DE102012203817A1 - Mounting arrangement for bearing rotor shafts in nacelle of wind-power plant, has hydraulic unit acting on auxiliary bearing and bringing rotor shaft from first level to second level during actuation of hydraulic unit on auxiliary bearing - Google Patents

Abstract

The arrangement has shaft bearings inserted in a separable connection construction part for bearing a rotor shaft (5). An auxiliary bearing (14) includes an inner ring (9) and an outer ring and a rolling body e.g. cylindrical roller, and is permanently or temporarily connected with the rotor shaft and partially supported on the construction parts with the outer rings. A hydraulic unit acts on the auxiliary bearing and brings the rotor shaft from first level to second level during actuation of the hydraulic unit on the auxiliary bearing. An independent claim is also included for a method for exchanging a shaft bearing.

Description

Field of the invention

The invention relates to the assembly and disassembly of bearings of rotor shafts in wind turbines, in particular with the assembly and disassembly of bearings on or in the nacelle of the wind turbine.

The structure and operation of wind turbines is well known, so that you can limit yourself to a brief description of such systems.

As 1 , which has a schematic representation of a wind turbine to the subject shows, this wind turbine 1 essentially from a mast 2 and a so-called gondola 3 formed on the mast 1 is attached. The gondola 3 at least take the generator 4 and the rotor shaft 5 on. At the front end of the rotor shaft 5 is a hub 6 and at least one wing 7 comprehensive rotor R arranged. The rotor shaft 1 is made up of two main bearings, which subsequently act as shaft bearings 8th be designated stored. The shaft bearings 8th are based as well as the generator 4 at the gondola 3 from. Just for the sake of completeness it should be noted that between rotor shaft 5 and generator 4 Also, a transmission can be arranged, which in 1 however not shown. Now, if the rotor R is rotated by the incoming wind in rotation, the rotor shaft 5 and about this the generator 4 driven.

In 2 is a conventional shaft bearing 8th shown. This shaft bearing 8th is from an inner ring 9 , one the inner ring 9 coaxially surrounding outer ring 10 and rolling elements 11 formed, wherein the rolling elements 11 between the inner ring 9 and the outer ring 10 are arranged. Even if in 2 a full complement shaft bearing 8th is shown, it is known in the art, this full complement shaft bearing 8th by replacing one in which the rolling elements 11 by a cage in mutual distance between inner and outer ring 9 . 10 being held.

Also is 2 removable, that the inner ring 9 from the rotor shaft 5 is penetrated and the outer contour 12 of the outer ring 10 in a connecting construction 13 , consisting of a lower and a upper part 13a . 13b , is arranged. To a sliding movement of the inner ring 9 on the rotor shaft 5 or the outer ring 10 in the adjacent construction 13 to exclude, are the inner ring 9 on the rotor shaft 5 and the outer ring 10 in the adjacent construction 13 . 13a . 13b established.

The definition of the inner ring 9 on the rotor shaft 5 is solved so that the inner ring 9 from two inner ring segments each describing a semicircle 9a . 9b is formed, wherein the two semicircular inner ring segments 9a . 9b be joined in a known manner so that they form a circular ring, which then by means of screws (not shown) in a rotor shaft 5 einklemmenden way is fixed to this.

Is the inner ring 9 As stated in the last paragraph, it is called a split inner ring 9 . 9a . 9b ,

Only for the sake of completeness, it should be noted that in a first storage because of the good accessibility of the rotor shaft 5 the inner ring 9 may also be integrally formed. In this case, then the one-piece inner ring 9 to bring about the clamping on the rotor shaft 5 shrunk.

As already in connection with the inner ring 9 is explained in 2 also the outer ring 10 divided educated. Specifically, this is solved so that two semicircular outer ring segments 10a . 10b be joined so that they form a circular ring. Depending on the training, these so joined outer ring segments 10a . 10b also still be interconnected by appropriate screws (not shown). The actual clamping of the outer ring 10 takes place by means of the split connection construction 13a . 13b , Is namely the shaft bearing 8th with its outer ring 10 in the lower part 13a the connection construction 13 used, the outer ring can 10 or that shaft bearing 8th - as can be easily seen - thereby in the adjacent construction 13 be jammed by the shell 13b the connection construction 13 on the lower part 13b is placed on and by means of screws (not shown) both parts 13a . 13b be bolted together.

