WO2018153523A1

WO2018153523A1 - A blade for a wind turbine

Info

Publication number: WO2018153523A1
Application number: PCT/EP2017/080460
Authority: WO
Inventors: Martin Bissenbakker HANSSON
Original assignee: Siemens Wind Power A/S
Priority date: 2017-02-24
Filing date: 2017-11-27
Publication date: 2018-08-30

Abstract

It is described a method for manufacturing a blade for a wind turbine and a device for manufacturing a blade for a wind turbine. Further, is described a blade for a wind turbine, comprising a main part (101) extending along a longitudinal axis (120) and an adapter part (102) extending along the longitudinal axis (120). The adapter part (102) has along the longitudinal axis (120) a first end (107) and an opposing second end (106). The first end (107) of the adapter part (102) is fixed to the main part (101) and the second end (106) of the adapter part is fixable to a rotor of the wind turbine.

Description

DESCRIPTION

A blade for a wind turbine Field of invention

The present invention relates to the field of wind turbines, in particular to the blades arranged at a wind turbine.

Specifically, the present invention relates to a method for manufacturing a blade for a wind turbine. Further, the pre^¬ sent invention relates to a blade which is separable into two parts. Furthermore, the present invention relates to a device for manufacturing a blade for a wind turbine. Art Background

Wind turbines are becoming longer and bigger. Therefore, the blades to be coupled to a wind turbine are also becoming larger for improving its efficiency. Because the blades be- come longer, more difficulties arise during the transporta^¬ tion of the blades to the destination where the wind turbine has to be equipped with the blades. This means, that the blade may be too heavy or too long for transportation. Furthermore, a plurality of local restrictions exist, which de- limit the allowable size of the blade to be transported. The local restrictions may vary depending on the country where the wind turbine is installed. For example, a restriction is the allowable length of the blade to be transported on a road .

There may be a need for providing a blade for a wind turbine which can be easily handled during transportation and which is further adaptable at the local transportation re^¬ strictions . Summary of the Invention

This need may be met by the subject matter according to the independent claims. Advantageous embodiments of the present invention are described by the dependent claims.

According to a first aspect of the invention a method for manufacturing a blade for a wind turbine is provided. The method comprises forming the blade in one piece, wherein the blade extends along a longitudinal axis. The blade comprises a main part extending along a longitudinal axis and an adapt^¬ er part extending along the longitudinal axis. The adapter part may has along the longitudinal axis a first end and an opposing second end, wherein the first end of the adapter part is fixable to the main part and the second end of the adapter part is fixable to a rotor of the wind turbine. The method further comprises cutting the one piece formed blade into the main part and the adapter part This aspect of the invention is based on the idea that a blade can be efficiently manufactured in one piece and which is afterwards easily handleable during transportation and which fulfils e.g. local restrictions regarding the transpor^¬ tation requirements. The manufacturing method will provide a blade which may be separable into the main part and the adapter part after being manufactured in one piece.

For this purpose and in accordance with an embodiment of the invention the blade is separated after being manufactured in one piece into two pieces, e.g. the main part and the adapter part, which both extend along the same longitudinal axis. By separating the blade in at least two pieces, the allowed length for transporting the blade can be adjusted, such that at least one part of the two pieces of the blade has the max- imum allowable length. The other part comprises the residual length . The "main part" may be the part which comprises the maximum allowable or maximum desired length of a blade part for transportation. Furthermore, the main part may be the part comprising the aerodynamic profile of the blade. In other words, the main part does not comprise a root of the blade and is therefore not directly coupleable to the hub of the wind turbine. Hence, the main part has a shape and dimensions which are required to efficiently extract energy from the wind. Because of this reason the main part is preserved as a whole, such that the aerodynamic design is not interrupted by dividing the main part. In other words, the main part is the part of the blade which extends along a longitudinal axis from a tip of the blade to the adapter part (i.e. into the direction of the hub, where the blade is attachable to the rotor of the wind turbine) .

The "adapter part" may be the part which is directly attacha^¬ ble to the rotor of the wind turbine, in particular which is directly attachable to the hub of the wind turbine. In other words, the adapter part forms a root section of the blade. The adapter part may comprise a cylindrical cross section. For example, the adapter part comprises two ends, i.e. the first end and the second end. Both ends are opposing each other along the longitudinal direction. The adapter part is attached to the main part by the first end. The adapter part is attachable to the rotor of the wind turbine, in particular to the hub of the wind turbine, by the second end.

