Abstract:
A method of mounting elements in a wind turbine tower, whereby the elements are partly or completely attached to the wind turbine tower by means of magnetic attraction forces. Wind turbine suspension units may be attached to the wind turbine tower, preferably completely or partly to the inner wall by means of magnetic attraction forces, which prevent structure-weakening attachments to the tower or part of the tower.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 10/157,286 filed on 29 May 2002 which claims priority to Danish Patent Application No. 2002 00178 filed on 6 Feb. 2002, both of which said applications are incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method of mounting elements in a wind turbine tower. 
       BRIEF DISCUSSION OF RELATED ART 
       [0003]    Typically, wind turbine towers carry the nacelle and the rotor of a wind turbine in order to allow for the desired rotation of the rotor and to raise the rotor into a position as far as possible from the ground as wind speeds typically increase with the distance to the ground. 
         [0004]    A wind turbine tower may be constructed according to several different principles, e.g. as tubular steel towers, lattice towers, or even concrete towers. 
         [0005]    Most large wind turbines are delivered with tubular steel towers manufactured in sections of 20-30 meters with flanges at both ends, and bolted together on the site. The towers are typically conical in order to increase strength and save materials at the same time. 
         [0006]    The steel towers typically comprise an interior with platforms, ladders, illumination means, power cables, lifts, etc. The components are welded to the inner surface of the steel tower section, or directly attached to the fittings, which have been welded and/or bolted to the tower. 
         [0007]    One of several problems associated with the above-described wind turbine towers is that the welding or bolting of especially the interior tower components such as ladders, cables, etc., weaken the tower structure. 
         [0008]    In some cases, it is possible to compensate for this problem by taking relatively costly precautions, which may, in turn, infer significant structural limitations for which no compensation is made. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The invention relates to a method of mounting elements in a wind turbine tower ( 10 ,  11 ,  12 ,  13 ,  14 ), whereby said elements are partly or completely attached to the wind turbine tower ( 10 ,  11 ,  12 ,  13 ,  14 ) by means of a magnetic attraction forces. 
         [0010]    According to the invention, the wind turbine suspension units may be attached to the wind turbine tower, preferably completely or partly to the inner wall by means of magnetic attraction forces, thereby preventing structure-weakening attachments to the tower or part of the tower. 
         [0011]    According to the invention, a wind turbine tower may comprise of a number of tower sections. 
         [0012]    When said magnetic attraction forces are established by means of permanent magnetic materials interacting with other magnetic materials, a further advantageous embodiment of the invention has been obtained. 
         [0013]    Permanent magnetic materials may be relatively stable and the necessary attraction forces may be maintained over a relatively long period. 
         [0014]    When said magnetic attraction forces are established by means of permanent magnetic materials comprised in a suspension unit ( 20 ;  60 ) interacting with magnetic materials forming at least a portion of a wind turbine tower, a further advantageous embodiment of the invention has been obtained. 
         [0015]    If the wind turbine tower comprises a magnetic material, steel or the like, a suspension unit according to the invention may advantageously be provided with permanent magnets. 
         [0016]    When said magnetic attraction forces are supplemented by mechanical fixation in at least one direction, a further advantageous embodiment of the invention has been obtained. 
         [0017]    According to the invention, a typical application of a suspension unit may be regarded as a “hybrid suspension” in the sense that the magnetic forces are primarily applied in order to handle the transverse suspension forces, whereas a mechanical suspension forces primarily deal with vertical suspension, i.e. typically gravity induced force on the suspended structure. 
         [0018]    Evidently, some suspension units may be applied for suspension based on magnetic attraction forces only if the magnetic attraction forces available are sufficient for the desired attachment. 
         [0019]    When said mechanical fixation in at least one direction is established by mechanical fixation to fixation areas ( 31 ,  32 ), said fixation areas being arranged in substantially the vertical direction of a wind turbine tower, a further advantageous embodiment of the invention has been obtained. 
         [0020]    According to the invention, wind turbine tower components may be mechanically suspended between the end point(s) of the turbine tower or wind turbine tower sections, thereby avoiding weakening of the wind turbine tower, which is typically caused by welding. 
         [0021]    The magnetic attachment means may then form a supplement to the mechanical suspension in areas in which no mechanical attachment is desired or possible. 
