Abstract:
A mechanical overrun clutch drive and driven assemblies are disengaged in an overrun situation by a magnetically biases pawl and ratchet assembly. The ratchet assembly is disposed between the drive and driven assembly, and is operative upon an overrun condition of the clutch driven assembly relative to the drive assembly to disengage the clutch drive and driven assembly and allow the driven assembly to freely rotate. Permanent magnets magnetically biased the pawls mounted on the drive assembly into unidirectional engagement with the saw teeth of an internal ring gear mounted to the clutch driven assembly. The magnets are isolated from physical contact with the pawls they bias by an air gap or airspace, to prevent the pawls from mechanically impacting the magnets during operation of the clutch.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Field of the Invention. The present invention is in the field of mechanical clutch mechanisms. More specifically, the present invention relates to combination magnetic-centrifugal clutches having pawl and ratchet structures where the interlocking engagement of the driving and driven clutch parts is effected in part by the magnetic attraction between the elements of the clutch.  
           [0002]    Mechanical pawl and ratchet clutches are known and used in unidirectional drive train, such a pumping unit on a well (e.g., an oil well), for transmitting rotational force (torque) from a drive mechanism to a driven mechanism. An “overrun” type clutch allows the drive and driven mechanisms of the drive train to disengage when the driven mechanism rotates faster then drive mechanism.  
           [0003]    An example of such a mechanical pawl and ratchet overrun clutch is disclosed in U.S. Pat. No. 4,914,906 to Burch. However, because of certain perceived limitations on Burch-type devices, others in the field were motivated to develop alternative clutch assemblies.  
           [0004]    For example, the U.S. Pat. No. 5,205,386 to Goodman et al. describes a type of pawl and ratchet clutch wherein the pawls are biased to engage the ratchet by a spring mechanism and simultaneously to disengage the ratchet by a magnet in combination with centrifugal force of rotation. When the rotation rate is over the optimized value, the pawl moves outwards radially under centrifugal force to contact the magnet. Once the pawl contacts the magnet, the ratchet pawl will be retained disengaged by the combination of the magnetic and centrifugal forces. The Goodman clutch uses a spring to bias the pawl into engagement with the ratchet. In applications where a drive train is frequently in the overrun condition, the spring of Goodman can become fatigued, and over time cause a change in the biasing force of the spring which alter the threshold rotation at which the clutch engages and disengages. Additionally, in a frequent overrun situation, during the operation of a Goodman-type clutch, the magnet and the pawl are constantly physically contacting and bumping against each other. This can result in the damage to the magnet or the magnet becoming overheated. The physical damage to the magnets can cause the malfunction of the clutch. Over heating a magnet can cause it shrink, and therefore may eventually loosen, resulting in the malfunction of the clutch.  
           [0005]    Therefore, it would be beneficial in the field to have an alternative mechanical overrun clutch that did not utilize a pawl and ratchet assembly having bias springs, or having bias magnets subject to physical impact during engagement and disengagement of the clutch in the drive train.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    The present invention is a mechanical overrun clutch having a ratchet assembly utilizing a magnetic field to bias the pawls and ratchet wheel of the assembly to engage. The present mechanical overrun clutch does not utilize bias springs. Additionally, the permanent magnets which generate the magnetic fields do not themselves physical contact the pawls during engagement and disengagement of the clutch. The mechanical overrun clutch comprising a clutch drive assembly, a clutch driven assembly, and a magnetically biased ratchet assembly to accomplish engagement of the drive and driven assemblies. The drive assembly is rotatable about an axis of rotation by a drive mechanism external to the clutch. The clutch driven assembly has an axis of rotation in common with the drive assembly and rotatable about the axis of rotation by the rotation of the clutch drive assembly when it is engaged via operation of the ratchet assembly. The ratchet assembly is disposed between the drive assembly and the driven assembly. The ratchet assembly is operative upon the appropriate rotation of the clutch drive assembly to engage the clutch drive assembly with the clutch driven assembly and to transfer torque to the clutch driven assembly. Further, the ratchet assembly disengages upon the inappropriate rotation of the drive assembly relative to the driven assembly, i.e., an “overrun” condition. An “overrun” situation occurs when the rotation rate of the driven assembly exceeds the rotation rate of the drive assembly. In such a situation, the present clutch disengages and the driven assembly is allowed to ratchet freely.  
