GB2377740A

GB2377740A - Electric motor actuator having planetary reduction gear

Info

Publication number: GB2377740A
Application number: GB0117564A
Authority: GB
Inventors: Viggo Norum; Gunter Hirt; Geir Huseby; Jan Oyen; Simen Ronne; Lars Lein
Original assignee: LuK Lamellen und Kupplungsbau Beteiligungs KG; LuK Lamellen und Kupplungsbau GmbH
Current assignee: Schaeffler Buehl Verwaltungs GmbH; LuK Lamellen und Kupplungsbau GmbH
Priority date: 2001-07-19
Filing date: 2001-07-19
Publication date: 2003-01-22
Also published as: GB0117564D0

Abstract

An electric motor actuator for control of a clutch or gear selector mechanism or brake of a motor vehicle; including an electric motor (10), an output shaft (18) of the electric motor (10), and an epicyclic drive mechanism comprising; a sun gear (20) secured to the output shaft (18) for rotation therewith, an annular planet gear carrier (22) mounted for rotation coaxially of the output shaft (18), a series of planet gears (24) mounted in angularly spaced relationship symmetrically of the planet carrier (22), the planet gears (24) meshing with first and second internal gear rings (30,34), one of the internal gear rings (30) being fixed and the other internal gear ring (34) being formed on an output member (36), the output member (36) being mounted for rotation coaxially of the output shaft (18), the number of teeth on the first internal gear ring (30,34) being different to the number of teeth on the second internal gear ring (34,30), in order to provide the required drive ratio.

Description

Clutch, Brake and Gear Actuators The present invention relates to clutch, brake and gear actuators and in particular electric motor actuators for control of a clutch, or a gear selector mechanism of an automated transmission systems or the brakes of a motor vehicle.

Electric motor actuators which are used to control the clutches and gear shifter mechanisms of automated transmission systems typically utilise a worm and worm gear drive mechanism to provide a high ratio reduction, for example as disclosed in GB2325036, GB2313885 and GB2309761 the disclosure of which explicit reference is made and whose content is expressly incorporated in the disclosure content of the present application, in order to convert the high speed, relatively low torque drive of the electric motor into a slow speed relatively high torque required to actuate a clutch or gear shifter mechanism. Typically the drive ratios of such mechanisms are of the order of 40: 1 to 60: 1. The requirements for an electric motor brake actuator are similar to those for clutch and gear actuators.

The worm gear drives utilised hitherto have the disadvantage that they are relatively large and present serious difficulties with regard to the packaging constraints encountered in automated transmission systems for motor vehicles.

The present invention utilises a high ratio drive of concentric construction to overcome the disadvantages of the worm drive used hitherto.

In accordance with one aspect of the present invention an electric motor actuator for control of a clutch or gear selector mechanism or brake of a motor vehicle ; includes an electric motor, an output shaft of the electric

motor, and an epicyclic drive mechanism comprising; a sun gear secured to the output shaft of the electric motor for rotation therewith, an annular planet gear carrier mounted for rotation coaxially of the output shaft of the electric motor, a series of planet gears mounted in angularly spaced relationship symmetrically of the planet carrier, the planet gears meshing with first and second internal gear rings, one of the internal gear rings being fixed and the other internal gear ring being formed on an output member, the output member being mounted for rotation coaxially of the output shaft, the number of teeth on the first internal gear ring being different to the number of teeth on the second internal gear ring, in order to provide the required drive ratio.

According to a preferred embodiment of the present invention, each planet gear is in the form of a double pinion, the planet gear defining a primary pinion which meshes with the sun gear and the first internal gear ring, and a secondary pinion which meshes with the second internal gear ring. In one version of this embodiment, the primary pinion meshes with the internal gear ring formed on the output member and the secondary pinion

meshes with the fixed internal gear ring. With the drive mechanism described above :-

.... 1 + N3N5 the drive ratio i =- 1 - N1N4/N2N3

where N1 = the number of teeth on the fixed internal gear ring; N2 = the number of teeth on the second internal gear ring; N3 = the number of teeth on the secondary pinion of the planet gear;

N4 = the number of teeth on the primary pinion of the planet gear; and N5 = the number of teeth on the sun gear; and :-

N1 = N3 + N4 + N5 ; and N2 = 2N4 + N5.

