FIELD OF THE INVENTION

The present invention relates to a brake disc for limiting the rotation speed of a shaft of an adjustment mechanism of an electrically driven clutch actuator. The invention also relates to a method for fabricating such a brake disc. Moreover, the invention relates to an electrically driven clutch actua ^¬ tor for engaging and disengaging a clutch.

BACKGROUND OF THE INVENTION

Clutch actuators are used in automotive vehicles such as trucks or cars in order to engage and disengage a clutch upon actuation by a driver. Such engagement or disengagement of a clutch is for example needed when the drive shaft of an engine has to be connected to or disconnected from an input shaft of a gearbox, e.g. in order to accelerate a vehicle from a stop position or in order to change a gear whilst driving. Clutch actuators used to be hydraulically coupled to a pedal posi ^¬ tioned in the cabin of the vehicle. Upon actuation of the pe ^¬ dal by the driver an actuation force was hydraulically trans ^¬ mitted to the clutch actuator. Nowadays, electrically driven clutch actuators are state of the art, which are connected to the pedal in the cabin via a cable or under use of a wireless connection. The force needed to actuate the clutch is generat ^¬ ed electronically by the clutch actuator upon actuation of the pedal by the driver.

Document US 4,865,173 discloses an electrically driven clutch actuator. The actuator comprises an electric motor which is positioned within the housing of the clutch. The motor includes an annular armature assembly which is positioned con ^¬ centrically around the clutch shaft. The armature assembly drives a nut member which in turn drives a screw member to move the release bearing linearly to engage and disengage the clutch .

According to customer safety regulations the clutch actuator has to be capable to automatically engage the clutch in case of a power failure. The safety regulations require that the clutch actuator is able to bring the clutch from a disengage state into an engaged state within a predefined time interval of 2 to 10 seconds.

US 2007/0251793 discloses an electric clutch actuator capable of operating in two power failure conditions. For a condition where the clutch is disengaged during a system power failure and the desired action is for the clutch to move to engaged positional state, a motor in the electric clutch actuator is used as a generator. The motor converts the potential energy of a clutch pressure spring into electrical energy to provide energy for powering a holding device to remain deactivated, permitting movement of the clutch actuator to the engaged po ^¬ sition.

However, conversion of the potential energy into electrical energy in case of a power failure is complex and cost inten ^¬ sive as a controller for controlling the generated power as well as additional wiring has to be installed.

It is an object of the present invention to provide a cost ef ^¬ fective and reliable solution for an automatic clutch engage ^¬ ment in case of a power failure. SUMMARY OF THE INVENTION

This object is achieved by a brake disc for limiting the rota ^¬ tion speed of a shaft in an adjustment mechanism of an elec ^¬ trically driven clutch actuator, comprising the features of claim 1. Moreover, this object is achieved by an electrically driven clutch actuator for engaging and disengaging a clutch having the features of claim 10 as well as a method for fabri ^¬ cating a brake disc comprising the features of claim 15.

According to the present invention the brake disc comprises a disc-shaped brake body radially extending from an axis of ro ^¬ tation and having an end face which is formed as a friction surface. The friction surface is designed such that a combina ^¬ tion of a friction surface and a corresponding counterpart rubbing against each other has a first coefficient of fric ^¬ tion. The brake body comprises recesses extending from the friction surface into the brake body along the axis of rota ^¬ tion. The recesses are configured to receive inserts which are designed such that a friction combination of the inserts and the corresponding counterpart of the friction surface of a brake body has a second coefficient of friction. The second coefficient of friction is lower than the first coefficient of friction .

