which is driven by a shaft, can for example be implemented by one of the ring gears, with the other ring gear acting as the output. Since the double planets each have one gear in contact with the first ring gear and the other gear in contact with the second ring gear, the movement of the ring gear acting as the drive is transferred to the ring gear acting as the output. However, other combinations are also possible.For example, a planetary carrier can be provided that is coupled to the double planetary gears and acts as the input. In this case, both ring gears can act as the output. A reverse arrangement, i.e., the ring gears as the input and the planetary carrier as the output, is also possible. A ring gear can also be fixed. In this way, 3-shaft transmissions can be realized in which all shafts can be freely driven, driven, driven, or fixed.

[0008] In order to enable cables, wires, pipes, and the like to be guided through the planetary gear, the number and size of the double planets are adapted to provide a predetermined size of the interior of the planetary gear. This means that the double planets are selected so that cables, etc. can be guided through the interior of the planetary gear. Alternatively, the number and size of the double planets can be adapted to provide a predetermined torsional rigidity and / or a predetermined load-bearing capacity. Preferably, the number and size of the double planets are adapted so that at least two, and most preferably all three, of these predetermined conditions are met. For example, the number and size of the double planets can be adapted to provide high torsional rigidity and / or load-bearing capacity of the planetary gear while simultaneously providing sufficient space for the passage of cables, wires, and the like.In this way, the planetary gear can be optimized with respect to one or more framework conditions.

[0009] According to one embodiment, the number and size of the double planets are adapted such that the predetermined size of the interior of the planetary gear set, the predetermined torsional rigidity and / or the predetermined load-bearing capacity is maximized and the outer diameter of the ring gears is minimized. This can be achieved, in particular, by appropriately adjusting the ratio of the inner diameter of the first and second ring gears to the number and size of the double planets. By taking the outer diameter of the ring gears into account, the available installation space, i.e. the maximum possible outer diameter of the planetary gear set, is also taken into account. In this way, in addition to the high load-bearing capacity, torsional rigidity and space for routing cables, etc., low demands are placed on the required installation space of the application in which the planetary gear set is used.

[0010] Preferably, the ratio of the inner diameter of the first and second ring gear to the number of double planets is optimized such that the diameter of the available interior space is sufficiently large to allow the cables, wires or other lines required in the application to pass through.

[0011] For example, when using four double planetary gears in a planetary gearset, these can each have a diameter of at least 18 teeth x module and a ring gear diameter of at least 48 teeth x module. If the ring gear diameter is increased to at least 60 teeth x module, the number of double planetary gears can be increased by two to six without increasing the diameter, especially the module, of the double planetary gears. This approach increases the bending and torsional rigidity of the gearset as well as the load-bearing capacity. In addition, the gearset now offers more space inside the gearset, concentric around the gearset axis, for example to route cables or pipes through the gearset.Increasing the ring gear diameter to at least 76 teeth per module allows for the arrangement of eight double planetary gears, increasing it to at least 92 teeth allows for the arrangement of ten double planetary gears, and increasing it to at least 104 teeth allows for the arrangement of twelve double planetary gears. With each increase in the number of teeth and thus the number of planetary gears, the bending and torsional rigidity, load-bearing capacity, and the ability to pass larger cables and pipes through the gearbox are significantly increased without changing the individual planetary gear.

[0012] This relationship also applies, for example, to a double planetary gear tooth count of 31, whereby the ring gear tooth counts for four, six, eight, ten, or twelve double planetary gears would then be 80, 100, 120, 140, and 160. Planetary tooth counts of more than 31, as well as an increase in the number of double planetary gears to more than twelve, are also possible.

[0013] Advantageously, the double planetary gears are identical to each other, and the ring gears can be adjusted in size depending on the application. The scaling of the planetary gear is then achieved solely by the size of the ring gears and the number of double planetary gears.

[0014] According to one embodiment, a tube is arranged in the interior of the planetary gear. The longitudinal axis of the tube is coaxial with the rotational axis of the ring gears and, in particular, coincides with the rotational axis of the ring gears. This tube can be a hollow shaft or a sleeve through which lines, such as cables, wires, etc., can be routed. By using such a tube, the lines can be protected from contact with the gearing of the double planetary gears and thus from damage.

