GB2354563A

GB2354563A - Lockable differential

Info

Publication number: GB2354563A
Application number: GB0020225A
Authority: GB
Inventors: Martin Budaker; Helmut Schwarzkopf; Dirk Graf
Original assignee: ZF Lenksysteme GmbH
Current assignee: Robert Bosch Automotive Steering GmbH
Priority date: 1999-08-20
Filing date: 2000-08-16
Publication date: 2001-03-28
Anticipated expiration: 2020-08-16
Also published as: GB0020225D0; FR2797671A1; GB2354563B; DE19939632A1

Abstract

A lockable differential gear comprises casing 1 having shaft wheels 6,7, each connected to an output shaft 5 and differential bevel pinions 8 in differential housing 3. Set of disks 10 enables locking a shaft wheel to the housing. Locking force is transmitted via a single-stage transmission gearing i.e. a worm gear 15, 17 from an electric motor (13 Fig 1) to the disks. Rotary motion of the worm gearing is converted by tread 18 or ball screw 20 into axial worm wheel motion acting upon the disks through bearing 22, thrust ring 23 and bolt 24. The disks are at the side of the shaft wheel adjacent the thread which becomes the lockable shaft wheel. The locking force transmitted from the thread to the disks is supported at the side of the lockable shaft wheel against the casing cover 2 through bearing 27.

Description

2354563 Lockable Differential Gear The invention relates to lockable

differential gearing for motor vehicles.

In vehicle differential gear bevel wheels, shaft wheels and two output shafts are disposed in a differential casing together with a set of disks, by means of which one of the shaft wheels is lockable relative to the differential casing. The set of disks may be acted upon from the outside by a locking force generated by an electric motor and transmitted via transmission gearing to the set of disks. For transmission of the locking force, a rotational motion delivered by the transrnission gearing is converted by a thread into an axial motion acting upon the set of disks.

The differential gear, often also known as a differential, allows the wheels to rotate at a different angular velocity relative to each other and distributes the driving torque between the individual wheels. This is necessary particularly during cornering when the wheels, in accordance with the distances covered, need to rotate at different speeds.' There are two basic types of differential gear: planetary gear mechanisms and bevel gear mechanisms. In a bevel gear differential, two output shafts are firmly connected to two mutually opposing conical shaft wheels, wherein one or more differential bevel pinions, the axes of which extend at right angles to the shaft axes, engage simultaneously into both shaft wheels. Differential bevel pinions, shaft wheels as well as axle shafts are usually supported in a differential cage or differential casing. When one output shaft rotates in one direction relative to the differential cage, the connection of the output shafts by the differential bevel pinions forces the other output shaft to rotate in the opposite direction relative to the differential cage by the same angular velocity. When the two output shafts are rotating at the same speed, they are stationary in relation to the differential cage. The differential cage is generally connected to a drive gear, e.g. a crown wheel or spur gear. Differential gears may be used for torque balancing between two wheels on one axle or for torque balancing between a plurality of driven axles. The former are known also as transverse differentials whereas the latter are known as longitudinal or central differentials.

lo A differential gear is known from US Patent No. 4,805,486 in which the locking force is generated by an electric motor and transmitted via a twostage spur transmission gearing from the electric motor to the set of disks. The locking force is introduced from outside the differential casing on one side of one of the shaft wheels and is transmitted via annular pressure plates and pistons to the set of disks disposed on the other shaft wheel. The two-stage spur transmission gearing and the transmission of the locking force transversely through the entire differential gear gives rise to unacceptable noise levels due to high friction at many points. The friction is increased by a high bearing load because the bearings are additionally required to take up the locking force. In a two-stage spur transmission gearing, there is an undesirable backlash. The overall result is low efficiency and high hysteresis.

It is an object of the present invention to provide a lockable differential gear whose overall efficiency is improved.

