GB2616877A - Linear actuator and braking system - Google Patents

Abstract

Linear actuator 10 for a hydraulic braking system, comprising an electric motor with a stator 6 and a rotor 2, a motor housing 7, and a nut 1 and screw 3 which are rotatable with respect to each other. The nut 1 is fixed to the rotor 2 and the screw 3 is fixed to the piston (20; fig. 4) which is axially moveable into a hydraulic pressure chamber (21; fig. 4). The axial length of the nut 1 may be longer than the axial length of the screw, e.g. three times longer. The nut and screw may form a ball screw drive, with ball roller bearings. The screw/spindle may include a rod with longitudinal channels (see figure 2) to prevent rotational movement of the screw relative to the piston. There may be a joint 14 to prevent axial deviation of the screw. There may be a rubber plug 16 between the screw and the housing for zero point protection.

Description

Description

Linear actuator and braking system The invention relates to a linear actuator for a hydraulic braking system, comprising an electric motor with a stator and a rotor, further comprising a motor housing, further comprising a nut and screw which are built rotatable with respect to each other, and with a piston axially moveable into a hydraulic pressure chamber. It also relates to a braking system.

In electrohydraulic braking systems, especially of brake-by-wire-type, braking pressure can be generated by an actuator without or in addition to the pressure created by muscle force of the driver. Depending on the driver braking wish and/or the desired value of the braking torque, the actuator pushes braking fluid into the brakes which are hydraulically connected to a fluid chamber of the actuator. In known braking systems, linear actuators with a drive train are employed.

The main components of the drive train of the linear actuator are the electric motor, which is typically built as a BLCD motor, a piston, a ball screw or ball screw drive for converting the rotational motion of the rotor of the motor into a linear motion of a piston, a torque support for the piston, and the piston housing.

To create pressure in the braking system, a torque is applied by the motor and the rotational movement is transferred using a coupling to the spindle of the ball screw.

To transform the rotational movement in translational movement, the nut of the ball screw is fixed rotationally by the torque support. The piston is fixed together with the ball screw and presents a translational movement inside the piston housing.

The pressure created by the braking system is created in a hydraulic cylinder which is mounted in a valve block or valve arrangement due to the axial displacement of a piston assembly comprising the piston and corresponding seals inside the cylinder. The piston assembly advances linearly and is pushed by a nut-profile pipe assembly of the ball screw drive which transforms the rotational movement of the BLCD motor into axial displacement. When the BLDG motor's stator is electrically powered, the rotor begins to rotate by operating with it an integrated interface in which the spindle screw of the ball screw drive is mounted. The rotation of the screw shaft is converted into the axial displacement of the profile pipe-nut assembly. The latter pushes the piston into the hydraulic cylinder. The screw rotates with the rotor and does not move axially. The nut profile assembly does not rotate due a torque support but moves axially along the screw.

The document DE 10 2012 213 216 Al discloses a linear actuator with an electric motor, a ball screw, a piston and a valve arrangement for providing individual braking pressures for the wheel brakes.

The linear actuators described above can lead to high costs due to the large number of complicated parts, large dimensions, and high assembly costs.

The object of the invention is to provide a linear actuator with reduced complexity, a smaller number of components and smaller dimensions. Furthermore, a corresponding braking system for a vehicle is provided.

With respect to the linear actuator, the object is solved according to the invention in that the nut is fixed to the rotor and the screw is fixed to the piston.

Preferred embodiments are described in the dependent claims and the figure description.

The invention is based on consideration that the volume occupied by a linear actuator should be as small as possible as the corresponding installation space is shared with other car equipment.

Applicant has recognized that these demands can be by replacing the current ball screw drive with a drive in which the nut is longer (depending on the required stroke), stiffened / fixed inside the rotor and rotating together with the rotor, which drives the screw, which has a threaded portion of smaller length, which moves axially without rotating due to its longitudinal channels and to the torque support. In contrast to new solutions, it is not the screw which rotates with the motor but the nut which thereby performs no axial motion.

Preferably, the axial length of the nut is larger than the axial length of the screw. This design is possible as the rotating component in the linear actuator according to the invention is the nut and not the screw. It allows to reduce axial length of the linear actuator and thereby to save installation space which becomes available for other components.

The axial length of the nut is preferably larger than the axial length of the screw by a factor between 2 and 4, especially 3. In an example embodiment, the screw has a length of ca. 30 mm while the nut has a length of ca. 100 mm. The outer diameter of the nut can be approx. 30 mm, with the ball splitting diameter being approximately 20 mm and the ball diameter being approximately 2.5 mm.

The screw is preferably arranged rotatable within the nut by means of a roller bearing. The rolling elements of the roller bearing are preferably built as balls.

Advantageously, the screw comprises a rod part with, especially four, longitudinal recirculation channels. The recirculation channels serve to recirculate the balls of the roller bearing.

In a preferred embodiment, a joint is mounted at the base of the screw. The joint serves to account for and compensate axial deviations between the axis of the hydraulic pressurization chamber and the nut axis, as these axes will not be 100% aligned. Hence a tolerance is required. Therefore, an articulation point between the screw and the 4-channel screw rod is advantageous.