[0015] When the planetary gear teeth—that is, the double planet gear teeth and the ring gear teeth—meet, backlash can occur between the teeth. This can be particularly detrimental when the gears change their direction of rotation, for example, when moving forward and backward, as the backlash between the gears can then cause their teeth to collide, possibly unevenly. This can lead to undesirable wear on the gears.

[0016] According to a further embodiment, two adjacently arranged double planets of the at least four double planets are therefore arranged such that the first gear of a first double planet of the two adjacent double planets is preloaded clockwise such that the toothing of the first gear of the first double planet is in contact with the toothing of the first ring gear, and that the second gear of the first double planet is preloaded counterclockwise such that the toothing of the second gear of the first double planet is in contact with the toothing of the second ring gear.Additionally, the first gear of a second double planet of the two adjacent double planets is preloaded counterclockwise such that the teeth of the first gear of the second double planet are in contact with the teeth of the first ring gear, and the second gear of the second double planet is preloaded clockwise such that the teeth of the second gear of the second double planet are in contact with the teeth of the second ring gear. In this way, the teeth of the gears of the first double planet are preloaded in two different directions, once clockwise and once counterclockwise against the ring gears (tangentially). As a result, due to the preload, there is already contact between the teeth of the ring gears and the teeth of the gears of the double planet.Additionally, this contact is maintained during operation by preloading in different directions, preventing backlash between the gears. At the same time, the gear teeth of the second double planetary gear are preloaded tangentially against the ring gears in opposite directions, i.e., once counterclockwise and once clockwise.

[0017] In particular, across the inner circumference of the ring gears, two adjacent double planetary gears are always preloaded in opposite directions. This results in a particularly good distribution of the preload force across the ring gears and thus a stable arrangement.

[0018] If the gears of the respective double planetary gears are preloaded, they can be fixed in their relative positions. This can be achieved, for example, by welding, soldering, gluing, radial or axial frictional engagement, or form-fitting.

[0019] According to a further embodiment, the first ring gear and the second ring gear each have helical gearing, wherein the first ring gear and the second ring gear are designed to rotate in a first direction of rotation and a second opposite direction of rotation, and wherein the gearings of the first and second gears of the at least four double planets are each helical gearing, wherein each helical gearing has a first tooth flank and a second tooth flank which is opposite to the first tooth flank, wherein a surface normal of the first tooth flank is directed in the first direction of rotation and a surface normal of the second tooth flank is directed in the second direction of rotation.

[0020] To prevent play between the toothings of the ring gears and the double planets, according to this embodiment, two adjacently arranged double planets of the at least four double planets are arranged such that a first force exerting element exerts a first force in a first direction, so that the first tooth flanks of the helical toothing of the first gear of a first double planet of the two adjacent double planets are in contact with the second tooth flanks of the helical toothing of the first ring gear, and that the second tooth flanks of the helical toothing of the second gear of the first double planet are in contact with the first tooth flanks of the helical toothing of the second ring gear, thereby providing a preload on the ring gears in the first and second directions of rotation.Furthermore, a second force-exerting element exerts a second force in a second direction opposite to the first direction such that the second tooth flanks of the helical gearing of the first gear of a second double planet of the two adjacent double planets are in contact with the first tooth flanks of the helical gearing of the first ring gear, and the first tooth flanks of the helical gearing of the second gear of the second double planet are in contact with the second tooth flanks of the helical gearing of the second ring gear, thereby providing a preload to the ring gears in the first and second rotational directions.

[0021] Due to the preload, contact between the helical gears of the ring gears and the helical gears of the double planetary gears already exists during start-up. Furthermore, this contact can be maintained during operation by means of the force-applying elements, maintaining the preload, thus preventing backlash between the helical gears.

[0022] The first gear and the second gear of each double planetary gear are fixed relative to each other. This fixation makes it possible to exert the first and second forces simultaneously on both gears of the double planetary gears, so that the first and second tooth flanks are in contact with the first and second tooth flanks of the helical gears of the first and second ring gears, respectively.