According to the invention there is provided a lockable differential gear for motor vehicles compn'smg:

a) a gearbox having mounted therein two shaft wheels, each connected to an output shaft, and at least two differential bevel pinions disposed in a differential casing; b) a set of disks disposed in the differential casing by means of which 5 one of the shaft wheels is lockable relative to said differential casing; C) an electric motor to generate and apply an external locking force on the set of disks, the locking force being transmitted via transmission gearing from the electric motor to the set of disks; d) the transmission gearing converting by means of a thread, a rotary 10 movement into an axial movement which acts upon the set of disks; e) the transmission gearing comprising a single transmission stage in the form of a worm gear with a worm and a worm wheel, the worm wheel of the worm gear having cylindrical external gearing and being displaceable in its axial direction with the aid of said thread through 15 rotation of the worm, the set of disks being situated on the shaft wheel which is adjacent to said thread and which therefore becomes the lockable shaft wheel, the locking force transmitted from the thread to the set of disks being supported at the side of the differential gear, where the lockable shaft wheel is situated, against 20 the gearbox.

In the preferred differential gear of the invention, the transmission gearing comprises a single transmission stage in the form of a worm gear having a worm and a worm wheel, the worm wheel of the worm gear having cylindrical external gearing. The set of disks is situated at the side of the shaft wheel, which is adjacent to the thread and which therefore becomes the lockable shaft wheel. The locking force transmitted from the thread to the set of disks is supported at the side of the lockable shaft wheel against the gearbox, thereby producing a closed power flow.

The preferred single-stage worm gear reduces the noise, friction and backlash in the transmission gearing. The cylindrical spur gearing of the worm wheel of the worm gear enables easy axial displaceability of the worm wheel. By virtue of the arrangement of the thread and the lockable shaft wheel with the set of disks at one side of the differential gear, a shorter power flow of the locking force in the differential gear is io achieved so friction is reduced. All of these features play a crucial part in increasing the overall efficiency of the differential gear of the invention.

DE-C2-39 20 861 discloses a differential gear in which a locking force is transmitted from an electric motor via a worm gear to the set of disks. However, the differential gear also requires two transmission stages and the power flow of the locking force extends through the entire differential gear. From there, the locking force is supported once more outside the differential casing at the side of the other shaft wheel against the gearbox.

In the preferred differential gear of the present invention, the short power flow of the locking force is achieved particularly easily because the locking force is supported via the lockable shaft wheel, the shaft on which the shaft wheel is mounted, and a thrust bearing against the gearbox.

When the locking force is controlled by the electric motor, it may be sustained by the electric motor. When the locking force is to be sustained for a prolonged period, this may be effected e.g. by a selflocking worm gear or by an electromagnetic brake.

When a vehicle travels over rough terrain, it is advantageous to provide a range in which the differential gear is locked in addition to the controlled locking range. This is possible by means of a self-locking worm or a holding brake. Locking force and/or locking moment are set lo so high that the locking moment clearly exceeds the driving torque. Consequently, the disks may be acted upon by a prestressing force which is substantially higher than the admissible prestressing force for the slip range.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a cross-section through a differential gear of the invention; and Figure 2 is a longitudinal section through the differential gear along the line II-II in Figure 1.

A differential housing 3 is rotatably supported in a differential casing 1 and carries a crown wheel 4 which is drivable by means of a drive shaft (not shown).

Two shaft wheels 6 and 7 are disposed in the differential housing 3 and are each connected to an output shaft 5, as well as at least two differential bevel pinions 8 cooperating with said shaft wheels.

Also disposed in the differential housing 3 is a set of disks 10, by means of which the one shaft wheel 6 may be locked relative to said housing 3. The set of disks 10 comprises inner disks 11 and outer disks 12, which are drivingly connected to the shaft wheel 6 and the differential housing 3 respectively. The set of disks 10 may be acted upon from the outside io by a locking force.

To said end, there is fastened to the differential casing 1 an electric motor 13, shaft 14 of which projects into the gearbox 1 (see Figure 1). The motor shaft 14 carries worm 15 of a worm gear 16 which forms a single-stage transmission gearing. A worm wheel 17 cooperates with the worm 15 and is supported via a thread 18 against the gearbox cover 2. The thread 18 may take the form of a single helical thread or a ball screw 20 (see the bottom half of Figure 2).

