Abstract:
A planetary gear including a stationary ring gear, a rotatable planet carrier having an even number of at least four mirror-symmetrically arranged planet wheels, and a sun gear and comprising two bearings disposed opposite of the sun gear and arranged between the planet wheels is provided. The bearings connect the input or output element of the planetary gear to the planet carrier in the center plane of the planet carrier, with the bearings forming an imaginary axis of symmetry, and at least two planet wheels having a lesser distance to each other than one of the planet wheels does to a further planet wheel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application represents a National Stage application of PCT/DE2009/001566 entitled “Planetary Gear” filed Nov. 5, 2009, pending. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a planetary gear with a stationary ring gear, a rotatable planet carrier having at least two planet wheels, and a sun gear. 
     The invention relates in particular to a gear for a wind power plant. 
     2. Discussion of the Prior Art 
     The trend that continues for wind power plants to construct relatively compact and light units leads to components installed in the wind power plants being exposed to increasingly larger deformations even in load-transmitting parts. In particular under dynamic loads, larger deformations above all of bearings and gear parts lead to locally excessive tensions that are partly inadmissibly strong in places, that again result in a marked reduction in the life of the components. For bearings and gear gears, a toothing offset or a trajectory deviation of the rolling elements of as little as 10 to 25 μm as a result of tilting due to external forces and moments can lead to an excessive load of such a magnitude that the components fail even after a very short time and thus lead to considerable financial damage. 
     On top of this, a wind power plant has an increased noise level due to the non-uniform engagement of the gear wheels into each other that is undesirable in particular in populated areas. 
     In particular for wind power plants of the 2.5 to 6 MW class, rotor bearings and ring gears of planet wheel have an outside diameter of approximately 2.5 to 3.5 m and widths of approximately 350 to 500 mm, the large teeth width leading to problems in terms of a uniform engagement of the toothing across the entire width of the teeth, in particular when outside loads act on this component. It is therefore necessary for the planet wheels to be able to give way to the deformation that is forced on them, above all tilting about the axes at right angles to the axis of rotation of the planet wheel, so that the teeth can engage as best as possible, that is to say uniform, across the tooth width in all load situations. 
     Due to the one-sided torque transmission via the planet wheel, the planet carrier is twisted. This leads to the axes of rotation of the planet wheel being shifted and thus to the tooth-engaging load to be less uniform across the entire tooth width, so that the gears can no longer transmit the forces in an evenly distributed manner. This leads to local excess loads and wear of the toothing that can be partly countered in the known planet wheels by tooth corrections. 
     As a further remedy, a planetary gear is for example known from DE 103 18 945 B3 where the planet axes are firmly connected to the gear wall and the planet wheels are mounted such on the planet axes that they tumble. 
     To mount the planet wheels, also the elements known from US 2008/0153657 A1 as “Flexpins” can be used. 
     However, this design of a planetary gear is also accompanied by increased assembly efforts despite increased manufacturing tolerances because each planet wheel has to be set up precisely and checked in terms of its functioning, i.e. its uniform engagement with the ring gear and the sun gear. 
     SUMMARY OF THE INVENTION 
     The object of the invention is therefore to provide a planetary gear where the probability of a non-uniform engagement of the gears and the noise level occurring on strong loads is reduced in comparison to the planet wheels known from the prior art, the intention being for it to be easier to manufacture and to assemble the planetary gear. 
     The basic idea of the invention is to apply the torque of the input or output element centrally to the planet carrier, in the center between the supporting discs. If the force is transmitted centrally from the rotor into the gear between the supporting discs of the planet carrier, twisting of the planet carrier is prevented. As a result, the inventive design of the planetary gear leads to a uniform contact pattern of the planet wheels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in more detail using a particularly preferred exemplary embodiment in terms of wind power plants and referring to the appended drawings. In the drawings: 
         FIG. 1  shows a schematic side view of the nacelle, tower, and rotor of a wind power plant; 
         FIG. 2  shows a cut perspective view of the planetary gear according to the invention; 
         FIG. 3  shows an exploded view of the planetary gear of  FIG. 2 ; 
         FIG. 4  shows a top view of the inventive planetary gear according to a particularly preferred exemplary embodiment; 
         FIG. 5  shows a perspective view of the planet carrier; 
         FIG. 6  shows a section through the planetary gear of  FIG. 4  along the line E-E; and 
         FIG. 7  shows a section through the planetary gear of  FIG. 4  along the line F-F in a slight modification. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a perspective view of a particularly compact design of a wind power plant. The wind power plant  100  exhibits a tower  110 , a head support  120  arranged thereon and generator housing  130  connected thereto, gear housing  140  and a hub  150  attached thereto. The head support  120 , the generator housing  130  and the gear housing  140  are preferably designed as load-transmitting components—as is known from WO 2008/113318 A2. 
       FIG. 2  shows a planetary gear according to the invention, installed in the gear housing  140  of the wind power plant  100  and of preferable design, in a perspective view,  FIG. 3  showing the same exemplary embodiment in an exploded view. The planetary gear  10  has a rotor bearing  20  with an outside ring  20   a  and an inside ring  20   b . Here the hub  150  of the wind power plant is connected to the inside ring  20   b . The ring gear  30  of the planetary gear is firmly connected to the gear housing. Four planet wheels  50   a ,  50   b ,  50   c ,  50   d  are rotatably mounted on the planet wheel  40  by means of screws and engage towards the outside the stationary ring gear  30  and towards the inside the sun gear  60 . 
