Abstract:
An improved magnetorheological (MR) fluid fan drive design is disclosed. A critical area in the design of such fan drives is the gap for MR fluid between the rotor ring and the corresponding structure of the stator. The stator member must confine the MR fluid in the gap and the applied magnetic field for the fluid and the field coil close to it. In this disclosure a ferrous alloy stator insert is made as one piece and cast within a larger aluminum alloy stator body. A slot for the rotor ring is cut through stator insert separating it into two pieces that remain supported by the aluminum body. The magnetically permeable insert pieces confine the fluid and the magnetic field at the fluid gap around the rotor, and one of the stator insert pieces supports the field coil next to the gap.

Description:
TECHNICAL FIELD 
     This invention pertains to making viscous clutch assemblies. More particularly, this invention pertains to the design of robust and more readily manufacturable, continuously controllable, magnetorheological fluid fan drive assemblies. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. Nos. 5,960,918; 6,032,772; 6,102,177; and 6,173,823, each entitled “Viscous Clutch Assembly,” describe clutches for a vehicle cooling fan assembly that use a magnetorheological (MR) fluid as the viscous material operating in a gap between the engine driven rotor and the fan carrying stator. The assembly further includes a coil for creating an electromagnetic field in the gap to vary the yield stress of the MR fluid and, thus, the speed of the fan. 
     U.S. Pat. No. 5,667,715 entitled “Magnetorheological Fluid” and U.S. Pat. No. 6,149,832 entitled “Stabilized Magnetorheological Fluid Compositions” describe viscous fluids suitable for use in the viscous clutch assemblies. Often these fluids comprise suitable, finely divided iron particles suspended in a nonpolar vehicle. The fluids are formulated to resist particle separation even under high separation force applications and they typically function as Bingham fluids. In an ambient gravitational field, but in the absence of a magnetic field, they display a shear stress that increases generally linearly as the shear rate on the fluid is increased. When magnetorheological Bingham fluids are subjected to a magnetic field, the shear stress vs. shear rate relationship is increased so that substantially more shear stress is required to commence shear of the fluid. This characteristic is very useful in controlling the transfer of torque between a rotor and stator in a viscous fluid clutch assembly. 
     In engine driven fan drive systems of the type described the speed of the fan is continuously variable by varying the magnetic flux in the MR fluid. Such variable speed fan drive assemblies provide vehicle fuel economy improvement, noise reduction, powertrain cooling improvement and cost reduction. After evaluation and testing of fan drive assemblies such as those described in the above four patents for a specific truck application it is realized that further improvements could be made. It is an object of this invention to provide improvements in the design of certain fan drives for the purpose of their ease of manufacture and robustness of operation. 
     SUMMARY OF THE INVENTION 
     As described in the “Viscous Clutch Assembly” patents identified above, a fan drive assembly has an engine driven input shaft with an attached hub and rotor assembly. This input assembly applies torque to a fan drive assembly using a viscous fluid, preferably a magnetorheological fluid such as those described in the above cited patents. Accordingly, the input and output structures are designed with complementary rotating portions that fit closely together with a thin layer of torque transmitting, MR fluid between them. 
     Also positioned close to this fluid gap is an electric coil for generating a variable magnetic field in the fluid to vary its yield stress and, thus, the torque transmitted from the input shaft/rotor assembly to the fan drive. A separate computer based controller determines the voltage or current flow applied to the coil. Experience with such fan drive mechanisms reveals the advantage of careful design of the complementary fluid gap forming portions of the input and fan drive assemblies and the means taken to seal in the MR fluid. This invention provides several such related improvements enhancing the ease with which the fan drive is made and the robustness of the drive. 
     In accordance with a preferred embodiment of the invention, a viscous fluid clutch for a vehicle cooling fan drive comprises a driving shaft/rotor assembly enclosed by a fan housing and a fan cover assembly. The fan housing is carried on the driving torque input shaft but separated from the shaft with respect to torque transmission by a bearing. A fan cover assembly that is attached to the fan housing includes a clutch stator insert that receives the rotor and the MR fluid for the transmission of torque. 
     The fan cover assembly includes a fan cover body, a fan cover insert and an annular coil body with coil windings. The coil body is carried on the circular, ferrous metal fan cover insert that, in turn, is preferably cast in place within the fan cover body. This assembly is co-axial with the input shaft. 
