WO2004034550A1

WO2004034550A1 - High axial force motor

Info

Publication number: WO2004034550A1
Application number: PCT/SE2003/001572
Authority: WO
Inventors: Lennart Stridsberg
Original assignee: Stridsberg Innovation Ab
Priority date: 2002-10-08
Filing date: 2003-10-08
Publication date: 2004-04-22
Also published as: AU2003267921A1

Abstract

A rotary electric motor has thrust bearings (411, 413) for supporting axial loads. To reduce the mass of the thrust bearings, the load carrying surfaces are permitted to flex, but in such a way that the deformation of the thrust carrying rollers remain acceptable. Parts (426) that transfer the axial forces between elements in the thrust bearings may also serve other purposes such as being back iron for permanent magnets of the rotor. The thrust bearings may be located at least partly radially inside the permanent magnets (409) of the rotor (426). This gives a compact motor that has a low mass and high torque and can be used in actuators such as for operating control surfaces or flaps of aircraft.

Description

HIGH AXIAL FORCE MOTOR RELATED APPLICATIONS

This application claims priority and benefit from Swedish patent applications Nos. 0202984- 1, filed October 8, 2002 and 0203139-1, filed October 22, 2002, the entire teachings of which are incoφorated herein by reference. TECHNICAL FIELD

The present invention is concerned with thrust bearing assemblies and electric motors comprising thrust bearing assemblies and adapted to withstand large axial forces. BACKGROUND AND PRIOR ART Axial thrust bearings are conventionally designed to have negligible deflection under a load. Electric motor systems for high axial loads conventionally include thrust bearing parts and motor parts having separated functions so that no component acts both as a significant part of the torque production and as a significant component in the handling of axial loads. For many applications, systems having separated torque producing parts and rigid axial force handling parts have turned out to be too heavy for applications such as the control of control surfaces in aircraft. SUMMARY

It is an object of the invention to provide an electric motor that can withstand large axial forces and yet have a low mass.

It is another object of the invention to provide a bearing system for axial loads with a low weight.

Generally, a thrust bearing assembly comprises a rotatable part that can rotate about a rotary axis, a stationary part and two thrust roller bearing sets connecting the rotatable and stationary parts to each other. Each of the two thrust roller bearing sets are located so that they face two support surfaces peφendicular to the rotary axis. One of the two support surfaces for each thrust roller bearing set is then a surface of the stationary part and the other one of the two support surfaces is a surface of the rotatable part.

The rotatable and stationary parts can in an advantageous way be arranged so that the support surfaces facing that one of the two thrust roller bearing sets that carries the dominating axial load, both flex under an axial load, this flexing causing a deviation of these support surfaces from a plane peφendicular to the rotary axis that would be unacceptable for a thrust roller bearing set facing both the flexing surface and another inflexible surface peφendicular to the rotary axis. The term "flex" is here taken to mean that the surfaces are deformed from an initial planar state in an unloaded condition to a state where a radial line on the surfaces takes an angle deviating from 90°. The two flexing surfaces are arranged to flex in such a way that the surfaces meeting any of the thrust roller bearing sets under a heavy axial load will remain substantially parallel in the sense that the resulting required thrust dependent deformation of the thrust roller bearing set is within a range which is permitted for the thrust roller bearing sets from a strength point of view such as within a range normally occurring in conventional roller bearing sets.

The support surfaces of the rotatable part can be opposite sides of part of the rotor of an electric motor. At least one of the support surfaces of the rotatable part can be a surface of an integrated part of the rotor of an electric motor, or a part integral with or supported by parts of the rotor of an electric motor. Forces transferred through this support surface can preferably pass through part of the rotor of the electric motor.

