Abstract:
A differential mechanism for transmitting power from an input to an output includes a case containing a side gear, a locking member rotatably secured to the case and axially displaceable relative to the case. The locking member alternately engages the side gear to limit rotation of the side gear relative to the case, and disengages the side gear to permit rotation of the side gear relative to the case. An electromagnetic coil assembly is supported on the case for movement toward and away from the locking member. A first actuator including an electromagnetic coil is supported on the case for moving the locking member toward engagement with the side gear in response to energizing the coil. A second actuator urges the locking member away from engagement with the side gear.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to an apparatus for alternately releasing and holding a side gear of a differential assembly against rotation relative to a case. More particularly, the invention pertains to electromagnetic actuation of a device for releasing and holding the side gear. 
   It is conventional to employ a hydraulically actuated clutch to permit limited wheel slip at an axle, i.e., to produce a rotational speed difference between the driven wheels. U.S. Pat. No. 4,265,143 discloses a hydraulic limited slip differential mechanism for locking up the differential gear set. A latch mechanism includes a latch member having a pair of latch surfaces, a frame member and a weighted member, which is oppositely disposed from the latch member about the axis of rotation of the gear set. The weighted member moves the latch mechanism, in opposition to the biasing force of a spring, along a straight, generally diametric path, in response to increasing rotational speed of the differential mechanism. This movement causes the latch to disengage the flyweights and prevents rotation of the flyweight. The position and mode of operation of the weighted member is effective to reduce missed engagements of the actuating mechanism. 
   A purpose of a locking differential is to prevent relative rotation of one driven wheel with respect to another driven wheel. This is usually accomplished by locking one differential side gear to a differential case, thereby preventing rotation of the side gear with respect to the differential case, and preventing relative wheel speed differential on any one axle. 
   Electronically-actuated locking differentials are well known in the automotive driveline industry. For example, U.S. Pat. No. 6,083,143 discloses a locking differential mechanism that includes a side gear having a set of teeth, and a locking member, also having a set of teeth for engaging the teeth on the side gear. A ball ramp actuator located adjacent the locking member is integral with an inner actuating plate. An outer actuating plate is located outside the case, and a set of cam balls operable with the actuating plates to cause ramp-up and engagement of the gear. An electromagnetic coil assembly is located adjacent the ball ramp actuator, operates to retard rotation of the outer actuating plate and to produce ramp-up in response to an electrical input signal. 
   A locking differential can also be used as an inter-wheel differential or as a center differential in 4×4 and AWD vehicles. In this case, the axis of the differential assembly is parallel to the longitudinal axis of the vehicle. The center differential allows drive shaft speed differences between the front and rear axles. But there are some cases where it is desired to lock the front and rear axle drive shafts together such that a single rotation speed is reattained. This condition is known as a locked center differential. 
   SUMMARY OF THE INVENTION 
   The present invention concerns an apparatus for alternately releasing and holding the side gear of a differential mechanism against rotation relative to a differential case. The differential is actuated into engagement by energizing an electromagnetic coil, and disengagement occurs upon deenergizing the coil by a spring. The engagement mechanism is reliable and uncomplicated by ball-and-ramp or cam-and-ramp mechanisms as are employed in the prior art. The electromagnetic coil does not rotate; therefore, it can be connected by reliable, conventional electric connectors to an electric power source without employing brushes, a slip ring, or another such device as would be required to connect the source of electric power to a rotating coil. 
   A moving coil electronic locking differential according to this invention will operate reliably at all normal operating temperatures in a front or rear axle differential or in a center differential, such as those used in 4×4 and AWD vehicles. 
   A differential mechanism according to this invention transmits power from an input to an output. The differential mechanism includes a case containing a side gear, a locking member rotatably secured to the case and axially displaceable relative to the case. The locking member alternately engages the side gear to limit rotation of the side gear relative to the case, and disengages the side gear to permit rotation of the side gear relative to the case. An electromagnetic coil assembly is supported on the case for movement toward and away from the locking member. A first actuator including an electromagnetic coil is supported on the case for moving the locking member toward engagement with the side gear in response to energizing the coil. A second actuator urges the locking member away from engagement with the side gear. 
