A linear electro-mechanical actuator for a parking brake

The present invention relates to a linear electromechanical actuator, in particular for a parking brake of a motor vehicle, comprising an electric motor, a screw mechanism and a gear reduction unit interposed between the electric motor and the screw mechanism.

In an actuator of this kind, rotary movement imparted to the rotor of the electric motor is reduced by the reduction unit and converted by the screw mechanism into a linear movement of a threaded rod or output screw which, pulling a Bowden cable, applies a braking force to the parking brakes of the vehicle. Considered as a whole, an actuator for the parking brake must be irreversible (or "non-backdrivable"), i.e. it must be able to maintain the braking force imparted in the absence of a release command, hi other words, as a result of the friction between the movable parts of the actuator, the output rod must be able to maintain the position reached until a new operating command is given.

The screw mechanisms proposed hitherto for actuating parking brakes generally make use of friction screw mechanisms in an attempt to achieve a compromise between the requirement for "irreversibility" and a "direct" efficiency which is acceptable in terms of friction, hi this context, the expression "direct efficiency" is understood as meaning the efficiency obtained by operating the electric motor, while "indirect efficiency" indicates the efficiency obtained by operating the actuator output member. The maximum direct efficiency which can be obtained with the friction screw mechanisms is in the order of 35- 40%. hi order to achieve a performance above this level it is necessary to use screw mechanisms which, however, are not irreversible or risk becoming so during the working life of the actuator, resulting in instability and incorrect functioning of the system.

U.S. patent No. 6,655,507 describes a screw actuator for a parking brake comprising an electric motor and a ball screw. The actuator requires an electromechanical brake in order to maintain the position reached by the ball screw when no power is supplied by the electric motor.

The object of the invention is to provide an actuator which combines a high direct

efficiency with the requirement for irreversibility, or negative indirect efficiency, without the addition of auxiliary devices for preventing undesirable movements of the output screw when no commands are given by the electric motor.

These and other objects and advantages, which will be understood more clearly in the following, are achieved by a transmission unit and by an actuator having the features set forth in the accompanying claims.

The structural and functional features of a preferred, but non-limiting embodiment of the invention will now be described. Reference is made to the accompanying drawing which is a schematic cross-sectional view of an actuator according to the invention.

In the drawing, 10 designates overall a linear electromechanical actuator according to the invention, for operating the parking brakes (not shown) of a motor vehicle. The actuator 10 includes an electric motor 20 which, by means of an epicyclic reduction unit 30 with spur gears and an associated ball screw 40, imparts linear movements to a component C for connection to the parking brakes by means of a Bowden cable (not shown). The assembly formed by the epicyclic reduction unit 30 and the ball screw forms the transmission unit of the actuator.

The electric motor 20 is preferably a direct current permanent-magnet motor which is able to rotate selectively in both directions of rotation depending on the command signal received from an electronic control module (not shown). The motor 20 rotationally drives, by means of a pair of spur gears 21, 31, a planet wheel carrier 32 mounted rotatably about an axis x coinciding with the actuating axis of the screw mechanism 40 and parallel to the axis X ₁ of the electric motor. Two bearings 51 are situated between the planet wheel carrier 32 and a tubular support portion 52 of the casing 50 of the actuator. Designated 33 are the planet wheels of the reduction unit, which are mounted on cylindrical pivot pins 34. The planet wheels mesh with a first fixed sun gear 35 mounted on the support 52 and with a second sun gear 36 rotationally integral with the screw nut 41 of the ball screw 40. Rotation of the screw nut 41 causes the displacement of a screw or threaded rod 42 along the axis x and, consequently, the connection component C connected to the screw 42.

While the ball screw 40 is per se a reversible mechanism with a high mechanical performance, the epicyclic reduction unit has a negative "indirect" efficiency when no commands are given by the electric motor. This special feature means that the transmission unit is generally irreversible, while offering a suitable direct performance able to produce high actuating forces for the brake.

It will be appreciated that the epicyclic reduction unit owing to its characteristic feature of irreversibility, also performs the function of an actuator locking device, avoiding the costs associated with an additional locking device, as discussed in the introduction.

Whilst it is not desired to be bound to any specific theory in this connection, experimental tests carried out by the Applicant have shown that the factor which plays a decisive part in ensuring the irreversibility of the epicyclic reduction unit 30 is the sliding friction which develops at the interface between the planet wheels and the pivot pins of the planet wheel carrier. Li particular, the tests carried out by the Applicant have shown that excellent results may be obtained if the planet wheels 33 and the pivot pins 34 are made of steel which has undergone a surface hardening treatment.

It is understood that the invention is not limited to the embodiment described and illustrated here, which is to be regarded as an example; instead the invention may be subject to modifications regarding the form and arrangement of parts and constructional and functional details, for example as regards the number of teeth which determines the different transmission ratios of the gears of the epicyclic reduction unit.