Abstract:
Integrated devices and methods for compensating electric grade steel lamination stack height for use in a conventional two-plate high pressure die cast tool used for casting aluminum induction rotors. These devices and methods allow for significant variation in the lamination stack height without associated failures related to stack height variation, and also ensure constant and accurate clamping pressure on both the OD and ID of the steel lamination stack which prevents electric insulation damage, metal flow between laminations, large casting metal flash, and tool damage for excessive height laminations stacks. The clamping pressure is adjustable and is actuated from a single hydraulic cylinder which allows for a wide range of pressures to accommodate fine adjustment of clamping pressure to insure no damage occurs to the laminations.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to integrated devices and methods for compensating electric grade steel lamination stack height for use in a two-plate high pressure die cast tool used for casting induction rotors. The devices allow for variation in lamination stack height and ensure constant clamping pressure on both the outside diameter (OD) and inside diameter (ID) of the steel lamination stack. 
       BACKGROUND OF THE INVENTION 
       [0002]    Increasing demands in fuel efficiency have made hybrid systems more attractive in the automotive industry. In addition to a conventional combustion engine, an electric motor is an important part of the hybrid system. Alternating current (AC) induction motors are commonly used because they offer simple, rugged construction, easy maintenance and cost-effectiveness. The AC induction motor includes two major assemblies: a stator and a rotor. The stator is the outermost component of the motor and is composed of steel laminations shaped to form poles, with copper wire coils wound around the poles. The primary windings are connected to a voltage source to produce a rotating magnetic field. The rotor (often referred to in one form as a squirrel cage rotor) is a cylinder that is mounted on a shaft or mandrel to electromagnetically cooperate with the stator. The rotor is formed of longitudinal conductor bars cast into generally peripheral slots and cast together at both ends with short rings forming a cage-like shape.  FIG. 1  shows an illustration of a notional induction motor  1  with a cast rotor  3  that may spin in response to changes in a magnetic field in stator  5 . The core of the rotor  3  is built with stacks of electrical grade steel laminations  4  and aluminum or copper alloy rotor bars  7  are cast through conducting bar slots formed in the laminations  4  and end rings  9  creating an integrated squirrel cage structure. 
         [0003]    As depicted in  FIG. 1 , a rotating magnetic field around the rotor  3  is generated from the field windings  11  in the stator  5  of an induction motor  1 . Electric current is generated in the conductor bars  7  from the relative motion between the rotor  3  and the rotating magnetic field around the rotor  3 . These lengthwise-flowing electric currents in the conductor bars  7  react with the magnetic field of the motor  1 , producing force acting at a tangent to the rotor  3 . This results in torque to turn the shaft or mandrel  20  and the rotor  3 . In operation, the rotor  3  is carried around with the magnetic field, but at a slightly slower rate of rotation. The difference between the speed of the rotor and the speed of the magnetic field is called slip, and the slip increases with load. 
         [0004]    Conductor bars  7  are usually skewed slightly along the length of the rotor  3  (i.e., the conductor bars  7  are not perpendicular to the plane of the end rings  9  where the end ring attaches to the conducting bars  7 ), as shown in  FIG. 1 . This results in the reduction of noise and also smoother torque fluctuations. Torque fluctuations can result in some speed variations due to interactions with the pole pieces of the stator  5 . The extent to which the induced currents are fed back to the stator field winding coils  11 , and thus the current through the coils, is determined by the number of conductor bars  7  on the squirrel cage. Constructions that use a prime number of bars offer the least feedback. 
         [0005]    The iron core (laminated steel stack) carries the magnetic field across the motor. The structure and materials for the laminated steel stack are specifically designed to minimize magnetic losses. The thin laminations (electrical steel sheets), separated by an insulating coating, reduce stray circulating currents that would result in eddy current loss. Further reducing eddy-current loss is the fact that the material for the laminations is a low carbon, high silicon steel with several times the electrical resistivity of pure iron. The low carbon content makes it a magnetically soft material with low hysteresis loss. 
         [0006]    The same basic design is used for both single-phase and three-phase motors over a wide range of sizes. However, the depth and shape of bars for three-phase motors will have variations to suit the design classification. 
