NO20230767A1

NO20230767A1 - Hollow actuator with strain wave gear and self-lock for emergency operation

Info

Publication number: NO20230767A1
Application number: NO20230767A
Authority: NO
Inventors: Jon Eirik Brennvall
Original assignee: Eltorque As
Priority date: 2023-07-07
Filing date: 2023-07-07
Publication date: 2025-01-08
Also published as: WO2025014374A1; TW202513993A

Description

Hollow actuator strain wave gear and self-lock for emergency operation

1 Technical field of invention

The presented invention relates to strain wave gear and a self-lock implemented in a hollow actuator for globe and gate valves. The strain wave gear is a new type and the self-lock is implemented in a new way.

2 Prior art

This is a presentation of prior art.

2.1 Introduction to gate and globe valve actuators

In gate valves the gate is connected to a stem. Part of the stem is a threaded rod which is screwed up to open the gate valve and down to close the gate valve. Gate valves are usually fitted with a hand wheel for operation. If the gate valve shall be operated with an actuator the actuator must have a design where there is space for the stem to move up and down.

One way to solve this problem is to make an actuator with a hollow planetary gear and a hollow motor as described in patent NO20220963 [1]. A problem with the solution described in patent NO20220963 [1] is that the sun gear must have a large diameter to make space for the stem. This leads to small gear ratios for each stage of the planetary gear, and consequently many stages, which makes the gear and the actuator expensive.

An actuator with a strain wave gear instead of a planetary gear which is emergency operated in a new way using the self-lock principle described in patent NO20130049 [2] and the fact that a strain wave gear is self locking, is presented

2.2 Introduction to the strain wave gearing

The strain wave gear was first patented in 1959 by Clarence Walton Musser in patent US2906143A [3]. The invention became known as under the trade mark ”Harmonic drive”.

A simplified strain wave gear is shown in figure 1. A strain wave gear 1 usually comprises a rigid internal-toothed gear 2, a flexible external toothed gear 3 and a wave generator 4 connected to the driving shaft 5. This is the same naming and numbering of parts as in patent US2015300475A1 [4] written by Harmonic drive system based in Japan, which is writing a lot of patents on strain wave gearing. The flexible external toothed gear 3 is sometimes called flexspline (Ueura et.al. [5]) or elastic gear, and the rigid internal-toothed gear 2 are sometimes called circular spline or inner toothed output gear (US11226027B2 [6]). Musser uses the terms strain gear about the flexible external toothed gear 3, strain inducer about the wave generator 4, and ring gear about the rigid internal-toothed gear 2. Musser call the concept strain wave gearing in his patent US2906143A [3].

The rigid internal-toothed gear 2 resembles a normal ring gear known from epicyclic gearing, but in optimized strain wave gears 1 the teeth 2.1 shape is diferent due to the diferent mode of operation. The flexible external toothed gear 3 is a thin walled tube. There is a (usually internal) flange in one end of the tube so the flexible external toothed gear 3 can be connected to the output shaft. The other end of the tube has a ring of external teeth 3.1 which interacts with the teeth 2.1 in the rigid internal-toothed gear 2. Also teeth 3.1 of the flexible external toothed gear 3 resemble gear wheel teeth, but has a diferent shape. The wave generator 4 is an elliptical cylinder connected to the input shaft. The rigid internal-toothed gear 2 has two more teeth than the flexible external toothed gear 3.

When the wave generator 4 rotates it changes the shape of the flexible external toothed gear 3 at the end with the teeth 3.1, so the external teeth 3.1 grips into the internal teeth 2.1 of the rigid internal-toothed gear 2 close the wave generators 4 widest point while the teeth 2.1 of the flexible external toothed gear 3 and the teeth 3.1 of the rigid internal-toothed gear 2 disengage at the wave generators narrowest point. This causes the flexible external toothed gear 3 to move on tooth every time the wave generator rotate half a revolution. The gear ratio is therefore The minus sign means the output shaft turns the opposite way of the input shaft. Usually the movement is transferred through the tube connecting the teeth 3.1 of flexible external toothed gear 3 to an internal flange which is connected to the output shaft, but it is possible to keep the flexible external toothed gear 3 fixed and let the rigid internal-toothed gear 2 rotate.