         [0022]    When said mechanical fixation in at least one direction is established by mechanical fixation to fixation areas ( 31 ,  32 ), said fixation areas being arranged at a certain distance from each other in the vertical direction of a wind turbine tower, said distances constituting approximately the vertical length of a wind turbine tower section ( 11 ,  12 ,  13 ,  14 ), a further advantageous embodiment of the invention has been obtained 
         [0023]    When said fixation areas comprise flanges ( 131 ,  142 ) adapted to joining the wind turbine tower sections ( 11 ,  12 ,  13 ,  14 ), a further advantageous embodiment of the invention has been obtained. 
         [0024]    According to the invention, the suspension method may be applied in more or less traditional wind towers or tower sections. 
         [0025]    Moreover, the invention relates to a wind turbine tower suspension unit ( 20 ;  60 ), said wind turbine tower suspension unit ( 20 ;  60 ) comprising magnetic attachment means ( 24 ). 
         [0026]    According to the invention, the wind turbine suspension unit may be attached to the wind turbine tower, preferably completely or partly to the inner wall by means of magnetic attraction forces. 
         [0027]    A unit according to the invention may advantageously be applied for by attaching the interior of a wind turbine tower, such as a ladder, ladder parts, illumination means, lifts, platforms, power cables, etc. 
         [0028]    According to the invention, improved suspension has been obtained since the desired attachment may be obtained by means of non-destructive attachment. In particular, the material properties of the tower or tower modules may remain substantially unaffected when attaching components or fittings to the surface of the tower. 
         [0029]    Moreover, the amount of steel used for e.g. a tubular steel tower may be reduced significantly when avoiding or minimizing structure weakening joining process(es). 
         [0030]    When said magnetic attachment means ( 24 ) are comprised by a permanent magnetic material, a further advantageous embodiment of the invention has been obtained. 
         [0031]    According to a preferred embodiment, the permanent magnetic material comprises at least one anisotropic magnet. 
         [0032]    When said permanent magnetic material comprises permanent Neodym magnets, a further advantageous embodiment of the invention has been obtained. 
         [0033]    According to a preferred embodiment of the invention, Neodym magnets are applied due to the fact that they feature strong magnetic attraction compared to other available magnetic materials. 
         [0034]    When said permanent magnetic material comprises permanent Samarium-Cobalt magnets, a further advantageous embodiment of the invention has been obtained. 
         [0035]    When said wind turbine tower suspension unit ( 20 ;  60 ) comprises mechanical fixating means ( 22 ;  62 ), a further advantageous embodiment of the invention has been obtained. 
         [0036]    According to the invention, the magnets applied should preferably feature a relatively high magnetic energy level and be resistant to demagnetizing, i.e. maintain the magnetic properties in different environments, both during mounting and during normal variations in e.g. temperature, humidity, etc. 
         [0037]    Interior elements may then be attached to the mechanical fixation means by means of traditional attachment precautions. 
         [0038]    When said wind turbine tower suspension unit ( 20 ) comprises at least one barrier ( 23 ) encapsulating said magnetic attachment means ( 24 ), a further advantageous embodiment of the invention has been obtained. 
         [0039]    When said barrier ( 23 ) prevents transportation of humidity to the magnetic means ( 24 ), a further advantageous embodiment of the invention has been obtained. 
         [0040]    According to the invention, degradation of especially the permanent magnetic material may be avoided simply by avoiding transportation of humidity into the magnet material. 
         [0041]    When said barrier layer comprises rubber, a further advantageous embodiment of the invention has been obtained. 
         [0042]    When said unit comprises means for receiving mechanical attraction forces in at least one first direction and means for establishment of magnetic attraction forces in at least one second direction, said at least one other direction being substantially transverse in relation to said at least one first direction, a further advantageous embodiment of the invention has been obtained. 
         [0043]    When said unit comprises ladder fixating means, a further advantageous embodiment of the invention has been obtained. 
         [0044]    When said unit comprises platform fixating means, a further advantageous embodiment of the invention has been obtained. 
         [0045]    When said unit comprises cable fixating means, a further advantageous embodiment of the invention has been obtained. 
         [0046]    When said unit comprises illumination means, a further advantageous embodiment of the invention has been obtained. 
         [0047]    When said unit comprises mechanical fixtures for receipt of standard mechanical fastening means, a further advantageous embodiment of the invention has been obtained. 
         [0048]    Moreover, the invention relates to a method of mounting suspension units whereby said units are attached to at least one wind turbine tower portion by means of computer-controlled positioning systems. 
         [0049]    Moreover, the invention relates to a method of mounting suspension units whereby an electromagnetically generated magnetic field is applied for complete or partly temporary demagnetization of the suspension when the suspension unit is positioned at the attachment location. 
