Abstract:
A magneto-rheological coupling (MRC) is provided having an input assembly operable to receive a torque input and an output member operable to selectively transmit torque to a driveshaft. A magneto-rheological fluid, or MRF, having a variable viscosity in response to a magnetic flux field is operable to vary the torque transmitted from the input assembly to the output member. At least one annular lip forming a magneto-rheological fluid retention pocket is provided on at least one of the input assembly and the output member of the MRC. The annular lip is operable to direct MRF away from a roller bearing, thereby reducing the likelihood of MRF fluid incursion within the roller bearing. Additionally, a labyrinth seal may be employed to provide additional protection to the bearing. The labyrinth seal may have an annular bushing disposed therein to reduce the clearances of the labyrinth seal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application 60/680,194, filed May 12, 2005, and which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to magneto-rheological couplings.  
       BACKGROUND OF THE INVENTION  
       [0003]     It is known to provide a power steering system for a vehicle such as a motor vehicle to assist a driver in steering the motor vehicle. Typically, the power steering system is of a hydraulic type. The hydraulic power steering system employs an engine driven hydraulic power steering pump for generating pressurized fluid, which is subsequently communicated to a hydraulic steering gear of the motor vehicle. Since the power steering pump is driven directly by the engine using a belt or other method, its rotational speed is determined by that of the engine and it operates continuously as long as the engine is running, resulting in continuous circulation of the hydraulic fluid through the steering gear. In addition, the power steering pump must provide the required flow and pressure for the worst case engine speed, which is typically near idle engine speed, under static steering conditions.  
         [0004]     More recently, electro-hydraulic power steering systems have been used to provide an on-demand hydraulic pressure using an electric motor to drive the hydraulic power steering pump. An example of such an electro-hydraulic power steering system incorporates a hydraulic power steering pump driven by a brushless direct current electric motor controlled by a pulse width modulated inverter. Also in use are electrically driven steering systems, which are operable to assist in steering the vehicle using purely electro-mechanical system components.  
         [0005]     Other devices, such as the one described in commonly assigned U.S. Pat. No. 6,920,753, provide a means to directly control the speed of the power steering pump by using a magneto-rheological clutch or coupling (MRC) disposed between the accessory drive belt and the power steering pump. The MRC provides a continuously variable speed by controlling the torque transmitted to the power steering pump. The MRC can be part of the pump assembly, a separate unit, an integral part of the pump pulley, etc. The viscosity of the magneto-rheological fluid, or MRF, contained within the MRC can be controlled by exposing the MRF to a magnetic flux field. As the viscosity of the MRF is increased, the torque transfer properties of the fluid are increased. Since a conventional electronic control unit (ECU) can control the intensity of the magnetic field, the duty cycle of the power steering pump may be varied independent of engine speed.  
         [0006]     The MRF contained within the MRC includes magnetically permeable particles, which tend to be highly abrasive and harmful to bearings. Although bearings within the MRC are typically sealed units, it is preferred that the MRF fluid should not be allowed to contact these seals.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, the present invention provides a magneto-rheological clutch or coupling (MRC) having improved sealing provisions such that the magneto-rheological fluid, or MRF, is substantially precluded from contacting bearings within the MRC.  
         [0008]     A magneto-rheological coupling, or MRC, is provided having an input assembly coaxially disposed and spaced from an output member such that a working gap is defined between the input assembly and the output member. A magneto-rheological fluid is at least partially disposed within the working gap. The magneto-rheological fluid exhibits a variable viscosity characteristic in the presence of a variable magnetic field. Also provided is at least one bearing operable to rotatably mount the input assembly with respect to the output member. Additionally, at least one annular lip is provided with respect to at least one of the input assembly and the output member. The annular lip is operable to direct the flow of the magneto-rheological fluid away from the bearing.  
         [0009]     At least one labyrinth seal may be provided that is operable to substantially restrict the flow of the magneto-rheological fluid from contacting the bearing. Additionally, an annular bushing may be disposed within the labyrinth seal. The annular bushing is operable to reduce the clearances of the at least one labyrinth seal by providing a predetermined amount of sacrificial material which is removed through wear during the operation of the MRC.  