     To be seen is furthermore the bearing  80   a ,  80   b , that is in each case illustrated as a cross-section of  FIG. 4 , that is designed, in the example shown, both as an elastomeric bushing  80   b  having a rectangular cross-section and also as a spherical plain bearing  80   a , these two elements of the bearing  80   a ,  80   b  being arranged, lying on one axis, on the sides of the planet carrier  40  opposite the sun gear  60  between the planet wheels  50   a  and  50   d  or the planet wheels  50   b  and  50   c . Of course, the plain bearings  80   a  and elastomeric bushings  80   b  can also be used as an alternative to each other. 
     The spherical plain bearing  80   a  is characterized by a high radial stiffness due to the large sliding surface and by low restoring moments when tilting due to the unimpeded rotatability. Use of the elastomeric bushings  80   b  is characterized by a slight radial correctability. Both bearing elements  80   a ,  80   b  permit the entire plant carrier to be tilted transversely to their axis of rotation, as a result of which they adjust to deformations without any problems without the components being subjected to larger component loads or without uneven tooth contact of the tooth engagements taking place with the disadvantages that have been described. 
     What catches the eye in  FIG. 4  is that the planet wheels  50   a ,  50   b ,  50   c ,  50   d  are located on an imaginary circle, but are not distributed uniformly. It is rather to be seen that the distance between the planet wheels  50   a  and  50   b  or  50   c  and  50   d  is smaller than the distance between the planet wheels  50   a  and  50   d  or the planet wheels  50   b  and  50   c . However, the planet wheels  50   a ,  50   b  and  50   c ,  50   d  are arranged mirror-symmetrically relative to each other, the axis on which the elements of the bearing  80   a ,  80   b  are arranged being the imaginary axis of symmetry. 
     This design is particularly advantageous in case the planetary gear is used for the power train of a wind power plant with a double-bladed rotor. In this case the inside ring  20   b  is directly connected to the hub of the double-bladed rotor and the bearing arrangement  80   a ,  80   b  is oriented at right angles to the axis of the rotor blades with the result that the forces acting on the screws of the planet wheels  50   a ,  50   b ,  50   c ,  50   d  and that have been introduced centrally from the rotor have as little influence as possible on the toothing of the planets  50   a ,  50   b ,  50   c ,  50   d  with the ring gear  30  or the sun gear  60  and even tooth engagement being guaranteed. 
       FIG. 5  shows the planet carrier  40  of the inventive planetary gear  10 . The planet carrier exhibits a rear and a front supporting disc  42 ,  44  that are interconnected by means of connecting elements  46 ,  48 . It is now essential for the invention that in the exemplary embodiment that has been shown two connecting elements  48  are designed as receptacles for the screws  80  that connect the input or the output element of the gear to the planet carrier, the forces being introduced centrally into the receptacles  48  between the supporting discs  42 ,  44 . 
       FIG. 6  shows the constructive relationships that are essentially known of the inventive planetary gear  10  of  FIG. 4  along line E-E in cross-section. The rotor bearing  20  consisting of the outside ring  20   a  and inside ring  20   b  can be seen clearly, the hub  150  being firmly connected to the inside ring  20   b  of the rotor bearing  20 . Arranged next to it are the planet wheels  50  that are mounted on the planet screws, and the sun gear  60 . 
     The planetary gear  10  that is shown here is designed as a two-stage planetary gear. However, it is obvious for the person skilled in the art that according to the invention also single-stage or multi-stage planet wheel can be designed in addition to a two-stage planetary gear. 
     According to the invention it is in particular provided, as is shown in  FIG. 7  in cross-section, that the hub  150  is connected to the inside ring  20   b  and via the connecting element  70  to the planet carrier  40 . This takes place for example by means of the spherical plain bearing  80   a  shown in  FIG. 4 , or the elastomeric bushing  80   b  (deviating therefrom,  FIG. 2  shows two spherical plain bearings  80 ). 
     It can be seen clearly in this context that the hub  150  as input element of the generator  10  is connected via the connecting element  70  and the screws  80  in the center plane M of the planet carrier  40  thereto so that the torque can be introduced distributed as evenly as possible into the planet carrier  40 . It can be gathered in particular from  FIG. 7  that especially the pivot heads  90  of the screws  80  that connect the connecting element  70  directly to the planet carrier  40  and thus indirectly the hub  150  to the planet carrier  40 , are arranged in the center plane M, that is to say centrally between the supporting discs  42 ,  44  of the planet carrier  40 . 
     It is obvious that the planet carrier  40  can be connected to a input element, in the example shown the hub  150  of a wind power plant  100 . In other cases, the inventive planetary gear  10  can also have such a design and be tied in functionally so that an output element is connected to the planet carrier. 
     It is further obvious that the input or output element can be connected directly or indirectly by means of further structures, for example the connecting element  70 , to the planet carrier. It is here essential that the force transmission between input element and planet carrier or planet carrier and output element takes place centrally in the center plane of the planet carrier.