Patent Publication Number: US-2017369012-A1

Title: Grounding of a shaft

Description:
This application claims priority from German patent application serial no. 10 2016 211 616.9 filed Jun. 28, 2016. 
     FIELD OF THE INVENTION 
     The invention concerns a transmission. 
     BACKGROUND OF THE INVENTION 
     Electric currents can give rise to decarburiza ion and sparking in bearings. The result is damage to the bearing, which can ultimately lead to failure. 
     To avoid the flow of current in a bearing, various solutions are known from the prior art which are designed to achieve potential equalization. These include carbon brushes, shaft grounding rings and telescopic brushes. Such means are suitable for avoiding currents in the range of a few milliamperes to several amperes. 
     However, more recent research has yielded the knowledge that even very small currents, i.e. currents in the microampere range, can cause damage to bearings. Such currents seem to be responsible for the production of so-termed White Etching Cracks (WEC). 
     The means described earlier are not suitable for avoiding currents in the microampere range when used in an oily environment. The oil produces an insulating film which prevents complete potential equalization. This problem becomes more acute, the faster the bearing to be grounded is rotating. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to overcome the inadequacies and disadvantages inherent in the solutions known from the prior art. In particular, damage to bearings due to White Etching Cracks should be avoided. 
     This objective is achieved by a transmission according the claims. 
     The transmission comprises a first component mounted so that it can rotate, a rotationally fixed second component and a third component. In particular, the first component can rotate relative to the second component. Preferably, the first component is also mounted to rotate in the second component. Thus, for example, the second component is a housing of the transmission. 
     The first component has a face that extends radially. This is a surface directed perpendicularly to a rotational axis of the first component. Preferably, the radially extending face is flat. The face is part of the surface of the first component. In particular, it can be a partial surface of a larger area. 
     The third component is fixed in the second component. Preferably, this is done by means of a solid connection which does not allow any translational or rotational degrees of freedom. In the case of a solid connection, at least part of the third component is attached in a fixed position relative to the second component. 
     According to the invention, the third component is braced against the face in such manner that by virtue of the third component an electrically conducting connection is formed between the face and the second component. Since the face is a face of the first component, there is therefore also an electrically conducting connection between the first component and the second component. 
     By virtue of the electrically conducting connection between the first component and the second component, potential equalization takes place. Since the face against which the third component is braced extends radially, the third component can be arranged close to the rotational axis of the first component. This reduces the relative speed with which the face slides on the third component. In turn, this prevents the formation of an insulating oil film between the face and the first component, which would oppose potential equalization due to increased Ohmic resistance. 
     In a preferred further development, the first component is electrically connected to at least part of a bearing, such as a roller bearing. In particular, by virtue of the bearing the first component can rotate—for example in the second component. This entails fixing the first component in an inner ring and an outer ring of the bearing. That fixing is electrically conducting. As a result of the potential equalization between the first component and the second component, there is also potential equalization relative to the bearing. 
     In an also preferred further development, the face is rotationally symmetrical relative to the rotational axis of the first component. Because of this, during rotation of the first component, the third component slides over the face. 
     The first component is preferably developed further as a shaft. Alternatively, the first component is an axle. According to this further development the face is an end-face of the shaft. 
     The end-face of a shaft is defined as a face that delimits the shaft in the axial direction. The shaft has two end-faces. An end-face of a shaft can be defined as the locus of all points on the surface of the shaft which can be projected by axial projection, i.e. parallel projection in the axial direction, onto a flat surface that extends radially, i.e. perpendicularly to the rotational axis of the shaft. 
     The third component is preferably developed further with an electric contact. That contact is braced against the radially extending face of the first component. 
     Preferably, the third component is also further developed by incorporating a spring. The spring is stressed against the contact so that a spring force acts on the contact. This results in the above-mentioned bracing of the contact against the face. 
     The contact can perhaps be in the form of a ball. In particular, as the third component a commercially available ball plunger can be used. 
     In an also preferred further development, the contact consists at least in part of hardened steel. Hardened steel can be produced by transformation hardening, precipitation hardening or cold-work hardening. The use of hardened steel is first enabled by the bracing of the contact against a radially extending surface. Compared with the solutions known from the prior art, this makes for improved wear behavior. 
     Relative to the first component, the third component can be positioned concentrically or eccentrically. In a concentric arrangement a central axis of the third component, preferably an axis of symmetry relative to which the third component is rotationally symmetrical, coincides with the rotational axis of the first component. In this case the face of the first component against which the third component is braced is preferably of circular shape. In particular, the surface can be punctiform. A concentric arrangement of the third component has the advantage that the wear that occurs due to friction between the third component and the surface is minimized. 
     In an eccentric arrangement of the third component, a central axis of the third component, preferably the symmetry axis relative to which the third component is rotationally symmetrical, extends a distance away from the rotational axis of the first component, the distance being different from zero. In this case the face of the first component against which the third component is braced is preferably shaped as a circular ring. The eccentric arrangement of the third component avoids the possibility that as a result of corrosion, the transfer resistance between the first and third components will increase. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred example embodiment of the invention is shown in the sole FIGURE, in which matching indexes denote similar or functionally equivalent features. In detail, the sole FIGURE shows an earthed shaft. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The shaft  101  of a transmission  115  has two end-faces with only one end-face  123  being shown in the FIGURE. The shaft  101  is mounted so that it can rotate about a rotational axis  103 . The shaft  101  shown in the FIGURE is electrically connected to at least part of a bearing  117 , such as a roller bearing. In particular, by virtue of the bearing  117 , the shaft  101  can rotate—for example in the housing  105 . This entails fixing the shaft  101  in an inner ring  119  and an outer ring  121  of the bearing  117 . Also shown in the FIGURE is a transmission housing  105 . To the transmission housing  105  are attached means  107  for grounding the shaft  101 . The means  107  are electrically connected to the transmission housing  105 . Furthermore, the means  107  comprise an electrically conductive pin  109 . This is loaded by a spring  111  and pressed against the shaft  101 , so that an electrically conductive connection is formed between the shaft  101  and the transmission housing  105 . 
     The means  107  with the pin  109  and the spring  111  are arranged offset relative to the rotational axis  103  by a radial distance  113 . This results in a self-cleaning effect of the electrically conductive connection between the pin  109  and the shaft  101 . 
     Compared with the radius of the shaft  101 , the distance  113  is small. For example, the distance  113  can be less than 20%, less than 15%, less than 10% or even less than 5% of the radius of the shaft  101 . 
     INDEXES 
     
         
         
           
               101  Shaft 
               103  Rotational axis 
               105  Transmission housing 
               107  Means for grounding the shaft 
               109  Pin 
               111  Spring 
               113  Radial distance