         [0050]    Moreover, the invention relates to a method of de-attaching suspension units whereby an electromagnetically generated magnetic field is applied for complete or partly temporary demagnetization of the suspension unit, thereby facilitating de-attachment of the suspension unit by means of reduced de-attachment forces. 
         [0051]    Moreover, the invention relates to a method of re-arranging suspension units whereby an electromagnetically generated magnetic field is applied for complete or partly temporary demagnetization of the suspension unit, thereby facilitating rearrangement of the suspension unit. 
         [0052]    Moreover, the invention relates to a system of mutually attachable members, said system comprising at least one magnetic wind turbine tower portion, said system comprising at least one suspension unit ( 20 ; 60 ), said at least one suspension unit ( 20 ; 60 ) comprising at least one permanent magnetic portion ( 24 ), said at least one permanent magnetic portion ( 24 ) facilitating magnetic attachment of at least one suspension unit ( 20 ;  60 ) to said at least one magnetic wind turbine tower portion. 
         [0053]    When said at least one magnetic wind turbine tower portion comprises a wind turbine tower ( 11 ,  12 ,  13 ,  14 ;  10 ), a further advantageous embodiment of the invention has been obtained. 
         [0054]    When said wind turbine tower portion comprises a magnetic material of magnetic steel, a further advantageous embodiment of the invention has been obtained. 
         [0055]    When said magnetic steel comprises steel S235 JR, JO, J2G3 or S 355 JO or J2G3, a further advantageous embodiment of the invention has been obtained. 
         [0056]    Suspension unit comprising permanent magnet(s) according to the invention is suitable for fastening to the above-mentioned materials. 
         [0057]    When said at least one permanent magnetic portion ( 24 ) comprises a Neodym magnetic material or Samarium-Cobalt magnetic material, a further advantageous embodiment of the invention has been obtained. 
         [0058]    When said system comprises at least one suspension unit ( 20 ; 60 ), a further advantageous embodiment of the invention has been obtained. 
         [0059]    When said tower comprises at least two tower sections ( 11 ,  12 ,  13 ,  14 ), a further advantageous embodiment of the invention has been obtained. 
         [0060]    When at least one of said tower sections is conical, a further advantageous embodiment of the invention has been obtained. 
         [0061]    When at least one of said tower sections comprises mechanical attachment areas ( 131 ,  142 ) arranged in the upper and lower parts of said at least one tower section, a further advantageous embodiment of the invention has been obtained. 
         [0062]    When at least one of said mechanical attachment areas ( 131 ,  142 ) facilitates mechanical attachment of at least one suspension unit ( 20 ;  60 ) and suspension to said at least one of said mechanical attachment areas ( 131 ,  142 ), a further advantageous embodiment of the invention has been obtained. 
         [0063]    When at least two of said mechanical attachment areas ( 131 ,  142 ) facilitate mechanical attachment of at least one suspension unit ( 20 ;  60 ) and suspension between said at least two of said mechanical attachment areas ( 131 ,  142 ), a further advantageous embodiment of the invention has been obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0064]    The invention will now be described in detail with reference to the drawings, in which 
           [0065]      FIG. 1   a  illustrates a wind turbine, 
           [0066]      FIG. 1   b  illustrates a wind turbine tower according to an embodiment of the invention, 
           [0067]      FIG. 1   c  illustrates a joining between two tower sections. 
           [0068]      FIGS. 2   a  and  2   b  illustrate a suspension unit according to a preferred embodiment of the invention, 
           [0069]      FIG. 3  illustrates fixation of a ladder within a wind turbine tower according to an embodiment of the invention. 
           [0070]      FIG. 4  illustrates assembly of a suspension unit according to one embodiment of the invention and a fitting, 
           [0071]      FIGS. 5   a  and  5   b  illustrate a further suspension unit according to the invention, 
           [0072]      FIG. 6  illustrates the main force directions of an applicable attachment according to one embodiment of the invention, and 
           [0073]      FIG. 7  illustrates the properties of permanent magnetic materials. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0074]    Most large wind turbines are delivered with tubular steel towers manufactured in sections of 20-30 meters with flanges at both ends, and bolted together on the site. The towers are conical (i.e. the diameter increases towards the base) in order to increase strength and save materials at the same time. 
         [0075]    The production costs of the wind turbine towers are relatively high compared with the manufacturing costs of the complete wind turbine construction. Moreover, these costs increase proportionately with the height and especially if it exceeds around fifty meters. 