A ball screw nut or roller screw nut for a screw can be attached to the rotatable part and can then be arranged to move so that part of the screw passes through the plane of at least one of or all of the four support surfaces for the two thrust bearing roller sets. Furthermore, generally as conventional, an electric rotary motor has a rotary axis and includes a rotor part, a stationary part and rotary bearings connecting the rotor part and the stationary part. Two of the bearings are thrust bearings for supporting axial loads, preferably thrust bearings capable of support only very small radial loads such as thrust roller bearing sets having support surfaces peφendicular to the rotary axis. The thrust bearings are mounted at the side of each other with a portion between them that belongs either to the rotor part or the stationary part. The two thrust bearings can thus be said to have lower or bottom support surfaces which face each other and which are over their whole area supported by opposite side surfaces of a portion of the rotor part or of the stationary part. This portion projects radially outwards, away from the rotary axis. The inner or bottom surfaces are preferably located radially between magnetically active portions of the rotor part and/or of the stationary part and the rotary axis, this meaning that at least part of the thrust bearings are also located radially between magnetically active portions of the rotor part and/or of the stationary part and the rotary axis. The active portions are here taken to in particular include permanent magnets and/or soft iron poles carrying and not carrying armature windings. The radially projecting portion supporting the thrust bearings can be a relatively thin disc portion. The thickness of disc portion can e.g. be substantially smaller than the axial length of said magnetically active portions of the rotor part and/or of the stationary part. In particular the thickness can be smaller than or substantially equal to said axial length. The radially projecting portion can in a first case project from an inner portion of the rotor part, this inner portion having the shape of cylindrical sleeve or shell receiving an end of a rotary shaft rigidly attached to the rotor part. Then a radial bearing is preferably mounted between the outermost portion of the end of the rotary shaft and a lower end shield included in the stationary part. The radially projecting portion can then at its radially outer region be attached to or continue into an outer portion of the rotor part that has the shape of a substantially cylindrical tube, this tube shape portion having an outer cylindrical surface that can carry permanent magnets which across an air-gap face soft iron portions of a stator included in the stationary part. The stationary part can include a top end shield and a bottom end shield attached to each other by screws and clamping between themselves a stator including the magnetically active parts. The top and bottom end shields then include support surfaces for supporting upper or top support surfaces of the thrust bearings. The screws can be located outside the magnetically parts of the stator or they can pass through the magnetically parts of the stator such as through balancing poles.

The radially projecting portion supporting the two thrust bearing can in a second case be part of the stationary part and can then have a free outer, cylindrical surface. Then the rotor part preferably includes two discs or annular portions projecting radially towards the rotary axis. The two discs or annular portions are then located axially at opposite sides of the radially projecting portion and also have support surfaces for supporting upper or top support surfaces of the two thrust bearings. The rotor part includes a rotor that can have an outer cylindrical surface carrying permanent magnets. A radial bearing can be mounted between the rotor part and an axially extending sleeve portion of the stationary part. The stationary part can include an axial bore for receiving a shaft that can move in the bore such as sliding axially in the bore. A screw nut can be rigidly attached to the rotor part so that a shaft having threads can cooperate with the screw nut and be displaced axially when the rotor part and the nut rotates.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

While the novel features of the invention are set forth with particularly in the appended claims, a complete understanding of the invention, both as to organization and content, and of the above and other features thereof may be gained from and the invention will be better appreciated from a consideration of the following detailed description of non-limiting embodiments presented hereinbelow with reference to the accompanying drawings, in which: - Fig. 1 is a sectional view of an electric motor in which the rotor is designed as a integral part of a thrust bearing and the motor case is designed as a bearing fixture,

- Fig. 2 is a schematic sectional view of the stator and assembly screws of the motor of Fig. 1,

- Fig. 3 is a schematic sectional view similar to Fig. 2 for a design having the assembly screws closer the position of the thrust bearings, - Fig. 4a is a sectional view of a complete high force actuator using both the motor rotor and the roller nut as part of the structure to stiffen the axial thrust bearing elements,

- Fig. 4b is a view of a detail of the design of Fig. 4a intended to obtain parallel deformation of roller surfaces,

- Fig. 5 is a schematic showing deformation of the design of Fig. 4a for elongating forces, and - Fig. 6 is a view similar to Fig. 5 showing the deformation of the design of Fig. 4a for compressing forces.