[0023] The backlash between the two ring gears and the double planetary gears is eliminated by using preloaded double planetary gears. Due to the preload of the second double planetary gear, which is in the opposite direction to the preload of the first double planetary gear, backlash between the gears can be avoided and further improved.

[0024] According to a further embodiment, a first half of the double planets is preloaded in the first rotational direction, and a second half of the double planets is preloaded in the second rotational direction, wherein the first half and the second half of the double planets are arranged alternately. In this way, the first and second forces, as well as the preload in the first rotational direction and the preload in the second rotational direction, can be evenly distributed and thus further improved.

[0025] According to a further embodiment, the first force and the second force are higher than an axial force exerted by the meshing of the helical gears of the double planets and the ring gears during rotation. Such a distribution between the first and second forces and the axial forces can reduce the elastic behavior of the system. This means that the double planets can be prevented from moving in the first and second directions during operation of the planetary gear. In the planetary gear, the first force and the second force are preferably equal.

[0026] According to a further embodiment, the force-applying element can be a spring element, a magnetic element, and / or a hydraulic element. Any other type of force-applying element may also be possible.

[0027] As already explained above, according to a further embodiment, the number of double planets is a multiple of two. This means that two double planets with opposing preloads are always present, whereby the different preloads are evenly distributed throughout the entire transmission. By using four or more double planets, the play between the two ring gears and the double planets, as well as any rotational rotation of the ring gears relative to each other, can be optimally prevented in all directions of rotation, i.e., forwards and backwards. Preferably, six or more double planets are provided. A higher number of double planets can, in particular, increase the load capacity of the planetary gear.

[0028] Further advantages and advantageous embodiments are set forth in the description, the drawings, and the claims. In particular, the combinations of features set forth in the description and the drawings are purely exemplary, so that the features may also be present individually or in other combinations. The invention will be described in more detail below with reference to exemplary embodiments illustrated in the drawings. The exemplary embodiments and the combinations shown in the exemplary embodiments are purely exemplary and are not intended to define the scope of the invention. This scope is defined solely by the appended claims.

[0029] They show:

[0030] Fig. 1 : a plan view of a first embodiment of a planetary gear;

[0031] Fig. 2: a plan view of a second embodiment of a planetary gear;

[0032] Fig. 3 : a plan view of a third embodiment of a planetary gear;

[0033] Fig. 4: a diagram showing a ratio of the number of teeth of the ring gears and double planets for different numbers of double planets in the planetary gears of Figures 1 to 3;

[0034] Fig. 5: a plan view of the planetary gear of Fig. 1 according to a first variant of a preload in the planetary gear;

[0035] Fig. 6: a plan view of the planetary gear of Fig. 1 according to a second variant of a preload in the planetary gear; and

[0036] Fig. 7: a sectional view of the planetary gear of Fig. 6.

[0037] In the following, identical or functionally equivalent elements are identified by the same reference symbols.

[0038] Fig. 1 shows a planetary gear 1 having a first ring gear 2 and a second ring gear (not shown in Fig. 1). Furthermore, the planetary gear 1 has a first double planet 6, a second double planet 8, a third double planet 10, and a fourth double planet 12. The double planets 6, 8, 10, 12 each have two gears, although only the first gear 14, 18, 22, 26 is visible in Fig. 1. The first gears 14, 18, 22, 26 are in contact with the first ring gear 2, and the second gears are in contact with the second ring gear.

[0039] To enable cables, wires, pipes, and the like to be guided through the planetary gear 1, the number and size of the double planets 6, 8, 10, 12 are adapted to provide a predetermined size of an interior space 30 of the planetary gear 1. This means that the double planets 6, 8, 10, 12 are selected such that cables, etc., can be guided through the interior space 30 of the planetary gear 1. In particular, the number and size of the double planets 6, 8, 10, 12 are adapted to provide high torsional rigidity and / or load-bearing capacity of the planetary gear 1 while simultaneously providing sufficient space for the passage of cables, wires, and the like.