   The scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples within the spirit and scope of the invention will become apparent to those skilled in the art. 

   
     DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
       FIG. 1  is an isometric cross section of a differential mechanism according to the present invention; 
       FIG. 2  is an isometric view of a locking ring; 
       FIG. 3  is a side view of the locking ring of  FIG. 2 ; 
       FIG. 4  is an isometric view of the case; 
       FIG. 5  is a side view of a field core coil assembly; 
       FIG. 6  is a graph showing the variation of magnetic force produced by the coil and an air gap; 
       FIG. 7  is a front view of the thrust plate shown in  FIG. 1 ; 
       FIG. 8  is a side view of the thrust plate of  FIG. 7 ; 
       FIG. 9  is a cross section through the case showing the actuators for engaging and disengaging the locking ring and side gear; 
       FIG. 10  is a graph showing the variation of the spring force applied to the locking ring and spring deflection; 
       FIG. 11  is a schematic diagram illustrating beveled surfaces on the case and coil assembly, clearance gaps and a bevel angle; and 
       FIG. 12  is a graph showing the variation of the air gap with displacement of the coil. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIG. 1 , a differential mechanism  8  according to this invention includes a differential case  10 , preferably of cast iron or steel, supported on a stationary housing (not shown) for rotation about a lateral axis  12 . A bevel ring gear, secured to the case at the attachment bore holes on the flange  11 , drives the case  10  in rotation about axis  12  from an output of a transmission or transfer case. 
   The case  10  provides an internal chamber  14 , which contains bevel pinions  16 , a right side gear  18  meshing with the pinions and driveably connected to a right output shaft  20 , which extends from the case  10  to a driven wheel of a motor vehicle, and a left side gear  22  meshing with the pinions and driveably connected to a left output shaft (not shown), which extends from the case to a driven wheel at the left side. The pinions  16  are each secured by pins  24  to the rotating case  10 , such that the pinions  16  rotate about the axis of pins  24  perpendicular to axis  12 , and the pinions and pins  24  rotate about axis  12 . 
   Also located in the case  10  is a locking ring  26 , secured to the case such that it rotates about axis  12  and moves axially relative to the case along the axis.  FIGS. 2 ,  3  and  9  show that ring  26  is formed with three posts  28 , each post extending axially through a hole in web  30 , which is formed in the case  10 ; a planar surface  32  facing the web  30 ; and a series of clutch teeth  34  and spaces  36  angularly arranged alternately about axis  12  on the axially opposite side of the locking ring from surface  32 . The clutch teeth and spaces are adjacent and face the side gear  22 . 
     FIG. 4  shows that side gear  22  is formed with a series of clutch teeth  38  and spaces  40  angularly arranged alternately about axis  12  on its axial outer face adjacent the clutch teeth  34  and spaces  36  of the locking ring  26 . The clutch teeth and spaces of the side gear  22  and locking ring  26  are mutually complementary such that they can engage and disengage as the locking ring moves toward and away from the side gear. The locking ring  26  is normally not engaged with the side gear  22  and permits the side gear to rotate with respect to the differential case  10  and the locking ring, thereby producing an unlocked or disengaged state. When the locking ring  26  is actuated to engage the side gear  22 , their clutch teeth and spaces mesh, thereby driveably connecting the side gear to the locking ring and case  10 , preventing the side gear from rotating relative to the case and locking ring, and producing a locked or engaged state. 
     FIGS. 1 and 5  show a field core coil assembly  42  supported on the case  10  outside the chamber  14 . The field assembly  42  includes an electromagnetic coil  44 , fitted into an annular recess  46 , formed in a ring  48 . The coil  44  produces a magnetic field when energized with electric current through the leads  50 . The field assembly is secured to the housing by brackets  52 , which prevent the coil assembly  42  and coil  44  from rotating. The magnetic field produces an axial force on the coil assembly  42 , whose magnitude varies with the width of an air gap  52  between the coil assembly and the case  10 . 