The blade is manufactured monolithically in one piece and is afterward separated in the main part and the adapter part.

The blade may be made of composite fibre materials. Further^¬ more, the blade may be manufactured by resin transfer mold^¬ ing. Fiber sheets are arranged within a mold and are flooded by resin material. Afterwards, the resin is cured. The manu- facturing of the blade may be conducted by using the half shelf principle (butterfly technique) , wherein two pre-casted blade shelves are attached to each other. The two pre-casted shelves may be attached together by an adhesive. For elimi- nating weaker areas at the glue joint the blade may be casted using a full cast method, wherein the whole blade body is formed in one piece. The cutting is performed in such a manner that the main part is separated by cutting and sawing, respectively, from the adapter part. The cutting may be performed at a predefined distance from the second side of the adapter part, wherein the predefined distance extends from second side of the adapter part to the first side of the adapter part. In other words, the predefined distance may define the length of the adapter part. The predefined distance may depend on the de^¬ sired length of the blade. Therefore, adapter parts for dif^¬ ferent blades may comprise a different length. For example, the whole blade may have a length of 57 meter but the allowa^¬ ble transportation length may be 54 meter, such that the predefined distance for the adapter part may be 3 meter. Hence both parts may be transported separately and may be reassem^¬ bled on site of the wind turbine.

The cutting may be performed after the casting of the blade in monolithically one piece.

According to a further embodiment of the invention the forming step further comprises arranging a main inlay inside a mold, wherein an outer profile of the main inlay defines an inner profile of the main part and the adapter part. The main inlay is arrangeable inside the mold such that the main part is formed between the outer profile of the main inlay and an inner profile of the mold. The method further comprises ar^¬ ranging a sealing element which seals a volume between the outer profile of the main inlay and the inner profile of the mold. The sealing element is arrangeable along the longitudi^¬ nal axis, such that the sealing element is arranged at the second end of the adapter part. According to a further aspect of the invention there is pro^¬ vided a device for manufacturing a blade for a wind turbine. The device comprises a mold having a first part which com^¬ prises an inner surface being an outer profile of a main part configured for forming the main part extending along a longitudinal axis. Further the mold has a second part which com^¬ prises an inner surface being an outer profile of an adapter part configured for forming the adapter part extending along the longitudinal axis and having along the longitudinal axis a first end and an opposing second end. Further, the mold comprises a main inlay, wherein an outer profile of the main inlay defines an inner profile of the main part and the adapter part. The main inlay is arrangeable inside the mold such that the main part is formed between the outer profile of the main inlay and an inner profile of the mold. Further^¬ more, the mold comprises a sealing element which seals a vol^¬ ume between the outer profile of the main inlay and the inner profile of the mold. The sealing element is arrangeable along the longitudinal axis, such that the sealing element is ar^¬ ranged at the second end of the adapter part.

The main inlay may be an inner part of the mold which forms the inner profile of the whole blade comprising the adapter part and the main part. In particular, the main inlay may be a casting core or an inflatable bag providing the inner shape of the blade, in particular of the adapter part and the main part. The mold may define an inner shell/profile forming a cast of the blade to be formed. Hence, the inner surface of the mold may define the outer profile of the blade to be formed, such that with the inner surface of the mold the aer^¬ odynamic profile of the blade is formed. Between the inner surface of the mold and the outer profile of the main inlay a casting volume is formed which volume is filled by the mate- rial, such as the composite sheets, prepregs, etc., used for forming the blade. The mold is closed at the tip section of the blade. For sealing the volume at the root end of the blade and the second end of the adapter part, respectively, the sealing element may be provided. The sealing element may be arranged between the inner surface of the mold and the outer profile of the main inlay. Further, the sealing element is arrangeable or adjustable along the longitudinal axis such that depending on the position where the sealing element is arranged the length of the blade is determined. In other words, the sealing element is arranged at a position between an end of the mold and the second end of the adapter part. For example, the sealing element may be arranged at the end of the mold which end is opposite to the end of the mold com^¬ prising the tip of the blade. Thus, the sealing element de^¬ fines the desired length of the blade.