[0040] If more space is needed to pass through cables and the like, the interior space 30 can be enlarged by using more double planets 6, 8, 10, 12 that have the same size or number of teeth x module. As shown in Figures 2 and 3, the number of double planets can be increased, for example, to six (Fig. 2) or eight (Fig. 3). If six double planets 6, 8, 10, 12, 32, 34 are used, the interior space is already larger. If eight double planets 6, 8, 10, 12, 32, 34, 36, 38 are used, the interior space becomes even larger. The size or number of teeth x module of the ring gears 2 is also increased accordingly. In all three variants of the planetary gear 1 shown in Figures 1 to 3, the same double planets 6, 8, 10, 12, 32, 34, 36, 38 can be used, which then increases the outer circumference accordingly.

[0041] If the outer circumference of the planetary gear 1 is limited by the available space in the application in which the planetary gear 1 is used, smaller double planets 6, 8, 10, 12, 32, 34, 36, 38 can be used alternatively to still maintain the interior space 30 at a predetermined size. The selection of the size and number of double planets 6, 8, 10, 12, 32, 34, 36, 38 can, as already explained, be based on the desired torsional rigidity, load capacity, and interior space.

[0042] To optimize torsional rigidity, load capacity, and interior space, a specific number of double planets can be used for a given ring gear size, which is defined by the ring gear diameter (number of teeth x module). Possible ratios of the ring gear diameter to the double planet diameter for different numbers of double planets are shown in Fig. 4. The x-axis represents the number of ring gear teeth z2, and the y-axis represents the number of planetary teeth zl.

[0043] As can be seen from Fig. 4, for example, when using four double planets in a planetary gear set (solid line), these can each be used with a diameter of at least 18 teeth x module and a ring gear diameter of at least 48 teeth x module. If the ring gear diameter is increased to at least 60 teeth x module, the number of double planets can be increased by two to six without increasing the diameter of the double planets.

[0044] A higher number of double planets 6, 8, 10, 12, 32, 34, 36, 38 can also be used. Preferably, a number of double planets 6, 8, 10, 12, 32, 34, 36, 38 is always provided that is a multiple of two. Such a number of double planets can achieve particularly good power distribution and reduce backlash between the toothings of the ring gears 2, 4 and the toothings of the double planets 6, 8, 10, 12, 32, 34, 36, 38, as described below with reference to Figures 5 to 7. In these figures, four double planets 6, 8, 10, 12 are present in each case. However, the same principles are also applicable to planetary gears 1 with six, eight, ten, ..., double planets.

[0045] A first variant of a backlash reduction mechanism is described in Figure 5. In the embodiment shown here, the double planets 6, 8, 10, 12 are connected to each other via a planet carrier 40. To allow the passage of cables and the like, the planet carrier 40 can have a through-hole (not shown).

[0046] In the embodiment shown in Fig. 5, the first ring gear 2 is larger than the second ring gear 4. Furthermore, the first ring gear 2 is fixed, with the planet carrier 40 acting as the input, rotating in the direction of the arrow, and the second ring gear 4 acting as the output. Other configurations and size ratios of the ring gears 2, 4 and double planets 6, 8, 10, 12 are also possible. The double planets 6, 8, 10, 12 each have two gears 14, 16 (first double planet 6), 18, 20 (second double planet 8), 22, 24 (third double planet 10), 26, 28 (fourth double planet 12). The first gears 14, 18, 22, 26 are in contact with the first ring gear 2 and the second gears 16, 20, 24, 28 are in contact with the second ring gear 4.

[0047] To prevent play between the teeth of the gears 14, 16, 18, 20, 22, 24, 26, 28 and the ring gears 2, 4, the gears 14, 16, 18, 20, 22, 24, 26, 28 of all double planetary gears 6, 8, 10, 12 are preloaded. For this purpose, the first gear 14 of the first double planetary gear 6 is preloaded clockwise against the teeth of the first ring gear 2, as indicated by the arrow. The second gear 16 of the first double planetary gear 6 is preloaded counterclockwise against the teeth of the second ring gear 4, as also indicated by an arrow.

[0048] Mirror-inverted, the first gear 20 of the second double planetary gear 8 is preloaded counterclockwise against the teeth of the first ring gear 2, as indicated by the arrow. At the same time, the second gear 22 of the second double planetary gear 8 is preloaded clockwise against the teeth of the second ring gear 4, as indicated by the arrow.