As already in connection with the inner ring 9 executed, because of the good accessibility of the rotor shaft 5 the outer ring 10 not necessary for the first storage as split, two segments 10a . 10b comprehensive outer ring 10 be executed, but may also be integrally formed.

Will on a clamping of the outer ring 10 in the adjacent construction 13 You can avoid laying down the shaft bearing 8th also screws (not shown) through the connecting structure 13 or the connecting structure 13 forming parts 13a . 13b in the outer ring 10 or its segments 10a . 10b be screwed.

Are the shaft bearings 8th worn and need to be replaced, according to the prior art, the procedure is that first the rotor shaft 5 from the generator 4 separated and the rotor R is removed. Then the corresponding tops 13b from the bottom part 13a the adjacent construction 13 removed and the rotor shaft 5 along with the shaft bearings 8th from a crane (not shown) from the divider 13a lifted and at ground level B ( 1 ) spent. Then the shaft bearings 8th swapped with known techniques and the combination of rotor shaft 5 and shaft bearings 8th from a crane (not shown) brought back to gondola level and in the lower parts 13a the connection construction 13 spent and the shells 13b to complete the connection construction 13 screwed.

Is the rotor R reattached and the rotor shaft 5 again with the generator 4 connected, is the wind turbine 1 ready for use again.

As you can see, there is an exchange of shaft bearings 8th in wind turbines 1 a very complicated matter. Not only because of modern wind turbines 1 Gondola heights of 50 and more meters must be overcome with high loads, but also because appropriate access for such complex maintenance work for truck cranes must be maintained permanently. Although the problem of accessibility is eliminated in offshore installations, the complete construction of rotor shaft and shaft bearings described above is no less expensive, because this work can be performed with appropriate ship cranes only at very low sea state.

Object of the invention

Therefore, the invention has the object to provide a mounting arrangement for bearings of rotor shafts of wind turbines, which avoids removal of the rotor shaft from the nacelle.

Summary of the invention

This object is achieved with the features of claim 1. A first embodiment of the invention is defined in claim 2 and a further embodiment of the invention is set forth in claim 3. Advantageous embodiments and further developments of the invention can be found in the claims 4 to 7. A method for carrying out the assembly of shaft bearings of rotor shafts of wind power is specified in claim 8. Advantageous embodiments of the method are set forth in claims 8 and 9. Basic knowledge of the invention is that the change of shaft bearings can then be carried out very easily when the rotor shaft resting in its normal position at a first level in the connecting structure or the shaft bearings is manipulated by means of auxiliary bearings so that the rotor shaft without use of cranes, etc. occupies a second level different from the first level.

In detail, according to claim 1 at least one an inner ring, an outer ring and rolling elements comprehensive auxiliary bearing, which is permanently or even temporarily connected to the rotor shaft and which is at least partially supported with the outer ring on the connecting structure, and means required on the Working auxiliary bearing and spend the effect of the auxiliary bearing, the rotor shaft from its first to a second level.

Such a means may be according to claim 2, a hydraulic arrangement which acts on on the outer contour of the outer ring of the auxiliary bearing or a connection part, in which the auxiliary bearing is incorporated, mounted projections and which is supported on or on the - remaining - adjacent construction.

According to the second, specified in claim 3 embodiment of the invention, the respective auxiliary bearing comprises first, a uniform diameter D1 having Wälzköper and means in the form of second rolling elements whose diameter D2 is greater than the diameter D1 of the first rolling elements.

A particularly simple process control is given according to claim 4 when the average diameter D2 of two side by side between the inner and outer ring of the auxiliary bearing arranged rolling elements is different in size. By the specified in claim 10 introducing second, a rising average diameter D2 rolling elements between the inner and outer ring of the auxiliary bearing ensures that by continuously increasing the average diameter D2 in the second rolling elements, the rotor shaft continuously from a first level on a second - required for disassembly - level spent. In particular, causes the slight increase in the average diameter D2 of the introduced second rolling elements, that they are retracted by rotation of the shaft by the prevailing friction as by itself between the inner and outer ring of the auxiliary bearing.

It is particularly advantageous if, according to claim 5, a shaft bearing at the same time assumes the function of an auxiliary bearing, since in this case no or only very small, beyond the axial extent of the respective shaft bearing areas must be provided. The advantages associated with the embodiment according to claim 5, however, are given only if anyway a double row bearing is used as the respective shaft bearing.