The values of N3, N4 and N5 must consequently be selected to provide the required drive ratio which will typically be from 40: 1 to 60: 1. With the sun gear and primary and secondary pinions of the planet gears being of substantially the same size, a difference in the number of teeth of 10% between N3 and N4 will give a drive ratio of the order of 50: 1.

In accordance with an alternative version of this embodiment the primary pinion meshes with the fixed internal gear ring and the secondary pinion meshes with the internal gear ring formed on the output member. With this drive mechanism :-

the drive ratio i = 1 + N1/Ns 1 - N1N4/N2N3 and ; N1 = 2N3 + Ns ; and N2 = N3 + N4 + N5.

Again the values N3, N4 and N5 must consequently be selected to provide the required drive ratio which will typically be from 40: 1 to 60: 1.

According to a further embodiment of the present invention, the planet gears are single pinions which are mounted on axes skewed to the axes of

the first and second internal gear rings, so that the planet gears mesh, at one end, with the internal gear ring having fewer teeth, on a smaller diameter; and, at the other end, with the internal gear ring having more teeth, on a larger diameter.

With this embodiment :-

1 + Ni/Ns the drive ratio i = 1-Ni/N2

where N1 = the number of teeth on the fixed internal gear ring ; N2 = the number of teeth on the second internal gear ring; and N5 = the number of teeth on the sun gear; As with the embodiments described above the values N1, N2 and N5 may be selected to provide the required drive ratio which will typically be from 40: 1 to 60 : 1.

The drive used in the present invention is particularly advantageous for use with shift drums for example as disclosed in GB2308874 and GB2311829 the disclosures of which explicit reference is made and whose content is expressly incorporated in the disclosure content of the present application, in which the electric motor and drive mechanism may be mounted coaxially within the shift drum. The drive mechanism described above may alternatively be used with linear actuators, for example ball and screw actuators or rack and pinion mechanisms.

The invention is now described by way of example only, with reference to the accompany drawings, in which:

Figure 1 illustrates diagramatically a drive mechanism utilised in clutch, brake or gear actuators according to the present invention; Figure 2 illustrates diagramatically an alternative drive mechanism utilised in clutch, brake or gear actuators according to the present invention; Figure 3 shows in sectional side elevation a shift drum actuator in accordance with the present invention; Figure 4 shows in sectional side elevation a ball and screw actuator in accordance with the present invention; Figure 5 shows in sectional side elevation a rack and pinion actuator in accordance with the present invention; Figure 6 shows in sectional side elevation a double shift drum actuator in accordance with the present invention; and Figure 7 shows in sectional side elevation an alternative double shift drum actuator in accordance with the present invention.

As illustrated in figure 1, an electric motor 10 has a motor flange 12 having three holes 14 located in lugs 16 spaced angularly from one another, by which the motor 10 may be bolted to, for example, a gear box housing of a motor vehicle. An output shaft 18 of the electric motor 10 extends through the motor flange 12.

A sun gear 20 is mounted on the output shaft 12 of an electric motor 14 for rotation with the output shaft 18. The sun gear 20 has N5 teeth.

A planet carrier 22 is located coaxially of the output shaft 1 8, the planet carrier 22 being located axially between the sun gear 20 and external surface of the motor flange 12. Three planet gears 24 are rotatably mounted on the planet carrier 22, the planet gears 24 being spaced at equal angles around the planet carrier 22. Each planet gear 24 comprises a double pinion, having a primary pinion 26 which meshes with the sun gear 20 and a secondary pinion 28. The primary and secondary pinions 26,28 are fixed rotationally with respect to one another. The primary pinion 26 has N4 teeth the a secondary pinion 28 has N3 teeth.

