Electronic locking differential

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.

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.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIGS. 1 and 2, a differential mechanism10includes a differential case11, preferably of cast iron or steel, supported on a stationary housing (not shown) for rotation about a lateral axis12. The case11is driveably connected through a bevel ring gear (not shown) to the output of a transmission or transfer case. The ring gear, secured to the case11at the attachment bolt holes on a flange13, is supported for rotation about axis12.

The case11provides an internal chamber14, which contains bevel pinions16,17. Chamber14contains a right-side bevel gear18meshing with the pinions16,17, driveably connected to an output shaft and secured by a spline to side gear18, which extends laterally at the right-hand side from the case11to a driven wheel of a motor vehicle. Chamber14contains a left-side bevel gear20meshing with the pinions16,17, driveably connected to a second output shaft and secured by a spline to side gear20, which extends laterally from the case11at the left-hand side to a driven wheel of the motor vehicle. A spindle22, is secured by a pin24to the rotating case11, supports the pinions16,17for rotation about the axis of spindle22perpendicular to axis12. The pinions16,17revolve about axis12.

Also located in case11is a lock ring26, which rotates with the case11about axis12due to contact with a differential case end cap27.FIGS. 3 and 4show that lock ring26is formed with angularly spaced arms28, each arm extending radially from axis12and extending circumferentially between angularly spaced posts30, formed on an inner surface of the end cap27. Case11is secured to the end cap27at attachment holes aligned with those on case flange13. Contact between the arm28and the posts30limits or prevents rotation of the lock ring26relative to the case11and end cap27. The axial inner or inboard surface of lock ring26is formed with a series of angularly spaced clutch recesses32, which are adjacent and face the axial outer or outboard surface of the side gear20.

The axial outer surface of side gear20is formed with a series of clutch teeth38angularly spaced about axis12, facing and adjacent the clutch recesses32of the lock ring26. The clutch teeth38of side gear20and the clutch recesses32of lock ring26are mutually complementary such that they can engage and disengage as the lock ring moves toward and away from the side gear.

The lock ring26is normally not engaged with the side gear20, permitting the side gear to rotate with respect to the differential case11and the lock ring, thereby producing an unlocked or disengaged state. When the coil44is energized with electric current it moves along axis12toward the case11, actuating lock ring26to engage the side gear20, and causing the clutch teeth38and recesses32mesh or engage mutually, thereby rotatably connecting the side gear to the lock ring and case11, preventing the side gear from rotating relative to the case and lock ring, and placing differential10in a locked or engaged state. When coil44is deenergized, the compression force of an annular Belleville spring40, located between the case11and lock ring26, forces the lock ring axially away from the side gear20, thereby returning the differential10to the unlocked or disengaged state.

FIGS. 1 and 2show a coil assembly42supported on the case11outside chamber14. The coil assembly42includes an electromagnetic coil44, fitted into an annular recess formed in a ring48, and a non-magnetic collar54press fitted into ring48. The coil44produces a magnetic field when energized with electric current. The magnetic field produces an axial force on the coil assembly42, whose magnitude varies with the width of an air gap between the coil assembly and the end cap27.

In operation when the coil44is energized, it is attracted to the differential end cap27due to the magnetic field generated by the coil. The coil assembly42is fixed against rotation with respect to the differential case11, but it can translate axially toward and away from the differential case. Axial displacement of the coil assembly42is transmitted to a collar54, which is secured to the end cap27by a snap ring58. Collar54allows rotation of the differential10with respect to the assembly42and provides a linear guide for the coil assembly42to translate axially.

When the coil44is energized, the sliding collar54applies an axial force directed rightward to a roller thrust bearing62and thrust plate or thrust washer64. Bearing62and thrust plate64are located in an annular recess formed in the end cap27. When coil44is energized, thrust plate64applies axial force to three angularly spaced balls66, each ball retained in a hole formed in the end cap27. AsFIGS. 3 and 4show, three angularly spaced levers68are pinned to lugs70formed on the end cap27, each lever located at the angular position of a ball66.

The mechanism comprising the balls66and lever68is located axially between the lock ring26and the case11. The levers68are actuated by the energized coil assembly42moving axially toward case11forcing thrust plate64against the balls66, causing the levers68to pivot about pivot axes72. The outboard end of each lever68contacts lock ring26as the lever pivots, thereby moving the lock ring clutch recesses32into engagement with clutch teeth38of the side gear20. The lock ring26moves into mechanical engagement with the side gear20to prevent rotation of the side gear relative to the case11.

Each ball66is located at a distance D1from the lever's pivot axis72. The lock ring26is moved due to contact with the end of the levers68, which end is located at a distance D2from the lever rotation axis72. Axial displacement of the coil assembly42due to energizing coil44is amplified at the locking ring28by the ratio D2/D1. For example, with an initial coil air gap of 1.0 mm and a final air gap of 0.5 mm when the differential10is fully locked, the coil44moves through a distance of 0.5 mm. Using a ball and lever D2/D1ratio of 2.3, the lock ring moves through a distance of 1.15 mm.

FIG. 5are 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 differential10unlocked when the coil44is deenergized.

FIG. 6shows the components of the mechanism for actuating lock ring26at a position when coil44is initially energized. Lever68contacts lock ring26at point A, which is closer to pivot point72than the point of contact between ball66and lever68at b point B. Therefore, the force applied to lock ring26by lever66at A is greater than force F1, which is applied to lever66at B by ball66. This arrangement actuates lock ring26with a greater force than the force that is applied to the ball66due to energizing coil44.

FIG. 7shows the components of the mechanism in an intermediate position later than that ofFIG. 6, wherein lever68contacts lock ring26at contact points A and B. In their positions inFIG. 7, coil44, ball66and lever68are in motion. Force applied to lock ring26by lever66is being transferred from point A to point C as the lever pivots about its pivot axis72.

A cam profile surface can be formed between contact points A and C on the upper surface of lever26or on the lower surface of lock ring26. The surface profile would match the coil force curve ofFIG. 5and the force-displacement relations of the return spring40and provide optimal displacement, engagement time and engagement force of lock ring26and electric current draw of the coil44.

FIG. 8shows the components of the mechanism in a final locked position later than that ofFIG. 7, wherein lever68contacts lock ring at contact point C. Axial displacement of lock ring26is greater the axial displacement of coil44because the lock ring contact point C is further from pivot axis72than ball contact point B.