         [0007]    A common aluminum squirrel cage induction rotor construction method with a conventional two-plate high pressure die casting tool starts with an iron core of stacked thin stamped coated steel laminations compressed to a specified height and clamp pressure. Importantly, the lamination stack must be held and accurately compressed. Without proper lamination stack height compensation an assembly of too many laminations could prevent full die closure resulting in a large casting flash. An assembly of too few laminations can result in low compression force on the lamination stack causing metal to penetrate between laminations and under the mandrel, potentially causing tooling damage. Furthermore, lamination stacks compressed below specified pressure allow for infiltration of molten aluminum between individual laminations resulting in increased eddy current losses thereby reducing motor efficiency. Lamination stacks compressed at too high of pressure can result in damage to lamination insulation, also resulting in increased eddy current loss thereby reducing motor efficiency. Additionally, lamination stacks compressed at too high of pressure can increase conducting bar tension stress resulting from lamination stack spring back causing rotor distortion and loss of durability during use. 
         [0008]    Therefore, there is a need for an integrated compensation device assembly for lamination stack height for use in a conventional two-plate high pressure die cast tool used for casting aluminum induction rotors that will allow for variation in lamination stack height and ensure constant clamping pressure on the steel lamination stack, as well as for improved methods of compensating for lamination stack height variation in the manufacture of die cast aluminum induction rotors. 
       SUMMARY OF THE INVENTION 
       [0009]    The invention relates to integrated devices and methods for compensating electric grade steel lamination stack height for use in a two-plate high pressure die cast tool used for casting aluminum induction rotors. These devices and methods allow for significant variation in the lamination stack height without associated failures related to stack height variation, and also ensure constant and accurate clamping pressure on both the OD and ID of the steel lamination stack which prevents electric insulation damage, metal flow between laminations, large casting metal flash, and tool damage for excessive height laminations stacks. The clamping pressure is adjustable and is actuated from a single hydraulic cylinder. The systems mechanical advantage allows for a very wide range of pressures to accommodate fine adjustment of clamping pressure to insure no damage occurs to the laminations. 
         [0010]    One aspect of the invention relates to a rotor die casting device comprising an integrated lamination stack height compensation assembly. In one embodiment the integrated lamination stack height compensation assembly includes: a moveable slider plate having at least one tapered surface; an annular die cast component arranged perpendicular to an axis of motion of the slider plate; a plurality of posts positioned about the perimeter of the annular die cast component, an end of each post in contact with a tapered surface of the slider plate; a mandrel positioned coaxial to the annular die cast component and comprising a stepped distal periphery such that a plurality of ferromagnetic laminations stacked may be stacked there between, the stack defining a lamination stack height, an outer diameter and an inner diameter; and a compensation ring disposed along the distal periphery, the compensation ring configured to interposition with the stepped distal periphery of the mandrel such that a clearance gap exists between the mandrel and the compensation ring, the assembly configured such that upon activation, the slider plate engages and guides the posts up the tapered surface to transfer a compressive force through the annular die cast component such that a clamping pressure is applied to the outer diameter of the lamination stack, the compressive force sufficient to deform the compensation ring to close the clearance gap a compensating degree sufficient to ensure that the compensation ring applies a clamping pressure to the inner diameter of the lamination stack. 
         [0011]    Another aspect of the invention relates to a method of compensating for lamination stack height variation in the manufacture of a die cast rotor. In one embodiment, the method includes providing a compensation assembly integrated with a die casting tool, said compensation assembly comprising a moveable slider plate having at least one tapered surface; an annular die cast component arranged perpendicular to an axis of the slider plate; a plurality of posts positioned about the perimeter of the annular die cast component, an end of each post in contact with a tapered surface of the slider plate; a mandrel positioned coaxial to the annular die cast component and comprising a stepped distal periphery such that a plurality of ferromagnetic laminations stacked may be stacked there between, the stack defining a lamination stack height, an outer diameter and an inner diameter; a compensation ring disposed along the distal periphery, the compensation ring configured to interposition with the stepped distal periphery of the mandrel such that a clearance gap exists between the mandrel and the compensation ring, the assembly configured such that upon activation, the moveable slider plate engages and guides the posts up the tapered surface to transfer a compressive force through the annular die cast component such that a uniform clamping pressure is applied to the outer diameter of the lamination stack, the compressive force sufficient to deform the compensation ring to close the clearance gap to a compensating degree to ensure that the compensation ring applies a uniform clamping pressure to the inner diameter of the lamination stack. 
         [0012]    Additional features and benefits of the invention will become apparent from the following detailed description with appropriate reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a perspective cutaway of a motor showing with particularity the relationship between a stator and a cast rotor. 
           [0014]      FIG. 2  is an illustration of how lamination stamping edge burrs can affect stack height. 
           [0015]      FIG. 3  illustrates two-plate die compensation devices for controlling OD stack compression. 
           [0016]      FIG. 4  illustrates a close-up of on key components of the two-plate die compensation devices for controlling OD stack compression. 