DETAILED DESCRIPTION

In the sectional view of Fig. 1 a rotary electric motor is illustrated in which the rotor is designed as an integral part of a thrust bearing and the motor case is designed as a bearing fixture.

The motor load is a ball or roller screw 101 extending with a cylindrical end portion into the motor. The rotor 102 of the electric motor has an inner cylindrical surface forming an axial hole that receives the end portion of the screw 101. The rotor is pressed or glued onto the end portion to obtain a rigid connection between the rotor and the screw. The rotor has a radially outer circular-cylindrical surface that carries permanent magnets 103. The radially outer portion of the rotor is thus the magnet back iron, supporting the magnet, that face soft iron portions of the stator 104. The soft iron portions or poles of the stator protrude radially inwards, directed towards the motor axis. Radially between the magnet back iron and the inner portion of the rotor annular recesses 121 are provided, these recesses being open in axial opposite directions and otherwise having the same shape and dimensions. The recesses have cylindrical sidewalls, the radially outer sidewalls having identical diameters and the inner sidewalls having identical diameters. The recesses 121 have bottom surfaces located in planes peφendicular to the motor axis. The cross-section of the rotor taken along a plane through the motor axis thus has, at each side of the axis, the shape of an H, the two legs of the H-shape passing in the axial direction and the web in plane peφendicular to the axis. The thrust bearings 105, 106 are located in the two opposite spaces of the H-shape. The outer leg is the magnet back iron of the rotor and the inner leg extends along the screw end portion and can be prolonged to extend some distance farther than the outer leg.

The recesses 121 receive the roller or rotatable parts of two axial thrust bearing roller sets 105 and 106. On the outer sides, the stationary parts of the roller sets are supported by annular, bearing steel inserts like 107, resting against side surfaces, peφendicular to the motor axis, of the insets. These insets are in turn supported, at their opposite side surfaces, by annular surfaces of metal supports 108 and 109, the metal support 108 being a motor end shield or outer bearing support located at the screw side of the motor and the other metal support 109 being the motor frame. The bearing lubricant is kept in place by seals. Seals 111 act between the rotor 102 and the outer cylindrical surface of the steel inserts 107. Another seal 110 acts between the motor end shield/bearing support 108 and the outer surface of a part of the inner portion of the rotor 102 that projects along the screw 101. An ordinary radial roller bearing 112 keeps the rotor 102 and the screw shaft 101 radially centered and can, if sealed, also act as a seal for the thrust bearing lubricant. To ensure that the thrust roller bearings 105 and 106 always obtain a required minimum load, six compression springs 122 are inserted in corresponding cavities in the motor shields 108, 109 for each steel insert 107. The motor end shield/outer bearing support 108 supports the outer thrust bearing 105 and is by screws 114 rigidly attached to the motor frame part/inner bearing support 109, that also acts as an opposite motor end shield. Between these two end shields 108, 109 the soft iron portions of the stator 104 and the motor armature are simultaneously clamped. The inner bearing support 109 acts as a support for the inner thrust bearing 106 and includes a fixture portion 115 for connection to a shaft. The fixture portion 115 permits the motor and roller screw assembly to wobble around a shaft passing through a bore 116, in the fixture portion, the bore having a direction peφendicular to the motor axis, allowing the motor and roller screw to act in the same way as a hydraulic cylinder and piston assembly or as a replacement for such a hydraulic assembly. It can e.g. be arranged to move a lever or crank, for example a crank that displaces a control surface or wing flap of an aircraft.

