FR2784163A1 - Limited slip differential for model motor vehicle has transmission ball bearings also acting as centrifugal control weights - Google Patents

Abstract

The limited slip differential for a model car has an input and two outputs with brakes to selectively act on them. Centrifugal controls (5,14) partially release the brakes when the rotational speed of the differential increases. The differential can have transmission balls acting also as centrifugal weights.

Description

LIMITED SLIP DIFFERENTIAL

  The present invention relates to a limited slip differential. We have known for a long time the motor problems linked to the classic differential. Indeed, a conventional differential which is located between two wheels of the same axle, by definition equitably transmits the input torque to the two output shafts. The problem is that when one of the outputs is deprived of resistant torque resulting from a loss of grip on its corresponding wheel, the other output can only transmit the equivalent torque. In the extreme case, if one of the two driving wheels of a vehicle is raised from the ground, it becomes impossible to transmit any torque on the axle

  when one of the two wheels is in contact with the ground.

  This is why we invented limited slip differentials that can transmit torque unevenly to two wheels on the same axle. These differentials generally operate according to a traditional gear train to which a friction device is added which acts when differential speeds appear by promoting the

  slowest exit, namely the one that does not skate.

  The present invention aims to provide a limited slip differential which is economical and space-saving for mass market applications, in particular in the field

scale models.

  To this end, the invention relates to a limited slip differential, of the type comprising three connecting members including an input member and two output members, and braking means for selectively connecting, at least with a certain slip , two of said three connecting members, characterized in that it comprises centrifugal means for at least partially releasing said braking means

  when the speed of rotation of the differential increases.

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  Thus, we will get transfer rates from one output to another increasingly lower as the speed increases. The braking means can be arranged to provide braking forces having at least one component perpendicular to the axes of the braking means. The braking surfaces can in this case be cylindrical or conical,

  coaxial with the differential output devices.

  Such an arrangement is particularly well suited to the invention, since the centrifugal forces can then directly oppose the braking means. However, it is possible to envisage other types of braking surfaces, in particular transverse to the axis, for example discs, subject to

  provide suitable means of effort referral.

  It will be observed that an arrangement of the braking means such that the latter provide braking forces having at least one component perpendicular to the axes of the differential output members, could be envisaged independently of the object of the present invention, such an arrangement may in itself present certain advantageous characteristics. In a particular embodiment, the differential according to the invention is a ball differential comprising two coaxial transverse plates between which are arranged transmission balls, cam surfaces being provided to bring said balls closer to the axis of rotation and the press towards braking devices when the differential is under torque. Ball differentials are known in themselves. Here, the balls are also used to press organs of

  braking towards the axis of rotation under the action of cam surfaces.

  The centrifugal force exerted on the balls and on the organs of

  braking therefore tends to oppose braking.

  More particularly, elastic means can be arranged to ensure an axial preload of the balls between

said plates.

  In another particular embodiment, the differential according to the invention is a train differential

  gears, and in particular bevel gears.

  In the latter case, the differential according to the invention may include axes of axially movable satellites under the action of centrifugal force, said axes being arranged to cooperate with cam surfaces to press a braking member axially inward. against surfaces of

  braking provided on said connecting means.

  Said braking member may be constituted by the

  radially inner ends of said satellite axes.

  In another embodiment, said braking member comprises an elastic ring disposed around said

braking surfaces.

  Also in a particular embodiment, means can be provided to limit the angular movement between the input member and the differential train. This ensures operation of the device even when clearances, due for example to the wear of the braking means,

  prevent correct operation of the braking means.