In a more practical strain wave gear, shown in figure 2, it is common to introduce either balls 6 or rollers between the wave generator 4 and flexible external toothed gear 3 to eliminate friction. The balls are kept in place by a flexible bearing cage 7 and flexible inner and outer raceways 8.1 and 8.2.

3D printed strain wave gears often replace the wave generator by two small eccentrically mounted bearings that roll against the inside of the flexible external toothed gear 3. Regular bearings 9 support driving shaft 5.

The harmonic drive has inspired many inventors. Search on ’’harmonic drive” at Espacenet give 150771 hits. Search on ’’strain wave gear” give 17075 hits. The share amount of patents granted in the field indicate that very little can be considered obvious to an expert on strain wave gears.

A common problem for inventors, is to create a strain wave gear with a hollow shaft. Patent US2015226302A1 [7], US2015300475A1 [4] and US2017051817A1 [8] describe such solutions. Patent US2017051817A1 [8] aims to solve the same problem as this patent, but because US2017051817A1 [8] is a strain wave gear variant with flexible external toothed gear 3 and internal wave generator 4 the solution in patent US2017051817A1 [8] becomes considerably more complex than the solution presented here. In all cited patents the flexible toothed gear 3 has external teeth. A strain wave gear where the flexible toothed gear has internal teeth and the strain wave generator is external is new. Patent US2017051817A1 [8] shows that the solution presented here with a flexible internally toothed gear 26 and external wave generator 25 is far from obvious.

3 Description of the invention

3.1 Description of major parts

Figure 3 show a hollow actuator 10 based on strain wave gearing. The major parts of the actuator are:

11. Bottom actuator housing

12. Main shaft

13. Valve flange (Connect the actuator to the valve.)

14. Valve nut (operate the stem of the valve)

15. Encoder plate

16. Stator

17. Magnets

18. Rotor back iron

19. Bottom motor lid

20. Top motor lid

21. Actuator housing

22. Top actuator housing

23. Emergency operation shaft

24. Emergency operation handle

25. External wave generator

26. Flexible internally toothed gear

27. Rotor shaft

28. Wires from motor and encoder

29. plug between actuator housing 20 and bracket for electronics 30 30. bracket for electronics

31. Shaft encoder gearwheel

Figure 4 is a detailed drawing of bottom actuator housing 11. 11.1 is groove for O-ring that seals against actuator housing 21. 11.2 is holes for screws for attaching actuator housing 21. 11.3 is steering edge for actuator housing 21.

11.4 is threaded holes for attaching encoder plate. 11.5 is threaded holes for attaching valve flange 13. 11.5 is steering edge for valve flange 13. 11.7 is space cut for encoders. 11.8 is steering edge for encoder plate. 11.9 is the cylindrical surface that give support for radial bearing for main shaft 12. 11.10 is the surface that give support for the axial bearing for the main shaft.

Figure 5 is a detailed drawing of main shaft 12. 12.1 is the rigid externallytoothed gear on the main shaft. 12.2 is surface for bearings and seals. 12.3 is edge that support the shaft encoder gearwheel 31. 12.4 is space for wave spring that pre-load the top axial shaft bearing. 12.5 is edge that support the bottom axial bearing. These axial bearings take up the large axial force from the stem on the main shaft 12 through the valve nut 14 when the valve nut move the gate up and down. 12.6 is threaded holes for attaching the valve nut 14 to the main shaft. 12.7 is surface for bearings and axial seals which shall keep sea water out.

12.8 is surface against seal between main shaft 12 and valve nut 14.

Figure 6 is a detailed drawing of valve flange 13 for attaching actuator to valve 13. 13.1 is hole for screws to attache the valve flange to the actuator. 13.2 is steering edge to make sure the the valve flange 13 and actuator 10 is aligned. The valve flange must be manufactured according to the valve the actuator 10 is going to be mounted on.

Figure 7 is a detailed drawing of valve nut 14 which move the stem of the valve 14. 14.1 is the valve nut treads which must be fitted to the valve. 14.2 is holes for attaching the valve nut 14 to the main shaft 12. 14.3 is surface against radial seal. 14.4 is surface against seal between main shaft 12 and valve nut 14.