As the friction surface together with the inserts are adapted to rub against one single counterpart, the overall coefficient of friction of the brake disc is lower than the first coeffi ^¬ cient of friction. The first and second coefficients of fric ^¬ tion are chosen such that the clutch is engaged within the predefined time interval of 2 to 10 seconds. At the same time the first and second coefficient of friction are chosen such that durability of the brake disc is guaranteed. In an embodiment of the present invention the inserts are sized and positioned such that abrasion of the inserts gener ^¬ ated by rubbing the inserts against the corresponding counterpart of the friction surface of the brake body is spread over the friction surface of the brake body thereby reducing the first coefficient of friction. By spreading the abraded insert material over the braking surface the first coefficient of friction and thus the overall coefficient of friction of the brake disc is further reduced. Thus, lifetime of the brake disc is further increased.

In another embodiment of the present invention the recesses are positioned at uniform angle distances in a circumferential direction of the brake body. This allows uniform distribution of the abraded inserts over the friction surface.

Preferably the brake disc comprises an attachment portion for attaching the brake disc to a carrier element, the attachment portion surrounding the brake body in circumferential direc ^¬ tion and extending radially thereto. Preferably, the attach ^¬ ment portion allows transfer of a brake torque in direction around the axis of rotation of the brake disc to the carrier element .

Also preferred is an embodiment according to which the attach ^¬ ment portion comprises radially extending teeth which are sized and positioned to form a form fitting connection with the carrier element. The radially extending teeth allow a uniform support of the brake body by the carrier element. Option ^¬ ally, the teeth are formed such that the attachment portion is additionally held by the carrier element via a frictional con ^¬ nection. This additionally allows a connection of the brake disc and the carrier element in a direction along the rota ^¬ tional axis of the brake disc and thus simplifies the fas ^¬ tening of the brake disc to the carrier element. In one embodiment the friction surface is oriented perpendicu ^¬ lar to the axis of rotation. This allows realization of a brake body with a radial extension that is larger than the ex ^¬ tension of the brake body along the axis of rotation.

In another embodiment of the present invention the brake body as well as the inserts are formed from plastic material, wherein the plastic material of the brake body differs from the plastic material of the insert. In combination with the present invention it has been found that certain plastic mate ^¬ rial provide the desired first and second coefficients of friction. Moreover, plastic materials do not damage other com ^¬ ponents of an electrically driven clutch actuator when being abraded. For example, if the brake disc would be formed from metal abrasion of the brake disc may lead to damages of gears, threads or bearings when being used in the clutch actuator.

In an embodiment of the present invention the brake body is formed of a polyimide. This class of materials has specially been designed for sliding applications without any lubrication (dry run) . It has a very low coefficient of friction when being rubbed against steel. Preferably, the material is formed from Vespel SP-211 having a coefficient of friction of 0,08- 0, 12.

Preferably, the inserts are formed from polytetrafluoro- ethylene (PTFE) . This plastic material has an even lower coef ^¬ ficient of friction when rubbed against steel compared to Vespel SP-211. Preferably, the coefficient of friction of PTFE is 0,05-0,08.

Besides the above mentioned ranges of the coefficients of friction of SP-211 and PTFE the requirements as claimed have to be fulfilled in connection with the present invention. In other words: If the first coefficient of friction is of the lower boundary of the range stated above, i.e. 0,08, the se ^¬ cond coefficient of friction has to be chosen from the given range of 0,05-0,08 to be below the upper boundary value such that the second coefficient of friction is lower than the first coefficient of friction and vice versa.

The object of the present invention as mentioned above is also achieved by an electrically driven clutch actuator for engag ^¬ ing and disengaging a clutch having the features of claim 10.

Due to customer safety regulations and due to customer needs the electrically driven clutch actuator according to the present invention has to be configured such that the clutch is automatically engaged in a predefined time interval in case of a power failure. The predefined time interval is between 2 and 10 seconds. This time interval was chosen such that a rapid engagement of the clutch and thus a rapid acceleration of the car upon engagement of the clutch is omitted. Thus, in case of a power failure and an automatic engagement of the clutch the car is easily controllable.