As specified in claim 6, the respective auxiliary bearing is not subject to any restrictions with respect to the shape of the rolling elements of the auxiliary bearing and may also include rolling elements in the form of cylindrical rollers, tapered rollers and also barrel rollers at the discretion of the person skilled in the art and the prevailing circumstances.

Because of the various usable Wälzkörperausführungen should be noted at this point that the term used in the application multiple times the mean diameter D of a rolling element is understood as one that adjusts itself centrally between the two end faces of the respective rolling element.

Since the shaft bearings anyway require a fixation on the rotor shaft in the axial direction, it is advantageous and space-saving if, according to claim 7, the auxiliary bearings or parts thereof simultaneously bring about the definition of the shaft bearing on the rotor shaft. In this case, the auxiliary bearing or the parts thereof are intended to remain permanently on the rotor shaft.

The specified in claim 8 method for mounting shaft bearings of rotor shafts in wind turbines is characterized in that according to this method no lifting of the rotor shaft of the nacelle is required when an auxiliary bearing is used, which on the one hand ensures a rotatable mounting of the rotor shaft and the Further, by the acting on the respective auxiliary bearing means is adapted to spend the rotor shaft from a first level to a second level. It is essential for the process management that the one or more auxiliary bearings are rolling bearings to ensure low-friction rotation of the rotor shaft.

The process control is particularly fast executable if according to claim 9 hydraulic arrangements are present, which act to bring the rotor shaft to a second level to the respective auxiliary bearing.

For a better understanding of the invention, the following explanations are given:
When referring to a first level N1 in the context of the invention, this refers to the distance A1 of the axis of rotation of the rotary shaft to the bottom of the nacelle and describes a state in which the wind turbine is operated to generate electricity. A second level N2 corresponds to a distance A2 of the axis of rotation to the bottom of the nacelle, which is slightly larger than the distance A1.

A connection construction is understood to mean a machine part which is suitable for receiving a shaft bearing with its outer ring and which supports the rolling bearing in the accommodated state with respect to the further component or the ground level B (FIG. 1 ) is supported. A shared connection construction is used when the respective shaft bearing receiving machine part comprises a remaining, even after dividing the ground support part and a removable part from the remaining part.

Short description of the drawing

Show it:

1 a wind turbine;

2 a shaft bearing for supporting a rotor shaft;

3 an auxiliary warehouse;

4 a partially dismounted shaft bearing;

5 an even further disassembled shaft bearing;

6 a second embodiment of the invention;

7 another illustration according to the second embodiment of the invention; and

8th another application of the invention.

Detailed description of the drawing

The invention will now be explained in more detail with reference to FIGS.

In 3 is an auxiliary camp 14 shown which is similar to the one in 2 shown shaft bearing 8th is formed and which also has an inner ring 15 , an outer ring 16 that the inner ring 15 Coaxially surrounds, and rolling elements 17 having. Through the inner ring 15 is the rotor shaft 5 guided. The outer ring 16 is in a connector part 18 used, which has the shape of an ellipse.

Furthermore, in 3 two hydraulic systems 19 shown, which is opposite to another machine part 20 support and which with their telescopic punches 21 on protrusions 22 act, in the present embodiment of the extensions of the long axis of the elliptical connection part 18 be formed.

At the in 3 shown embodiment auxiliary bearing 14 is this one, which only temporarily with the rotor shaft 5 is connectable. That's why the auxiliary camp is 14 divided constructed and includes two semicircular inner rings 15a . 15b and two semicircular outer rings 16a . 16b , If necessary, in a known manner to a rotor shaft 5 comprehensive auxiliary camp 14 can be mounted. With regard to this divided training of the auxiliary camp 14 is also the connection part 18 formed divided and comprises a first and second connection part 18a . 18b , In this context, it should be noted that the inner ring 15 even at a merely temporary with the rotor shaft 5 connectable auxiliary bearing 14 does not necessarily have to be formed in two parts, but can already in the original equipment as a one-piece inner ring 15 with the rotor shaft 5 can be connected. In such a case, the permanent remaining on the rotor shaft 5 provided, integrally formed inner ring 15 at the same time also for the axial fixing of the actual shaft bearing 8th on the rotor shaft 5 be used.

