Abstract:
The invention discloses an electromagnetic friction clutch for driving a fan impeller in the engine compartment of a vehicle. The clutch includes a disk rotor consisting of a plane flat part, a further clutch element which is supported so that it is freely rotatable and which can be driven by the rotor through controllable friction contact, and elements for transmitting a rotational movement according to a principle of an eddy current drive with an eddy current ring fitted to the outer edge area of the disk rotor of the clutch.

Description:
This application claims the benefit under 35 USC §119(a)-(d) of German Application No. 10 2009 033 179.4 filed Jul. 13, 2009, the entirety of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to an electromagnetic friction clutch. 
     BACKGROUND OF THE INVENTION 
     Electromagnetic friction clutches of many different types are known, particularly in the field of vehicle construction. They are often used to control the distribution of a proportion of the mechanical power output of a main drive unit to one or more auxiliary units. Some designs are also known which combine an electromagnetic friction clutch with an eddy current drive. This makes it possible to avoid shutting down an auxiliary unit or to transmit a lower rotational speed than that of the drive for energy-saving operation. In order to support the transmission of rotational movements according to an additional operating principle, further components must be accommodated and catered for in the design of an electromagnetic friction clutch. At the same time further simplification and reduction in the size of known designs face the problem that the heat generated by eddy currents in the clutch has to be dissipated to the surroundings. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a comparatively simple and compact clutch. In particular, the friction clutch according to the invention is intended to allow cost-effective manufacture for applications in vehicle construction, for example, as a fan clutch, even in small quantities. 
     The invention proceeds from an electromagnetic friction clutch, which comprises a disk rotor consisting of a plane flat part having two planar main sides, a further clutch element which is supported so that it is freely rotatable and which can be driven by the rotor through controllable friction contact, and elements for transmitting a rotational movement according to the principle of an eddy current drive. The essence of the invention resides in the fact that an eddy current element is attached to an outer edge area of the disk rotor. 
     The clutch according to the invention may be embodied in such a way that the armature disk is rotationally fixed to the freely rotatable clutch element, for example, and can be mechanically engaged by electrical current flow in the electromagnetic coil. An embodiment of the invention in which the armature disk is rotationally fixed to the disk rotor, for example, is equally possible, the disengaged state being brought about by an energizing of the magnetic coil. The disk rotor can be manufactured as a circular disk, for example, blanked from sheet steel or produced in plane disk form through a laser cutting process. Apart from the eddy current element fitted thereto, the disk rotor consists exclusively of the disk-shaped, plane flat part. The disk rotor is preferably composed of soft-magnetic material. To enhance the force of attraction to an armature disk, a plurality of annular segments can be magnetically divided from one another on the disk rotor concentrically with the rotor axis of rotation. 
     According to the invention an area, on which an eddy current element, particularly one composed of a highly electrically conductive metal, is fitted to the disk rotor, is provided at the outer edge of the disk rotor. A suitable fixing of the eddy current element to the disk rotor can advantageously be selected according to the material combination of the disk rotor and eddy current element. For example, the two components can be non-positively connected together, for example by bolting or riveting on, or by adhesive bonding, or they can be connected together through positive interlock, for example, by casting on or extrusion. 
     The eddy current element may be of any design dimensions extending radially from the edge area of the disk rotor. In this way the eddy current element can advantageously be adapted to a rotational speed that is to be transmitted. In order to dissipate the heat generated by eddy currents to the surroundings, the eddy current element may project beyond the annular zone in which eddy currents are generated. In an axial direction the eddy current element may extend into the area of a housing of an electromagnet. The eddy current element may substantially overlap the housing. 
     In a preferred embodiment of the invention, the eddy current element is fitted to an outer edge area of a planar main side of the disk rotor. It is especially preferable to fit the eddy current element to that main side of the disk rotor which is situated further away from the electromagnet, in order to avoid a detrimental conduction of the magnetic flux. Connecting the components via sides, the surfaces of which are perpendicular to the clutch axis of rotation, affords the advantage that the size of the connecting surface can be adapted relatively easily to the forces acting on the connection. A further advantage is that the outer circumferential surface of the disk rotor remains, at least in part, fully accessible, so that this external face can be used to conduct the magnetic flux. 