Control feedback of the motor position can be obtained from an optical code disc 117 attached to the end surface of the roller screw 101 an placed between two optical and electronic transmitter and receiver boards 118 and 119 and enclosed by a cover 120. Fig. 2 is a schematic sectional view of a section through the stator 104 and assembly screws 114 of the motor of Fig. 1. The soft iron portions of the stator include 18 wound poles and six balancing poles such as 201, as also disclosed in the published International patent application WO 02/063760. The distance 202 from the thrust bearing inserts or washers 107 to the enclosing screws 114, that are located outside the soft iron portions of the stator and keep the two supports 108, 109 for the thrust bearing washers 107 together is rather large.

Fig. 3 is a schematic sectional view of a section through a modified stator 104 and assembly screw set having a significantly shorter distance 303 between the assembly screws 302 and the thrust bearing washers 107. This structure permits more compact support designs and is achieved by using wider balancing poles 302 in which axial bores for the clamping screws 114 are made.

Fig. 4a is a sectional view of a complete high force actuator using both the motor rotor and the roller nut as part of the structure to stiffen the axial thrust bearing elements.

The roller screw is shown in two positions, the topmost position to the right as 401 and the lowest position to the left as 402. The actuator is connected to a load though two bolts, not shown, one passing through a lower bore 405 and another passing through an upper bore made in the roller screw 401, 402 and shown in two positions 403 and 404. The lower hole 405 passes through an axially projecting part 406 of a trust bearing center disc 412. A roller nut 419 contains rollers 407 that forces the roller screw 401, 402 to move vertically if the nut 419 rotates. The bolts together prevents the roller screw from rotating together with the nut.

A flange of the roller nut 419 is attached to the rotor 426 by screws that also keep the upper rotating thrust bearing disc 410 in position, attached to the rotor. Between the two sets of thrust roller bearings 411 and 413 the radially projecting, stationary thrust bearing disc 412 is located. The lower rotating trust bearing disc 414 is a portion of the motor rotor 414 that projects radially inward toward the motor axis. Permanent magnets 409 are as above carried on an outer surface of the motor rotor. Between the rotor 414 and the static, central thrust bearing disc carrier 406 an ordinary radial roller ball bearing 415 is inserted to ensure a correct radial position of the rotor. All three thrust bearing disc portions are supported by tube shaped devices that also have other essential functions. The rotatable top disc 410 is supported by the outer tube of the roller nut 419. The stationary center disc 412 is supported by and preferably integral with the projecting part 406 having at its end the lower connecting bore 405. The lower disc 414 is supported by the tube shaped motor rotor 426 and is preferably an integrated portion thereof. It can be observed that the roller screw in its lower position 402 passes through the plane of the thrust bearing surface between the rotor part 414 and the lower roller set 413. This permits a more compact design with a shorter total length for a given stroke of the device. The rotating nut 419 is protected by a thin static shell 427. To ensure the centering of the of the shell in relation to the nut, a PFTE bearing 428 is inserted between the two parts. Elastic bellows 416, 417 protect the screw threads from contamination. Lubrication grease is kept in place by seals like 418 in the roller nut and by seals in the ball bearing 415 and a seal 420 located between a sleeve portion projecting axially from the center disc 412 and the top disc 410. As an example of a feedback device, a printed circuit board 421 carries light receiving parts of an optical encoder, an encoder disc 422 is rigidly attached to the rotor and a printed circuit board 423 carries the light emitting elements of the encoder. The circuit boards are attached to a stationary housing that also carries the stator part 408 of the motor and the bellows 416, 417 and is rigidly attached to the lower projecting portion 406. In Fig. 4b a detail in a larger scale of the actuator of Fig. 4a is shown that is intended to assist in obtaining parallel deformation of roller surfaces for loads acting on the roller screw 401, 402. By selecting suitable combinations or material thickness, radii of supporting surfaces such as 424 shown in Fig.4b and other dimensions affecting the deformation of thrust bearing roller surfaces 410, 412 and 414, the deformations can be arranged to keep the thrust carrying roller surfaces substantially parallel regardless of load.