         [0076]    It is therefore quite important with respect to the final cost of energy to build towers as optimally as possible. 
         [0077]      FIG. 1   a  illustrates a typical wind turbine. The wind turbine comprises a tower  10  founded to the ground by means of a foundation. The tower  10  carries a nacelle  1  and a rotor  2 . 
         [0078]    Today, most modern wind turbine towers are conical tubular steel towers. 
         [0079]      FIG. 1   b  shows the cross section of such an exemplary tubular steel tower. 
         [0080]    The illustrated wind turbine tower  10  comprises a number of conical tower sections  11 ,  12 ,  13  and  14 . The length of the sections  11 ,  12 ,  13  and  14  are h 1 , h 2 , h 3 , h 4 , respectively. 
         [0081]    The complete tower is conical and each of the sections  11 ,  12 ,  13  and  14  is conical and the upper diameter is d 1  and the lower diameter is d 4 . 
         [0082]    Hence, the towers may be assembled from these smaller, conical subsections  11 ,  12 ,  13  and  14  cut and molded into the right shape, and subsequently welded together. 
         [0083]    The towers sections  11 ,  12 ,  13 ,  14  are usually manufactured in h 1 , h 2 , h 3 , h 4 =20 to 30 meter sections, the limiting factor being transportation on e.g. roads or by rail. 
         [0084]    The illustrated tower sections  11 ,  12 ,  13 ,  14  have flanges  131 ,  142  at both ends, and may be bolted together on the site. The towers are conical (i.e. the diameter increases towards the base) in order to increase strength and save materials at the same time. 
         [0085]    Typically, modern tower weights are 40 metric tonnes for a 50 m tower for a turbine with a 44 m rotor diameter (e.g. a 600 kW wind turbine), and 80 metric tonnes for a 60 metres tower for a 72 m rotor diameter (e.g. a 2000 kW wind turbine). 
         [0086]    The illustrated tower may e.g. be an 80 (eighty) metric tonnes tower. The lower and upper diameters d 4  and d 1  may e.g. be approximately 4.2 meters and 2.3 meters, respectively. 
         [0087]    It should be emphasized that the illustrated wind turbine tower and the illustrated choice of dimensions should in no way be regarded as restriction of the invention to the illustrated tower. 
         [0088]      FIG. 1   c  illustrates a detailed view of a joining of two sections  13 ,  14 . 
         [0089]    A lower flange  131  of the tower section  13  is bolted together with an upper flange  142  of the tower section  14 . Evidently, the other sections may be joined in the same way. 
         [0090]    The interior components of the above-described tower of  FIG. 1   b  may typically be comprised by platforms  1110 ,  1120 ,  1130 ,  1140 , ladders  111 ,  112 ,  113  and  114  illumination means  121 ,  122 ,  123 , power cables (not shown), lifts (not shown), etc (not shown). 
         [0091]    The interior components are attached to the tower e.g. by means of suitable fittings attached to the inner surface of the tower. 
         [0092]    The platforms  1110 ,  1120 ,  1130 ,  1140  may e.g. be suspended to attachment areas located near the assembly joints of the tower fittings, e.g. the upper and lower flanges of the tower sections  11 ,  12 ,  13 ,  14 . 
         [0093]    The platform may simply be attached to these joinings, e.g. flanges, by means of wires fixed to the platform at the lower end and to the flanges at the upper end. Thus, vertically, the platform is mechanically attached to the flange and hanging in the wires, whereas the platform comprises permanent magnetic suspension units providing magnetic fixation of the platform with respect to non-vertical excitation forces of the platform. 
         [0094]      FIG. 2   a  illustrates a wind turbine tower suspension unit  20  comprising a main body  21  and a mechanically connectable member or section  22  to which mechanical structures may be fixated by suitable fixation means. 
         [0095]    The dimensions of the main body  21  of the illustrated unit may e.g. be approximately 200 mm×50 mm×25 mm and the weight may be approximately ½ kilogram. 
         [0096]      FIG. 2   b  shows a cross-section of the above-described wind turbine tower suspension unit  20 . 
         [0097]    The unit comprises a magnet  24  comprising one or a group of magnets. The magnet is a permanent magnet. 
         [0098]    The characteristics of the magnet are described in detail below. 
         [0099]    The magnet  24  is encapsulated by a barrier material  23  of e.g. rubber. 