For example, the sealing element may be a ring which may be adjustable along the longitudinal direction at different po^¬ sitions along the main inlay.

Furthermore, the mold may comprise additionally an adapter inlay, wherein an outer profile of the adapter inlay may de- fine an inner profile of the adapter part. The adapter inlay may also be arranged inside the mold such that between the inner surface of the mold and the outer profile of the adapt^¬ er inlay the adapter part is formed. In between the inner surface of the mold and the outer profile of the adapter in- lay the volume for the material for forming the adapter part may be provided. In this embodiment, the sealing element may be used for separating the adapter inlay from the main inlay such that inside the mold the two parts of the blade may be formed monolithically in one piece. The cutting may be per- formed when the main inlay and the adapter inlay are still within the inner volume of the monolithically formed blade. The cutting tool (such as a laser cutter or a saw device) cuts the blade along a cutting line between the adapter inlay and the main inlay.

According to a further embodiment of the invention the forming step further comprises forming respective holes for re^¬ spective fixing elements, wherein the holes are formed from the second end of the adapter part through the adapter part and into the main part. The holes are extending along the di- rection of the longitudinal axis. The respective holes com^¬ prise a predefined length depending on the length of the fixing elements which will be inserted into the blade. For exam^¬ ple, the respective holes for the respective fixing element may be formed using respective fixing element inlays, wherein the fixing element inlays may be arrangeable inside the mold. The fixing elements may be removable bolts, screws and the like .

The holes for the fixing elements may be formed integrally during the forming process of the blade itself and before the step of cutting the blade in the main part and the adapter part. The fixing element inlays may extend inside the volume of the mold from the second end of the adapter part (or from the second end of the adapter part inlay) to the main part (or the main part inlay) . The hole provided by the fixing el^¬ ement inlays through the adapter part may be a through-hole. The hole extending into the main part extends into the main part with a determined length and not through the whole main part .

According to another embodiment the holes may be formed me^¬ chanically in particular using a drilling device. For example the holes may be drilled by a machine inside the formed adapter part and in the main part located in longitudinal di^¬ rection behind the adapter part. Therefore, the holes may be provided after the forming process of the blade, i.e. after the forming process of the adapter part and the main part.

According to a further embodiment of the invention the method further comprises coupling the main part and the adapter part for forming the blade, wherein the main part and the adapter part are fixed together via fixing elements. The fixing ele- ments extend for example through the adapter part into the main part. This means, after forming the blade comprising the main part and the adapter part, the main part and the adapter part are separated by cutting. Then, the adapter part and the main part may be transported without failing local transpor- tation limitations. After the adapter part and the main part have been transported to the desired location, they can be coupled together for forming the complete blade which may be attached to the rotor of the wind turbine. The adapter part is coupled to the main part using the fixing elements. Fur^¬ thermore, the adapter part and the coupled main part are at^¬ tached together to the rotor of the wind turbine. As described above, according to a further embodiment of the device, the sealing element is adjustable along the longitu^¬ dinal axis, such that the sealing element is arranged at the second end of the adapter part. By using an adjustable seal^¬ ing element the length of the blade may be adjustable, in particular the length of the adapter part may be adjustable. In particular, the sealing element may be arrangeable at dif^¬ ferent positions depending on the desired length of the blade . In a further exemplary embodiment of the invention, the device for manufacturing the blade may comprise a further mold. The mold described above may only comprise the main inlay which defines the inner profile of the main part. The further mold may only comprise the adapter inlay which defines the inner profile of the adapter part. In other words the device for manufacturing the blade comprises at least two molds which are arranged one after another along the longitudinal direction. The blade is monolithically formed by the two molds. The main part and the adapter part are afterwards sep- arated by cutting after the forming of the blade. By using a further mold for the adapter part different adapters compris^¬ ing different length may be applied easily. Therefore, the overall length of the blade may be easily adjusted by apply^¬ ing different adapter molds having a different mold length.