3.1.1 Encoder system

Figure 8 is a detailed drawing of the encoder plate 15. The encoders which keep track of the mains shafts 12 position are attached to this plate together with encoder gearing. Multi-turn actuators for gate valves need to rotate typically 30-100 turns to move the stem on a gate valve from fully open to fully closed. It is therefore possible to attach two encoders to the encoder plate 15 so the absolute position of the two encoders can be used to determine how many turns the valve nut has rotated. 15.1 is holes for bearing for the encoder shaft. 15.2 is threaded holes for attaching the encoder. 15.3 is holes for attaching the encoder plate to he bottom actuator housing 11. 15.4 is holes for bearings for the encoder gearing.

Figure 9 shows the encoder system.

Figure 10 is closeup of section A in figure 9. 32 is the magnetic encoder. 33 first gearwheel in the encoder gearing. 34.1 and 34.2 are the gearwheels attached to the encoder shafts 35. Gearwheel 34.1 has one tooth more than gearwheel 34.2. This ensures that the main shaft 12 can rotate many turns before the gearwheels 34.1 and 34.2 are back in the same position. 35 is the encoder shaft. 36 is the encoder gearing shaft. 37 is bolts that attache the bottom actuator housing 11 and the actuator housing 21. 38 are bolts that holds bottom actuator housing 11, encoder plate 15 and bottom motor lid 19 together.

Figure 11 show two cross sections of the encoder system which show encoder bearings 39 and the second gearwheel in the encoder gearing 40 which is not visible in figure 10.

3.1.2 Hollow actuator which hollow motor and new type hollow strain wave gearing with external wave generator

Figure 12 shows a cross section of the actuator through the strain wave gear 52 with external wave generator 25 and flexible internally toothed gear 26. Here the strain wave gear rollers 41 and the flexible roller cage 42 are shown. Bolts 43 that hold the motor together are also shown. Figure 12 illustrate how the rollers 41 roll between the wave generator 25 and the flexible internally toothed gear 26 and transfer the force which deform the flexible internally toothed gear 26 from a circle in not strained form to an ellipse in strained form. Figure 12 also show how the deformation of the flexible internally toothed gear 26 make it interact with the teeth 12.1 on the main shaft 12. Like other stain gears the flexible internally toothed gear 26 will move the main shaft 12 two teeth for each rotation of the wave generator 25. Since the main shaft has 62 teeth in the presented figures, the gear ration will be 1:31. Opposite to normal strain gears, the teeth 12.1 of the main shaft 12 and the teeth 26.4 of the flexible internally toothed gear 26 interacts at the wave generators 25 narrowest point. In ”normal strain wave gears” this interaction happens at the strain wave generators widest point.

It should be noted how simple and compact the gear and motor have become, and how it is possible to make it even more compact by merging the rotor back iron 18 and the wave generator 25 into one part. This can be done if the wave generator is made of a ferromagnetic material like steel. Figure 3 and figure 12 show in fact how the entire strain wave gear fits inside the length of the motor and between the inner diameter of the rotor back iron 18 and the surface of the main shaft, something that makes it possible to have a hollow main shaft with a large diameter. The tube length of the the flexible internally toothed gear 26 is in fact longer than it needs to be, because of the length of the electric motor chooses for the shown actuator 10. The flange end 26.6 of the flexible internally toothed gear 26 and the top actuator housing 22 can also be merged into one part in many designs, but in this design they are two parts because of the emergency operating mechanism described later.

The electric motor in the presented actuator is a hollow PM-motor. However any hollow motor electric or otherwise would be suitable for the actuator. Figure 13 shows the bottom motor lid 19. The bottom motor lid 19 has a holes 19.1 used to fix the bottom motor lid 19 to the encoder plate 15 and bottom actuator housing 11, with bolts 38. 19.2 are threaded bolt holes for bolts 43 which hold the top motor lid 20, the bottom motor lid 19 and the motor 16, 17 ,18 together. 19.3 is removed material for taking wires out from the motor. Figure 14 shows the top motor lid 20. 20.1 is holes for bolts 38. 20.2 is and edge which is a part of the self lock in the emergency operation mechanism. Figure 15 shows the emergency handle shaft 23. 23.1 is polygon to transfer torque from emergency operation handle 24. 23.2 is surface for bearings and seals against top actuator housing 22. 23.3 is surface for seals against main shaft 12. 23.5 is holes for self lock release pins. 23.6 are holes for torque transfer pins. 23.7 is surface for bearing against the main shaft 12.

Figure 16 shows the emergency shaft system 89.