  We will now describe, by way of nonlimiting example, two particular embodiments of the invention, with reference to the appended schematic drawings in which: - Figure 1 is an axial sectional view of a differential according to a first embodiment production; - Figure 2 is a cross-sectional view along the line A-A of Figure 1 in the rest position; - Figure 3 is a view similar to Figure 2 with the differential in the torque operating position; - Figure 4 is an enlarged view of part of Figure 3; Figure 5 is an exploded perspective view of the differential of Figures 1 to 4 - Figure 6 is an axial sectional view along line C-C of Figure 7, of a differential according to a second embodiment; - Figure 7 is a cross-sectional view along line D-D of Figure 6, in the rest position; - Figure 8 is a view similar to Figure 7 with the differential in the torque operating position; Figure 9 is an enlarged view of part of Figure 8 - Figure 10 is an enlarged view of part of Figure 6; and - Figure 11 is an exploded perspective view of the

differential of Figures 6 to 10.

  The assembly of the limited slip differential of FIGS. 1 to 5, of ball type usable for example for reduced models, is maintained by bearings 1 and 2 which are housed

  in a casing 3 closing the assembly.

  The drive torque coming more or less directly from the motor is transmitted in this particular embodiment by means of a not shown toothed belt which meshes with the toothed crown 4 provided with teeth 4 '. The ring gear 4 rotates along the XY axis of the differential, a set of 8 balls 5a, 5b, 5c, 5d, 5e, 5f, 5h and i by means of recesses 6a, 6b, 6c, 6d, 6e, 6f, 6h and 6i. The eight balls 5a to 5i are in contact with the left output universal joint 7 and right output universal joint 8 of the differential, by means of annular plates 7 ′ and 8 ′ respectively secured to these outputs. The set of balls 5, the left outlet 7 and the right outlet 8 are put under axial stress by an assembly composed of Belleville washers 9a, 9b, 9c and 9d forming springs, washers 11a and 11b, of a ball bearing. 12 and a circlip 13, put in compression by an axial screw 10. Four braking segments 14a, 14b, 14c and 14d are arranged axially between the two plates 7 'and 8', and radially between on the one hand the balls 5a to 5i and on the other hand two cylindrical shoulders 17 and 18 secured respectively to the plates 8 'and 7' and provided with cylindrical braking surfaces 17 'and 18'

coaxial at outputs 7 and 8.

  In this embodiment, the differential function is obtained in a conventional manner by the rotation of the balls 5a to 5i respectively along the axes 15a to 15i, the transmission of

  torque between the different elements by adhesion.

  When the differential transmits a positive torque, the balls 5a to i come into contact with slopes 16a to 16i inclined with respect to the axis XY, formed on the radially internal surface of

  the crown 4, opposite the teeth 4 '.

  The radial components Fpa to Fpi of the resultants Fra to Fri of these contacts respectively press the balls 5a to 5i against the segments 14a to 14d. The segments 14a to 14d are therefore pressed in proportion to the input torque by the forces Fpa to Fpi against the shoulders 17 and 18 thus ensuring a more or less important connection between the two outputs 7 and 8. When a negative torque is applied to the differential, the balls 5a to 5i respectively come into contact with the slopes 19a to 19i. The radial components of these contacts press the balls 5a to 5i respectively proportionally against the segments 14a and 14d. The segments 14a and 14d are likewise pressed in proportion to the input torque against the shoulders 17 and 18 ensuring a more or less connection

  less important between the two outputs 7 and 8.

  It will be noted that the angle of the slopes 16a to 16i and 19a to 19i is involved in the operation of the differential and in particular in the transfer rate. Similarly, it will be noted that the friction diameter of the two shoulders 17 and 18 will be involved in the calculation of the transfer rate in the same way as the coefficient of friction obtained between the shoulders 17 and 18

  on the one hand and the segments 14a to 14d on the other hand.

  For the explanation of the creation of Fpa forces at Fpi, we will limit ourselves to the study of the Fpa force. When applying a positive torque to the differential, the ball 5a comes into contact with the slope 16a. This contact creates a Fra force that decomposes

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  in two forces Fea and Fpa. The force Fpa is applied in a direction passing through the point of contact between the ball 5a and the slope 16a on the one hand and the center of the differential on the other hand. We can apply the same reasoning for all the forces Fpb to Fpi as well as for the application of negative couples. The direction of the pressing efforts Fpa to Fpi oppose these efforts to the centrifugal force to which each of the balls is subjected and in this embodiment each of the segments 14a to 14d. By this opposition, the forces due to the centrifugal force more or less partially oppose the pressing forces and thus tend to decrease the transfer rate in a manner proportional to the square of the speed of rotation of the differential and, consequently, to the square of vehicle speed. It will be recalled that the high transfer rates are useful

  at low speed when the torque at the wheels is high.