In an embodiment of the present invention the clutch actuator further comprises a threaded spindle shaft configured to be driven by an electric motor and a slide configured to be driv ^¬ en by the spindle shaft from a clutch disengaging to a clutch engaging position and vice versa thereby engaging and disengaging the clutch. The slide is configured to set the spindle shaft into rotation in case of a power failure to automatical ^¬ ly engage the clutch. The brake disc is connected to the spin ^¬ dle shaft and is configured to limit a rotation speed of the spindle shaft during movement of the slide in case of a power failure. Preferably, the spindle shaft and slide form an ad ^¬ justment mechanism of the clutch actuator. Using a brake disc for limiting the transferred load from the clutch of the vehicle to the electric motor during movement of the slide in case of a power failure is an easy, a purely me ^¬ chanical and thus reliable as well as cost effective way to control the time needed by the clutch actuator in order to en ^¬ gage the clutch in case of a power failure. In connection with the present invention it has been found that is advantageous to fabricate the brake disc according to the present invention from plastic material. Plastic materials allow realization of the first and second coefficients of friction which enable an engagement of the clutch within the predefined time interval. Moreover, plastic materials are advantageous as abraded parti ^¬ cles from these materials do not damage other components of the clutch actuator.

In an embodiment of the present invention the brake disc is connected to the spindle shaft via a one way clutch. The one way clutch is configured to provide a torque proof connection between the brake disc and the spindle shaft when the slide moves from the clutch disengaging position to the clutch engaging position. The one way clutch is further configured to allow free rotational movement of the spindle shaft relative to the brake disc when the slide moves from the clutch engag ^¬ ing position to the clutch disengaging position.

During movement of the slide from the clutch disengaging to the clutch engaging position the slide sets the spindle shaft into rotation. The electric motor is adapted to limit this ro ^¬ tation, in particular a rotational speed, of the spindle shaft by applying a drive torque, in particular a brake torque, onto the spindle shaft, the drive torque acting in a direction around the axis of rotation counter the rotation of the spindle shaft. As the one way clutch provides a torque proof con ^¬ nection between the brake disc and the spindle shaft when the slide moves from the clutch disengaging position to the clutch engaging position the brake disc causes a brake torque thus supporting the electric motor in limiting the rotational speed of the spindle shaft. Accordingly, compared to an embodiment in which the electric motor alone controls the rotation of the spindle shaft, a less powerful model of the electric motor can be chosen which is cheaper and less power consumptive. During movement of the spindle from the clutch engaging position to the clutch disengaging position the one way clutch allows free rotational movement of the spindle shaft relative to the brake disc. Thus, when turning the spindle shaft the electric motor does not have to overcome an additional brake torque generated by the brake disc.

In another embodiment of the present invention the brake disc is further connected to the spindle shaft via an axial bearing which is configured to transmit a force onto the brake disc, the force acting in a direction along the axis of rotation of the brake disc. Preferably, the clutch actuator is configured to move the slide from the clutch engaging position to the clutch disengaging position counter to a restoring force acting from the clutch onto a push rod of the clutch actuator and pushing the slide towards the clutch engaging position. The restoring force increases during movement of the slide towards the clutch disengaging position and decreases towards the clutch engaging position. At its peak the restoring force may amount to 7 kN. Optionally, the restoring force is transmitted from a release bearing of the clutch to the slide and thus to the spindle shaft. The release bearing is actuated by the clutch actuator in order to disengage and engage the clutch. Preferably, the axial bearing is configured to at least par ^¬ tially transmit the restoring force onto the brake disc. The restoring force is acting at least partially on the brake disc along the axis of rotation and thus defines amongst others the braking torque of the brake disc. Preferably, the electric motor is connected to the spindle shaft via a planetary gear. The planetary gear provides a gear ratio in between the electric motor and the spindle shaft. Thus, the planetary gear transforms the rotation speed and force of the electric motor to suitable values for the rota ^¬ tion of the spindle shaft in order to drive the slide from the clutch disengaging position to the clutch engaging position and vice versa.

