Patent Publication Number: US-2023148245-A1

Title: Method for producing a pole core, pole core, electromagnetic actuator and vibration damper

Description:
The present invention relates to a method for producing a pole core, an electromagnetic actuator with improved properties having the features of claim  1 , a pole core having the features of claim  10 , and an electromagnetic actuator having a pole core having the features of claim  12 , and a vibration damper having such an electromagnetic actuator with the features of claim  13 . 
     Pole cores are already known from the prior art in different configurations and are produced from a magnetic material, in particular ferromagnetic metal. Such pole cores are widely used in electromagnetic actuators for guiding a magnetic flux, which are used, for example, in damper valves for controlling the damper characteristics of a vibration damper. Between an inlet and an outlet, damper valves have an outlet opening that can be closed more or less. By opening or closing the outlet opening is throttled and thus the damping performance of a vibration damper is set. If the vehicle is to have a “hard” chassis, the throttle effect is increased or the outlet opening is closed more, while the outlet opening is opened further with a “soft” chassis. The damper valve is opened and closed by an electromagnetic actuator system, which closes a valve body when an excitation coil is energized accordingly. 
     Damping valves should be designed to be “fail-safe” so that, for example, if the electromagnetic actuator fails, the motor vehicle can be operated safely with such a vibration damper. 
     In the prior art, various types of pole cores have been proposed for this purpose, the pole core being a flux-carrying component on the one hand and sealing off components of the electromagnetic actuator from the medium in the vibration damper on the other. This seal is necessary to prevent contaminants from penetrating into the components of the electromagnetic actuator and possibly interfering with the function of the electromagnetic actuator or vibration damper. 
     In the past, a pole core with a flange was proposed for this purpose, which is divided into a first section and a second section by a gap. A non-magnetic insert is placed in the gap bonding the first section to the second section. The insert is integrally bonded to both the first section and the second section, typically by brazing, and seals the gap. A pole core configured in this way is gas- and liquid-tight and has proven itself in the past. However, it has been shown that the integrally bonding between the first section, the insert and the second section is subject to wear due to internal stresses and different coefficients of thermal expansion and the tightness of the gap is not permanently guaranteed, which can lead to early failure of the electromagnetic actuator or of the damper valve as well as possibly the vibration damper. 
     This is where the present invention comes in. 
     It is the object of the present invention to propose an improved method for producing a pole core, a pole core, an improved electromagnetic actuator, and an improved vibration damper which expediently eliminates the disadvantages known from the prior art and enables the production of a long-term stable pole core taking into consideration economic aspects. 
     These objects are achieved by a method for producing a pole core with the features of claim  1 , a pole core with the features of claim  10 , an electromagnetic actuator with the features of claim  12 , and a vibration damper with the features of claim  13 . 
     Further advantageous configurations of the present inventions are specified in the dependent claims. 
     According to the invention, the method for producing a pole core with the features of claim  1  comprises the following method steps:
         providing a magnetic workpiece which has a first side and a second side, the first side having a groove directed towards the second side,   inserting at least two inserts and at least one bonding means into the groove, the at least one bonding means being arranged between two inserts, and   integrally bonding the at least two inserts to each other and/or the at least two inserts to the workpiece by preferred melting of the bonding means.       