The secondary pinion 26 meshes with a fixed internal gear ring 30 which is formed on an annular flange 32 which extends from the external face of the motor flange 12 coaxially of the output shaft 18 and radially outwardly of the planet carrier 22. The fixed internal gear ring 30 has Ni teeth which is equal to N3 + N4 + N5.

The primary pinion 26 also meshes with an internal gear 34 formed by an output member 36 which is mounted for rotation coaxially of the output shaft 18. The internal gear ring 34 has N2 teeth equal to 2N4 + N5

The drive ratio i for the gear mechanism described above ;

1 + N1N4/N3N5 I 1 - N1N4/N2N3

The values of N3, N4 and N5 are selected to provide an appropriate gear ratio which is preferably of the order of 40: 1 to 60: 1. As indicated in Table 1 below, when the sun gear 20 and primary and secondary pinions 26,28 are of similar size, drive ratios of the order of 50: 1 may be achieved by making the number of teeth of the primary and secondary pinions 26 an 28 differ by about 10%. Variation of the size of the sun gear has a lesser effect on the drive ratio. A smaller or greater difference

in the number of teeth on the primary and secondary pinions 26,28, may however be accommodated by relatively significant changes in the number of teeth on the sun gear 20.

Table 1

Number of Teeth Drive ratio i N1 N2 N3 N4 N5 58 56 20 18 20 53. 20 59 57 20 18 21 51. 57 60 58 20 18 22 50. 09 116 112 40 36 40 53. 20 116 120 36 40 40-57. 00* 139 138 20 19 100 53. 82 96 90 40 34 22 50. 45 86 78 40 32 14 50. 14 80 70 40 30 10 49. 00 * negative value signifies rotation of output member in opposite direction to motor shaft.

In the alternative drive mechanism illustrated in figure 2, the primary pinion 26 meshes with the fixed internal gear ring 30 and the secondary pinion 28 meshes with the internal gear ring 34, on the output member 36.

The drive ratio i for this drive mechanism equals :-

1 + N1/N5 i = 1 - N1N4/N2Na

The values of N3, N4 and N5 are again selected to provide an appropriate gear ratio which is preferably of the order of 40: 1 to 60: 1. Examples are given below in Table 2 :- Table 2

Number of Teeth Drive ratio i N1 N2 N3 N4 N5 60 58 20 18 20 58. 00 53 50 20 17 13 51. 28 98 90 40 32 18 50. 00 56 58 18 20 20 -52. 20* 8894344020-53. 27* 140 139 20 19 100 55. 60 * negative value signifies rotation of output member in opposite direction to motor shaft.

Figure 3 illustrates a shift drum assembly for a gear shift mechanism of a motor vehicle, utilising the drive mechanism described above. The electric motor 10 is mounted in a cylindrical housing 40, the motor 10 being secured to an internal flange 42 located adjacent one end 44 of the housing 40. An external flange 45 at the other end of housing 40, is adapted to be secured to, for example, a gearbox housing. The fixed internal gear ring 30 is provided at end 44 of housing 40.

The sun gear 20 is mounted on the output shaft 18 of the electric motor 10.

The planet carrier 22 is located coaxially of the output shaft 18, axially between the sun gear 20 and the internal flange formation 42. Three planet gears 24 are rotationally mounted on the planet carrier 22, the primary pinions 26 of the planet gears 24 meshing with the sun gear 20

and an internal gear ring 34 formed on the internal diameter of a shift drum 50. The secondary pinions 28 of planet gears 24 mesh with a fixed internal gear ring 30 formed on the internal diameter of the housing 40, adjacent the end 44 thereof.

The shift drum 50 is rotatably mounted, by means of rolling bearings 52, on the external diameter of the cylindrical housing 40. The end of the shift drum 50 remote from flange 46 of housing 40 is closed, the closed end of the shift drum 50 being rotatably mounted on the output shaft 18 of electric motor 10, by rolling bearing 54.