           [0007]    The clutch drive assembly has an axis of rotation, about which it is rotatable by a drive mechanism, and having a ratchet plate, the ratchet plate being substantially circular and having a center on and radii perpendicular the axis of rotation, at which center is disposed a means for attaching the drive assembly to a drive mechanism. The clutch drive assembly comprises a substantially circular ratchet plate. The ratchet plate has its center on the axis of rotation and the plane of the plate is perpendicular to the axis. At the center of the ratchet plate is disposed a connecting means for receiving and fixing the drive assembly in rotational communication a drive mechanism. Typically, this connecting means is a shaft bore for receiving and rotationally communicating with the axial shaft. To accomplish this, the shaft bore is configured to compliment and closely receive the axial shaft. Complimentary configuration of the axial shaft and shaft bore are known to and readily practicable in the present invention by the ordinary skilled artisan. Examples include, keyed and splined shafts and complimentary bores.  
           [0008]    The clutch driven assembly comprises a housing having a substantially cylindrical interior and a substantially cylindrical interior wall. The housing has an axis of rotation in common with the drive assembly and is rotatable about the axis of rotation. The housing receives and contains the ratchet assembly and the clutch drive assembly. The housing is in rotational communication with an external driven mechanism to which the rotation of the housing is imparted.  
           [0009]    Being held fixed in the drive assembly housing, the ratchet wheel is also subject to rotational communication with the drive mechanism and rotates about the axis of rotation with the housing.  
           [0010]    The ratchet assembly of the present invention comprises a ratchet wheel and an associated plurality of ratchet pawls and biasing magnets. The ratchet wheel is received and held fixed in the housing of the clutch drive assembly. The ratchet wheel comprises an internal toothed ring gear with the teeth being ratchet teeth and configured to unidirectionally engage the ratchet pawls of the ratchet assembly. The engagement is unidirectional in that the ratchet pawls engage the gear teeth of the ratchet wheel when the relative rotation (of the drive to the driven assembly) is in one direction, and do not engage the gear teeth when relative rotation is in the other direction. The ratchet pawls and biasing magnets are mounted on the ratchet plate. More specifically, the plurality of ratchet pawls are each pivotably mounted on a separate pawl axle. The pawl axles are fixed to the ratchet plate of the clutch driven assembly at equal radial distances from the axis of rotation of the driven assembly. An equal number of permanent magnets are fixed to the drive assembly for biasing the ratchet pawls into the engaged position. This is accomplished by positioning a magnet relative to each ratchet pawl to have its magnetic field affect a portion of the associated ratchet pawl and bias the pawl to pivot on its pawl axle into the engagement position.  
           [0011]    To accomplish the magnetic biasing of the ratchet pawls, the pawls are composed of a para-magnetic material susceptible to a magnetic field. The ratchet pawls have a gearing or gear engaging section, a mid-section and a tail section. The mid-section has an axle bore for receiving and pivotably mounting the ratchet pawl to the pawl axle. Distal to the mid-section, the gearing section has a gearing end for engaging with the ratchet wheel fixed in the housing of the clutch drive assembly. Distal to the mid-section, the tail section has a tail end. The tail section is acted on by the magnetic field of the associated biasing magnet, to pivot the ratchet pawl and bias its gearing end into an engagement position with the drive assembly.  
           [0012]    The distance from the gearing end to the center of the axle bore of a ratchet pawl is larger than the distance from its tail end to the center of the axle bore. This allows a movement of the tail end of the ratchet pawl to impart a greater arc of movement to the gearing end. A ratio of the distances between the gearing end to the center of the axle bore and the tail end to the center of the axle bore of about 1.1 to 1.0 has been beneficially utilized in the ratchet pawls.  
           [0013]    The biasing magnets are permanent magnets. Each biasing magnet is located on a radius of the axis of rotation intersecting the tail end of the pawl with which it is associated. The permanent magnets are made of a permanent magnet material such as an iron-oxide material or a neodymium-iron-boron material. The biasing magnets are isolated from direct physical contact with the ratchet pawls and with the clutch drive assembly.  
           [0014]    Isolation from the drive assembly is accomplished by having each magnet first fixed to a non-magnetic material, and then having the non-magnetic material fixed to the clutch drive assembly. For example, each magnet may be received in a sleeve made of a non-magnetic material, such as stainless steel, copper or aluminum, and the sleeve then being fixed in a recess on the clutch drive assembly to isolate the magnet from direct contact with the drive assembly.  