     The fan cover insert has a larger diameter than the coil and the outer region of the insert contains a slot in which the rotor is received during assembly of the drive. The slot and the rotor ring leave gaps on both sides of the rotor for the MR fluid. And the magnetic permeability of the ferrous composition on both sides of the slot confines the magnetic field of the coil on the MR fluid in the gaps. 
     One important feature of the invention is the method by which the fan cover assembly is made. The fan cover insert is made as a single round, disc-like precursor piece, preferably by hot forging a steel billet. The fan cover body is cast around the hot forged insert precursor using a suitable aluminum alloy. Anchoring features are formed on the fan cover insert to prevent separation from the cover body. Further processing of this composite part includes machining a circular slot through the insert for the rotor. This operation divides the round insert into two parts, both of which are anchored in the aluminum body portion of the composite. The separated portions of the insert define a slot for the rotor and MR fluid and provide magnetically permeable regions to concentrate the magnetic field in the fluid. 
     Additional machining of the fan cover insert provides a circular channel to receive the coil body and to provide a passage for the coil leads to a non-rotating assembly for supplying power to the coil. Additional machining of the fan cover body provides for improved sealing engagement with the fan housing member. 
     Other objects and advantages of the invention will become more apparent from a detailed description of preferred embodiments which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view, partly in section, of a fan drive in accordance with the invention. 
     FIG. 2 is a side view, in section, of the ferrous metal (steel) insert, as initially made, of the stator portion of the drive shown in FIG.  1 . 
     FIG. 3 is a side view in section of the insert of FIG. 2 cast into an aluminum cover assembly. 
     FIG. 4 is a side view, in section, of the cover member of FIG. 3 after machining. 
     FIG. 5 is an assembly view of the fan drive shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference is first made to FIG. 1 in which a fan drive assembly is shown in side sectional view. 
     The complete drive assembly is indicated at  10 . It comprises an input shaft  12 . Input shaft  12  is suitably made of steel and it is driven by the engine of the vehicle, e.g., using the water pump pulley. Accordingly, the speed of the input shaft is engine speed or proportionately higher in accordance with the pulley ratio for the water pump. 
     Affixed to the end  14  of input shaft  12  is a hub  16  for rotor  18 . Hub  16  has a central rim  20  tightly engaging the end  14  of input shaft  12 . Hub  16  then extends radially to enclose the rim portion  22  of rotor  18 . Rotor  18  is preferably made of a magnetically permeable ferrous material such as a low carbon steel alloy. The hub is made of aluminum and, as will be described further below, is cast so that it encloses the rim portion  22  of rotor  18 . The hub  16  also has a plurality of holes  23  to reduce the weight of the hub. 
     Thus, input shaft  12 , hub  16  and rotor  18  constitute an input shaft/rotor assembly for the vehicle fan drive  10 . 
     Ball bearing  26  is affixed at its inner race  28  to shaft  12 . Affixed to the outer race  30  of ball bearing  26  is an aluminum fan housing  31 . Fan housing  31  has a radially inner rim portion  32  which extends around the front edge of bearing  26 . Additionally, the housing  31  has a neck portion  34  that lies radially and axially close to, but spaced from, the rim portion  20  of hub  16  to form a labyrinth sealing path to prevent MR fluid from entering the bearing  26 . 
     The fan housing  31  is isolated by bearing  26  from torque application by input shaft  12 . 
     Still referring to FIG. 1 a fan cover assembly  38  includes a cast aluminum fan cover body  40 , a fan cover insert  42 , coil body  44  and coil cover  46 . 
     Fan cover insert  42  is preferably made of magnetically permeable ferrous material such as a low carbon steel alloy. As seen in FIG. 1 it includes an inner wheel portion  48  and an outer ring portion  50 . As will be described in more detail in connection with FIGS. 2,  3 , and  4 , fan cover insert  42  is preferably initially made as one piece and then machined so that ring portion  50  is separated from wheel portion  48 . 