         [0010]     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]      FIG. 1  is a cross sectional side elevational view of a magneto-rheological fluid clutch or coupling (MRC) of the present invention, shown at rest and adapted to operate a vehicular power steering pump. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     Referring now to  FIG. 1 , there is shown a magneto-rheological fluid clutch or coupling (MRC), generally indicated at  10 , having an input assembly  12  rotatably mounted with respect to a drive shaft  14  of a hydraulic power steering pump  16  and adapted to be driven by an engine accessory drive belt  18 . Those skilled in the art will recognize additional methods of providing drive to the input assembly  12 , such as a gear drive. The MRC  10  is adapted to provide variable rotational speed to the drive shaft  14  of the power steering pump  16 . The rotational speed of the drive shaft  14  may be varied from a zero rotational speed condition to a maximum of the rotational speed of the input assembly  12 . The input assembly  12  includes a generally cylindrical magnetically permeable ring  20  coaxially located with respect to, and radially spaced from, the drive shaft  14 . Secured to the magnetically permeable ring  20  is a non-magnetic first cover member  22  that extends radially inward toward a central axis of the driveshaft  14 . The magnetically permeable ring  20  pilots a non-magnetic second cover member  24 , having a generally L-shaped partial cross section, to the first cover member  22 . The axially extending portion of the second cover member  24  is secured to the first cover member  22  via a plurality of fasteners  26 . A pulley member  28  is secured to the second cover member  24  via a plurality of fasteners  30 . The outer circumferential surface of the pulley member  28  has a plurality of radially extending ribs  32  defined thereon. The ribs  32  are operable to provide a surface upon which the accessory drive belt  18  may frictionally engage.  
         [0013]     The second cover member  24  is secured, via a plurality of fasteners  34 , to a magnetically permeable core  36  disposed coaxially with respect to, and spaced from, the drive shaft  14 . The core  36  has an annular channel  38  with a wire coil  40  disposed therein. An outer surface  42  of the core  36  forms an inner boundary, while an inner surface  44  of the magnetically permeable ring  20  forms an outer boundary of a working gap  46 . The wire coil  40  is operable to provide a magnetic flux field  48  when energized with electrical current. The core  36  has a low magnetically permeable portion  50  formed centrally thereon. The portion  50  is filled with a high-temperature resistant epoxy or other suitable non-magnetic material, and operates to shape the magnetic flux field  48  of the core  36  and ensures proper distribution through the working gap  46 . Additionally, the interstices of the wire coil  40  within the channel  38  may be filled with a high-temperature resistant epoxy similar to that of the portion  50 . A seal  52 , such as an elastomeric o-ring, is disposed between the second cover member  24  and the core  36 . Likewise, a seal  54 , such as an elastomeric o-ring, is disposed between the second cover member  24  and the first cover member  22 . The seals  52  and  54  operate to prevent leakage of magneto-rheological fluid (MRF)  56  from the MRC  10 .  
         [0014]     The MRF  56  contains magnetizable particles such as carbonyl iron spheroids of about half (½) to twenty five (25) microns in diameter dispersed in a viscous fluid such as silicon oil or synthetic hydrocarbon oil. The MRF  56  may also contain surfactants, flow modifiers, lubricants, viscosity enhancers, and other additives.  
         [0015]     A slip ring assembly  58  is mounted with respect to the MRC  10 . The slip ring assembly  58  includes spring-biased brushes  59  and  60 , which are operable to communicate electrical current to and from a first ring  62  and a second ring  64 , respectively. The first and second rings  62  and  64  are secured to the core  36  and are in electrical communication with the coil  30  though conductors  66  and  66 ′, respectively. A carrier assembly  68  is provided to secure the brushes  59  and  60  with respect to a power steering pump housing  70 . The brushes  59  and  60  are in electrical communication with the electrical system of the vehicle and are provided with operating signals from a conventional electronic control module (ECU), not shown. The ECU preferably includes a programmable digital computer that contains stored data for establishing the operational criteria of the MRC  10  during operation of the vehicle.  
         [0016]     An inner rotor or output member  72  includes a non-magnetic drive portion  74  secured to the drive shaft  14  through an interference fit or other method. A conventional fastener  76 , such as a hex head bolt, is employed to fixedly retain the output member  72  in relation to the drive shaft  14 . A non-magnetic hub portion  78  extends generally radially from the drive portion  74 , while a substantially cylindrical magnetically permeable drum portion  80  extends generally axially from the hub portion  78 . The magnetically permeable drum portion  80  bisects the working gap  46 , thereby creating a first working gap  46 A and a second working gap  46 B. The drum portion  80  has a first surface  82  and a second surface  84  in contact with MRF  56  contained within the working gaps  46 A and  46 B, respectively. The drum portion  80  has a low magnetic permeability portion  86  to ensure that the magnetic flux field  48  of the core  36  is properly distributed through the working gaps  46 A and  46 B. The core  36 , the magnetically permeable ring  20 , the drum portion  80 , and the MRF  56  disposed within the working gaps  46 A and  46 B form the magnetic circuitry of the MRC  10 . The dual working gap geometry of the MRC  10  is suited to reduce the axial length of the MRC  10 , thereby minimizing the cantilevered loading on the driveshaft  14 . The first surface  82  and second surface  84  may have a roughness to reduce the surface sliding friction of the MRF  56 , thereby increasing the shear forces of the MRF  56  on the drum portion  80 .  