[0049] This is also done for the double planets 10, 12: The first gear 22 of the third double planet 10 is preloaded clockwise against the teeth of the first ring gear 2, as indicated by the arrow. The second gear 24 of the third double planet 10 is preloaded counterclockwise against the teeth of the second ring gear 4, as also indicated by an arrow. In a mirror image, the first gear 26 of the fourth double planet 12 is preloaded counterclockwise against the teeth of the first ring gear 2, as indicated by the arrow. At the same time, the second gear 28 of the fourth double planet 8 is preloaded clockwise against the teeth of the second ring gear 4, as indicated by the arrow.

[0050] In this way, the double planetary gears 6, 8, 10, and 12 are preloaded alternately and in opposite directions. This ensures that the preload is evenly distributed around the circumference and prevents backlash between the gears 14, 16, 18, 20, 22, 24, 26, and 28 and the ring gears 2, 4, which would have a detrimental effect on the planetary gear 1 during operation.

[0051] Another variant for reducing backlash is shown in Figures 6 and 7. In the embodiment shown in Figure 6, the first ring gear 2 is larger than the second ring gear 4. The first ring gear 2 can act as an input, and the second ring gear 4 can act as an output. Other embodiments and proportions of the ring gears 2, 4 and the double planets 6, 8, 10, 12 are also possible.

[0052] As in the embodiment shown in Fig. 5, the double planetary gears 6, 8, 10, 12 each have two gears 14, 16; 18, 20; 22, 24; 26, 28. The first gears 14, 18, 22, 26 are in contact with the first ring gear 2 and the second gears 16, 20, 24, 28 are in contact with the second ring gear 4. Due to this contact between the double planetary gears 6, 8, 10, 12 and the ring gears 2, 4, a movement of the first ring gear 2 in a first rotational direction 46 or a second rotational direction 48 is transmitted to the second ring gear 4 and vice versa. The rotational directions 46, 48 of the ring gears 2, 4 are indicated by arrows 46, 48 in the figures.

[0053] Fig. 7 shows sectional views of each double planetary gear 6, 8, 10, 12 in combination with the ring gears 2, 4. As can be seen in Fig. 7, the ring gears 2, 4 and the double planetary gears 6, 8, 10, 12 each have a helical gearing 50. Each helical gearing 50 has a first tooth flank and a second tooth flank opposite to the first tooth flank. A surface normal of the first tooth flanks is directed in the first rotational direction 46 of the first and second ring gears 2, 4, and a surface normal of the second tooth flanks is directed in the second rotational direction 48 of the ring gears 2, 4.

[0054] When the gears (the ring gears 2, 4 and thus also the gears 14, 16; 18, 20; 22, 24; 26, 28 of the double planetary gears 6, 8, 10, 12) change their direction of rotation 46, 48, i.e., perform a forward and backward movement and / or perform an oscillating movement between the two directions of rotation, a tooth clearance between the toothings 50 can cause an uneven movement profile of the gears. In order to minimize the tooth clearance between the helical toothings 50 of the gears 14, 16; 18, 20; 22, 24; 26, 28 of the double planetary gears 6, 8, 10, 12 and the ring gears 2, 4, one or more force-exerting elements 52, 54 are provided in this variant. A first force applying element 52 applies a first force in a first direction and a second force applying element 54 applies a second force in a second direction opposite to the first direction.The first direction and the second direction are perpendicular to the first and second rotation directions 46, 48.

[0055] In the embodiment shown in Figures 6 and 7, the first force is exerted on the first and fourth double planets 6, 12 and the second force is exerted on the second and third double planets 8, 10. Due to the first force, the first tooth flanks of the helical gears 50 of the first gears 14, 26 of the first and fourth double planets 6, 12 are in contact with the second tooth flanks of the helical gearing of the first ring gear 2. In addition, the second tooth flanks of the helical gears 50 of the second gears 16, 28 of the first and fourth double planets 6, 12 are in contact with the first tooth flanks of the helical gearing of the second ring gear 4. Due to this contact, a preload is provided to the ring gears 2, 4 in the first rotational direction 46.