     Fan cover body  40  comprises a plurality of boltholes  52  for attachment of fan hub  180  and bolts  182  (see also FIG. 5) and fan blades which are not shown in these figures. Fan housing  31  and fan cover body  40  each contain corner structures at their area of contact, generally  54 , so as to provide a labyrinth sealing surface between them and to provide a pilot surface for radial alignment of fan cover assembly  38  with fan housing  31 . Further, fan cover body  40  has a circular pocket  56  to provide for a polymeric sealant  57  to be molded in pocket  56  between fan cover body  40  and fan housing  31 . 
     In the operation of fan drive assembly  10 , rotor  18  rotates in a slot  60  through fan cover insert  42 . The inner wheel portion  48  of fan cover insert  42 , thus, defines a gap  62  with the inner surface of rotor  18 , and the outer ring portion  50  of fan cover insert  42  defines a gap at  64  with the outer surface of rotor  18 . 
     In the FIG. 1 sectional view, coil body  44  sits in a circular channel  66  in fan cover insert  42 . The other side of coil body  44  is enclosed by a round coil cover  46 . The coil body  44  is clamped by fan cover insert  42  at area of contact  69  and by coil cover  46  at area of contact  67 . Areas of contact  67  and  69  must form a seal to prevent leakage of MR fluid. A suitable electrically insulating, non-magnetic encapsulating material  65  (a commercial material from DuPont called Zenite is suitable) applied around the coil body  44  has a protrusion  68  that prevents a shunt in the magnetic field between the coil cover  46  and the inner wheel portion  48 . Coil cover  46  is spot welded to inner wheel portion  48  at locations not shown. 
     In order to simplify its manufacture and to prevent a shunt in the magnetic field, a series of discontinuous slots  70  are formed in rotor  18 . Furthermore, to improve the bond between the rotor  18  and the cast hub  16  a series of holes  71  are formed in the rim portion  22  of rotor  18  to provide interlocking contact between the cast aluminum hub  16  and the rotor  18  which increases the mechanical strength between them. Joining these parts by insert casting, rather than pressing and pinning, for example, improves heat transfer between them by essentially eliminating contact thermal resistance. 
     Coil body  44  has a diametric arm  72  through which coil leads  73  are led into the central post region  74  of insert  42 . Post  74  also carries a bearing, not shown, which carries a non-rotating slip ring assembly  76 . Slip ring assembly includes slip rings, brushes, brush holders, a retaining clip and a Hall effect sensor and target to measure fan speed. These features are known and used in the art and, therefore, are not illustrated to simplify FIG.  1 . 
     Leading to the slip ring assembly  76  is a tether  78  terminated by an electrical connector  80  through which electrical power is conducted to slip assembly  76  and the leads  73  of the coil body  44 . The electrical connector  80  interfaces to the vehicle electrical harness. When electrical power is applied to the coil body  44  a magnetic field is formed in the gaps  62  and  64  between the rotor  18  and the insert wheel portion  48  and insert ring portion  50 . A suitable MR fluid  88  is provided in gaps  62 ,  64  and its yield stress increased for torque transmission by the application of the magnetic field. Accordingly, input shaft  12  and the connected hub  16  and rotor  18  are turning at an input speed determined by the engine or the water pump pulley system. As power is provided to the coil body  44 , the formation of the magnetic field causes a yield stress increase in the MR fluid  88  enabling torque to be transferred between the rotor  18  and fan cover insert portions  50  and  48  to thereby drive the fan cover assembly  38 . 
     In addition to the design provisions that provide improved sealing of the MR fluid  88  within the drive assembly there are features of the invention which improve the manufacturability of the fan cover assembly  38 . 
     A fan cover insert precursor piece  142  is shown at succeeding stages of its transformation to a fan cover insert  42  in FIGS. 2,  3 , and  4 . In FIG. 2 the insert precursor  142  is shown in its initially formed condition. Preferably, the insert precursor  142  is formed as a single wheel-like piece by hot forging a suitable steel alloy blank. As thus formed, insert precursor  142  includes a central hub portion  174  which ultimately becomes the coil lead post  74  of fan cover insert  42 . Insert precursor  142  also includes a lip portion  176  for later contact with a cast around fan cover body  40 . Following hot forging, lip  176  is further formed or machined outwardly to anchor fan cover body  40  from axial separation. 