     According to the invention, the method for producing a pole core is based on the finding that, when the bonding means is melted, capillary forces distribute the melted bonding means on the contact areas between the workpiece and the inserts, and the contact areas are completely wetted, bringing about intermetallic bonding between the inserts and the workpiece. The at least two inserts inserted into the groove partially compensate for material stresses caused by the distribution of the force vectors, and a pole core with a closed and integral bonding can be provided, which eliminates the problems known from the prior art. It has also proven to be essential that the flow paths of the melted bonding means are reduced. 
     Here and in the following, in connection with this invention, a magnetic workpiece is to be understood as a workpiece that is made of a soft-magnetic material that can be easily magnetized in a magnetic field. Such a material is, for example, carbon steel, iron or so-called soft iron, steel with added silicon, a nickel-iron alloy, a nickel-cobalt alloy or similar. 
     Furthermore, a preferred development of the production method provides that the at least two inserts are made of a non-magnetic material, in particular made of high-grade steel or stainless steel. 
     It has also been shown to be advantageous when carrying out the production method that the at least one bonding means comprises at least one isolated copper body which is inserted into the groove. Copper is excellently suited for producing an integral bond in the form of a closed soldered bond and, due to its material properties, has a high degree of elasticity, which means that stresses due to different thermal expansions of the inserts and the workpiece can be compensated for. Furthermore, copper has a low viscosity in the molten state, which is why the melt of the bonding means completely wets the inserts in the groove due to capillary forces and, after cooling, enables a resilient, brazed and integral bond. The at least one isolated copper body can preferably be a copper body adapted to the shape of the groove. For example, the copper body can be a copper wire, a molded part or bulk material. The copper body can also be formed from a copper alloy. Alternatively, instead of copper as the bonding means, another non-magnetic bonding means can be used that contains silver, brass, or similar, and that is suitable for brazing. By definition, brazing takes place in a temperature range above 723 K. 
     A development of the production method provides that the groove is machined into the first side of the workpiece by a machining manufacturing method, with the groove preferably being formed in the first side as an annular groove with a U-shaped cross section, having an inner side, a bottom side and an outer side. The respective insert is cuboid in cross section and can be adapted to the shape of the groove. The insert has a first area and a second area. If the groove is an annular groove, the respective insert can also be annular, with the first area corresponding to an inner lateral area and the second area to an outer lateral area. 
     According to a further preferred embodiment of the production method, the first area and/or the second area of at least one of the two inserts is designed as a transition fit, which is why the respective insert can be pressed into the groove with a moderate pressing force. It can be advantageous in this case if the insert has a transition fit only on the first side or on the second side, while a clearance fit can be provided on the other side, which takes into account the different thermal expansion of the materials used for the workpiece and the inserts. When the required temperature is reached, the bonding means melts and can flow a capillary gaps of approximately the same size both on the first side and on the second side. Due to capillary forces, the melt flows and is distributed therein. 
     As provided by a further advantageous embodiment of the present production method, the workpiece is positioned during integrally bonding in such a way that at least one of the inserts is pressed into the groove by gravity. As soon as the bonding means has liquefied when the melting temperature is reached, at least one of the inserts can exert a compressive force on the liquefied bonding means due to gravity and displace the liquefied bonding means between the at least two inserts in order to promote the distribution of the melted bonding means on the contact areas in the capillary gaps. In this case, the corresponding insert sinks, preferably so far that the at least two inserts in the groove—apart from a thin layer of the melted bonding agent—abut against each other. When the workpiece cools down, the bonding means cures and forms a closed integral bond both between the workpiece and the inserts and preferably also between the at least two inserts. 
     According to a preferred development of the production method, heating the workpiece and liquefying the bonding means takes place until the bonding means is almost completely displaced by capillary transport in the groove between the inserts, and the two inserts abut against each other in the groove. Excess melt of the bonding means can either escape from the groove or settle in a recess in the bottom side. An approximately constant thickness of the bonding means after cooling both in the capillary gaps and between the inserts prevents blowholes or cavities from forming, which on the one hand could impair the tightness of the integral bond and on the other hand promote internal stresses. 
     A development of the production method provides that after the workpiece has cooled down, material is removed from the first side and/or from the second side. In particular, it is preferred if after the cooling of the workpiece, the first side and/or the second side are/is reworked by means of a machining manufacturing method, such as lathing, milling or grinding. 
     Material is preferably removed at least in regions from the second side in order to expose an end face of one of the at least two inserts—or the bottom side of the groove—on the second side, whereby the magnetic workpiece is divided into a first section and a second section. After material has been at least partially removed from the second side and at least one of the inserts has been exposed, the groove forms a gap between the first section and the second section, the first section and the second section being integrally bonded in the gap by means of the at least two inserts and the bonding means. 
     A further aspect of the present invention relates to a pole core produced using the production method according to the invention. The pole core is preferably divided into at least two sections by a gap, with at least two non-magnetic inserts being arranged in the gap, via which the at least two sections are bonded by an integral bond. 
     Furthermore, it is preferred that the at least two non-magnetic inserts are made of stainless steel or high-grade steel and are bonded by means of a brazed integral bond, with copper being able to serve as the bonding means. The gap is sealed gas-tight and liquid-tight by the inserts and the integral bond, which means that no substance transport through the gap can take place from the first side to the second side, or vice versa. 
     A further aspect of the present invention relates to an electromagnetic actuator with such a pole core. 
     A third aspect of the present invention relates to a vibration damper with such an electromagnetic actuator. 
     A fourth and final aspect of the present invention relates to a motor vehicle with at least one such vibration damper. 
    
    
     
       An exemplary embodiment of a pole core according to the invention is described in detail with reference to the accompanying drawings, in which 
         FIG.  1    shows a sectional representation of a workpiece with a groove into which two inserts are pressed and which are bonded to the workpiece by means of a brazed integral bond in the groove, and 
         FIG.  2    shows a schematic representation of the method for producing a pole core. Identical or functionally identical components are identified below with the same reference numerals. For the sake of clarity, not all identical or functionally identical parts are given a reference numeral in the individual figures. 
     