           [0017]      FIG. 5  illustrates the angled (tapered) slider plate of the two-plate die compensation devices for controlling OD stack compression. 
           [0018]      FIG. 6  illustrates a cut-section through the two-plate die compensation devices for controlling OD stack compression. 
           [0019]      FIG. 7  illustrates the center gate mandrel and compensation ring providing tuned ring deflection pressure for controlling ID stack compression. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    It is important to have adequate and accurate pressure on the lamination stack  2  to hold it in position with known stack height and clamp compression pressure. As shown in  FIG. 2 , laminations  4  typically exhibit a small edge burr  6  caused during stamping and the edge burrs  6  continue to get worst with increased stamping die use which can lead to increased lamination stack height variation (note: lamination drawings do specify a maximum burr height). An exaggerated example of lamination edge burrs  6  which can influence core stack height is shown in  FIG. 2 . As shown in  FIG. 2 , additional problems can arise if there is not adequate and accurate pressure on the lamination stack  2 . During the stamping process and/or manual stacking of laminations  4  they can occasionally become flipped, resulting in additional stack height variation. Furthermore, laminations  4  can have a slight shift from center during stamping, resulting in an increased burr  6  effect. The devices and methods described allow for significant error in the assembly of the laminations stack height without associated failures related to stack height variation. An assembly of too many laminations  4  could prevent full die closure resulting in large casting flash. An assembly of too few laminations  4  can result in low compression force on the lamination stack  2  causing metal to penetrate between laminations  4  and under the mandrel  20  potentially causing tooling damage. The devices and methods described improve stack height compensation related to lamination stamping variation and inconsistent lamination assembly count, and are designed to allow variance on +/− five lamination  4  plus individual lamination burr  6  height. Of note, greater stack lamination count and height variance can be designed into the system if desired. 
         [0021]    Referring to  FIGS. 3-7 , the devices and methods in accordance with the present invention provide accurate compression pressure on both the OD  8  and the ID  10  of the steel lamination stack  2  during high pressure metal casting in a two-plate tool. The devices and methods in accordance with the present invention include a hydraulically activated slider plate  12  having at least one tapered surface  14 , and an annular die cast component  16  arranged perpendicular to an axis of motion of the slider plate  12 . As shown in the figures, the axis of motion is preferably along the vertical axis of the slider plate  12 , although it will be appreciated by those skilled in the art that other axes of movement are also contemplated, depending on the orientation of the stack height compensation assembly in general and the slider plate  12  in particular. A plurality of posts  18  are positioned and attached about the perimeter of the annular die cast component  16 , with an end of each post  18  in contact with a tapered surface  14  of the slider plate  12 . As shown in the figures, the plurality of posts  18  are preferably positioned and attached substantially equidistant and circumferentially about the perimeter of the annular die cast component  16 . A mandrel  20  is positioned coaxial to the annular die cast component  16 , with the mandrel  20  comprising a stepped distal periphery  22  such that a plurality of ferromagnetic laminations  4  may be stacked therebetween, the lamination stack  2  defining a lamination stack height, OD  8  and an ID  10 . A compensation ring  32  is disposed along the distal periphery  22 . The compensation ring  32  is configured to interposition with the stepped distal periphery  22  of the mandrel  20  such that a clearance gap  38  exists between the mandrel  20  and the compensation ring  32 . The clearance gap  38  is designed to be greater than the allowable lamination stack variation (resulting from extra laminations  4  or burrs&#39;  6  related expansion). Through the use of the slightly tapered  42  wedge sliding mechanism, the slider plate  12  engages and guides the posts  18  up the tapered surface  14 , transferring pressure to the multiple guided posts  18  in the two-plate die cast tool. The posts  18  push directly on the annular shaped die component  16 , transferring a compressive force through the annular die cast component  8  that exerts uniform clamping pressure on the OD  8  of the lamination stack  2 . The slight taper  42  provides accurate OD  8  clamp pressure. Simultaneously, the mandrel  20  and the compensation ring  32  assist in providing accurate clamping pressure to the ID  10  of the lamination stack  2 . The two-plate die exerts the compressive force on the compensation ring  32  that is designed to deflect (deform)  44  and provide a uniform specific pressure on the ID  10  of the lamination stack  2 , as illustrated in  FIG. 7 . The compressive force deforms  44  the compensation ring  32  to close the clearance gap  38  a compensating degree such that the compensation ring  32  applies a uniform specific clamping pressure to the ID  10  of the lamination stack  2 . As the two-plate die closes, the compensation ring  32  deflects at a precise pressure based on the thickness, geometry, and the tool steel (material) modulus. The mandrel  20  is of known length and the center portion of the mandrel  20  surface acts as a stop  30  and ring defection  44  is limited providing the desired ID stack clamping pressure. Thus, the clearance gap  38  determines how much variability can occur in the lamination stack  2 , caused by burrs, flipped laminations, or additionally laminations. Concurrently, the compensation ring  32  controls and provides fine adjustment of the clamping pressure exerted through the annular die cast component  8  that exerts uniform clamping pressure on the OD  8  of the lamination stack  2 . 