According to a further aspect of the invention there is pro^¬ vided a blade for a wind turbine, wherein the blade is manu^¬ factured by the method described above. The blade comprises a main part extending along a longitudinal axis and an adapter part extending along the longitudinal axis. The adapter part has along the longitudinal axis a first end and an opposing second end, wherein the first end of the adapter part is fixed to the main part and the second end of the adapter part is fixable to a rotor of the wind turbine.

This aspect of the invention is based on the idea that a bet- ter handling of the blade may be achieved, such that the transportation can be made easier. Simultaneously, local re^¬ strictions for the transportation can be fulfilled.

According to a further embodiment of the invention, the main part and the adapter part are attachable to each other by

(e.g. removable) fixing elements. The fixing elements are ex^¬ tendable from the second end of the adapter part through the whole adapter part into the main part. By using the fixing elements the adapter part is attachable to the rotor.

The adapter part and the main part are attachable to each other by at least two fixing elements. More fixing elements may also be applicable. For example, the adapter part and the main part are attachable to each other by a plurality of fix^¬ ing elements. The plurality of fixing elements may be ar- ranged circumferentially around the longitudinal axis. The plurality of fixing elements may be equally spaced apart to each other with a same distance.

On the other hand, the main part and the adapter part are at- tachable to each other by fixing elements, which do not ex^¬ tend through the whole adapter part. This means, the first end of the adapter part is fixable by fixing elements to the main part, such that the first end of the adapter part and the corresponding end of the main part to be fixed to the first end comprise respective fixing elements. Furthermore, the second end of the adapter part comprises fixing elements for fixing the adapter part to the rotor of the wind turbine.

According to a further embodiment of the invention the fixing element is a bolt extending from the second end through the adapter part into the main part. Furthermore, the fixing ele^¬ ment comprises a bushing configured for being arrangeable (e.g. mounted at) at the adapter part of the blade or at the abutting side of the main part, such that the bolt is fixable to the bushing.

The bolt may be a metal bolt. For inserting the bold into and/or through the adapter part a respective hole is provided inside the adapter part. Further, also in the main part a re^¬ spective hole for the bolt is provided, such that the bolt is insertable into the main part. Furthermore, the fixing ele^¬ ment may comprise a plurality of bolts which are configured for being insertable through the adapter part into the main part. For the plurality of bolts also respective holes (e.g. including respective bushings) may be provided in the adapter part and in the main part. By using a plurality of bolts the connection between the adapter part and the main part can be ensured.

To each bolt a respective bushing is provided in the main part. If the bolt extends through the adapter part, a bushing is only necessary in the main part. If the fixation of the adapter part and the main part is carried out by fixing ele- ment which does not extend through the adapter part, a first bolt may be arranged at the first end of the adapter part and at the main part. Further, a second bolt may be arranged at the second end of the adapter part for fixing the adapter part to the rotor. Therefore, a bushing may be used at the second end of the adapter part, the first end of the adapter part and in the main part, respectively.

According to a further embodiment of the invention the first end of the adapter part has an abutting side and a corre- sponding end of the main part has an abutting side along the longitudinal axis, wherein the respective abutting sides abut at each other.

The abutting side provides a connection between the first end of the adapter part and the corresponding end of the main part. The corresponding end of the main part may be the end opposite to the blade tip. Both, the abutting side of the adapter part and the abutting side of the main part extend along the longitudinal axis of the blade. Abutting means that the abutting sides are adjacent to each other and contacts each other when the adapter part is fixed to the main part. In particular the abutting sides are connectable or are con- nected to each other.

According to a further embodiment of the invention the respective abutting sides of the main part and the adapter part are mechanically or chemically roughened.

The term "mechanically roughened" may particularly denote a machined surface finish. For example the abutting sides may be mechanically roughened by sandblasting or grinding. The term "chemically roughened" may particularly denote a chemi- cally surface finish for example a chemically etching pro^¬ cess. By roughening the abutting sides a friction between the abutting sides may be provided. The friction may ensure a better bonding between the abutting sides, such that the adapter part and the main part are reliably connected.

According to a further embodiment of the invention between the abutting sides of the main part and the adapter part a friction element is arranged for improving a friction between the abutting sides. The friction element may be an element made of metal, e.g. steel. The friction element is insertable between main part and the adapter part. By using the friction element a better connection between the main part and the adapter part may be provided. The inserted friction element may provide a surface for the respective abutting sides. The distribution of the surface pressure caused by the connection (attachment) of the main part and the adapter part may be im^¬ proved .