The present invention also relates to a method for fabricating a brake disc of the present invention comprising the features of claim 15. By injection molding of the brake disc comprising the recesses an additional machining step of machining the re ^¬ cesses into the brake body is omitted. Thus, the brake disc may be produced in a cheaper way the same applies to the in ^¬ jecting molding of the inserts into the recesses as the manual processing step of inserting the inserts into the recesses is also omitted.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be described in connection with one exemplary embodiment shown in the figures in which:

Figure 1 shows a sectional view of an electrically driven clutch actuator,

Figure 2 shows a detailed view of section A in Figure 1 and Figure 3 shows an exploded view of the brake disc and the ad ^¬ joining carrier element.

Figures 1 and 2 show an electrically driven clutch actuator 1 for engaging and disengaging a clutch (not shown) . The clutch actuator 1 comprises a housing 2 in which a threaded spindle shaft 3 is rotatably mounted. The spindle shaft 3 is config ^¬ ured to be driven by an electric motor 4 such that the spindle shaft 3 rotates around an axis of rotation 5. The electric mo ^¬ tor 4 is connected to the spindle shaft 3 via a planetary gear 6.

The clutch actuator 1 further comprises a slide 7 which is formed as a nut. The slide 7 is configured to be driven by the spindle shaft 3 from a clutch disengaging position to a clutch engaging position and vice versa.

The slide 7 has an inner thread 8 which is configured to en ^¬ gage with the threaded spindle shaft 3 such that upon rotation of the spindle shaft 3 the slide 7 moves along the axis of ro ^¬ tation 5 of the spindle shaft 3 from the clutch disengaged po ^¬ sition into the clutch engaging position and vice versa.

Moreover, the threaded spindle shaft 3 as well as the inner thread 8 of the slide 7 are configured such that translational movement of the slide 7 along the axis of rotation 5 causes the spindle shaft 3 to rotate. Such a translational movement of the slide 7 may occur when the slide 7 is pushed along the axis of rotation 5. The electric motor 4 is configured to con ^¬ trol any rotational movement of the spindle shaft 3 which is initiated by the slide 7 by being pushed along the axis of ro ^¬ tation 5. The electric motor 4 is configured to control this rotational movement of the spindle shaft 3 by generating a drive torque M _dr i _ve , in particular a break torque, acting in a direction around the axis of rotation 5 counter to the rota ^¬ tion of the spindle shaft 3. The electric motor 4 is thus con ^¬ figured to limit a rotation speed of the spindle shaft 3 and/or to stop the same.

In Figure 1 the slide 7 is shown in the clutch disengaging position. In order to arrive at the clutch engaging position the slide 7 has to be moved in Figure 1 to the right side along the axis of rotation 5 of the spindle shaft 3. The slide 7 is connected to a push rod 9 which upon movement of the slide 7 from the clutch disengaging position to the clutch engaging position and vice versa interacts with the clutch (not shown) thereby engaging and disengaging the clutch .

The clutch actuator 1 is configured to move the slide 7 from the clutch engaging position to the clutch disengaging position counter to a restoring force F _rest acting from the clutch onto the push rod 9 and pushing the slide 7 towards the clutch engaging position. The restoring force F _rest is transmitted from a release bearing (not shown) of the clutch to the slide 7 and thus at least partially to the spindle shaft 3. The restoring force F _rest increases during movement of the slide 7 towards the clutch disengaging position and decreases towards the clutch engaging position. At its peak the restoring force may amount to 7 kN.

The clutch actuator 1 further comprises a safety mechanism 10 for automatically engaging the clutch in a predefined time in ^¬ terval in case of a power failure. The safety mechanism 10 comprises a brake disc 11 which is attached to a carrier ele ^¬ ment 12 and which is connected to the spindle shaft 3 via a one way clutch 13. The spindle shaft 3 and the brake disc 11 are positioned concentrically to each other such that the axis of rotation 5 of the spindle shaft 3 is identical to the axis of rotation 5 of the brake disc 11.