When the electric motor 10 drives shaft 18, the sun gear 20 drives the planet gears 24, so that they roll around the fixed internal gear ring 30.

Engagement of the primary pinion 26 with the internal gear 34 will cause the shift drum 50 to rotate, the drive ratio typically being of the order of 40: 1 to 60: 1.

The linear drive illustrated in figure 4 may typically be used to control movement of a piston of an hydraulic master cylinder of the type disclosed in GB2325036, GB2313885 and GB2309761, which in turn will supply hydraulic pressure to a clutch slave cylinder to control engagement and disengagement of a clutch. Alternatively linear actuators of this type may be used to control engagement and disengagement of a clutch, or selection of a gear ratio, via a suitable mechanical linkage mechanism or cable drive.

In the linear actuator illustrated in figure 4, the output member 36 of the epicyclic drive mechanism is rotationally mounted on the annular flange 32, by means of rolling bearings 62 and the output shaft 18 by rolling bearing 64.

The end 66 of output member 36 remote from motor 10 defines the internal threaded portion of a ball screw actuator 68. An outer threaded portion 70 of the ball screw actuator 68 is mounted coaxially of the inner portion 66, with a series of balls 72 located therebetween, in the thread formation defined by the inner and outer portions 66,70. The outer portion 70 of the ball screw actuator 68 has a plunger formation 74 which extends through an end wall 76 of housing 78, the plunger formation 74 being moveable axially of the housing 78 but constrained from rotation relative thereto. The plunger formation 74, which may be connected directly or indirectly to the piston of an hydraulic master cylinder, will thereby be moved axially by rotation of the output member 36, when driven by the electric motor 10.

A compensating spring 80 acts between the output member 36 and outer portion 70 of the ball screw actuator 68, urging the outer portion 70 towards the end wall 76 of housing 78. The compensating spring 80 will thereby oppose the load applied by the clutch spring. Typically, the compensating spring 80 will be arranged to be under compression when the clutch is fully engaged and the ball screw actuator 68 is at a limit of its movement away from the closed end of housing 78. The load applied by compensating spring 80 will thereby assist the electric motor 10 as the ball screw actuator 68 is driven to disengage the clutch. In this manner a smaller electric motor 10 may be used, than would be required if a compensating spring 80 were not present.

Figure 5 illustrates a rack and pinion drive actuator, similar to the actuator illustrated in figure 4, in which the output member 36 defines a pinion 90 which engages a rack 92 which extends transversely of the housing 94.

In the double shift drum arrangement illustrated in figure 6, a first shift drum 100 is closed at one end and is rotationally mounted on a first cylindrical motor housing 102, coaxially thereof, by means of rolling

bearings 104. A seal 106 is provided between the internal diameter of the shift drum 100 and the external diameter of the housing 102, adjacent to open end of the shift drum 100.

A second shift drum 110 is rotationally mounted on a second cylindrical motor housing 112, in similar manner to the first shift drum 100.

The first and second motor housings 102,112 are adapted to be secured, coaxially of one another, one to, for example a gearbox housing 120 and the other to, for example a clutch housing 122, so that the double drum assembly is located inside the gearbox housing defined therebetween.

The adjacent closed ends of the shift drums 100,110 are provided with a socket 124 and a spigot 126 respectively, the spigot 126 engaging in the socket 124 when the first and second shift drums 100,110 are assembled coaxially of one another, a rolling bearing 128 being provided therebetween. An axial rolling bearing 129 is also provided between adjacent end faces of the first and second shift drums 100,110.

Electric motors 130 are mounted in the motor housings 102, 11 2, coaxially thereof. Each electric motor 130 has an output shaft 1 32 on which a sun gear 134 is mounted for rotation therewith. A planet carrier 136 surrounds each of the output shafts 132 and carries three planet gears 138, the planet gears 138 being spaced evenly about the planet carrier 136. The planet gears 138 are in the form of double pinions, having a primary pinion 140 and a secondary pinion 142, the primary and secondary pinions 140,142 having different numbers of teeth.