           [0015]    Isolation of the biasing magnets from the ratchet pawls is accomplished by limiting the pivotal travel of the ratchet pawl and having each magnet is positioned relative to the ratchet pawl to avoid contacting the pawl at the point of closest approach of its tail end toward the magnet. Although the tail end of the ratchet pawl never contacts the magnet, the biasing magnet still magnetically affects the ratchet pawl during rotation of the drive and driven assemblies relative to each other. Each magnet is positioned relative to its associated ratchet pawl to maintain an air gap or air space separation of at least about 1 mm at the point of closest approach of the tail end of the ratchet pawl toward the magnet.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a partial cross-sectional view down the axis of rotation of the clutch.  
         [0017]    [0017]FIG. 2 is a cross-sectional view along the axis of rotation of the clutch.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring now to the drawings, the details of preferred embodiments of the present invention are graphically and schematically illustrated. Like elements in the drawings are represented by like numbers, and any similar elements are represented by like numbers with a different lower case letter suffix. As shown in FIGS. 1 and 2, the present invention is a mechanical overrun clutch  10 . The clutch  10  comprises: a clutch drive assembly  12 ; a clutch driven assembly  14 ; a ratchet assembly  16  and an axis of rotation  20  common to both the drive assembly  12  and the driven assembly  14 .  
         [0019]    The clutch drive assembly  12  is rotatable about the axis of rotation  20  by a drive mechanism (not shown). The drive assembly further comprises a ratchet plate  24 , the ratchet plate  24  being substantially circular and having its center on the axis of rotation and extending radially and perpendicularly therefrom (see FIGS. 1 and 2). At the center of the ratchet plate is a drive hub  28 , disposed to include an attaching means  32  for attaching the ratchet plate  24  of the drive assembly  12  to the drive mechanism. In a preferred embodiment, the attaching means  32  of the drive hub  28  a shaft bore  32  through the center of the hub  28  along the axis of rotation  20 . The shaft bore  32  mates with the output shaft  34  of the drive mechanism and fixes the two together so that rotation of the output shaft  34  is directly communicated to the clutch drive assembly  12  via the drive assembly hub  28 . Means for fixably mating an output shaft  34  with the shaft bore  32  are known to the ordinary skilled artisan and readily practicable in the present invention. For example, the hub bore  32  and the output shaft can be complementary threaded. Alternatively, as shown in the preferred embodiment of FIG. 1, the hub bore  32  includes a key-way  36 , for mating the cross-section of the hub bore  32  with a complimentary cross section of the output shaft  32  of the drive mechanism.  
         [0020]    The clutch driven assembly  14  is also rotatable about the axis of rotation  20 . The clutch driven assembly  14  comprises a housing  40  having a substantially cylindrical interior space and a substantially cylindrical interior wall  44 . The housing  40  is disposed to receive and contain the ratchet assembly  16  and the clutch drive assembly  14 .  
         [0021]    The ratchet assembly  16  is disposed in mechanical communication with both the drive assembly  12  and the driven assembly  14 . The ratchet assembly  16  is operative upon appropriate rotation of the clutch drive assembly  12  relative to the driven assembly  14  to engage the clutch drive assembly  12  with the clutch driven assembly  14 , to rotate the clutch driven assembly  14 . Additionally, the ratchet assembly  16  disengages the drive  12  and driven  14  assemblies upon the inappropriate rotation (“overrun” condition) of the driven assembly  14  relative to the drive assembly  12 . In such a situation, the present clutch  10  disengages and the driven assembly  14  is allowed to ratchet freely.  
         [0022]    As shown in FIG. 1, the ratchet assembly comprises a ratchet wheel  50  fixedly received inside the housing  40  the driven assembly  14 , and a plurality of ratchet pawls  56  pivotably mounted on pawl axles  66 , and an equal plurality of permanent magnets  80  fixed to the outer surface  29  of the drive hub  28 .  
         [0023]    The ratchet wheel  50  is configured as an internal toothed ring gear with the gear teeth  52  of the ring gear disposed to unidirectionally engage the ratchet pawls  56  of the ratchet assembly  16 . The ratchet wheel  50  may be a separate component and fixed to an interior surface of the housing  40 , such as the interior cylindrical wall  44 , using a means known to and practicable by the ordinary skilled artisan, such as threaded fasteners. Alternatively, the ratchet wheel  50  may be cast or milled into integral to an interior surface of the housing  40 , such as the interior cylindrical wall  44 .  