Fig. 4b also shows an arrangement to ensure that the thrust roller bearing sets 411 and 413 obtain a minimum load. A spring washer 425 acts to lift the rotor 426 and nut 419 and to press down the top thrust bearing disc 410, thereby causing a pressure force over the thrust roller bearings 411 and 413. This minimum load will always be present for the top roller set 411 but will be absent from the bottom roller set 413 if a downward pressure on shaft 402 is approximately equal to the force from disc spring 425. In applications where this is not acceptable, a loose lower thrust bearing disc arrangement similar to that of the upper disc 410 can be used.

Fig. 5 is a schematic diagram illustrating the deformation of the actuator of Fig. 4a for elongating forces, i.e. forces that try to move the screw shaft 401, 402 away from the electric motor. The thrust roller bearings, not shown in this figure, transfer forces shown as 501 and 502 to the surfaces 503 and 504 respectively. For the dimensions shown in Fig. 4a, the surfaces 503 and 504 are both considerably bent. The angular difference between the two surfaces remains however very small, creating a distance difference of less than 1 μm. This difference is smaller than the permitted variation of roller bearing diameter, which suggests that the elasticity of the roller bearings in conventional bearing accommodate such dimensional variations.

Fig. 6 is a schematic diagram illustrating the deformation of the actuator of Fig. 4a for compressing forces, i.e. forces that tend to press the screw shaft 401, 402 farther into the electric motor. The thrust roller bearings, not shown in this figure, transfer forces shown as 601 and 602 to the surfaces 603 and 604 respectively. For the dimensions shown in figure 4a, the surfaces 603 and 604 are both considerably bent. The angular difference between the two surfaces remains however very small, creating a distance difference of less than 1 μm. The angular deflection of surface 603 depends for example of the radius of the supporting edge shown as 424 in Fig. 4b. In Figs. 5 and 6 the deformations are dramatically exaggerated for the sake of clarity. It should be understood herein and in the claims hereof that such terms as "top", "bottom", "upper", "lower" and the like have been used for illustration puφoses only, in order to provide a clear and understandable description and claiming of the invention. Such terms are not in any way to be construed as limiting, because the devices of invention are omni-directional in use as can be understood by their various uses in different application fields.

While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous additional advantages, modifications and changes will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within a true spirit and scope of the invention.

Claims

1. An electric motor having a rotary axis and comprising a rotor part, a stationary part and rotary bearings connecting the rotor part and the stationary part, characterized in that at least two of the bearings are thrust bearings for supporting axial loads which have lower or bottom support surfaces that face each other and are over their whole area supported by opposite side surfaces of a portion of the rotor part or of the stationary part that projects radially outwards, away from the rotary axis.

2. An electric motor according to claim 1, characterized in that the inner or bottom surfaces are located radially between magnetically active portions of the rotor part and/or of the stationary part and the rotary axis, the active portions in particular including permanent magnets and/or soft iron poles carrying and not carrying armature windings.

3. An electric motor according to claim 1, characterized in that the radially projecting portion is relatively thin disc portion, having a thickness substantially smaller than the axial length of magnetically active portions of the rotor part and/or of the stationary part, in particular a width smaller than or substantially equal to said axial length.

4. An electric motor according to claim 1, characterized in that the radially projecting portion projects from an inner portion of the rotor part having the shape of cylindrical sleeve or shell receiving an end of a rotary shaft rigidly attached to the rotor part.

5. An electric motor according to claim 4, characterized by a radial bearing mounted between the outermost portion of the end of the rotary shaft and a lower end shield included in the stationary part.

6. An electric motor according to claim 1, characterized in that the radially projecting portion is part of the rotor part and at its radially outer region is attached to or continues into an outer portion of the rotor part that has the shape of a substantially cylindrical tube.

7. An electric motor according to claim 6 characterized in that the outer portion of the rotor part has an outer cylindrical surface carrying permanent magnets facing, across an air-gap, soft iron portions of a stator included in the stationary part.