[0056] In addition, due to the second force, the second tooth flanks of the helical gears 50 of the first gears 18, 22 of the second and third double planetary gears 8, 10 are in contact with the first tooth flanks of the helical gearing of the first ring gear 2. In addition, the first tooth flanks of the helical gears 50 of the second gears 20, 24 of the second and third double planetary gears 8, 10 are in contact with the second tooth flanks of the helical gearing of the second ring gear 4. Due to this contact, a preload is provided to the ring gears 2, 4 in the second rotational direction 48.

[0057] By pressing the first and fourth double planetary gears 6, 12 in the direction of the first ring gear 2, the helical gears 50 of the first and fourth double planetary gears 6, 12 are in contact with the helical gears of the first and second ring gears 2, 4. By pressing the second and third double planetary gears 8, 10 in the direction of the second ring gear 4, the helical gears 50 of the second and third double planetary gears 8, 10 are in contact with the helical gears of the first and second ring gears 2, 4. Due to this preload, contact already exists between the helical gears 50 of the ring gears 2, 4 and the helical gears 50 of the gears 14, 16; 18, 20; 22, 24; 26, 28 of the double planetary gears 6, 8, 10, 12 during start-up.In addition, this contact can be maintained during operation by means of the force-applying elements 52, 54 by maintaining the preload, and thus a tooth clearance between the helical gears 50 can be avoided.

[0058] The force-exerting elements 52, 54 can be implemented, for example, as springs, magnetic elements, hydraulic elements, or any other type of element capable of exerting a force on the double planetary gears 6, 8, 10, 12. This force could also be generated by a different gearing angle between the ring gears 2 and 4. Preferably, the total amount of the first force exerted by the first force-exerting elements 52 and the total amount of the second force exerted by the second force-exerting elements 54 are equal.

[0059] The planetary gear system described above makes it possible to optimize the number and size of the double planets in relation to the required internal space, possibly taking into account the dimensions of the ring gear, a predetermined torsional stiffness, and / or load capacity. Furthermore, backlash between the ring gears and the double planets can be avoided.