The one way clutch 13 is configured to provide a torque proof connection between the brake disc 11 and the spindle shaft 3 when the slide 7 moves from the clutch disengaging position to the clutch engaging position. The one way clutch 13 is further configured to allow free rotational movement of the spindle shaft 3 relative to the brake disc 11 when the slide 7 moves from the clutch engaging position to the clutch disengaging position .

The brake disc 11 is further connected to the spindle shaft 3 via an axial bearing 14. The axial bearing 14 is configured to transmit the restoring force F _rest acting from the clutch (not shown) onto the pushrod 9 and thus onto the slide 7 at least partially onto the brake disc 11 in a direction along its axis of rotation 5.

The brake disc 11 is configured to rub against a corresponding counterpart which is a steel plate 15 that is installed in the housing 2 and is positioned perpendicular to the axis of rota ^¬ tion 5. Upon at least partial transmittance of the restoring force F _rest onto the brake disc 11 the brake disc 11 is pressed onto the steel plate 15. Upon additional rotation of the brake disc 11 caused by the rotating spindle shaft 3 being torque proof connected to the brake disc 11 via the one way clutch 13 the brake disc 11 generates a brake torque M _brake around the ax ^¬ is of rotation 5.

Figure 3 shows an exploded view of the brake disc 11 and the carrier 12 in detail. The brake disc 11 comprises a disc ^¬ shaped brake body 16 radially extending from the axis of rota ^¬ tion 5 and having an end phase which is formed as a friction surface 17. The friction surface 17 is oriented perpendicular to the axis of rotation 15. The friction surface 17 is de ^¬ signed such that a friction combination of the friction surface 17 and the steel plate 15 (Figure 1) rubbing against each other has a first coefficient of friction.

The brake body 16 further comprises recesses 18 extending from the friction surface 17 into the brake body 16 along the axis of rotation 5. The recesses 18 are positioned at uniform angle distances in circumferential direction of the brake body 16. The brake disc 11 further comprises inserts 19 which are posi ^¬ tioned in the recesses 18. The inserts 19 are designed such that a friction combination of the inserts and the steel plate 15 (Figure 1) has a second coefficient of friction, wherein the second coefficient of friction is low than the lower than the first coefficient of friction.

The inserts 19 are sized and positioned such that abrasion of the inserts 19 generated by rubbing the inserts 19 against the steel plate 15 (Figure 1) is spread over the friction surface 17 of the brake body 16, thereby reducing the first coeffi ^¬ cient of friction.

The brake disc 11 further comprises an attachment portion 20 for attaching the brake disc 11 to the carrier element 12. The attachment portion 20 surrounds the brake body 16 in circum ^¬ ferential direction and extends radially thereto. The attach ^¬ ment portion 20 comprises radially extending teeth 21, which are sized and positioned to form a form fitting connection with the carrier element 12.

The carrier element 12 has an attachment surface 22 oriented perpendicular to the axis of rotation 5. The carrier element 12 further comprises protrusions 23 protruding from the at ^¬ tachment surface 22 along the axis of rotation. The protru ^¬ sions 23 are positioned at uniform angle distances in circum ^¬ ferential direction of the carrier element 12 such that the teeth 21 of the brake disc 11 are received in between the pro ^¬ trusion 23. The protrusions 23 are configured to transmit any rotational movement of the carrier 12 to the brake disc 11 such that the friction surface 17 is rubbed against the steel plate 15 (Figure 1) .

The brake body 16 as well as the inserts 19 are formed from plastic material, wherein the plastic material of the brake body 16 differs from the plastic material of the inserts 19. The brake body 16 is formed of a polyimide, for example Vespel Sp-211 which may be purchased by the company Dupont . The in ^¬ serts are formed of polytetrafluoroethylene (PTFE) .