The primary pinions 140 of the planet gears 138 mesh with the sun gears 134 and with a fixed internal gear ring 144 formed on the internal diameter of the motor housing 102,112. The secondary pinions 142 of

the planet gears 138 mesh with an internal gear ring 146 formed on an output gear ring 148.

The output gear rings 148 are drivingly connected to the shift drums 100, 110 by means of resilient bushes 150, which are located under radial compression between the output gear rings 148 and the respective shift drums 100, 110. The resilient bushes 150 will thereby provide radial compliance between the shift drums 100, 110 and the epicyclic drive mechanisms.

Axial bearings 152 are provided between the output gear rings 148 and the ends of the associated motor housings 102, 112 an a thrust spring 154 acts between the closed end of shift drum 100 and the associated output gear ring 148, in order to axially load the axial bearings 130 and 152.

The double shift drum assembly described above provides a compact arrangement which may be located within the gearbox between the gear housing and the clutch housing of a motor vehicle. The electric motors and drive mechanisms are sealed from the gearbox oil by seals 106.

Position sensors may furthermore be provided in the double shift drum mechanism for measuring the angular movement between the motor housings 102, 112 and shift drums 100, 110.

The double shift drum actuator illustrated in figure 7, first and second shift drums 200,202 are rotatably mounted coaxially of one another on a shaft 204. The shaft 204 is mounted at one end in a spigot formation 206, provided in the gearbox housing 208, the other end of the shaft 204 being secured to a clutch housing 210, by means of nuts 212 and 214.

The outer ends of shift drums 200,202 are closed and are mounted on the shaft 204 by means of sealed rolling bearings 216. The inner ends of

shift drums 200,202 are mounted by rolling bearings 220 on an annular member 218 which is secured non-rotatably, centrally of the shaft 204, seals 222 being provided between an internal diameter of the shift drums 200,202 and an external diameter of the annular member 218.

Permanent magnet electric motors 230 are mounted on the shaft 204, the motors 230 being located internally of the shift drums 200,202, coaxially and adjacent the closed outer ends thereof. The motors 230 comprise an annular stator 232 which is non-rotatably mounted on the shaft 204 and a rotor 234, rotatably mounted on the stator 232 by sealed rolling bearings 236. Annular sun gears 238 are secured to the inner ends of rotors 234 for rotation therewith.

A planet carrier 240 is mounted coaxially of each of the sun gears 238, the planet carriers 240 each carrying four planet gears 242, the planet gears 242 being evenly spaced around the planet carrier 240. The axes of the planet gears 242 are inclined to the axis of the shaft 204, so that as the planet carrier 240 rotates about the shaft 204, the outer ends of the planet gears 242 will describe a larger diameter than the inner ends.

The teeth on the sun gear are inclined similarly to those on the planet gears 242 and mesh therewith.

The planet gears 242 also mesh with an internal gear ring 244 formed on an internal diameter of the annular member 218; and an internal gear ring 246 defined by an output gear ring 248, the teeth on internal gear rings 244 and 246 being similarly inclined to those on the planet gears 242.

The external gear rings 244 and 246 are spaced axially so that the planet gears 242 engage the internal gear ring 244 on a smaller diameter than gear ring 246, gear ring 244 having fewer teeth than gear ring 246. The output gear ring 248 is drivingly connected to the shift drum 200,202 by means of a resilient bush 250 which is mounted under compression between an internal diameter of the shift drum 200,202 and an external

diameter of the output gear ring 248, in order to provide compliance in the drive mechanism.

An axial groove 252 is provided in the shaft 204 which may serve as a conduit for the electrical connections to the motors 230 and also as a key to prevent rotation of the shaft 204, annular member 218 and stators 232.