  Conversely, it will be recalled that the transfer rate can be

  weak or even zero for high speeds.

  The second embodiment will now be described with reference to FIGS. 6 to 11, which represents a differential at

bevel gears according to the invention.

  The differential assembly rotates on two bearings 30 and 31 resting in a casing 33. A housing 34 associated with a cover 35 contains the mechanism of the differential. The housing 34, the cover 35 and the ring gear 38 are held tight and secured by means of the two assembly screws 36 and 37. The ring gear 38 is capable of transmitting the torque

  drive coming more or less directly from the engine.

  An entry plate 39 is held laterally in

  the housing 34 and the cover 35 by means of spacers 40 and 41.

  The input plate 39 is rotated by the two screws 36 and 37 engaged in two notches 42 and 43 which are formed in the input plate 39, and in two angular recesses 42 'and 43' which are formed in a planet carrier 39 '. The recesses 42 'and 43' allow an angular offset in one direction or the other between the input plate 39 and the planet carrier 39 'when the differential is under

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  couple. In addition, their end stops allow degraded operation of the differential, without limiting the

  slip, when the braking surfaces are worn.

  The assembly of the differential gear train, composed of the two planets 44 and 45 and the four satellites 46, 47, 48, 49, is driven by the axes of satellites 50, 51, 52 and 53 carried by the satellite carriers 39 '. These satellite axes are respectively in contact, when a positive torque is applied to the entire differential, with slopes 54, 55, 56 and 57 inclined relative to the axis of exit of the device and formed on the radially inner surface of the input plate 39. The axes of the satellites are respectively in contact with the slopes 58, 59, 60 and 61 during an application

  of a negative torque on the entire differential.

  Output shafts 62 and 63 are linked in rotation to the planets 44 and 45 respectively by means of grooves 64 and 65. The output shafts 62 and 63 have shoulders 66 and 67, which are conical in this embodiment, being made facing the center of the differential. A connecting ring 68 made of an elastic material takes place between the two

  conical shoulders 66 and 67 of shafts 62 and 63.

  In this embodiment, the differential function is conventionally obtained by the gear train composed of the two planets 44 and 45 and the four satellites 46, 47, 48

and 49.

  When the differential transmits a positive torque, the axes, 51, 52 and 53 come into contact with the slopes 58, 59, 60 and 61. The forces resulting from these contacts Fpk, Fpl, Fpm and Fpn respectively support the axes of satellites 50, 51, 52 and 53 against the elastic ring 68. The elastic ring 68 is thus pressed in a manner proportional to the input torque against the two conical shoulders 66 and 67 thus ensuring a more or less important connection between the two shafts output 62 and 63. On the other hand, the contact between the elastic ring 68 and the two output shafts 62 and 63 taking place on conical surfaces, lateral forces Fpkg, Fpkd, Fplg will be obtained,

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  Fpld, Fpmg, Fpmd, Fpng and Fpnd. The lateral forces Fpkg Fpkd, Fplg, Fpld, Fpmg, Fpmd, Fpng and Fpnd are

  respectively proportional to the forces Fpk, Fpl, Fpm and Fpn.

  The lateral forces Fpkg, Fplg, Fpmg and Fpng press in proportion to the input torque the output shaft 62 and the sun gear 44 which is associated with it against the cover 35. The lateral forces Fpkd, Fpld, Fpmd and Fpnd support proportional to the input torque the output shaft 63 and the sun gear 45 associated therewith against the housing 34. As a result, the lateral forces Fpkg Fpkd, Fplg, Fpld, Fpmg, Fpmd, Fpng and Fpnd combined with the coefficients friction between the sun gear 44 and the cover 35 on the one hand, and between the sun gear and the housing 34 on the other hand, create a more or less connection

  important between the two output shafts 62 and 63.