8. An electric motor according to claim 1, characterized in that the stationary part includes a top end shield and a bottom end shield attached to each other by screws and clamping between themselves a stator including magnetically active parts.

9. An electric motor according to claim 8, characterized in that the top and bottom end shields have support surfaces for supporting upper or top support surfaces of the thrust bearings.

10. An electric motor according to claim 8, characterized in that the screws are located outside the magnetically parts of the stator.

11. An electric motor according to claim 8, characterized in that the screws pass through the magnetically parts of the stator, in particular through balancing poles.

12. An electric motor according to claim 1, characterized in that the radially projecting portion is part of the stationary part and has a free outer surface.

13. An electric motor according to claim 12, characterized in that the rotor part includes two discs projecting radially towards the axis, the two discs located axially at opposite sides of the radially projecting portion and having support surfaces for supporting upper or top support surfaces of the thrust bearings.

14. An electric motor according to claim 12 characterized in that the rotor part includes a rotor having an outer cylindrical surface carrying permanent magnets.

15. An electric motor according to claim 12, characterized by a radial bearing mounted between the rotor part and an axially extending sleeve portion of the stationary part.

16. An electric motor according to claim 12, characterized in that the stationary part includes an axial bore for receiving a shaft movable in the bore, in particular capable of sliding in the bore in an axial direction.

17. An electric motor according to claim 16, characterized by a screw nut rigidly attached to the rotor part, the shaft having threads cooperating with the screw nut to be displaced axially when the rotor part rotates.

18. A thrust bearing assembly having a rotary axis and comprising a rotatable part, a stationary part and two thrust roller bearing sets (411, 413) connecting the rotatable and stationary parts, each of the two thrust roller bearing sets facing two support surfaces peφendicular to the rotary axis, one of said two support surfaces for each thrust roller bearing set being a surface of the stationary part and the other one being a surface of the rotatable part, characterized in that the rotatable and stationary parts are arranged in such a way that the support surfaces facing that one of the two thrust roller bearing sets that carries the dominant axial load flex under an axial load, this flexing causing a deviation of said support surfaces from a plane peφendicular to the rotary axis that would be unacceptable for a thrust roller bearing set facing both the flexing surface and another inflexible surface peφendicular to the rotary axis, the two flexing surfaces flexing in such a way that the surfaces meeting any of the thrust roller bearing sets under a heavy axial load will remain substantially parallel in the sense that the resulting required thrust dependent deformation of the thrust roller bearing set is within a range permitted for the thrust roller bearing sets, in particular within a range not damaging the material of the thrust bearing roller sets or within a range normally occurring in conventional roller bearing rollers.

19. A thrust bearing assembly according to claim 18, characterized in that the support surfaces of the rotatable part are opposite sides of a portion of the rotor (102) of an electric motor.

20. A thrust bearing assembly according to claim 18, characterized in that at least one of the support surfaces (414) of the rotatable part is a surface of an integrated portion of the rotor (426) of an electric motor.

21. A thrust bearing assembly according to claim 18, characterized in that at least one of the support surfaces (410) of the rotatable part is a surface of a component of the rotor (426) of an electric motor.

22. A thrust bearing assembly according to claim 18, characterized in that at least one of the support surfaces (414) of the rotatable part are is a surface of a part integral with or supported by portions of the rotor (426) of an electric motor and that forces transferred through said at least one of the support surfaces pass through said part.

23. A thrust bearing assembly according to claim 18, characterized in that the rotor of an electric motor is assembled on the rotatable part.

24. A thrust bearing assembly according to claim 18, characterized by a ball screw nut or roller screw nut attached to the rotatable part, the ball screw or roller screw intended to cooperate with a screw and arranged to be capable of moving so that part of the screw passes through the plane of at least one of the four support surfaces for the two thrust bearing roller sets.

25. A thrust bearing assembly according to claim 18, characterized by a ball screw nut or roller screw nut attached to the rotatable part, the ball screw or roller screw intended to cooperate with a screw and arranged to be capable of moving so that part of the screw passes through the planes of the four support surfaces for the two thrust bearing roller sets.