The inserts 19 are sized and configured such that they extend beyond the friction surface 17 of the brake body 16 along the axis of rotation 5 upon insertion into the recesses 18. When being pressed against the steel plate 15 (Figure 1) upon at least a part of the restoring force F _rest (Figure 1) the inserts 18 are deformed such that they are flush with the friction surface 17 of the brake body 16.

In the following the operation of the clutch actuator 1 shall be explained with reference to Figures 1 and 2. The term "standard mode" is used hereinafter to describe the operation mode of the electrically driven clutch actuator 1 when power is supplied to the electric motor 4. The term "power failure mode" refers to the operation mode of the electrically driven clutch actuator 1 when no power is supplied to the electric motor 4 because of a power failure. As a basis for the follow ^¬ ing description of operation it is assumed that the clutch is initially engaged and the slide 7 is located in the clutch en ^¬ gaging position in proximity to the right end of the spindle shaft in Figure 1.

In order to disengage the clutch in the standard mode of the clutch actuator the electric motor 4 generates a drive torque M _dr ive driving the spindle shaft 3 via the planetary gear 6 in a counter clockwise direction thereby moving the slide 7 from the clutch engaging position to the clutch disengaging position. The push rod 9 moves together with the slide 7 thereby disengaging the clutch. During movement of the slide 7 towards the clutch disengaging position the electric motor 4 has to overcome the restoring force F _rest acting from the clutch to the push rod 9 and thus on the slide 7. In particular the electric motor 4 has to overcome a restoring torque M _rest acting on the spindle shaft 3 counter to the drive torque M _drive of the electric motor 4. The restoring torque M _{re st} is caused by the restoring force F _{re st} pushing against the slide 7 and trying to set the spindle shaft 3 into rotation due to the engagement of the inner thread 8 of the slide 7 and the threaded spindle shaft 3. The restoring force F _rest and thus the restoring torque M _rest in ^¬ crease during movement of the slide 7 from the clutch engaging position to the clutch disengaging position and reach their peaks at the clutch disengaging position.

During the movement of the slide 7 from the clutch engaging position to the clutch disengaging position and thus during the counter clockwise rotation of the spindle shaft 3 the one way clutch 13 allows free rotational movement of the spindle shaft 3 relative to the brake disc 11. Accordingly, the spin ^¬ dle shaft 3 rotates whereas the brake disc 11 stands still.

In order to keep the slide 7 in the clutch disengaging position the electric motor 4 has to continuously drive the spin ^¬ dle shaft 3 with the drive torque M _dr i _ve which compensates the restoring force F _rest and the restoring torque M _rest , respective ^¬ ly, pushing the slide 7 along the axis of rotation 5 and trying to set the spindle shaft into clockwise rotation. In other words, the electric motor 4 blocks the rotation of the spindle shaft 3.

In order to engage the clutch in standard mode by moving the slide 7 from the clutch disengaging position to the clutch engaging position the electric motor 4 is driven such that the drive torque M _dr i _ve acting onto the spindle shaft 3 is smaller than the restoring torque M _rest from the slide 7. Accordingly, due to the restoring torque M _rest the spindle shaft 3 is rotated clockwise, whereas the speed of rotation of the spindle shaft 3 is limited by the drive torque M _dr i _ve of the electric motor 4.

