Abstract:
A differential mechanism includes a case, a gear rotatable about an axis, a lock ring held against rotation relative to the case, a lever contacting the lock ring, and an electromagnetic coil that is displaced axially when energized, pivoting the lever, engaging the lock ring with the side gear, and preventing the gear from rotating relative to the case.

Description:
BACKGROUND OF THE INVENTION 
     1. Field 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. 
     2. Description of the Prior Art 
     A locking differential is used 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 a wheel speed differential on any one axle. 
     A locking differential employs hydraulic pressure or an electromagnet to actuate a mechanism that alternately holds a side gear against rotation and releases the side gear to rotate freely. Due to packaging constraints, however, certain vehicle applications require a small electromagnetic coil whose size and number of windings may not provide an engagement force of sufficient magnitude to lock the differential. In such instances, a technique is required to amplify the actuating force produce by the coil to a magnitude that is sufficient to produce reliable, axial displacement of the coil. 
     The actuating force produced by the coil varies non-linearly and inversely with air gap. Thus for a given coil size, the initial air gap should be kept as small as possible in order to maximize the force that actuates the differential to the locked condition. 
     A need exists in the industry for a locking differential actuated by a small axially displaceable electromagnetic coil having a minimum air gap such that displacement of the coil is amplified producing greater displacement for a locking mechanism that secures one of the side gears of the differential against rotation on a differential case. 
     SUMMARY OF THE INVENTION 
     A differential mechanism includes a case, a gear rotatable about an axis, a lock ring held against rotation relative to the case, a lever contacting the lock ring, and an electromagnetic coil that is displaced axially when energized, pivoting the lever, engaging the lock ring with the side gear, and preventing the gear from rotating relative to the case. 
     A method for locking a differential includes supporting a gear for rotation, holding a lock ring against rotation, placing a lever in contact with the lock ring, energizing an electromagnetic coil causing the lever to pivot, engaging the lock ring with the side gear, and preventing the side gear from rotating relative to the lock ring. 
     The locking differential employs a relatively small coil having a small copper winding, thereby reducing its weight and cost. 
     The locking differential amplifies displacement of the energized coil, thereby allowing the coil to move a short distance while providing a large movement for the lock ring and ensuring its full engagement with the side gear. 
     Due to the small coil, a small air gap produces an axial force that is able to move the coil to the engaged or locked position, thereby allowing use of a large return spring, which keeps the differential unlocked when the coil is deenergized. 
     The moving coil locking differential operates 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. 
     The scope of applicability of the preferred embodiment 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 will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a cross section taken at a diametric plane through a locking differential mechanism; 
         FIG. 2  is an isometric cross section showing the lock ring, side gear and a lever installed at the inner axial face of the end cap; 
         FIG. 3  is an isometric view showing the inboard side of the end cap with the lock ring and return spring installed; 
         FIG. 4  is an isometric view of the inboard side of the end cap showing the lock ring and return spring in spaced-apart relationship; 
         FIG. 5  are graphs showing the non-linear relation between axial force of the coil and air gap for various magnitudes of electric current applied to the coil; 
         FIG. 6  is a side view showing the lock ring actuating mechanism when the coil is initially energized; 
         FIG. 7  is a side view showing the lock ring actuating mechanism in an intermediate position later than that of  FIG. 6 ; and 
         FIG. 8  is a side view showing the lock ring actuating mechanism in a final locked position. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIGS. 1 and 2 , a differential mechanism  10  includes a differential case  11 , preferably of cast iron or steel, supported on a stationary housing (not shown) for rotation about a lateral axis  12 . The case  11  is driveably connected through a bevel ring gear (not shown) to the output of a transmission or transfer case. The ring gear, secured to the case  11  at the attachment bolt holes on a flange  13 , is supported for rotation about axis  12 . 
     The case  11  provides an internal chamber  14 , which contains bevel pinions  16 ,  17 . Chamber  14  contains a right-side bevel gear  18  meshing with the pinions  16 ,  17 , driveably connected to an output shaft and secured by a spline to side gear  18 , which extends laterally at the right-hand side from the case  11  to a driven wheel of a motor vehicle. Chamber  14  contains a left-side bevel gear  20  meshing with the pinions  16 ,  17 , driveably connected to a second output shaft and secured by a spline to side gear  20 , which extends laterally from the case  11  at the left-hand side to a driven wheel of the motor vehicle. A spindle  22 , is secured by a pin  24  to the rotating case  11 , supports the pinions  16 ,  17  for rotation about the axis of spindle  22  perpendicular to axis  12 . The pinions  16 ,  17  revolve about axis  12 . 
     Also located in case  11  is a lock ring  26 , which rotates with the case  11  about axis  12  due to contact with a differential case end cap  27 .  FIGS. 3 and 4  show that lock ring  26  is formed with angularly spaced arms  28 , each arm extending radially from axis  12  and extending circumferentially between angularly spaced posts  30 , formed on an inner surface of the end cap  27 . Case  11  is secured to the end cap  27  at attachment holes aligned with those on case flange  13 . Contact between the arm  28  and the posts  30  limits or prevents rotation of the lock ring  26  relative to the case  11  and end cap  27 . The axial inner or inboard surface of lock ring  26  is formed with a series of angularly spaced clutch recesses  32 , which are adjacent and face the axial outer or outboard surface of the side gear  20 . 