     In a further preferred embodiment of the invention, magnets are fitted to the freely rotatable clutch element, in such a way that under a rotational movement relative to the eddy current element the magnets induce eddy currents therein. This advantageously avoids either having to fit the magnets to a separate component or increasing the inertial forces under axial movement, if the magnets are fitted to the armature disk. 
     The eddy current element preferably comprises cooling elements for the dissipation of heat. It is particularly advantageous if cooling elements are integrally formed on the eddy current element, for example. The cooling elements advantageously increase the area for the dissipation of heat, which is generated, in particular for example, by the eddy currents. 
     In a further preferred embodiment of the invention, an electromagnetic coil is at least partially enclosed by a housing of magnetically conductive material, in such a way that one face of the housing at least partially encloses an outer circumferential surface of the disk rotor without touching it. A passage for the magnetic lines of force can thereby be created, so that a considerable proportion of the magnetic flux passes through the circumferential surface. This affords the advantage that the magnetic flux is conducted mainly radially, and axial forces together with stresses thereby generated can be avoided. 
     An electromagnetic coil is preferably enclosed at least partially by magnetically conductive material, the housing being coordinated with the disk rotor and an element between the housing and the clutch axis of rotation that bears flatly against the disk rotor such that the magnetic flux is for the most part conducted via the element between the housing and clutch axis of rotation. The element may be a soft-magnetic annular disk, for example, which close to the clutch axis of rotation bears against the disk rotor, so that the magnetic flux from the disk rotor can enter directly into the annular disk and by way of an outer circumferential surface of the annular disk, for example, and a minimal gap can be conducted further radially to the magnetically conductive housing of the clutch without any contact occurring. This serves to further reduce axial forces acting on the disk rotor. 
     In a further preferred embodiment of the invention, the eddy current element comprises areas of magnetically conductive material. In particular, the magnetically conductive material is preferably fitted to the eddy current element in such a way that areas of the eddy current element composed of electrically highly conductive material are situated axially between the magnetically conductive areas and the magnets capable of exciting eddy currents. In this way the eddy current element affords a magnetic yoke for the magnetic flux produced by the eddy current-generating magnets. 
     In an especially preferred embodiment of the invention, the further clutch element comprises a device, so that a cylindrical or truncated cone-shaped bore or recess of a machine part to be driven can be fitted to the clutch element, centered in relation to the clutch axis of rotation by a bore diameter which is smaller than a diameter on which the magnets of the eddy current drive are situated. The centering device may be embodied, for example, as an annular axial elevation or extension, for example on the freely rotatable clutch element. The centering device is preferably made to fit the recess in the machine part, for example a fan impeller hub, precisely. The socket radially inset relative to the eddy current drive allows the clutch to have an axially compact construction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Further features and advantages of the invention will be described with reference to an exemplary embodiment of the friction clutch according to the invention represented schematically in the drawing. 
         FIG. 1  shows a schematically represented section along the axis of rotation through a friction clutch according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  depicts an electromagnetic friction clutch  1 , which is designed to drive a fan impeller (not shown). The clutch  1  comprises an annular housing  2 . Walls  2   a ,  2   b ,  2   c  enclose an annular chamber  2   d  in the housing  2 , in which an electromagnetic coil  3  is accommodated. The electromagnetic coil is connected to an electrical current supply via a lead-through bushing  15 . On a fourth side, the face of which is perpendicular to a clutch axis of rotation  17 , the annular chamber  2   d  is covered by a disk rotor  4 . 
     The disk rotor  4  is pushed onto a rotary shaft  10 . So that it is freely rotatable, the rotary shaft  10  is fitted to the housing  2  by means of the ball bearing  13   a , which is seated on the inside diameter of the annular housing  2 . The rotary shaft  10  in turn carries a clutch element  6 , which by means of a ball bearing  13   b  is supported so that it is freely rotatable in the bearing sleeve  6   d  concentrically both with the housing  2  and the disk rotor  4 . The clutch element  6  is intended as driven flange for the fitting of a fan impeller. 
     At the side of the housing wall  2   c  a journal  10   a  of the rotary shaft  10  protrudes from the clutch housing  2 , to which a drive element (not shown) such as a belt drive wheel, for example, can be fitted. The rotary shaft  10  is embodied as a hollow shaft and comprises a thread  10   b , into which a clamping bolt  11  is screwed. By means of a locking ring  12   a  and the spacer rings  12   b  and  12   c , the disk rotor  4  and the ball bearings  13   a  and  13   b  are arranged axially in relation to one another so that the clutch housing  2 , the disk rotor  4  and the clutch element  6  are supported in such a way that without touching they are freely rotatable relative to one another. 