Similar to the previous embodiment position sensors may furthermore be provided in the double shift drum mechanism for measuring the angular movement of the shift drums 200,202.

With the actuator described with reference to figure 7, when the electric motors 230 are energised, rotation of the sun gears 238 will cause the planet gears 242 to roll around the fixed internal gear ring 244. Because of the difference in the number of teeth of internal gear ring 244 and 246, this in turn will cause the output gear ring and the shift drum 200,202 attached thereto, to rotate.

The drive ratio of this drive mechanism :-

1 + Ni/Ns the drive ratio i = 1-Ni/N2

where N1 = the number of teeth on the fixed internal gear ring 244 ; N2 = the number of teeth on the internal gear ring 246 on the output gear ring 248; and N5 = the number of teeth on the sun gear 238;

In a typical example, the number of teeth N1 on the fixed internal gear ring 244 is 60, the number of teeth N2 on the internal gear ring 246 on the output gear ring 248 is 65 and the number of teeth N5 on the sun gear 238 is 20, giving a drive ratio of 52: 1.

The double shift drum assemblies described with reference to figures 6 and 7 are particularly suitable for twin clutch transmission systems of the type disclosed in co-pending UK Patent Applications GB 0028310 and GB 0103312 the disclosures of which explicit reference is made and whose content is expressly incorporated in the disclosure content of the present application where, for example, shift drum 100 may be arranged to control shifts from R - 15t - 3rd - 5th - R in association with one clutch and shift drum 110 may be arranged to control shifts from 2nd - 4th - 6th - 2nd in association with the other clutch.

Various modifications may be made without departing from the invention.

For example with regard to the ball screw actuator described with reference to figure 4, the output member of the drive mechanism may define to outer threaded portion of the ball screw rather than the inner portion.

The patent claims submitted with the application are proposed formulations without prejudice to the achievement of further patent protection. The applicant reserves the right to submit claims for further combinations of characteristics, previously only disclosed in the description and/or drawings.

References back used in sub-claims refer to the further development of the subject of the main claim by the characteristics of the respective subclaim ; they are not to be understood as a waiver with regard to achieving

independent item protection for the combination of characteristics in the related sub-claims.

Since the subject of the sub-claims can form separate and independent inventions with reference to the prior art on the priority date, the applicant reserves the right to make them the subject of independent claims or of division declarations. Furthermore, they may also contain independent inventions which demonstrate a design which is independent of one of the objects of the preceding sub-claims.

The embodiments are not to be considered a restriction of the invention.

Rather, a wide range of amendments and modifications is possible within the scope of the current disclosure, especially those variations, elements and combinations and/or materials which, for example, the expert can learn by combining individual ones together with those in the general description and embodiments in addition to characteristics and/or elements or process stages described in the claims and contained in the drawings with the aim of solving a task thus leading to a new object or new process stages or sequences of process stages via combinable characteristics, even where they concern manufacturing, testing and work processes.

Claims

CLAIMS 1. An electric motor actuator for control of a clutch or gear selector mechanism or brake of a motor vehicle ; including an electric motor, an output shaft of the electric motor, and an epicyclic drive mechanism comprising; a sun gear secured to the output shaft of the electric motor for rotation therewith, an annular planet gear carrier mounted for rotation coaxially of the output shaft of the electric motor, a series of planet gears mounted in angularly spaced relationship symmetrically of the planet carrier, the planet gears meshing with first and second internal gear rings, one of the internal gear rings being fixed and the other internal gear ring being formed on an output member, the output member being mounted for rotation coaxially of the output shaft, the number of teeth on the first internal gear ring being different to the number of teeth on the second internal gear ring, in order to provide the required drive ratio.
2. An electric motor actuator according to claim 1 in which the drive mechanism has a drive ratio of from 40: 1 to 60: 1.
3. An electric motor actuator according to claim 1 or 2 in which each planet gear is in the form of a double pinion, the planet gear defining a primary pinion which meshes with the sun gear and the first internal gear ring, and a secondary pinion which meshes with the second internal gear ring.
4. An electric motor actuator according to claim 3 in which the primary pinion meshes with the internal gear ring formed on the output member and the secondary pinion meshes with the fixed internal gear ring.