     The axial outer surface of side gear  20  is formed with a series of clutch teeth  38  angularly spaced about axis  12 , facing and adjacent the clutch recesses  32  of the lock ring  26 . The clutch teeth  38  of side gear  20  and the clutch recesses  32  of lock ring  26  are mutually complementary such that they can engage and disengage as the lock ring moves toward and away from the side gear. 
     The lock ring  26  is normally not engaged with the side gear  20 , permitting the side gear to rotate with respect to the differential case  11  and the lock ring, thereby producing an unlocked or disengaged state. When the coil  44  is energized with electric current it moves along axis  12  toward the case  11 , actuating lock ring  26  to engage the side gear  20 , and causing the clutch teeth  38  and recesses  32  mesh or engage mutually, thereby rotatably connecting the side gear to the lock ring and case  11 , preventing the side gear from rotating relative to the case and lock ring, and placing differential  10  in a locked or engaged state. When coil  44  is deenergized, the compression force of an annular Belleville spring  40 , located between the case  11  and lock ring  26 , forces the lock ring axially away from the side gear  20 , thereby returning the differential  10  to the unlocked or disengaged state. 
       FIGS. 1 and 2  show a coil assembly  42  supported on the case  11  outside chamber  14 . The coil assembly  42  includes an electromagnetic coil  44 , fitted into an annular recess formed in a ring  48 , and a non-magnetic collar  54  press fitted into ring  48 . The coil  44  produces a magnetic field when energized with electric current. The magnetic field produces an axial force on the coil assembly  42 , whose magnitude varies with the width of an air gap between the coil assembly and the end cap  27 . 
     In operation when the coil  44  is energized, it is attracted to the differential end cap  27  due to the magnetic field generated by the coil. The coil assembly  42  is fixed against rotation with respect to the differential case  11 , but it can translate axially toward and away from the differential case. Axial displacement of the coil assembly  42  is transmitted to a collar  54 , which is secured to the end cap  27  by a snap ring  58 . Collar  54  allows rotation of the differential  10  with respect to the assembly  42  and provides a linear guide for the coil assembly  42  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 end cap  27 . When coil  44  is energized, thrust plate  64  applies axial force to three angularly spaced balls  66 , each ball retained in a hole formed in the end cap  27 . As  FIGS. 3 and 4  show, three angularly spaced levers  68  are pinned to lugs  70  formed on the end cap  27 , each lever located at the angular position of a ball  66 . 
     The mechanism comprising the balls  66  and lever  68  is located axially between the lock ring  26  and the case  11 . The levers  68  are actuated by the energized coil assembly  42  moving axially toward case  11  forcing thrust plate  64  against the balls  66 , causing the levers  68  to pivot about pivot axes  72 . The outboard end of each lever  68  contacts lock ring  26  as the lever pivots, thereby moving the lock ring clutch recesses  32  into engagement with clutch teeth  38  of the side gear  20 . The lock ring  26  moves into mechanical engagement with the side gear  20  to prevent rotation of the side gear relative to the case  11 . 
     Each ball  66  is located at a distance D 1  from the lever&#39;s pivot axis  72 . The lock ring  26  is moved due to contact with the end of the levers  68 , which end is located at a distance D 2  from the lever rotation axis  72 . Axial displacement of the coil assembly  42  due to energizing coil  44  is amplified at the locking ring  28  by the ratio D 2 /D 1 . For example, with an initial coil air gap of 1.0 mm and a final air gap of 0.5 mm when the differential  10  is fully locked, the coil  44  moves through a distance of 0.5 mm. Using a ball and lever D 2 /D 1  ratio of 2.3, the lock ring moves through a distance of 1.15 mm. 
       FIG. 5  are graphs showing the non-linear relation between axial force of the coil and air gap for various magnitudes of electric current applied to the coil. For a given coil size it is desirable to keep the initial air gap as small as possible in order to maximize the differential lock force, thereby allowing use of a large return spring, which acts to keep the differential  10  unlocked when the coil  44  is deenergized. 
       FIG. 6  shows the components of the mechanism for actuating lock ring  26  at a position when coil  44  is initially energized. Lever  68  contacts lock ring  26  at point A, which is closer to pivot point  72  than the point of contact between ball  66  and lever  68  at b point B. Therefore, the force applied to lock ring  26  by lever  66  at A is greater than force F 1 , which is applied to lever  66  at B by ball  66 . This arrangement actuates lock ring  26  with a greater force than the force that is applied to the ball  66  due to energizing coil  44 . 
       FIG. 7  shows the components of the mechanism in an intermediate position later than that of  FIG. 6 , wherein lever  68  contacts lock ring  26  at contact points A and B. In their positions in  FIG. 7 , coil  44 , ball  66  and lever  68  are in motion. Force applied to lock ring  26  by lever  66  is being transferred from point A to point C as the lever pivots about its pivot axis  72 . 
     A cam profile surface can be formed between contact points A and C on the upper surface of lever  26  or on the lower surface of lock ring  26 . The surface profile would match the coil force curve of  FIG. 5  and the force-displacement relations of the return spring  40  and provide optimal displacement, engagement time and engagement force of lock ring  26  and electric current draw of the coil  44 . 
       FIG. 8  shows the components of the mechanism in a final locked position later than that of  FIG. 7 , wherein lever  68  contacts lock ring at contact point C. Axial displacement of lock ring  26  is greater the axial displacement of coil  44  because the lock ring contact point C is further from pivot axis  72  than ball contact point B. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.