Will that be just temporary joining the auxiliary camp 14 with the rotor shaft 5 considered too expensive, the corresponding auxiliary storage 14 or just parts of it (such as the inner ring 15 ) during the original equipment of the rotor shaft 5 be installed.

For the sake of completeness it should be noted that in another - not shown - embodiment of the connecting part 18 can be omitted if the outer ring 16 at the same time also projections 22 includes, for example, with the outer ring 16 are integrally connected.

Shall now a shaft bearing 8th the in 2 shown type under action of auxiliary bearings 14 from the rotor shaft 5 solved and through a new shaft bearing 8th be replaced, is initially the shared trained connecting structure 13 dissolved by the one or more shells 13b from the corresponding parts 13a is lifted. Then the upper outer ring segment 10b and the rolling elements 11 removed as far as these are accessible. Then or later, the upper inner ring segment 9a from the rotor shaft 5 separated. After the completion of this work, conditions arise that in 3 are shown. It is clear from the illustration according to 3 removable, that even after removal of the tops 13b . 10b and 9b and some rolling elements 11 the rotor shaft 5 still on a combination of lower outer ring segment 10a , Rolling elements 11 and lower inner ring segment 9b in the lower part 13a the connection combination 13 rests. This position of the rotor shaft 5 based on the lower part 13a the connection combination is referred to in connection with the application as the first level N1.

Are in addition the auxiliary storage or the 14 axially offset to the shaft bearings 8th with the rotor shaft 5 connected and extend the hydraulic assemblies 19 between the projections 22 and a machine part 20 , can with the lifting of the rotor shaft 5 be started on a second level N2. These are in a known manner, the hydraulic systems 19 with - preferably - applied oil, whereby their telescopic rods 21 in the direction of arrow P ( 3 ) and the auxiliary camp 14 and the rotor shaft mounted in it 5 Lift. Such conditions are 5 shown in FIG 5 only a particularly large distance between the first and second level N1, N2 is shown for better representability. In practice, by lifting the rotor shaft 5 in the lower part 13a the adjacent construction 13 corresponding 4 remaining parts 9a . 10a , and 11 of the auxiliary camp 14 only relieved.

Just for the sake of completeness it should be noted that not necessary with the lifting of the rotor shaft 5 at the same time a lifting of the lower inner ring segment 9b he follows.

Is the rotor shaft 5 brought to the second level N2, can with the expansion of the lower part 13a the adjacent construction 13 remaining parts 9a . 10a , and 11 be started by rotating the rotor shaft 5 the lower inner ring 9a and the remaining rolling elements 11 be removed in a known manner and then still the lower outer ring 10a is removed.

Everything is removed, can build a new shaft bearing 8th can be started by executing the above-described sequence of steps exactly the opposite.

Finally, it should be added that it is not necessary that the inner and outer rings 9 and 10 the shaft bearing 8th even with the original equipment of the rotor shaft 5 also have to be formed in one piece. If this is the case, - after the top 13b the adjacent construction 13 from the lower part 13a has been lifted - the inner and outer rings 9 . 10 first be separated by using known techniques.

In the following, a further embodiment of the invention will now be described. This is first a state of the shaft bearing 8th related to 4 has already been explained.

Is a state according to 4 made, the rotor shaft 5 set in rotation (indicated in 4 by the curved arrow), the rolling elements which have the remaining mean and a first average diameter D1 can be characterized by the prevailing friction 11 at the first end of the gap 23a be removed.

The details are 6 removable.

By the rotation of the rotary shaft 5 not just rolling elements 11 with the mean diameter D1 at the first column end 23a taken, but also at the same time rolling elements 11 at the end of the gap 23b be introduced. It has been found that not only rolling elements 11 with a mean diameter D1 at the column end 23b , but also second rolling elements 24 with a diameter D2 slightly above the diameter D1 at the second column end 23b between the inner and outer ring 9 . 10 to be brought. By a continuous increase in the diameter D2 of the second rolling elements 24 (indicated by the rolling elements 24.1 - 4 or the two the rolling elements 24.1 - 4 Tangentially touching and enclosing an angle lines) can be the rotor shaft 5 also bring from a first level N1 to a second level N2. These relationships are in 7 illustrates where between the inner ring 9 and the outer ring 10 three second rolling elements 24 are arranged.