         [0024]    The ratchet pawls  56  are made of a para-magnetic material. Each pawl  56  has a gearing section  58 , a mid-section  60  and a tail section  62 . In the mid-section  60  of the pawl  56  is disposed an axle bore  64 . The axle bore  64  receives and pivotably mounts the ratchet pawl  56  to a pawl axle  70 . The gearing section  58  of the pawl  56  has a gearing end or gearing surface  66  for engaging with the gear teeth  52  of the ratchet wheel  50 . The tail section  62  of the pawl  56  extends away from the mid-section  60  opposite the gearing section  58 . The para-magnetic material of the tail section  62  is acted on by the magnetic field of a magnet  80  to pivot the ratchet pawl  56  and bias the gearing end  66  of the gearing section  58  into a position to engage with the gear teeth  52  of the ratchet wheel  50 . The distance from the gearing end  66  of the pawl  56  to the center of the axle bore  64  is greater than the distance from the tail end  68  of the tail section  62  to the center of the axle bore  64 . In the preferred embodiment, the ratio of the distances between the gearing end  66  of the gearing section  58  to the center of the axle bore  64  and the tail end  68  of the tail section  62  to the center of the axle bore  64  was about 1.1 to 1.0. The pawl axles  70  are fixed to the ratchet plate  24  of the drive assembly  12  at equal radial distances  72  from the axis of rotation  20  of the drive assembly. The angular separation  74  of the pawl axles  70  may be the same, or as shown in FIG. 1, they may be different.  
         [0025]    A plurality of permanent magnets  80  equal to the number of ratchet pawls  56  are fixed to the outer surface  29  of the drive hub  28 . In the preferred embodiment shown in the figures, the magnets  80  were columnar shaped. Permanent magnets and the materials for making them are known in the art. Such materials include iron-oxide and neodymium-iron-boron materials. The magnets  80  are disposed on the drive hub outer surface  29  to have it magnetic field impinge on the associated ratchet pawl  56  to bias the pawl  56  to pivot on its pawl axle  70  into an engagement position relative to the gear teeth  52  of the ratchet wheel  50 . The permanent magnets  80  are each located on a radius of the axis of rotation  20  intersecting the tail section  62  of the associated ratchet pawl  56 . In the preferred embodiment shown in FIG. 1, the magnets  80  are isolated from direct physical contact with their associated ratchet pawls  56  by an air space, and also from the drive hub  28  of the clutch drive assembly  12  by a non-magnetic material  84 .  
         [0026]    Each magnet  80  was isolated from direct contact with drive hub  28  by first attaching the magnet  80  to a non-magnetic material  84 . After the magnet  80  was fixed to a non-magnetic material  84 , and the non-magnetic material  84  was in turn fixed to the drive hub  28  of the clutch drive assembly  12  to isolate the magnet  80  from direct physical contact with the drive hub  28 . In the preferred embodiment shown in FIG. 1, each magnet  80  was inserted into a non-magnetic material  84  formed as a closed end sleeve  84   a.  Non-magnetic materials for constructiong the sleeves  84   a  are known in the art and include stainless steel, copper and aluminum. In turn, the sleeve  84   a  containing the magnet  80  was fixed in a recess  88  on the drive hub  28  of the clutch drive assembly  12  to isolate the magnet from direct physical contact with the drive hub  28 .  
         [0027]    The airspace  82  which separates each magnet  80  from its associated pawl  56  is accomplished by positioning each magnet  80  relative to its associated ratchet pawl  56  to avoid physically contacting the pawl  56  during rotation of the drive assembly  12 , while still magnetically affecting the tail section  62  of the pawl  56 . The size of the magnet  80  (and hence the strength of its magnetic field), the para-magnetic mass of the tail section  62  and the desired minimum and maximum range of the airspace are all considered by the ordinary skilled artisan to select the size and shape of the magnets  80 , the tail sections  62  to achieve the desired biasing of the pawls  56  into engagement with the ratchet wheel  50 . In the preferred embodiment shown in the figures, each magnet  80  was positioned relative to the associated ratchet pawl  56  to maintain a minimum air space of at least about 1 mm, while still magnetically biasing the ratchet pawl  56  during rotation of the drive  12  and driven  14  assemblies relative to each other.  
         [0028]    While the above description contains many specifics, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of one or another preferred embodiment thereof. Many other variations are possible, which would be obvious to one skilled in the art. Accordingly, the scope of the invention should be determined by the scope of the appended claims and their equivalents, and not just by the embodiments.