[0060] List of reference symbols

[0061] 1 planetary gear

[0062] 2 first ring gear

[0063] 4 second ring gear

[0064] 6 first double planet

[0065] 8 second double planet

[0066] 10 third double planet

[0067] 12 fourth double planet

[0068] 14 first gear of the first double planet

[0069] 16 second gear of the first double planet

[0070] 18 first gear of the second double planet

[0071] 20 second gear of the second double planet

[0072] 22 first gear of the third double planet

[0073] 24 second gear of the third double planet

[0074] 26 first gear of the fourth double planet

[0075] 28 second gear of the fourth double planet

[0076] 30 Interior

[0077] 32 fifth double planet

[0078] 34 sixth double planet

[0079] 36 seventh double planet

[0080] 38 eighth double planet

[0081] 40 planet carriers

[0082] 46 first direction of rotation

[0083] 48 second direction of rotation

[0084] 50 helical gearing

[0085] 52 first force-exerting element

[0086] 54 second force-exerting element

Claims

P a t e n t a n s p r ü c h e Planetary gear (1) with a first and a second ring gear (2, 4), and at least four double planetary gears (6, 8, 10, 12), wherein each double planetary gear (6, 8, 10, 12) has a first and a second gear (14, 16; 18, 20; 22, 24; 26, 28) which are rotatably arranged about a common shaft, wherein the teeth of the first gear (14; 18; 22; 26) engage with the teeth of the first ring gear (2) and wherein the teeth of the second gear (16; 20; 24; 28) engage with the teeth of the second ring gear (4), characterized in that the number and size of the double planetary gears (6, 8, 10, 12) are adapted such that a predetermined size of an interior space (30) is achieved. a predetermined torsional stiffness and / or a predetermined load-carrying capacity of the planetary gear (1) is given.A planetary gear set according to claim 1, wherein the number and size of the double planetary gears (6, 8, 10, 12) are adapted, in particular the ratio of the inner diameter of the first and second ring gears (2, 4) to the number and size of the double planetary gears (6, 8, 10, 12) is selected such that the predetermined torsional stiffness and / or the predetermined load-carrying capacity is maximized and the outer diameter of the ring gears (2, 4) is minimized. A planetary gear set according to claim 1 or 2, wherein a tube is arranged in the interior (30) of the planetary gear set (1). Planetary gear (1) according to one of the preceding claims, wherein two adjacent double planets (6, 8; 10, 12) of the at least four double planets (6, 8, 10, 12) are arranged such that the first gear (14; 22) of a first double planet (6; 10) of the two adjacent double planets (6, 8; 10, 12) is biased clockwise such that the teeth of the first gear (14; 22) of the first double planet (6; 10) are in contact with the teeth of the first ring gear (2), and that the second gear (16; 24) of the first double planet (6; 10) is biased counterclockwise such that the teeth of the second gear (16; 24) of the first double planet (6; 10) are in contact with the teeth of the second ring gear (4) is, and that the first gear (18; 26) of a second double planet (8; 12) of the two adjacent double planets (6, 8; 10, 12) is biased counterclockwise such that the toothing of the first gear (18;26) of the second double planet (8; 12) is in contact with the toothing of the first ring gear (2), and that the second gear (20; 28) of the second double planet (8; 12) is pre-tensioned clockwise such that the toothing of the second gear (20; 28) of the second double planet (8;12) is in contact with the teeth of the second ring gear (4). Planetary gear according to claim 4, wherein each pair of adjacent double planets (6, 8, 10, 12) of the at least four double planets (6, 8, 10, 12) are oppositely biased. Planetary gear according to claim 4 or 5, wherein the gears (8, 10) of each double planet (6, 8, 10, 12) are fixed in their position relative to each other. Planetary gear according to one of claims 1 to 3, wherein the first ring gear (2) and the second ring gear (4) each have helical teeth (50), wherein the first ring gear (2) and the second ring gear (4) are configured to rotate in a first direction of rotation (46) and a second opposite direction of rotation (48), and wherein the teeth of the first and second gear (14, 16; 18, 20; 22, 24;26, 28) of the at least four double planets (6, 8, 10, 12) each constitute a helical gear (50), wherein each helical gear (50) has a first tooth flank and a second tooth flank opposite to the first tooth flank, wherein a surface normal of the first tooth flank is directed in the first rotation direction (46) and a surface normal of the second tooth flank; in the second direction of rotation (48), wherein two adjacent double planets (6, 8; 10, 12) of the at least four double planets (6, 8, 10, 12) are arranged such that a first force-exerting element (52) exerts a first force in a first direction, such that the first tooth flanks of the helical gear (50) of the first gear (14; 22) of a first double planet (6; 10) of the two adjacent double planets (6, 8; 10, 12) are in contact with the second tooth flanks of the helical gear (50) of the first ring gear (2), and that the second tooth flanks of the helical gear (50) of the second gear (16; 24) of the first double planet (6;10) are in contact with the first tooth flanks of the helical gearing (50) of the second ring gear (4), thereby providing a preload on the ring gears (2, 4) in the first and second directions of rotation (46, 48), and wherein a second force-exerting element (54) exerts a second force in a second direction opposite to the first direction, such that the second tooth flanks of the helical gearing (50) of the first gear (18; 26) of a second double planet gear (8; 12) of the two adjacent double planet gears (6, 8; 10, 12) are in contact with the first tooth flanks of the helical gearing (50) of the first ring gear (2), and that the first tooth flanks of the helical gearing (50) of the second gear (20; 28) of the second double planet gear (8;12) are in contact with the second tooth flanks of the helical gearing (50) of the second ring gear (4), thereby providing a preload on the ring gears (2, 4) in the first and second directions of rotation (46, 48). Planetary gear according to claim 7, wherein a first half of the at least four double planetary gears (6, 8, 10, 12) is preloaded in the first direction of rotation (46), and wherein a second half of the at least four double planetary gears (6, 8, 10, 12) is preloaded in the second direction of rotation (48), the first half and the second half of the double planetary gears (6, 8, 10, 12) being arranged alternately. Planetary gear according to any of the preceding claims, wherein the number of double planetary gears (6, 8, 10, 12, 32, 34, 36, 38) is a multiple of two. Planetary gear set according to one of the preceding claims, wherein the number of double planets (6, 8, 10, 12, 32, 34, 36, 38) is greater than or equal to six.

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