The embodiment of the invention according to the 6 and 7 is not just on auxiliary storage 14 limited but can also be used with shaft bearings 8th be executed when the corresponding shaft bearing 8th is formed at least two rows. In this case, takes over a row of the multi-row shaft bearing 8th at the same time the function of an auxiliary bearing 14 ,

Such a double row shaft bearing 8th is in 8th shown. The two rows of the shaft bearing 8th are with 8a and 8b designated. The inner rings 9 ' . 9 '' the two shaft bearings 8a . 8b be from the rotor shaft 5 penetrated. The two outer rings 10 ' . 10 '' the two shaft bearings 8a . 8b are in a connecting construction 13 arranged. Are both rows of shaft bearings 8a ; 8b as well as in 4 shown disassembled, can by applying the related to the 6 and 7 technology shown the rotor shaft 5 from a first level N1 to a second level N2, by rotating the rotor shaft 5 for example, in the rolling bearing row 8b their rolling elements 11 '' with a mean diameter D1 through second, a rising average diameter D2 having rolling elements 11 '' be replaced. To the introduction of the second having a larger average diameter D2 rolling elements 11 '' To facilitate, these rolling elements should 11 '' first in the rolling bearing row 8a ; 8b to be introduced, by all Wälzkörperreihen 8a ; 8b has the greatest wear. Are the second, a larger average diameter D2 have rolling elements 11 '' between the inner and outer ring 9 '' . 10 '' Inserted, has by the larger average diameter D2 of the second rolling elements 11 '' the rotation shaft 5 moved to its second level N2, leaving the shaft bearing row 8a then easily disassembled and replaced afterwards. Is the shaft bearing 8a replaced, the rotor shaft 5 Turned until the shaft bearing row 8b free of rolling elements 11 '' of the larger diameter. In this state, then the bearing rings 9 '' . 10 '' replace with new bearing rings. Then, then new rolling elements 11 '' as described 6 in the shaft bearing 8b introduced, these rolling elements 11 '' have a mean diameter equal to the mean diameter of the rolling elements 11 ' corresponds to that in the rolling bearing row 8a have been newly used. To the introduction of new rolling elements 11 '' in the rolling bearing row 8b To facilitate, the first new rolling elements can 11 '' also have a reduced mean diameter D1. If this is the case, but must be ensured that these rolling elements 11 '' be removed again.

Is appropriate 8th the bearing shown not as a double row shaft bearings 8th . 8a . 8b but like in the 3 to 7 described as a combination of shaft bearings 8a and auxiliary storage 14 built up, can the 8th It can be easily seen that at least the inner ring 9 '' of the auxiliary camp 14 if he is constantly on the rotor shaft 5 remains, with replacement of the otherwise used components also very good for the axial fixing of the actual shaft bearing 8a can be used.

LIST OF REFERENCE NUMBERS

1

 Wind turbine

2

 mast

3

 gondola

4

 generator

5

 rotor shaft

6

 hub

7

 wing

8th

 shaft bearing

8a, 8b

 Bearing rows of a double row shaft bearing

9, 9 ', 9' '

 inner ring

9a, 9b

 Inner ring segment

10, 10 ', 10' '

 outer ring

10a, 10b

 Outer ring segment

11, 11 ', 11' '

 rolling elements

12

 outer contour

13

 port construction

13a, 13b

 Lower and upper part of the adjacent construction

14

 auxiliary bearing

15

 Inner ring auxiliary bearing

15a, 15b

 Lower and upper part of inner ring (auxiliary bearing)

16

 Outer ring auxiliary bearing

16a, 16b

 Lower and upper part of outer ring (auxiliary bearing)

17

 Rolling element auxiliary bearing

18

 connector

18a, 18b

 first and second connection part

19

 hydraulic arrangement

20

 machinery

21

 telescopic ram

22

 projections

23a, 23b

 cleaving

23

 Rolling elements of diameter D2

R

 rotor

N

 level

D

 diameter

B

 ground level

Claims (10)