Figure 17 shows the top actuator housing 22. 22.2 is holes for fixing the top actuator housing to the actuator housing 21 with bolts. 22.2 is surface for seals against emergency operation shaft 23. 22.3 is surface for bearing against emergency operation shaft 23. 22.4 is groove for O-ring that seals against the actuator housing 21. 22.5 is steering edge against actuator housing 21.

Figure 18 show the external wave generator 25. 25.1 is surface for normal bearing. 25.2 is surface which is fixed against rotor 18 back iron with glue, press pass or similar. 25.3 is the inner surface for the rollers 41 in the flexible roller cage 42. Surface 25.3 has the shape of an ellipse.

Figure 19 show rotor 88 comprising of external wave generator 25, rotor back iron 18, rotor shaft 27 and magnets 17. These parts rotate as one part. 27.1 is a surface for radial bearing against top motor lid 20.

Figure 20 shows the flexible internally toothed gear 26. 26.1 is surface for self lock rollers 44. 26.2 is hole for torque transfer pins 45. 26.3 is surface for radial bearing against main shaft 12. 26.4 is the teeth that interacts with the teeth 12.1 on the main shaft. 26.5 is the felxible tube that transfer torque from the teeth 26.4 to the flange 26.6. The flange 26.6 has bearing surfaces 26.7 and 26.8 is against slide bearings.

The gear ratio is of the new type of strain wave gear is:

Since the number of external teeth 12.1 in the main shaft 12 is 62 and the number of internal teeth 26.4 in the flexible internally toothed gear 26 is 64 for the presented strain wave gear 52 the gear ratio is This means the main shaft 2 rotate opposite way of the wave generator 25 at 1/31 of the speed of the speed of the wave generator 25.

3.1.3 New use of self- lock mechanism to allow emergency operation of actuator with strain wave gearing

Figure 21 shows a cross section of the self- lock mechanism 50 in the actuator 10. The first purpose of the self-lock mechanism 50 is to keep the flexible internally toothed gear 26 still when the actuator is operated normally by the electric motor. The second purpose is to allow the flexible internally toothed gear 26 to be rotated by the emergency handle 24. The self-lock comprise of self lock rollers 44, torque transfer pins 45 which are fixed bye glue, press pass or similar in the holes 23.6 in the emergency handle shaft 23, self-lock release pins 46 which are fixed bye glue, press pass or similar in the holes 23.5 in the emergency handle shaft 23. Springs 47 make sure the self lock rollers 44 are in lock position as shown in figure 21. 48 and 49 are seals for the bracket for electronics 30 and not related to the self-lock 50. 51 is an axial bearing between the flexible internally toothed gear 26 and the main shaft 12.

When the emergency handle 24 is not rotated, the self lock rollers 44 are pressed into lock position by the spring 47 as shown in figure 21. If the flexible internally toothed gear 26 is attempted rotated from the gear side the edge 26.1 will press the self lock rollers 44 against the wall 20.2 in the top motor lid 19. This will create a large normal force and therefore large friction that prevent the flexible internally toothed gear 26 from moving.

Figure 22 shows how the self-lock release pins 46 press the self lock rollers out of position if the emergency handle 24 is twisted clockwise. Then, when the torque transfer pins 45 get in contact with the walls in hole 26.2 in the flexible internally toothed gear 26 the emergency handle can rotate the flexible internally toothed gear 26 whit out being blocked by the friction from the rollers. Because the strain wave gear is self locking the torque will be transferred through the flexible internally toothed gear 26 and the main shaft 12 to the valve nut 14 which then moves the stem of the valve to open or close it.

Figure 23 shows counterclockwise rotation of the emergency handle 24. There are many ways to apply this principle. Patent N020130049 provides more details. The new idea in this patent application is to use the self-lock to keep the flexible internally toothed gear 26 of a strain wave gear stationary unless the actuator 10 is emergency operated. The advantage is that the parts of the selflock 50 only move if the actuator is emergency operated, which usually happens rarely. This means that the self- lock 50 will not wear out. Wear is a problem with the self-lock described in patent N020130049 because this self lock rotates under normal operation.