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5. An electric motor actuator according to claim 3 in which the primary pinion meshes with the fixed internal gear ring and the secondary pinion meshes with the internal gear ring formed on the output member.
6. An electric motor actuator according to any one of claims 3 to 5 in which the sun gear and the first and second pinions of the planet gears are of similar size, the number of teeth on the primary pinion differing from the number of teeth on the secondary pinion by of the order of 10%.
7. An electric motor actuator according to claim 1 or 2 in which the planet gears are single pinions which are mounted on axes skewed to the axes of the first and second internal gear rings, so that the planet gears mesh, at one end, with the internal gear ring having fewer teeth, on a smaller diameter; and, at the other end, with the internal gear ring having more teeth, on a larger diameter.
8. An electric motor actuator according to any one of the preceding claims in which the output member is a shift drum which is adapted to control movement of a gear shift mechanism.
9. An electric motor actuator according to claim 8 in which the electric motor is mounted inside a cylindrical housing, the cylindrical housing defining the fixed internal gear ring, the shift drum being rotatably mounted on the external diameter of the cylindrical housing, the relatively rotatable internal gear ring being provided on an internal diameter of the shift drum.
10. An electric motor actuator according to claim 9 in which the fixed internal gear ring is defined by the cylindrical housing, the relatively rotatable internal gear ring being defined by an output gear ring, the output gear ring being secured to the shift drum in compliant manner.

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11. An electric motor actuator according to claim 10 in which an elastomeric bush is located under radial compression between an external diameter of the output gear ring and an internal diameter of the shift drum.
12. An electric motor actuator according to any one of claims 9 to 11 in which seal means is provided between an external diameter of the cylindrical housing and an internal diameter of the shift drum.
13. An electric motor actuator according to any one of claims 9 to 12 in which two shift drum/motor assemblies are mounted coaxially of one another.
14. An electric motor actuator according to claim 13 in which a spigot formation on one shift drum engages a socket formation on the other shift drum.
15. An electric motor actuator according to claim 14 in which axial bearings are provided between the shift drums.
16. An electric motor actuator according to claim 15 in which means is provided for resiliently loading the axial bearings.
1 7. An electric motor actuator according to claim 13 in which two electric motor shift drum assemblies are mounted coaxially of one another on a central shaft.
18. An electric motor actuator according to claim 17 in which an axial recess is provided in the central shaft.

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19. An electric motor actuator according to claim 17 or 18 in which a stator of the motor is secured to the central shaft, a rotor being rotatably mounted radially outwardly of the stator.
20. An electric motor actuator according to claims 9 to 19 in which the shift drum/motor assembly is adapted to be located within a gearbox housing.
21. An electric motor actuator according to any one of claims 1 to 7 in which the output member drives a screw actuator.
22. An electric motor actuator according to claim 21 in which the output member drives ball screw actuator, the ball screw actuator comprising an inner and an outer threaded portion with a series of balls located in the thread formation defined therebetween, the output member defining one of the threaded portions of the ball screw actuator, the other threaded portion if the ball screw actuator being moveable axially but constrained rotationally with regard to the output member.
23. An electric motor actuator according to claim 21 or 22 in which a compensating spring is provided between inner and outer threaded portions of the screw actuator.
24. An electric motor actuator according to any one of claims 1 to 7 in which the output member drives a rack and pinion mechanism.
25. An electric motor actuator according to claim 24 in which the output member defines a pinion, said pinion meshing with a rack mounted transversely of the axis of rotation of said pinion.

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26. An electric motor actuator substantially as described herein with reference to and as shown in figures 1 to 7 of the accompanying drawings.