   When the coil  44  is energized, it is attracted to the differential case due to the magnetic field generated by the coil. The coil assembly  42  is fixed against rotation with respect to the differential case  10 , but it can translate axially toward and away from the differential case. Axial translation of the coil assembly  42  is transmitted to a sliding collar  54 , which is secured to the coil assembly  42  by a press fit and an overlapping rim  58 . A bushing  60 , which is press fit onto the inside diameter of the sliding collar  54 , allows rotation of the case with respect to the sliding collar  54  and coil assembly  42 . The bushing  60  also provides a linear guide for the sliding collar  60  and coil assembly  42 , allowing them to translate axially. 
   When the coil  44  is energized, the sliding collar  54  applies an axial force directed rightward to a roller thrust bearing  62  and thrust plate or thrust washer  64 . Bearing  62  and thrust plate  64  are located in an annular recess formed in the case. Thrust plate  64  applies axial force to the lock ring  26  through the posts  28  on the locking ring.  FIGS. 7 and 8  show that the thrust plate  64  is annular. The posts  28  extend through the axial holes in web  30 , causing the locking ring  26  to rotate with the case  10  and allowing the locking ring to move axially relative to the case. The post surfaces  70  are located at the left side of the web  30  adjacent the thrust plate lugs  68 .  FIG. 9  illustrates this alTangement in greater detail. 
   The locking ring  26  moves into mechanical engagement with the side gear  22  to prevent rotation of the side gear. Springs  80  and  82  are located adjacent to the locking ring  26  and are arranged in series such that spring  80  contacts and applies resilient force to the locking ring, and spring  82  is secured to the case  10  by a snap ring  84  and applies resilient force to spring  80 . Preferably springs  80 ,  82  are wave springs having corrugations directed radially from axis  12  to their radial outer peripheries, the corrugations being formed with alternating radial ridges and grooves. The springs  80 ,  82  are separated by a flat plate  86 , located axially between the springs, such that the ridges of each spring corrugation contact the plate, thereby preventing mutual contact of the springs. The springs continually apply resilient axial force directed leftward to the locking ring  26  to oppose movement of the locking ring toward the locked position with the side gear  22  in response to the magnetic force produced by the coil  44 . When the coil culTent is removed, the springs  80 ,  82  return the locking ring  26  to the disengaged position. The force applied by the springs is sufficient to prevent inadvertent locking of the differential during normal driving conditions when the coil is deenergized. Furthermore, spring  80  has a much lower spring rate than that of spring  82 , such that a nonlinear spring force curve is generated, as shown in  FIG. 10 . The spring arrangement ensures that the spring force is always lower than the force applied to the locking ring  26  by coil assembly  42  when the coil  44  is energized. Since the force produced by the coil assembly  42  when coil  44  is energized is nonlinear, springs  80 ,  82  are selected so that the magnitude of the spring force applied to the locking ring  76  is less than the force applied by the coil assembly when energized. 
     FIGS. 1 ,  5  and  9  show that a beveled surface  90  is formed near the outer diameter of the coil assembly  42 , and parallel beveled surface  92  is formed on the differential case  10  adjacent the beveled surface on the coil assembly. When the coil is energized, there must be clearance between the coil and the differential case  10  so that the coil does not contact the rotating differential case. This clearance is established by the measurement between the differential case web and the thrust plate face in the engaged state. This clearance must be less than the clearance between the coil  42  and differential case  10  in the disengaged state. 
     FIG. 11  illustrates beveled surfaces  90 ,  92 , a clearance gap B normal to the beveled surfaces, a clearance gap A parallel to the longitudinal axis  12 , and angle b. Gap A varies linearly with axial movement of coil  42 , but gap B varies as the product of gap A and cosine b. The beveled surfaces  90 ,  92  permit gap B initially to be smaller than gap A, and gap B decreases more slowly than gap A after the coil is energized and the coil assembly  42  moves axially in response to the energizing electric culTent applied to the coil. The force produced by energizing the coil is a function of the air gap between the coil and the differential case. 
     FIG. 12  shows the variation of the air gap with coil displacement. 
   In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.