         [0017]     The first cover member  22  and the core  36  cooperate to form a storage cavity  88  for the MRF  56  that recedes from the working gap  46  when the MRC  10  is idle. The first cover member  22  has an inner cavity  90  that is a portion of the storage cavity  88 . The inner cavity  90  has a wall  92  that diverges toward the working gap  46 . Centrifugal forces acting on the MRF  56  in the inner cavity  90  promote the return of the MRF  56  to the working gap  46  during operation of the MRC  10 .  
         [0018]     The first cover member  22  and the core  66  are rotatably supported on the output member  74  by bearings  94  and  96 , respectively. The bearings  94  and  96  are preferably ball-type or roller-type bearings. Labyrinth seals  98  and  100  have tight radial clearances that cooperate with the high viscosity of the MRF  56  to substantially prevent the MRF  56  from reaching the roller bearings  94  and  96 , respectively. Disposed within the labyrinth seals  98  and  100  are annular bushings  102  and  104 , respectively. The bushings  102  and  104  have a generally C-shaped cross section that closely matches the dimensions of the labyrinth seals  98  and  100 , respectively. The bushings  102  and  104  are preferably made from a low friction material such as a carbon based material, or may be made from polytetrafluoroethylene (PTFE) or other suitable polymer. The bushings  102  and  104  operate to reduce the need for precise machining and assembly tolerances of the labyrinth seals  98  and  100  by providing a predetermined amount of sacrificial material, which will be removed through wear during the operation of the MRC  10 .  
         [0019]     A generally radially extending annular lip  106  is provided on the first cover member  22  and a partially radially and partially axially extending annular lip  108  is provided on the hub portion  78  forming pockets  110  and  112 , respectively. The pocket  110  is formed at the inner radial boundary of the storage cavity  88 . The pockets  110  and  112  operate to capture and redirect MRF  56  that may recede from the working gap  46 B; thereby, preventing MRF  56  from migrating to the labyrinth seal  98  when the MRC  10  is at rest or idle. By redirecting the MRF  56  away from the labyrinth seal  98 , the likelihood of exposing the bearing  94  to MRF  56  is minimized. Additionally, a generally radially extending annular lip  114  is provided on the hub portion  78  forming a pocket  116  thereon. The pocket  116  is operable to capture MRF  56  that may recede from the working gap  46 A to prevent MRF  56  from migrating to the labyrinth seal  100  and possibly the bearing  96  when the MRC  10  is idle. The pockets  110  and  112  operate to extend the life of the MRC  10  by preventing incursion of MRF  56  within the bearings  94 . Likewise, pocket  116  operates to extend the life of the MRC  10  by preventing incursion of MRF  56  within the bearings  96 . While the bearings  94  and  96  are sealed units, it is preferred to maintain the MRF  56  out of contact with the bearing seals. Those skilled in the art will recognize that the annular lip  106  may be separate piece attached to the first cover member and may be formed from various non-magnetic materials such as rubbers or polymers. Likewise, the annular lips  108  and  114  may be separate pieces attached to the first cover member and may be formed from various non-magnetic materials such as rubbers or polymers.  
         [0020]     A toothed wheel  118  is secured to the drive shaft  14  and cooperates with a sensor  120  to provide the ECU, not shown, with a rotational speed value of the power steering pump  16 . The preferred sensor  120  is a Hall Effect sensor; however, those skilled in the art will recognize that other types of sensors may be employed.  
         [0021]     During operation, the coil  40  is selectively and variably energized with electrical current, thereby creating the magnetic flux field  48  that passes through the MRF  56  contained within the working gap  46 . As is well known, when the MRF  56  is exposed to the magnetic flux field  48 , the magnetizable particles therein will align with the magnetic flux field  48  and increase the viscosity of the MRF  56 . The increased viscosity will therefore increase the shear strength of the MRF  56  resulting in torque transfer from the input assembly  12  to the output member  72  causing rotation of the drive shaft  14 , which operates the power steering pump  16 . The torque transfer ability or characteristic of the MRF  56  varies with the intensity of the magnetic flux field  48 .  
         [0022]     Although the description has detailed the MRC  10  application within a power steering system, those skilled in the art will recognize that the present invention may be incorporated into other clutches employing MRF, such as fan clutches. Additionally, while the foregoing description describes an MRC  10  with a rotating coil  40 , the invention herein described may be used in a stationary coil-type MRC. While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.