Linear electromechanical screw actuator for parking brake

A linear electromechanical actuator of the screw type for a parking brake including a tubular nut (22) coupled by means of an external thread to the casing (12) of the actuator and operated to perform a screwing movement about and along a longitudinal axis (x). The nut has an internal thread (23) opposite the external thread, for engaging a threaded rod (11) which is prevented from rotating with respect to the casing. Operation of the electrical motor (13) brings about a telescopic movement of the nut (22) and the rod (11) with respect to the casing (12), with a rapid linear translation of the rod (x).

This is a National Stage entry of International Application PCT/EP2004/007316, with an international filing date of Jul. 5, 2004, which was published under PCT Article 21(2) as WO/2005/005212 A1, and the complete disclosure of which is incorporated into this application by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a linear electromechanical screw actuator for a parking brake of a motor vehicle.

Instead of the traditional manually operated lever which imparts a traction movement to one or two Bowden cables connected to the parking brakes of a motor vehicle, it has recently been proposed to effect that traction movement by means of an electromagnetic actuator (see, for example, US2002/0100647 A1).

SUMMARY OF THE INVENTION

The object of the invention is to provide a linear actuator capable of imparting to the Bowden cables a traction movement for a maximum travel of approximately 40 mm rapidly (less than 1.0-1.5 seconds), in a non-backdrivable manner (that is to say, capable of maintaining the imparted braking force in the absence of an unlocking command) and capable of exerting a high traction force of the order of approximately 3600 N.

Another object of the invention is to provide a strong and reliable linear actuator which is easy to mount and which is constituted by a minimum number of components.

A further particular object of the invention is to provide an actuator which is such that, when the braking force is applied, the path of forces and reactions which passes through that actuator involves a minimum number of components.

Those and other objects and advantages which will be better understood hereinafter are achieved according to the invention by a braking system and by an actuator according to the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Referring first of all toFIG. 1, a braking system of a motor vehicle comprises parking brakes B1and B2associated with the rear wheels W1and W2of a motor vehicle V. Each parking brake is operated by a respective Bowden cable C1, C2constrained on a distributor R which, in order to operate the brakes, has to be pulled in the direction of the arrow A by a rod transmission element11at the output side of a linear electromechanical actuator generally indicated10and illustrated inFIGS. 2 to 4.

The actuator10has a substantially cylindrical rigid casing12defining a central longitudinal axis x which coincides with the direction of operation of the rod11. The casing12is advantageously formed by joining two bodies12a,12b,of which the one body12alocated on the side where the rod11emerges is rigid while the complementary body12b,which does not have to be subjected to appreciable stresses during use, can be thinner.

Mounted inside the casing is an electrical motor13with an associated reduction unit14whose rotary member at the output side comprises a radial flange15which is rotatable about the central axis x of the actuator. The flange15carries a plurality of axial pins16, for example three or four parallel pins arranged in an angularly equidistant manner around the axis x.

The head or output portion12aof the casing12has an end transverse terminal wall17having a central opening18through which the rod11extends. Secured to the inside of the head portion12aof the casing is a metal bush19which is threaded internally with a trapezial thread20with which engages the external thread21of a tubular nut element22having a trapezial internal thread23opposite the external thread21and coaxial therewith. The nut element22co-operates in a threaded manner with the rod11, which has an axially internal (or proximal) portion11ahaving a trapezial thread24which is congruent with the internal thread23of the nut, and an axially more external (or distal) portion11bhaving a non-circular cross-section. For example, as shown inFIG. 4, the distal portion11bwhich extends through the opening18in the transverse wall17of the casing has a prismatic shape or has one or more flat surfaces11c,whose function is described hereinafter, and a terminal portion11dfor connection to the distributor R. As will be seen, the rod and the nut co-operate by performing a telescopic movement with respect to the fixed casing. As an alternative to the example illustrated, the thread20which is fixedly joined to the casing12could be formed in a single piece with the casing instead of being formed by a separate element (the bush19).

The nut element22has a base portion in the form of a radial flange25in which are formed axially oriented through-openings26in which the pins16carried by the rotary member15engage slidingly.

In the region of the outlet opening18for the rod11, the casing has means for preventing the rotation of the rod about the longitudinal axis of the actuator. Those anti-rotation means may be constituted by the opening18itself, which may have a non-circular shape corresponding to the cross-section of the distal portion11bof the rod11. Alternatively, the anti-rotation means could comprise one or more grub screws27(FIG. 2) which, by co-operating with the flat surfaces11cof the rod11, prevent the rotation thereof.

In the alternative embodiment illustrated inFIG. 5, the rotary member15at the output side comprises a cylindrical seat having internal axial grooves15a.A grooved peripheral portion26aof the flange25of the nut22engages slidingly along those grooves.

When the electrical motor13is activated, the flanged member15causes the nut22to rotate relative to the threaded bush19and to the casing12, so that the nut performs a screwing movement about and along the longitudinal axis x. Owing to the anti-rotation engagement between the rod and the casing, and owing to the internal threaded coupling between the nut and the rod, which, as stated, have respective threads23and24opposite those20,21of the external threaded coupling between the nut22and the casing, the rotation of the nut brings about a rapid linear translation (without rotation) of the rod11, which is rapidly retracted towards the inside of the actuator, thus operating the Bowden cables C1, C2which control the parking brakes B1, B2. During the screwing movement about the axis x, the nut22, with its flange25, approaches (or moves away from, depending on the sense of rotation imparted by the motor) the rotary flange15, by sliding on the axial pins16(or, in the variant ofFIG. 5, by sliding along the axial grooves15a).

It will be appreciated that, owing to the contribution made by the two threaded couplings, the speed of linear translation of the rod11is very high and therefore the parking brakes can be activated rapidly, despite the fact that the threads have small angles of inclination (preferably less than 14° and more preferably of approximately 8°) in order to render the system non-backdrivable. It is desired that, owing to the wear between the mobile portions of the actuator, the rod should be capable of maintaining the retracted position reached and avoiding a situation where, in the absence of an operating command, the rod may be removed from the casing, for example if the vehicle V is parked on a slope.

The reaction forces transmitted in return to the actuator during braking may be discharged to the outside (onto a fixed component of the vehicle) by way of a flange28formed near the region where the rod emerges from the rigid portion12aof the casing or, alternatively, as illustrated with a broken line inFIG. 3, by way of the transverse end wall17of the casing, which acts as a check surface for a rigid sheath S which surrounds the rod and which is secured at its opposite end to the vehicle at a point (not illustrated) remote from the actuator. That variant advantageously enables the actuator to be arranged at any point in the vehicle, that is to say, even where there are no particularly rigid anchoring points present.

Owing to the proximity of the threaded members to the check portions or surfaces17and28, the path of the forces acting on the actuator when the latter is activated affects few members (the rod, the nut, the threaded bush and the casing) and those forces are discharged to the outside of the actuator without stressing the reduction unit14or the electrical motor13. This is advantageous if it is considered that the braking forces (of the order of approximately 3600 N) would inevitably increase the clearance between the transmission members of the reduction unit and the motor, thus reducing the service life of the actuator. The small number of members affected by the braking forces instead defines a very short kinematic chain which guarantees the reliability of the actuator over time.