Mounting arrangement for bearings of rotor shafts of wind turbines, with a rotor shaft ( 5 ), whose axis of rotation is at a first level N1, with at least one rotor shaft ( 5 ) bearing shaft bearings ( 8th ), each of these shaft bearings ( 8th ) a split or divisible inner ring ( 9 ), a split or divisible, the inner ring ( 9 ) coaxially surrounding outer ring ( 10 ) and a plurality of between inner and outer ring ( 9 . 10 ) arranged rolling elements ( 11 ), and with a divisible connecting structure ( 13 . 13a . 13b ) into which the respective shaft bearings ( 8th ) for supporting the rotor shaft ( 5 ) are inserted, characterized in that at least one an inner ring ( 15 ), an outer ring ( 16 ) and rolling elements ( 17 ) comprehensive auxiliary storage ( 14 ) is provided, which permanently or even temporarily with the rotor shaft ( 5 ) and which is connected to the outer ring ( 16 ) on the connecting structure ( 13 . 20 ) is at least partially supported, and that means are provided which on the auxiliary bearing ( 14 ) and which act on the auxiliary storage ( 14 ) the rotor shaft ( 5 ) from their first to a second level N2. Mounting arrangement according to claim 1, characterized in that on the outer ring ( 16 ) of the auxiliary storage ( 14 ) or a connection part ( 18 . 18a . 18b ), in which the auxiliary storage ( 14 ), projections ( 22 ) are formed and that the means one on the projections ( 22 ) acting hydraulic arrangement ( 19 ). Mounting arrangement according to claim 1, characterized in that the auxiliary bearing ( 14 ) first rolling elements having a uniform mean diameter D1 ( 17 ) and that the means which the rotor shaft ( 5 ) from a first to a second level N1, N2, second rolling elements ( 24 ) whose average diameter D2 is greater than the mean diameter D1 of the first rolling elements ( 17 ). Mounting arrangement according to claim 3, characterized in that the diameter D2 of two side by side between inner and outer ring ( 15 . 16 ) arranged second rolling elements ( 24 ) is different in size. Mounting arrangement according to claim 3 or claim 4, characterized in that the auxiliary bearing ( 14 ) at the same time a shaft bearing row ( 8a ; 8b ) of a multi-row shaft bearing ( 8th ). Mounting arrangement according to one of claims 3 to 5, characterized in that the rolling bodies ( 17 . 24 ) of the auxiliary storage ( 14 ) are either cylindrical rollers, tapered rollers or barrel rollers. Mounting arrangement according to one of claims 1 to 6, characterized in that the auxiliary bearing ( 14 ) or parts ( 15 . 16 ) of which at the same time make determinations to a shaft bearing ( 8th ) in the axial direction on the rotor shaft ( 5 ). A method of replacing a shaft bearing according to claim 1, characterized in that in a first step, the split connection construction ( 13a . 13b ) of the shaft bearing or bearings ( 8th ) is dissolved, so that the outer rings ( 10 . 10a ) of the respective shaft bearings ( 8th ) only with the remaining adjacent construction ( 13a ) are in physical contact, that in a subsequent step, the rotor shaft ( 5 ) by already existing or still on the rotor shaft ( 5 ) auxiliary bearing ( 14 ) acting means the rotor shaft ( 5 ) is moved from a first level N1 to a second level (N1). A method according to claim 8, characterized in that the level change by hydraulic arrangements ( 19 ), which are connected to the adjacent construction ( 13 . 13a ) and on the outer rings ( 10 ) of auxiliary storage ( 14 ) Act. A method according to claim 8, characterized in that in a first step, the one or more auxiliary storage ( 14 ) are or are reduced so far that in the area of the respective auxiliary storage ( 14 ) the rotor shaft ( 5 ) only via the inner ring ( 15 ), first Wälzköper ( 17 ) and the semicircular, in the remaining adjacent construction ( 13 . 13a ) remaining part of the outer ring ( 16a ) of the auxiliary bearing (s) ( 14 ) is supported, in a second step, the first rolling bodies having a mean diameter D1 ( 17 ) by rotation of the rotor shaft ( 5 ) and that in a third step second rolling elements ( 24 ) whose mean diameter D2 is greater than the mean diameter D1 of the first Wälzköper ( 17 ), wherein the average diameter D2 of the second Wälzköper ( 24 ) is different from each other, that the second rolling elements ( 24 ) by rotation of the rotor shaft ( 5 ) into a column end ( 24b ) between inner ring ( 15 ) and outer ring ( 16 ) of the auxiliary storage ( 14 ), wherein with the smallest mean diameter D2 having second rolling elements ( 23.1 ) is started and then subsequently the average diameter D2 of the second rolling elements ( 24 ) is increased.

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