     The outer ring portion  150  of insert precursor  142  is formed with four lugs  152  (two shown in the sectional views of FIGS. 2-4) spaced at ninety degrees on its circumferential portion for subsequent interlocking mechanical engagement with cast aluminum cover body  40 . Also formed on the body of fan cover insert precursor  142  are four fillets  184  (two shown in FIGS. 2-4) spaced at ninety degrees at the base of lip  176 . Like lugs  152 , fillets  184  are for locking the later cast, fan cover body precursor  140  from separation from fan cover insert  42 . 
     Referring now to FIG. 3 the aluminum fan cover body precursor  140  is seen cast in place around the steel insert precursor  142 . The fan cover body precursor  140  is seen cast around the lugs  152  on the ring portion  150  of insert precursor  142 . The fan cover body precursor is also cast over fillets  184 . Thus, cast-in-place fan cover body precursor  140  is anchored against axial and radial separation from fan cover insert precursor  142  by lugs  152  and fillets  184 . The slight outward curvature of lip  176  also serves to retain fan cover body  40  against fan cover insert  42 . 
     FIG. 4 shows the fan cover insert  42 /fan cover body  40  composite structure after machining of the corresponding precursor portions of FIG.  3 . An important difference is that slot  60  has been cut completely through the single piece precursor body  142  insert body to form fan cover insert ring portion  50  and fan cover insert wheel portion  48  as separate pieces. The advantage of this separation is to prevent shunting of the magnetic field in the gaps  62 ,  64  and to provide end clearance for the rotor  18 . As seen in FIGS. 1 and 4, the now separate ring  50  and wheel  48  portions of the fan cover insert  42  are held in place by the cast around fan cover body  40 . Upon assembly of the fan drive, slot  60  receives the rotor structure  18  as shown in FIG.  1 . 
     The shape of fan cover insert  42  and its higher melting composition permits the fan cover body  40  to be cast around portions of it. This is an important feature of the invention because fan cover insert precursor  142  is formed as a single piece and fan cover body precursor  140  is cast around it before the insert precursor  142  is machined into two pieces. The manufacture of the fan cover insert wheel  48  and insert ring  50  are greatly simplified because they remain held in place by fan cover member  40  and do not require handling or complex handling effort during assembly of the fan drive  10 . The four lugs  152  help to retain insert ring  50  in the fan cover member  40  and fillets  184  and lip  176  help to retain fan cover insert wheel  48  in the fan cover member  40 . Also, cutting slot  60  after the insert casting process allows for more precise geometric tolerances to be maintained with respect to the relative positions of the slot  60 , insert wheel portion  48  and insert ring portion  50 . 
     A further comparison of FIGS. 3 and 4 shows the machining of the precursor composite  142 ,  140  to form the lead post  74 , a circular channel  66  in insert wheel  48  for the coil body  44  and sealant pocket  56  in cover  40 . 
     FIG. 5 is an exploded assembly view of the fan drive assembly  10 . FIG. 5 thus complements the FIG. 1 side sectional view of the assembly  10 . FIG. 5 includes fan hub  180  and self tapping bolts  182  for attaching hub  180  and blades, not shown, to fan cover assembly  38 , specifically fan cover body  40  at bolt holes  52  (FIG.  1 ). 
     FIG. 5 better illustrates the cooling fins  33  on fan housing  31  and cooling fins  45  on fan cover body  40 . Although a “lockup” condition between rotor and stator is possible the viscous fluid clutch typically operates with a difference in speed (termed slip) between the input shaft/rotor assembly and the fan housing/cover assemblies. When slipping, the shearing of the MR fluid in the gaps produces heat in the drive assembly and the cooling fins help to dissipate it. 
     During assembly and operation of the fan drive assembly  10  the MR fluid  88  typically stays in gaps  62  and  64  since the MR fluid  88  is a Bingham fluid which has a non-zero yield stress with no fluid shear. However, in the event the MR fluid  88  slumps when the fan drive assembly is not rotating, the reservoir  86  is sized such that there will not be MR fluid intrusion into the bearing  26 . 
     Thus, a vehicle fan drive assembly has been developed and described that is relatively easy to manufacture. It also provides improved sealing of the MR fluid within the drive and better confinement of the magnetic field in the region between rotor and stator where the fluid is intended to function. The invention has been described in terms of specific embodiments but it is apparent that other forms of the invention could be adapted by those skilled in the art. Accordingly the scope of the invention is to be considered limited only by the following claims.