During movement of the slide 7 from the clutch disengaging position to the clutch engaging position the one way clutch 13 is configured to provide a torque proof connection between the spindle shaft 3 and the brake disc 11. Thus, during movement of slide 7 from the clutch disengaging position to the clutch engaging position the brake disc 11 rotates together with the spindle shaft 3 and is pressed at least partly by the restor ^¬ ing force F _rest against the steel plate 15 thereby generating a brake torque M _brake in circumferential direction around the axis of rotation 5. In other words, the brake disc 11 generates a brake torque M _brake and thus supports the electric motor 4 by controlling (limiting) the rotation speed of the spindle shaft 3. During movement of the slide 7 to the clutch engaging posi ^¬ tion the restoring force F _rest decreases and amounts to zero when the slide 7 reaches the clutch engaging position. Accordingly, the same applies to the restoring torque M _{re st} -

Due to law regulations and due to customer needs the clutch has to be engaged upon a power failure in order to allow a safe control of the car by the driver. In case the clutch is already engaged upon a power failure no action of the clutch actuator is needed. In case the clutch is disengaged when a power failure occurs the clutch has to be engaged in a prede ^¬ fined time interval of preferably of 2 to 10 seconds as stated in the law regulations.

In the following the automatic engagement of the clutch by au ^¬ tomatically moving the slide into the clutch engaging position in a power failure mode of the clutch actuator is explained. As a basis for the following description it is assumed that - li ^¬

the clutch is initially disengaged and the slide 7 is thus in a clutch disengaging position when a power failure occurs.

As described above, the restoring force F _rest acting on the push rod 9 and thus on the slide 7 causes a restoring torque M _rest acting on the spindle shaft 3 in a clockwise direction. As the electric motor 4 is no longer supplied with power, the elec ^¬ tric motor 4 is no longer able to generate the drive torque drive onto the spindle shaft 3. Consequently, the restoring force F _rest pushes the slide 7 from the clutch disengaging posi ^¬ tion to the clutch engaging position, thereby automatically disengaging the clutch in case of a power failure. In other words: The slide is configured to set the spindle shaft 3 into rotation in case of a power failure to automatically engage the clutch.

As the one way clutch 13 is configured to provide a torque proof connection between the brake disc 11 and the spindle shaft 3 when the slide 7 moves from the clutch disengaging position to the clutch engaging position the brake disc 11 is rotated together with the spindle shaft 3 around the axis of rotation 5 and thus provides a brake torque M _brake . This brake torque M _brake acts in a direction around the axis of rotation 5 counter to the rotation of the spindle shaft 3. Thus, the brake disc 11 is configured to limit a rotation speed of the spindle shaft 3 during movement of the slide in case of a pow ^¬ er failure.

The first coefficient of friction of the friction surface 17 and the second coefficient of friction of the inserts 19 are chosen such that the brake torque M _brake generated by the disc brake 11 is sufficient to limit the rotation speed of the spindle shaft 3 such that the slide 7 is moved from the clutch disengaging position to the clutch engaging position in the predefined time interval of preferably 2 to 10 seconds. Finally, a method for fabricating the brake disc 11 as shown in Figure 2 is explained hereinafter.

In a first step the brake disc comprising the recesses 18 ex ^¬ tending from the friction surface 17 along the axis of rotation 5 is injection molded. In other words: The brake disc 11 comprises the recesses extending from the friction surface 17 along the axis of rotation 5 upon being released from the mould (not shown) . A machining of the recesses 18 is not nec ^¬ essary .

In a second step the inserts 19 are inserted into the recesses 18. Alternatively, the inserts 19 are injection molded into the recesses 18 such that the brake body 16 and the inserts 19 are integrally formed.

The brake disc 11 is then attached to the carrier 12 and mounted via the one way clutch 13 onto the spindle shaft 3 in the housing 2 of the clutch actuator 1.

Reference numerals

1 clutch actuator

2 housing

3 spindle shaft

4 electric motor

5 axis of rotation

6 planetary gear

7 slide

8 inner thread

9 push rod

10 safety mechanism

11 brake disc

12 carrier element

13 one way clutch

F _res t restoring force

14 axial bearing

15 steel plate

M _brake brake torque

16 brake body

17 friction surface

18 recesses

19 insert

20 attachment portion

21 teeth

22 attachment surface

23 protrusions

M _drive drive torque

M _res t restoring torque