  Similarly, when the differential transmits a negative torque, the axes 50, 51, 52 and 53 come into contact with the slopes 54, 55, 56 and 57. The forces resulting from these contacts Fpk ', Fpl', Fpm 'and Fpn 'respectively support the axes of satellites 50, 51, 52 and 53 against the elastic ring 68. The elastic ring 68 is thus pressed in a manner proportional to the input torque against the two conical shoulders 66 and 67 thus ensuring a more or less between the two trees

62 and 63.

  On the other hand, the contact between the elastic ring 68 and the two output shafts 62 and 63 taking place on conical surfaces, lateral forces Fpk'g, Fpk'd, Fpl'g, Fpl'd will be obtained, Fpm'g, Fpm'd, Fpn'g and Fpn'd. The lateral forces Fpk'g Fpk'd, Fpl'g, Fpl'd, Fpm'g, Fpm'd, Fpn'g and Fpn'd are respectively proportional to the forces Fpk ', Fpl', Fpm 'and Fpn'. The lateral forces Fpk'g, Fpl'g, Fpm'g and Fpn'g press in proportion to the input torque the output shaft 62 and the sun gear 44 which is associated therewith against the cover 35. The lateral forces Fpk 'd, Fpl'd, Fpm'd and Fpn'd press proportional to the input torque the output shaft 63 and the sun gear 45 which is associated with it against the housing 34. As a result, the lateral forces Fpk 'g Fpk'd, Fpl'g, Fpl'd, Fpm'g, Fpm'd, Fpn'g and Fpn'd combined with the coefficients of friction which prevail between the sun gear 44 and the cover 35 on the one hand, and between the sun gear 45 and the housing 34 on the other hand create a more

  or less between the two output shafts 62 and 63.

  It will be noted that the angles of the slopes 54 to 57 and 58 to 61 are involved in the operation of the differential and in particular in the transfer rate. It will also be noted that the friction diameter of the shoulders 66 and 67 will be involved in the calculation of the transfer rate in the same way as the friction coefficient obtained between the conical shoulders 66 and 67 and the elastic ring 68. It will further be noted that the slope of the conical shoulders will be used in the calculation of the transfer rate. Finally, it will be noted that the radii and the coefficients of friction which appear between the sun gear 44 and the cover 35 on the one hand, and between the sun gear 45 and the housing 34 on the other hand,

  will intervene in the calculation of the transfer rate.

  For the explanation of the creation of the forces Fpk at Fpn and Fpk 'and Fpn', we will limit ourselves to the study of the force Fpk. When applying a positive torque to the differential, the satellite axis 50 is in contact with the slope 58. This contact creates a resultant force Frk which breaks down into two forces Fea and Fpk. The force Fpk is applied in a direction passing through the point of contact between the satellite axis 50 and the slope 58 on the one hand and the center of the differential on the other hand. We can apply the same reasoning for all the forces Fpl to Fpn and Fpk '

at Fpn '.

  The pressing forces Fpk to Fpn and Fpk 'to Fpn' are opposed to the centrifugal force to which the satellite axes 50 to 53 and the elastic ring 68 are subjected. By this opposition, the forces due to the centrifugal force are opposed more or less partially to the pressing efforts and thus tend to decrease the transfer rate in a manner proportional to the square of the speed of rotation of the differential, and consequently to the square of the

vehicle speed.

  Furthermore, the pressing efforts will also have to overcome the elasticity of the elastic ring 68 before it is in contact with the conical shoulders 66 and 67, which means that the differential will remain completely free up to a

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  some input torque sufficient to create pressing efforts

  greater than the elastic force of the ring 68.