     By means of springs  9  allowing it to move axially, an armature disk  8  is fitted to the clutch element  6  on the side facing the disk rotor  4 . With a current flowing through the coil  3 , the armature disk  8  is therefore attracted axially towards the disk rotor  4  against the force of the springs  9 . Both the rotor  4  and the armature disk  8  comprise a plurality of annular segments  4 ,  4   a ,  4   b ,  8 ,  8   a , which are arranged concentrically with the clutch axis of rotation  17  and which are radially offset in relation to one another so that the magnetic flux generated by the electromagnets  3  flows alternately through the armature disk  8  and the disk rotor  4 . 
     In an axial direction the outer annular wall  2   b  of the clutch housing  2  is drawn higher than the inner annular wall  2   a , so that the annular wall  2   b  encloses the circumferential surface of the rotor  4  without touching it. This advantageously ensures that on actuation of the clutch the magnetic flux is conducted substantially in a radial direction and only slight forces occur in an axial direction. On actuation of the clutch the outermost edge of the rotor  4  is therefore scarcely moved in an axial direction and in the engaged state mechanical stresses in the disk rotor  4  are largely avoided. 
     An annular eddy current element  5  is fitted to the side of the rotor  4  facing the armature disk  8 , on the outermost segment  4   a , concentrically with the clutch axis of rotation. The eddy current ring  5  may be adhesively bonded or bolted to the rotor segment  4   a , for example. 
     Because the eddy current ring  5  protrudes radially beyond the outside diameter of the clutch housing  2  to a considerable extent, cooling elements  5   a  are formed on the eddy current ring  5  in such a way that they extend from the eddy current ring  5  in an axial direction along the clutch housing  2  to the outside thereof, without touching it. Due to the comparatively simple geometry of the eddy current ring  5  and the cooling elements  5   a , these can advantageously be manufactured together, for example by inexpensive molding. Said example of a manufacturing method moreover affords the possibility of additionally molding a plane steel ring (not shown) into the eddy current ring  5 , in order to boost the excitation of eddy currents through an improved magnetic yoke. 
     At an axial interval from the eddy current ring  5 , magnets  7  are fitted to the clutch element  6  on an outer edge area  6   a , which is situated inside a radial area, defined by the eddy current ring  5 , to the clutch axis of rotation  17 . As soon as the disk rotor  4  and the clutch element  6  rotate relative to one another, for example in the mechanically disengaged state, the magnets  7  induce eddy currents on the eddy current ring  5 , through the magnetic reaction to which a torque is transmitted from the eddy current ring  5  to the clutch element  6 , for example. 
     At its side facing axially outwards from the clutch, the clutch element  6  is configured as a driven flange. On an annular area  6   b  a fan impeller, for example, may be fixed to the clutch element  6  by means of the bolt  16 . A centering ring  6   c  assists the attachment of the hub of a fan impeller coaxially with the clutch axis of rotation, in order to avoid any imbalance and the associated oscillations. Since only minor heat dissipation requirements, if any, apply to the clutch element  6 , the centering ring  6   c  can run relatively close around the bearing sleeve  6   d , so that fan impellers of correspondingly small centering diameter can be fitted. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  friction clutch 
           2  housing 
           2   a  inner annular wall 
           2   b  outer annular wall 
           2   c  wall 
           2   d  chamber 
           3  electromagnetic coil 
           4  disk rotor 
           4   a  outer annular segment 
           4   b  middle annular segment 
           4   c  inner annular segment 
           5  eddy current ring 
           5   a  cooling element 
           6  driven element 
           6   a  outer annular segment 
           6   b  annular segment for fixing 
           6   c  centering ring 
           6   d  bearing sleeve 
           7  magnet 
           8  armature disk 
           8   a  outer annular segment of the armature disk 
           9  spring 
           10  shaft 
           10   a  journal for belt wheel 
           10   b  thread 
           11  clamping bolt 
           12   a  lock washer 
           12   b  spacer washer 
           12   c  spacer washer 
           13   a  ball bearing 
           13   b  double-row ball bearing 
           14   a  gap 
           14   b  gap 
           15  lead 
           16  bolt 
           17  clutch axis of rotation