         [0022]    In other embodiments, the slider plate  12  is comprised of a plurality of grooves  46 , with each grove  46  having a tapered surface  14  such that upon hydraulic activation  40  of the slider plate  12  a groove  46  engages and guides a post  18  up a tapered surface  14 . In further embodiments, the slider plate  12  comprises a brass wear surface. In additional embodiments, the slider plate  12  is hydraulically activated  40 . 
         [0023]    In certain embodiments, the stepped distal periphery  22  of the mandrel  20  comprises an outer planar surface  24 , a recessed surface  26 , and inner planer surface  28 , with the inner planar surface  28  being a compression stop surface  30 . 
         [0024]    In other embodiments, the compensation ring  32  comprises an outer surface  34  that is substantially co-planar with the inner planar surface  28  of the mandrel  20  and which extends beyond the outer planar surface  24  of the mandrel  20  forming an overhang  36  and wherein the clearance gap  38  exists between the outer planar surface  24  and the compensation ring  32 . In certain embodiments, upon activation the slider plate  12  engages and guides the posts  18  up the tapered surface  14  transferring a compressive force through the annular die cast component  16  and applying a clamping pressure to the OD  8  of the lamination stack  2 , said compressive force deforming the compensation ring  32  to close the clearance gap  38  a compensating degree such that the compensation ring overhang  36  applies a clamping pressure to the inner diameter  10  of the lamination stack  2 . With continued die closure the other half of the die bottoms out on mandrel  20  stop surface  30 . 
         [0025]    According to additional embodiments, the compensation ring  32  is fabricated from a deformable material having an elasticity modulus such that the clamping pressure applied to the ID  10  of the lamination stack  2  is tunable by selection of deformable material according to a desired elasticity modulus. 
         [0026]    In certain embodiments, the compensation ring  32  possesses a ring geometry, see  FIG. 7 , such that the clamping pressure applied to the ID  10  of the lamination stack  2  is tunable by adjusting the ring geometry. In a more specific embodiment, the adjustable ring geometry comprises length and thickness. Other embodiments comprise adjusting a lateral side length of either or both of the mandrel  20  and compensation ring  32 , while further embodiments comprise adjusting a lateral side thickness of either or both of the mandrel  20  and compensation ring  32 . 
         [0027]    In other embodiments, the clearance gap  38  is maximum and the compensating degree is zero at a resting state, and the maximum clearance gap  38  is set to be greater than a permissible lamination stack height variation. In a more specific embodiment, permissible lamination stack height variance is defined as plus or minus five lamination  4  plus a single lamination burr  6  height. 
         [0028]    According to additional embodiments, the annular die cast component  16  comprises a casting cavity  48  and the mandrel  20  is configured to position the lamination stack  2  within the casting cavity  48 . Thus, the mandrel  20  is also used to properly position the steel lamination stack  2  within the casting cavity  48  so no additional complexity is needed. The casting method utilizes a center shot position and therefore the projected area of the die cast cavity is minimized resulting in the ability to use smaller casting machine for manufacturing than would be required for a conventional method. The small projected areas allow for use of extremely high cavity pressure to further enhance the rotor casting quality. An additional advantage of this method is that the assembled steel lamination stack  2  and mandrel  20  can be preheated prior to casting enhancing thermal control and repeatability. 
         [0029]    It is noted that terms like “generally,” “commonly,” and “typically,” when utilized herein, are not utilized to limit the scope of the claimed embodiments or to imply that certain features are critical, essential, or even important to the structure or function of the claimed embodiments. Rather, these terms are merely intended to identify particular aspects of an embodiment or to emphasize alternative or additional features that may or may not be utilized in a particular embodiment. 
         [0030]    For the purposes of describing and defining embodiments herein it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0031]    Having described embodiments of the present invention in detail, and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the embodiments defined in the appended claims. More specifically, although some aspects of embodiments of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the embodiments of the present invention are not necessarily limited to these preferred aspects.