  It will be noted that the lateral forces Fpkg Fpkd, Fplg, Fpld, Fpmg, Fpmd, Fpng and Fpnd are respectively proportional to the forces Fpk, Fpl, Fpm and Fpn. For the demonstration of the creation of efforts Fpkg to Fpng and Fpkd to Fpnd and Fpk'g to Fpn'g and Fpk'd to Fpn'd, we will limit ourselves to the explanation of the efforts Fpkg and Fpkd. For simplicity, it will be considered that the torque is applied to the differential at the instant of starting, that is to say for a rotation speed equal to zero. In this situation, no part is therefore subject to the effects of centrifugal force. We will not return to the creation of the Fpk force created by the setting

under differential torque.

  The force Fpk is opposite to the elastic force Fr68K of the elastic ring 68. In this example, we will assume the input torque applied to the input of the differential sufficient for

  create an effort Fpk greater than the effort Fr68k. The resulting Fpk-

  Fr68k is distributed uniformly over the two conical shoulders 66 and 67, giving rise to the two forces Fpckg and Fpckd normal to the contact surfaces. The two forces Fpckg and Fpckd break down respectively into two forces Fpkzg and Fpkg on the one hand and Fpkzd and Fpkd on the other hand. The forces Fpkg and Fpkd respectively press the planetary 44 and shaft 62 assemblies against the cover 35 on the one hand, and the planetary 45 and the shaft 63 against the housing 34 on the other hand, which

  causes a more or less partial bond between these sub-

  together. As a variant, the elastic ring 68 could be omitted, the axes 50 to 53 directly ensuring braking on the conical surfaces 67 and 67. Of course, such a differential would no longer benefit from the threshold effect linked to the elasticity of the ring 68. Two embodiments of the invention have been described above, respectively integrated into a ball differential and a bevel gear differential. Other embodiments could be envisaged in other types of differentials, it for example in differentials with planetary gear. Arrangements similar to those described above could

  be used in such a differential.

  Moreover, the various arrangements described with reference to one or the other of the two embodiments shown in the drawings could be applied to one or the other of the different cases. Thus, for example, the braking segments of the first embodiment could be replaced by a member of the type of the elastic ring of the second embodiment. Similarly, weights independent of the balls, of the type of the axes of satellites of the second embodiment could be added to the first embodiment. Likewise also, a preload could be envisaged in the second embodiment, while the elastic ring

provides the opposite result.

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Claims (5)

  1 - Limited slip differential, of the type comprising three connecting members including an inlet member (4; 38) and two outlet members (7, 8; 62, 63) and means (7 ', 8', 14 ; 68, 66, 67) for braking to selectively connect, at least with a certain slip, two of said three connecting members, characterized in that it comprises centrifugal means (5, 14; 50-53, 68) for at least partially releasing said braking means when the speed of rotation of the differential increases. 2 - Limited slip differential according to claim 1, wherein the braking means are arranged to provide braking forces having at least one component   perpendicular to the axes of the braking means.   3 - Limited slip differential according to any one   of claims 1 and 2, comprising two plates   transverse coaxial (7 ', 8') between which are arranged   transmission balls (5), cam surfaces (16a-16i, 19a-   19i) being provided for bringing said balls closer to the axis of rotation and pressing them towards braking members (14a-14d)   when the differential is under torque.   4 - Limited slip differential with bevel gears according to claim 1, comprising axes (50-53) of satellites (46-49) axially movable under the action of centrifugal force, said axes being arranged to cooperate with surfaces of cams (54-57, 58-61) for pressing a braking member (68) axially inwards against braking surfaces (66, 67) formed on said connecting means (62.63).   - Limited slip differential according to claim 4, wherein said braking member is constituted by the radially inner ends of said axes (50-53) of satellites (46-49). -13 - 2784163   6 - Limited slip differential according to any one   claims 4 and 5, wherein said braking member   comprises an elastic ring (68) disposed around said braking surfaces (66, 67).   7 - Limited slip differential according to any one   of claims 1 to 6, in which means (42 ', 43') are   designed to limit the angular movement between the member   input and differential gear.