26. A thrust bearing assembly having a rotary axis and comprising a rotatable part, a stationary part and two thrust roller bearing sets (411, 413) connecting the rotatable and stationary parts, each of the two thrust roller bearing sets facing two support surfaces peφendicular to the rotary axis, one of said two support surfaces for each thrust roller bearing set being a surface of the stationary part and the other one being a surface of the rotatable part, characterized in that those support surfaces that are surfaces of the rotary part are surfaces of integrated parts of the rotor (102) of an electric motor.

27. A thrust bearing assembly having a rotary axis and comprising a rotatable part, a stationary part and two thrust roller bearing sets (411, 413) connecting the rotatable and stationary parts, each of the two thrust roller bearing sets facing two support surfaces peφendicular to the rotary axis, one of said two support surfaces for each thrust roller bearing set being a surface of the stationary part and the other one being a surface of the rotatable part, characterized in that at least one of those support surfaces that are surfaces of the rotary part is a surface of an integrated part of the rotor (426) of an electric motor.

28. A thrust bearing assembly having a rotary axis and comprising a rotatable part, a stationary part and two thrust roller bearing sets (411, 413) connecting the rotatable and stationary parts, each of the two thrust roller bearing sets facing two support surfaces peφendicular to the rotary axis, one of said two support surfaces for each thrust roller bearing set being a surface of the stationary part and the other one being a surface of the rotatable part, characterized in that at least one of those support surfaces that are surfaces of the rotary part is a surface of parts of the rotor (426) of an electric motor.

29. A thrust bearing assembly having a rotary axis and comprising a rotatable part, a stationary part and two thrust roller bearing sets (411, 413) connecting the rotatable and stationary parts, each of the two thrust roller bearing sets facing two support surfaces peφendicular to the rotary axis, one of said two support surfaces for each thrust roller bearing set being a surface of the stationary part and the other one being a surface of the rotatable part, characterized in that at least one of those support surfaces that are surfaces of the rotary part is integral with or supported by parts of the rotor (426) of an electric motor and that forces transferred through said at least one of the support surfaces pass through part of the rotor (426) of the electric motor.

30. A thrust bearing assembly having a rotary axis and comprising a rotatable part, a stationary part and two thrust roller bearing sets (411, 413) connecting the rotatable and stationary parts, each of the two thrust roller bearing sets facing two support surfaces peφendicular to the rotary axis, one of said two support surfaces for each thrust roller bearing set being a surface of the stationary part and the other one being a surface of the rotatable part, characterized in that the rotor of an electric motor is assembled on the rotary part.

31. A thrust bearing assembly having a rotary axis and comprising a rotatable part, a stationary part and two thrust roller bearing sets (411, 413) connecting the rotatable and stationary parts, each of the two thrust roller bearing sets facing two support surfaces peφendicular to the rotary axis, one of said two support surfaces for each thrust roller bearing set being a surface of the stationary part and the other one being a surface of the rotatable part, characterized by a ball screw nut or roller screw nut which is intended to cooperate with a screw, is attached to the rotating part of the bearing and is arranged to be capable of moving so that part of the screw passes though the plane of at least one of the four support surfaces for the two thrust bearing roller sets.

32. A thrust bearing assembly having a rotary axis and comprising a rotatable part, a stationary part and two thrust roller bearing sets (411, 413) connecting the rotatable and stationary parts, each of the two thrust roller bearing sets facing two support surfaces peφendicular to the rotary axis, one of said two support surfaces for each thrust roller bearing set being a surface of the stationary part and the other one being a surface of the rotatable part, characterized by a ball screw nut or roller screw nut which is intended to cooperate with a screw, is attached to the rotary part and is arranged to be capable of moving so that part of the screw passes though the planes of the four support surfaces of the two thrust bearing roller sets.