The worm wheel 17 has cylindrical external gearing 21 which, depending on the angle of intersection between the axes of the worm 15 and the worm wheel 17, may take the form of spur or helical gearing. The important point is that because of its cylindrical external gearing 21, the worm wheel 17 may be displaced in its axial direction through rotation of the worm 15. In order to sustain the locking force produced by the electric motor 13, the worm gear 16 may be of a self-locking design. To said end, the lead angle of the worm gearing is generally less than approximately 50.

A needle bearing 22 is disposed on the worm wheel 17 in the direction of the shaft wheel 6 and its side facing towards the shaft wheel 6 is supported against a thrust ring 23. At least one boh 24 and at least one compression spring 25 are disposed between the thrust ring 23 and the set of disks 10.

There now follows a description of the mode of operation of the illustrated lockable differential gear. When the electric motor 13 is lo activated in dependence upon at least one vehicle parameter, e.g. wheel speed difference, steering angle, travelling speed, engine speed, transverse acceleration or the like, the motor shaft 14 and the worm 15 are rotated, said rotational movement being transmitted by the worm gear 16 to the worm wheel 17. By means of the thread 18 or the ball screw 20, this rotational movement is converted into an axial motion of the worm wheel 17 which is transmitted via the needle bearing 22, the thrust ring 23 and the boh 24 to the set of disks 10. A locking force therefore acts upon the set of disks 10 and brakes rotation of the shaft wheel 6.

The preferred differential gear just described has a very short power flow due to the locking force produced by the electric motor 13 passing from the worm wheel 17 via the needle bearing 22, the thrust ring 23, the boh 24 and through the set of disks 10 to the lockable shaft wheel 6 and from there via a screw 26, by means of which the shaft wheel 6 is locked in its axial direction on the output shaft 5, as well as via a thrust bearing 27, which is disposed between a collar of the output shaft 5 and the cover 2, to the cover 2 and hence back to the differential casing 1.

Claims

1. A lockable differential gear for motor vehicles comprising:

a) a casing having mounted therein two shaft wheels, each connected to an output shaft, and at least two differential bevel pinions disposed in a differential housing; b) a set of disks disposed in the differential housing by means of which one of the shaft wheels is lockable relative to said casing; C) an electric motor to generate and apply an external locking force on the set of disks, the locking force being transmitted via transmission gearing from the electric mo tor to the set of disks; d) the transmission gearing converting by means of a thread, a rotary movement into an axial movement which acts upon the set of disks; e) the transmission gearing comprising a single transmission stage in the form of a worm gear with a worm and a worm wheel, the worm wheel of the worm gear having cylindrical external gearing and being displaceable in its axial direction with the aid of said thread through rotation of the worm, the set of disks being situated on the shaft wheel which is adjacent to said thread and which therefore becomes the lockable shaft wheel, the locking force transmitted from the thread to the set of disks being supported at the side of the differential gear, where the lockable shaft wheel is situated, against the casing.

2. A lockable differential gear according to claim 1 wherein the locking force is supported against the casing by means of the lockable shaft wheel, the output shaft on which the shaft wheel is mounted and a thrust bearing.

3. A lockable differential gear according to claim 1 or 2 wherein the 5 locking force is controllable by the electric motor.

4. A lockable differential gear according to claim 3 wherein the locking force is controllable by the electric motor and the worm gear is of a self-locking design so that the locking force may be sustained by the lo worm gear.

5. A lockable differential gear according to claim 3 wherein the locking force is controllable by the electric motor and sustained by an electromagnetic brake.

6. A lockable differential gear according to any preceding claim wherein two ranges are provided for locking the differential in the locking force in the first range being controllable by the electric motor and the differential gear being locked in the second range.

7. A lockable differential gear substantially as herein described with reference to the accompanying drawings.