    
    
       FIG.  1    shows a workpiece  10  which is designed to be approximately rotationally symmetrical about a longitudinal axis X-X. Workpiece  10  has a sleeve section and a flange section, the flange projecting perpendicularly from the longitudinal axis X-X from the sleeve section. 
     The flange section has a first side  11  and a second side  12  spaced along the longitudinal axis X-X on opposite sides of the flange section. 
     A groove  15  with a U-shaped cross-section is arranged annularly about the longitudinal axis X-X, preferably in a rotationally symmetrical manner, in the flange section of first side  11  and directed towards the second side. Groove  15  has an inner side  15   a , a bottom side  15   b  and an outer side  15   c , inner side  15   a  and outer side  15   c  being arranged parallel to the longitudinal axis X-X. Bottom side  15   b  can be arched in the direction of the second side  12  and can have a cone or spherical countersink. 
     It can also be seen from  FIG.  1    that two inserts  20  are inserted or pressed into groove  15 . Each insert  20  is made of a non-magnetic material such as stainless steel. The respective insert  20  is ring-shaped and has two end faces  20   b , a first area  20   a  formed by an inner lateral area and a second area  20   c  formed by an outer lateral area. 
     The respective insert  20  can be inserted into groove  15  coaxially to longitudinal axis X-X, with first area  20   a  and second area  20   c  of insert  20  being adapted to inner side  15   a  and outer side  15   c . Inner side  15   a  of groove  15  and first area  20   a  of the respective insert  20  may be designed as a transition fit and their dimensions may be within a tolerance band of ±0.05 mm, more preferably ±0.025 mm, and particularly more preferably ±0.015 mm. Outer side  15   c  of groove  15  and second area  20   c  of respective insert  20  may also be designed as a transition fit and the dimensions thereof may be within a tolerance range of ±0.025 mm, more preferably ±0.015 mm. 
       FIG.  1    shows that a further pole core section  40  made from a magnetic material may be bonded to workpiece  10 . Pole core section  40  is bonded to workpiece  10  via a non-magnetic sleeve  30 , preferably made of stainless steel, wherein non-magnetic sleeve  30  may also be bonded to bonding means  25  by a brazed integral bond. Workpiece  10  can preferably have a cone-shaped bevel  35  on the side facing sleeve  30 . 
     In groove  15 , the two inserts  20  are integrally bonded to the workpiece, the integral bond being established by brazing with a bonding means  25  at temperatures above 723 K, which, according to a preferred embodiment of the production method described in detail later, contains copper. 
     For this purpose, workpiece  10  made of a magnetic material with first side  11  and second side  12  is first provided in a method step  100  according to  FIG.  2   . 
     Then, in method step  120 , groove  15  is incorporated into workpiece  10  on the first side  11 —preferably by lathing or milling. 
     In a subsequent method step  130 , at least two inserts  20  and bonding means  25  are inserted into groove  15 , bonding means  25  being arranged between the two inserts  20 , preferably as at least one isolated body. In this state, insert  20  last inserted or pressed into groove  15  can protrude from groove  15  in some areas. 
     In a subsequent method step  140 , the arrangement formed in method step  130  is heated until the bonding means melts for integrally bonding inserts  20  to workpiece  10  and inserts  20  to one another. 
     If copper is used as bonding means  25 , the arrangement is heated to at least the melting temperature of copper, which is approx. 1358 K. The copper liquefies and flows due to capillary forces around inserts  20  in groove  15 , particularly in capillary gaps between the inner side  15   a  and the first side  20   a  and between the outer side  15   c  and the second side  20   c.    
     When heating the arrangement formed in method step  130 , it is advantageous if the workpiece  10  is arranged such that the groove  15  is arranged in the vertical direction so that the inserts  20  are pressed into the groove  15  by gravity. Accordingly, workpiece  10  shown in  FIG.  1    is to be arranged in such a way that the longitudinal axis X-X is aligned vertically and groove  15  points upwards with respect to the ground. 
     The arrangement from method step  130  is heated to a temperature above the melting temperature of bonding means  25  until bonding means  25  between the two inserts  20  has completely melted and the two inserts  20  abut against each other with the exception of a remaining film layer of bonding means  25 . A pressing force can also press the two inserts  20  into groove  15  in the heated state of the arrangement. 
     After cooling, both first side  11  and second side  12  can be mechanically reworked. In order to form pole core  1  according to the invention, material is removed in method step  150  from second side  12  in order to expose groove  15  or insert  20  first inserted in groove  15  and to divide the workpiece into two areas  16 ,  18 . The exposure of insert  20  is preferably done by milling or lathing, whereby workpiece  10  is divided in the two sections  16 ,  18  by a gap filled by groove  15  and inserts  20  integrally arranged in groove  15 . The two sections  16 ,  18  are bonded exclusively by non-magnetic material, namely two inserts  20  and bonding means  25 . 
     LIST OF REFERENCE NUMERALS 
     
         
           1  pole core 
           10  workpiece 
           11  first side 
           12  second side 
           15  groove 
           15   a  inner side 
           15   b  bottom side 
           15   c  outer side 
           16  sections 
           17  gap 
           18  section 
           20  insert 
           20   a  first area of  20   
           20   b  end face of  20   
           20   c  second area of  20   
           25  bonding means 
           30  sleeve 
           35  bevel 
           40  pole core section 
         X-X longitudinal axis