It has to be noted that embodiments of the invention have been described with reference to different subject matters.

In particular, some embodiments have been described with ref^¬ erence to method type claims whereas other embodiments have been described with reference to apparatus type claims. How- ever, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features re- lating to different subject matters, in particular between features of the method type claims and features of the appa^¬ ratus type claims is considered as to be disclosed with this document . The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodi- ment but to which the invention is not limited.

Brief Description of the Drawing

Figure la shows a blade comprising a main part and an adapter part according to an embodiment of the present invention.

Figure lb shows a Detail of Figure la according to an embodi^¬ ment of the present invention. Figure 2 shows a blade comprising a main part and an adapter part according to another embodiment of the present inven^¬ tion .

Detailed Description

The illustration in the drawing is schematical. It is noted that in different figures, similar or identical elements or features are provided with the same reference signs or with reference signs, which are different from the corresponding reference signs only within the first digit. In order to avoid unnecessary repetitions elements or features which have already been elucidated with respect to a previously de- scribed embodiment are not elucidated again at a later posi^¬ tion of the description.

Further, spatially relative terms, such as "front" and

"back", "above" and "below", "left" and "right", et cetera are used to describe an element's relationship to another el^¬ ement (s) as illustrated in the figures. Thus, the spatially relative terms may apply to orientations in use which differ from the orientation depicted in the figures. Obviously all such spatially relative terms refer to the orientation shown in the figures only for ease of description and are not nec^¬ essarily limiting as an apparatus according to an embodiment of the invention can assume orientations different than those illustrated in the figures when in use.

Fig. la shows a blade 100 according to an exemplary embodi^¬ ment of the invention. The blade 100 comprises a main part

101 and an adapter part 102. Both the main part 101 and the adapter part 102 extend along a longitudinal axis 120. The adapter part 102 comprises a first end 107 and a second end 106, wherein the first end 107 of the adapter part 102 is fixed to the main part 101 and the second end 106 of the adapter part 102 is fixable to a rotor of the wind turbine. The main part 101 and the adapter part 102 are attachable to each other by fixing elements 103. As can be seen in Fig. la, the fixing elements 103 extend from the second end 106 through the adapter part 102 into the main part 101. By the fixing element 103 the adapter part 102 is attachable to the rotor. The fixing element 103 comprises a bolt 105 and a bushing 104.

The bolt 105 is configured for being insertable through the adapter part 102 into the main part 101. Further, the bushing 104 is configured for being arrangeable at the adapter part

102 of the blade or at the abutting side of the main part 101. The first end 107 of the adapter part 102 has an abut^¬ ting side and a corresponding end of the main part 101 has an abutting side along the longitudinal axis 120. The respective abutting sides abut at each other. In Fig. la the abutting sides of the adapter part 102 and of the main part 101 are laying at the line of 110. In Fig. la, the line 110 is used to illustrate the cutting line between the main part 101 and the adapter part 102.

Fig. lb shows a detail A of Fig. la, wherein the separation area between the adapter part 102 and the main part 101 is shown in an enlarged view. In this detail A, a mechanically or chemically roughened surface of the abutting sides of the main part 101 and the adapter part 102 is illustrated. In be^¬ tween the abutting sides of the main part 101 and the adapter part 102 a friction element 108 is arranged for improving a friction between the abutting sides.

As can be seen from Fig. la, the used bolt 105 may have a length of about 3500 millimeters. The bolt 105 has a length LI which is longer than the length L2 of the adapter part 102. The bolt 105 extending through the adapter part 102 is received in the main part 101 by the bushing 104. The adapter part 102 may comprise a length L2 of about 2500 millimeters, such that the bolt 105 extends through the adapter part 102 into the main part 101. The length L2 of the adapter part 102 may vary depending on the desired length of the blade 100.