3.2 Minor parts

Figure 24 is the same as Figure 3 but here the cross section 52.1 of the strain wave gear 52 in figure 12 and the position of the cross section 50.1 of selflock mechanism 50 in figure 21, 22 and 23 are shown. Figure 25 is the same as Figure 3 but here all minor parts not yet mentioned are numbered. 53 is a screw which lock the emergency hand wheel 24 to Emergency operation shaft 23. 54 is protection for the external emergency shaft seals 55. 57 is flexible top lid that protects the hollow shaft. The flexible top lid 57 must be removed if the actuator is placed on valves with very long stem. 56 is internal emergency shaft seals that seal against the main shaft 12. 58 is O-ring that seals between top actuator housing 22 and actuator housing 21. 59 is an axial bearing between emergency operation shaft 23 and top actuator housing 22. 60 is bearing between main shaft 12 and emergency operation shaft 23. 61 and 62 are self-lock slide bearings that allow the self-lock rollers 44 to move freely. 63 is a top motor bearing. 64 is a wave spring that takes up slack so the motor stay in one position axially. 65 is the top of the flexible roller cage 42. The two parts 42 and 65 are joined with glue, press fit or similar after the rollers 41 are put in. The flexible roller cage 42 can also be manufactured in one part with tolerances to the rollers can be pressed into their slots. 66 is bottom motor bearing. 67 is the top trust bearing with race ways that take up trust from the valve nut 14. 68 is a wave spring that pre-load the trust bearing and take up slack in axial direction. 69 is an O-ring that seals between actuator housing 21 and bottom actuator housing 11.

70 is trust bearing with raceways. 71 is axial bearing between the main shaft 12 and the bottom actuator housing 11. 72 are bottom seals between bottom actuator housing 11, main shaft 12 and valve nut 14. 73 is the seal between main shaft and valve nut. 74 are bolts that locks the valve nut 14 to the main shaft 12. These bolts 74 make it relatively easy to change the valve nut 14.

75 are bolts that attach the valve flange 13 to the bottom actuator housing 11. The bolts make it relatively easy to change the valve flange for attaching actuator to valve 13. By changing valve flange 13 and valve nut 14 the actuator 10 can operate almost all valves, including gate-, globe-, knife-, butterfly- and ball-valve as long as the hole in the main shaft 12 is big enough for the stem. (Spindle on butterfly and ball-valves.)

Figure 26 shows a 3D-printed version of the flexible bearing 76 for the wave generator 25. The rollers 41 are in steel bought from the a local retail bearing supplier. The flexible bearing is placed between the wave generator 25 and the flexible internally toothed gear (26,79,84) to eliminate the friction that would be between these parts if the flexible bearing 76 was omitted.

3.3 Definition of input and output shaft

In a gear at least one part is fixed relative to the gear housing, but it can often wary which part is fixed. In a strain wave gear the input shaft (from the motor or other driving device) is always connected to the wave generator 25. It is then possible to fix either the flexible internal toothed gear (26,79,84), or the main shaft main shaft (12, 78). If the flexible internal toothed gear (26,79,84) is fixed, the main shaft (12, 78) become output shaft. If the the main shaft (85) is fixed, the flexible internal toothed gear (84) become output shaft. Because the strain wave gear is self-locking (not possible to run in reverse) the main shaft (12, 78, 85) and the flexible internal toothed gear (26,79,84) cannot be input shaft.

3.4 Two simplified actuators

The actuator 10 has all functionality to function as a valve actuator for gate valves. To show variants of the invention and to show the advantage of combining a hollow motor with the new type of strain wave gear 52 two simplified actuators are shown in figure 27 and 28.

Figure 27 shows actuator 77. Actuator 77 is functional and the same as actuator 10, but it is simplified down to motor and strain wave gear only. The main shaft 78 is the same as main shaft 12, but main shaft 78 is fitted with only two axial bearings 81 and 82. Flexible internally toothed gear 79 is the same as flexible internally toothed gear 26, but joined with top motor lid 20 into one part. Bottom motor lid 80 is a variant of bottom motor lid 19 but modified with hole 80.1 for fixing actuator 77 to other equipment. This way the actuator 77 can be a substitute for a motor with high torque.

Figure 28 shows actuator 83. In this actuator the main shaft 12 has been joined with bottom motor lid 18 into main shaft 85. Here the flexible internally toothed gear 84 is the output shaft. Top motor lid 20 is modified into top motor lid 87 with a bearing 86 which allow the flexible internally toothed gear 84 to rotate.

Cross section through the strain wave gear in figure 27 and 28 is the same as figure 12 without the actuator housing 21, plug 29, bracket 30 and wire 28.