         [0100]    Advantageously, the barrier material should feature a relatively high friction coefficient, thereby maximizing the transversal conservative forces between the unit and the tower inner surface when the unit is attached to an inner surface by magnetic force in the usual direction. 
         [0101]    The barrier material protects the magnet from humidity and dust, thereby preventing corrosion or other physical degradation of the magnet. 
         [0102]    Moreover, a mechanically connectable member or section  22  is fastened to the main body of the unit. In this case, the magnet and the member or section  22  extend to the exterior of the unit and form a bolt, e.g. M10×30, at the end. 
         [0103]    The bolt  22  may be fitted to a suitable nut, thereby providing fixation of a suitable fitting to the unit. 
         [0104]    Please note that the lower part of the rubber sealing is quite thin in order to obtain the strongest possible attraction forces. 
         [0105]      FIG. 2   c  shows a further embodiment of the invention. 
         [0106]    The illustrated suspension unit  200  comprises a U-shaped steel cover  250  arranged to be in galvanic contact with a permanent magnet  24 , such as Neodym. The assembly has been formed with an upper sealing  230 . The sealing is provided with a lip  231  adapted to engage with an underlying surface, thereby providing air-tight and water-proof enclosure around the magnet  24  once it is mounted. 
         [0107]    The U-shape of the magnet provides an improved magnetic coupling to the underlying surface. 
         [0108]    Evidently, other standard types of magnet configurations may be applied within the scope of the invention. 
         [0109]      FIG. 2   d  illustrates a further embodiment of the invention in which a suspension unit  200  comprises a magnet  24  arranged with a U-shaped magnetic conducting cover  250 . 
         [0110]    When the unit  200  has been magnetically attached to a surface  232 , the entire assembly is subsequently covered by a suitable sealing means  231 , e.g. tectyle, etc. 
         [0111]      FIG. 3  illustrates an advantageous embodiment of the invention. 
         [0112]    The figure illustrates a cross-section of the tower section  13  illustrated in  FIG. 1   b . According to the illustrated embodiment, a ladder  113  is fixated to the inner surface  30  of the wind turbine tower section  13 . 
         [0113]    The ladder is suspended mechanically in two suspensions to the tower section, an upper mechanical suspension  31  and a lower mechanical suspension  32 . 
         [0114]    Bolting, welding, etc may e.g. obtain the suspension of the ladder to the tower. 
         [0115]    Moreover the ladder is suspended in the tower by means of a number of wind turbine suspension units  20 , e.g. illustrated in  FIGS. 2   a  and  2   b , or derivatives thereof. 
         [0116]    The units  20  are attached to the inner surface by means of magnetic forces established by the magnets of the units  20 , and the ladder may be attached to the units  20  e.g. by means of the fittings  22  of the units  20 . 
         [0117]    Basically, it may be appreciated that the main fixation in the vertical direction is obtained by means of mechanical end-point fixation to the tower, the upper flange  132  and lower flange  131  of the tower section  31 , while the transverse fixation (i.e. the non-vertical) is obtained magnetically by means of the units  20 . 
         [0118]    According to the illustrated embodiment the vertical mechanical stress is primarily created by traditional mechanical attachment to the tower, e.g. by welding and/or bolting, while fixation of the ladder  31  is primarily obtained by magnetic fixation to the inner surface  30  of the tower section  13 . 
         [0119]    The distance between the suspension unit  20  in the vertical direction may e.g. be between 0.5 meter and 4 meters depending on the requirements to the fixation. 
         [0120]    A preferred vertical distance is between 1.5 to 2.3. metres. 
         [0121]    Evidently, a suspension unit according to the invention may be applied for fixation of several other interior components than the illustrated ladder. 
         [0122]    According to a further embodiment of the invention, a ladder construction may e.g. be fixated to the tower by the above-described provisions and the further interior components such as lamps, power cables, etc. may then be fixated to the ladder construction itself by traditional attachment mechanics, welding, bolting etc. 
         [0123]      FIG. 4  illustrates a further embodiment of the invention in which a tube  41  (e.g. of a ladder, a platform fitting, etc.) comprises a fitting  42  which is attached to a suspension unit  20  by means of a nut (not shown) fitted to the bolt  22  of the suspension unit. 
         [0124]    The unit  20  may be magnetically attracted to an underlying surface, e.g. the above-described inner surface of a steel tower section wall, thereby establishing attachment by normal forces between the underlying surface and the tube  41 . 