Fig. 2 shows a blade 100 according to an exemplary embodiment of the invention. The blade 100 also comprises a main part 101 and an adapter part 102 which both extend along the lon- gitudinal axis 120. The blade 100 further comprises the fix^¬ ing element 103 having a bolt 105 and a bushing 104. Further, the blade 100 comprises a further fixing element 203 which may comprise a further bolt 205 and the further bushing 204. As can be seen in Figure 2 the fixing element 103 does not extend through the adapter part 102. The fixing element 103 is arranged in the adapter part 102 such that by the fixing element 103 the adapter part 102 is attachable to the rotor of a wind turbine. The further fixing element 203 may be used for coupling the main part 101 with the adapter part 102.

It should be noted that the term "comprising" does not ex- elude other elements or steps and the use of articles "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

1. A method for manufacturing a blade for a wind turbine, the method comprising

forming the blade in one piece,

wherein the blade extends along a longitudinal axis,

wherein the blade comprises a main part extending along a longitudinal axis and an adapter part extending along the longitudinal axis, wherein the adapter part having along the longitudinal axis a first end and an opposing second end, wherein the first end of the adapter part is fixable to the main part and the second end of the adapter part is fixable to a rotor of the wind turbine,

cutting the one piece formed blade into the main part and the adapter part.

2. The method according to claim 1, wherein the forming step further comprises

arranging a main inlay inside a mold, wherein an outer profile of the main inlay defines an inner profile of the main part and the adapter part,

wherein the main inlay is arrangeable inside the mold such that the main part is formed between the outer profile of the main inlay and an inner profile of the mold,

arranging a sealing element which seals a volume between the outer profile of the main inlay and the inner profile of the mold,

wherein the sealing element is arrangeable along the longitu^¬ dinal axis, such that the sealing element is arranged at the second end of the adapter part.

3. The method according to claim 1 or 2, wherein the forming step further comprises

forming respective holes for respective fixing elements, wherein the holes are formed from the second end of the adapter part through the adapter part and extend into the main part.

4. The method according to one of the claims 1 to 3, wherein the method further comprises

coupling the main part and the adapter part for forming the blade, wherein the main part and the adapter part are fixed together via fixing elements,

wherein the fixing elements extend through the adapter part into the main part.

5. A device for manufacturing a blade for a wind turbine, comprising a mold having

a first part which comprises an inner surface being an outer profile of a main part configured for forming the main part extending along a longitudinal axis and

a second part which comprises an inner surface being an outer profile of an adapter part configured for forming the adapter part extending along the longitudinal axis and having along the longitudinal axis a first end and an opposing se^¬ cond end,

a main inlay, wherein an outer profile of the main inlay defines an inner profile of the main part and the adapter part,

wherein the main inlay is arrangeable inside the mold such that the main part is formed between the outer profile of the main inlay and an inner profile of the mold

a sealing element which seals a volume between the outer profile of the main inlay and the inner profile of the mold, wherein the sealing element is arrangeable along the longitu^¬ dinal axis, such that the sealing element is arranged at the second end of the adapter part

6. The device according to claim 5, wherein the sealing element is adjustable along the longitudinal axis, such that the sealing element is arranged at the second end of the adapter part .

7. A blade for a wind turbine manufactured according to a method of claims 1 to 4, comprising

a main part extending along a longitudinal axis and an adapter part extending along the longitudinal axis and having along the longitudinal axis a first end and an op^¬ posing second end,

wherein the first end of the adapter part is fixed to the main part and the second end of the adapter part is fixable to a rotor of the wind turbine.

8. The blade according to claim 7,

wherein the main part and the adapter part are attachable to each other by fixing elements,

wherein the fixing elements extends from the second end through the adapter part into the main part,

wherein the adapter part is attachable to the rotor by the fixing elements.

9. The blade according to claim 8,

wherein the fixing element is a bolt extending from the second end through the adapter part into the main part, wherein the fixing element further comprises a bushing con- figured for being arrangeable at the adapter part of the blade or at the abutting side of the main part, such that the bolt is fixable to the bushing.

10. The blade according to claim 7 to 9,

wherein the first end of the adapter part has an abutting side and a corresponding end of the main part has an abutting side along the longitudinal axis, wherein the respective abutting sides abut at each other.

11. The blade according to any of the claims 7 to 10, wherein the respective abutting sides of the main part and the adapter part are mechanically or chemically roughened.

12. The blade according to any of the claims 7 to 11, wherein between the abutting sides of the main part and the adapter part a friction element is arranged for improving a friction between the abutting sides.