It will obviously be complicated to put the strain wave gear inside a hollow motor as shown in figure figure 27 and 28 without the external wave generator 25 and the flexible internally toothed gear 26,79,84 presented in this patent.

4 Conclusion

It is possible to put a wheel motor inside the wave generator 4 presented in figure 1 and 2 which could be useful in e.g. automotive applications. A hollow motor with the strain wave gear presented here is the opposite and useful when you need a hollow moving shaft in the center of a valve actuator. The presented combined hollow motor and strain wave gear 52 give a very compact valve actuator design.

References

[1] Brennvall, J. E., “Actuator for a Valve,” Patent application NO20220963, Sep. 2022.

[2] Sør ̊as, T., Brennvall, J. E., and Naebb, T. E., “Self lock mechanism for valve actuator,” Patent application NO20130049, Jul. 2016.

[3] Musser, C. W., “Strain wave gearing,” Patent application US2906143A, Sep.

1959.

[4] Yuya, M. and Yoshihide, K., “Hollow-type strain wave gearing unit,” Patent application US2015300475A1, Jun. 2013.

[5] Ueura, K., Kiyosawa, Y., Kurogi, J., Kanai, S., Miyaba, H., Maniwa, K., Suzuki, M., and Obara, S., “Development of strain wave gearing for space applications,” Proceding 12th Euro Space Mechanisms and Tribology Symp (ESMATS) in Liverpool UK , August 2007.

[6] Hain, B. and Zierer, P., “Gearing having an elastic gear,” Patent application US11226027B2, Jan. 2022.

[7] Jun, H. and Yoshihide, K., “Hollow strain wave gearing,” Patent application US2015226302A1, Jul. 2013.

[8] Toshiki, M., “Hollow strain wave gearing and holow actuator,” Patent application US2017051817A1, Feb. 2017.

Claims

1. A strain wave gear comprising a wave generator (25), a flexible gear (26,79, 84) and a main shaft (12, 78, 85) characterized by the flexible gear (26, 79, 84) being a flexible internally toothed gear (26, 79, 84) having internal teeth (26.4), the main shaft (12) having external teeth (12.1) arranged to interact with the internal teeth (26.4), and the wave generator (25) surrounding the flexible internally toothed gear (26, 79, 84).

2. Strain wave gear according to claim 1, characterized by the external wave generator (25) having an inner elliptical surface (25.3) pressing on the outside of the flexible internally toothed gear (26,79,84) so the flexible internally toothed gear (26,79,84) is interacting with the external teeth (12.1) of the main shaft (12,78,85) at the points where the distance across the inner elliptical surface (25.3) of the external wave generator (25) is shortest causing strain wave gearing action when the external wave generator (25) rotates.

3. Strain wave gear according to one of the claims above, characterized by the flexible internal toothed gear (79) being fixed relative to gear housing, making the main shaft (78) an output shaft.

4. Strain wave gear according to claim 1 or 2, characterized by the main shaft 85 being fixed relative to gear housing, making the flexible internally toothed gear (84) an output shaft.

5. Strain wave gear according to one of the claims above, characterized by a flexible bearing (76) arranged between the external wave generator (25) and the flexible internal toothed gear (26,79,84).

6. Strain wave gear according to one of the claims above, characterized by a self-lock mechanism (50) keeping the flexible internal toothed gear (26) fixed in normal operation while allowing operation by a secondary shaft in alternative operation, like e.g. emergency operation by manually turning an emergency operation handle (24).

7. Strain wave gear according to claim 6, characterized by an emergency operation handle (24) attached to an emergency operating shaft (23), where the emergency operating shaft (23) has self-lock release pins (46) which press self-lock rollers (44) in the self-lock mechanism (50) out of lock position so rotation of the emergency operation handle (24) to torque transfer pins (45) to the flexible internally toothed gear (26) and further to the main shaft (12) and valve nut (14) utilizing that the strain wave gear is self-locking.

8. An actuator characterized by comprising a strain wave gear according to one of the claims above and a motor with an internal hollow rotor (88), where the strain wave gear is arranged inside the hollow rotor (88), and where the external wave generator (25) is moving together with or being part of the hollow rotor (88) which comprises the external wave generator (25), a rotor back iron (18), a rotor shaft (27) and magnets (17).