         [0125]    Moreover, it should be noted that the fitting facilitates a degree of tolerance in the direction of the illustrated arrow between the fitting  42  and the bolt  22  of the suspension unit  20 . 
         [0126]    The freedom of tolerance provides a certain degree of tolerance when attaching the suspension units  20  to a tower in the manner suggested by the invention. 
         [0127]      FIG. 5   a  illustrates a further embodiment of the invention. 
         [0128]    The illustrated suspension unit  60  comprises a main body  61  and two bolts  61 . 
         [0129]    Basically, the illustrated suspension unit  60  is constructed according to the principles outlined in the single-bolt version of the unit in  FIG. 2   b.    
         [0130]      FIG. 5   b  illustrates the fitting of e.g. a steel bar  65  to the suspension unit  60 . 
         [0131]    The steel bar  65  is mechanically fixated to the suspension unit  60  by means of a suitable fitting  66  by means of nuts (not shown). 
         [0132]    It should be noted that the illustrated assembly features a certain degree of tolerance in both the “vertical” and “horizontal” directions as illustrated by the arrows. 
         [0133]      FIGS. 6   a  and  6   b  illustrate a method of mounting a suspension unit  60  according to the invention to a magnetic surface (not shown), e.g. a steel wind turbine tower. 
         [0134]    Initially, in  FIG. 6   a  a suspension unit is arranged on e.g. a steel surface. A nylon sheet  65  is located between the underlying surface and the suspension unit  60 . Due to the thickness of the sheet and the relatively low friction, the suspension unit  60  and the sheet  65  may be displaced relatively easily to the exact desired position. 
         [0135]    In  FIG. 6   b , the desired position has been reached, and the sheet  65  is removed, whereby stronger attachment forces have been obtained. 
         [0136]      FIGS. 7   a  and  7   b  illustrate the main force directions F V  and F N  acting on a suspendible element  90  in an attachment according to one embodiment of the invention. 
         [0137]    Vertical mechanical fixation ensures that the interior structure to be suspended is fixed in relation to the tower, at least with respect to gravity. These forces are primarily dealt with as vertical forces F V . Basically, the vertical forces should primarily be handled by traditional mechanical attachment so to speak. 
         [0138]    Moreover, the magnetic suspension provides transverse fixation of the suspended element established by means of normal force F N  acting in the illustrated transverse direction. This normal force F N  may be exploited further, if friction members are applied between the suspension unit and the corresponding magnetic tower portion. Hence, according to well-known relationships between normal force F N  and the frictions, transverse force is established to counteract e.g. rotational movements (tangential movement tm) of e.g. a platform or a ladder suspension. 
         [0139]      FIG. 8  illustrates some of the properties of different permanent magnetic materials. 
         [0140]    The illustrated diagram illustrates the behavior of six groups of magnetic materials, G 1  to G 6  at a temperature of approximately 20 degrees Celsius. 
         [0141]    The x-axis represents HcJ (Coercivity [kA/m] and the y-axis represents maximum energy product |BH| max [kJ/m 3 ] 
         [0142]    |BH| max  represents the energy potential of the material, whereas HcJ represents the material&#39;s ability to resist de-magnetizing fields. 
         [0143]    Group G 1  comprises so-called Neodym magnets, NdFeB. The Neodym magnets are some of the most powerful permanent magnets on the market. 
         [0144]    Groups G 2  and G 3  comprise Sm x Co y  magnets characterized by a high degree of stability with respect to variation in temperature. Group G 2  refers to Sm 2 Co 17  and Group G 3  refers to SmCo 5 . 
         [0145]    Group G 4  refers to a further group of so-called low H c Sm x Co y  magnets, Sm 2 Co 17 . 
         [0146]    Group G 5  comprises a group of plastic-bounded Neodym magnet material NdFeB. 
         [0147]    Finally, group G 6  comprises more traditional magnet materials such as Alnico and FeSr/FeBa materials. 
         [0148]    Different magnetic materials may be applied in a suspension unit according to the invention. 
         [0149]    According to a preferred embodiment of the invention, the applied permanent magnetic materials are primarily selected from materials featuring a high energy potential |BH| max . Hence, according to a preferred embodiment of the invention, the materials are chosen from group G 1 , i.e. the Neodym-magnets. 
         [0150]    Evidently, if for instance temperature variations become critical, Samarium-Cobalt magnets may be preferred. 
         [0151]    According to the invention, suspension units may e.g. be combined by units having magnets from different groups, thereby creating a degree of redundancy.