Preferably, a plug is arranged in the motor housing between the screw and the motor housing which prevents damage of the housing during the zero point detection (ZPD) manoeuvre. The stroke of the piston is given by the stroke of the screw in the nut. How far the piston advances depends on the need for braking (a higher stroke represents a higher pressure, so a "firmer braking") therefore, the stroke is given from a lower point (which must be determined very precisely) to a certain position. The zero position (bottom point) can be checked when the screw retracts until it hits the housing. In order to prevent possible shocks of this manoeuvre, the plug can is inserted The plug is preferably made of rubber. Alternative materials are materials that withstand repeated blows! compressions and are chemically resistant to possible contamination with grease, etc. With respect to the braking system, the invention is solved by a linear actuator described above.

The invention has especially the following advantages. Due to the small dimensions of several components of the proposed linear actuator, low manufacturing prices can be realized. The elimination of some components yields lower complexity and low costs of manufacturing and assembly.

The final product comprises a reduced volume which allows for more space for the other components of the car. The main usage of the invention is in automotive industry and other areas where it is necessary to reduce the space in screw drives.

A preferred embodiment of the invention is described in connection with a schematic drawing in which FIG. 1 a linear actuator in a referred embodiment in an explosive view; FIG. 2 components of the linear actuator according to FIG. 1; FIG. 3 the linear actuator according to FIG. 1 an explosive and perspective view FIG 4 the linear actuator according to FIG. 1 in a lateral cut view; FIG. 5 an enlarged part of FIG. 4, and FIG. 6 the part shown in FIG. 5 with a rubber plug.

Same parts are labelled in all figures with identical reference numbers.

In FIG. 1, a linear actuator 10 in a preferred embodiment is shown. The linear actuator 10 comprises a nut 1, a screw 3, a torque support 4, a bearing 5, a motor housing 7 in which an electric motor 12 is arranged which comprises a stator 6 and a rotor 2 arranged rotatable inside the stator 6.

The linear actuator 10 differs from known linear actuators in that the ball screw drive (BSD) is replaced by a different drive. To this end, the nut 1 is built longer (depending on the required stroke) compared to a nut of a regular BSD and is stiffened / fixed inside the rotor 2 and rotating together with the rotor 2, which drives the screw 3, which has a threaded portion of smaller length, which moves axially without rotating due to its longitudinal channels and to the torque support 4. The screw 3 is fixed to a piston 20. In contract to known linear actuators, it is not the screw which rotates with the rotor 2 of the motor 12 but the nut 1.

When the rotor 2 is rotated, the nut 1 is rotated together with the rotor 2. Thereby, the screw 3, which is connected to a piston 20, is moved axially. In this way, the piston 20 is moved into a hydraulic pressure chamber 21 filled with braking fluid. A valve arrangement 22 or valve block comprises several valves and allows to increase the pressure in wheel brake sin a target-oriented way by opening or closing the corresponding valve.

In FIG. 2, the nut 1 and the screw 3 are shown in a detailed perspective view. The nut 1, which has a larger length compared to nuts of conventional linear actuators, rotates with the rotor 2 and does not move axially. The screw 3 does not rotate due to the torque support piece 4 but moves axially along the nut 1. This new ensemble or drive is henceforth called a "Ball Nut Drive" (BND). It comprises a roller bearing 23, whereby the balls (return mechanism) are recirculated by the screw 3 through recirculation channels formed inside screw 3.

The screw 3 therefore rotates within nut by means of a roller bearing 23 in which balls 24 are used as rolling elements. The screw 3 comprises a rod part 25 in which four channels 26 are built in equal distance around the circumference of the rod part 25. These channels 24 cooperate with pins of torque support 4 which slide inside these channels, thereby preventing the rotation of the screw 3.

In FIG. 3, the linear actuator 10 is shown in an explosive and perspective view and in FIG. 4 in a sectional view. The design of the linear actuator 10 allows to reduce the size of the rotor 2, the stator 6, the bearing 5 and a further bearing 8 and the motor housing 7. The proposed linear actuator 10 compared to known linear actuators no longer includes a coin assembly at the end of the ball screw drive. A washer, a rubber, a friction disc, circlips, and a spiral snap ring are not needed in the proposed linear actuator 10 according to the invention. The length of the screw 3 is reduced, eliminating the end that is machined hexagonally.

Another aspect of the proposed variant is that he transmission of the rotational movement from the outside to the inside is fluent. In known linear actuators, the rotor transmits the rotational movement to the rotor tube, then through the coin to the screw 3, then to the nut 1. As seen in FIG. 5, in the preferred embodiment of the linear actuator 10 shown, the rotational movement is transferred from the rotor 2 to the nut 1 (large diameter -step A) and then from the nut 1 to the screw 3 (small diameter -step B).

In order to keep or maintain some mechanical properties of known solutions, in the linear actuator 10 a joint 12 can be inserted at the base 15 of the screw 3 to account for any axial deviations between the axis of the hydraulic pressure chamber 21 and the nut 1. A rubber plug 16 can be arranged at the opposite end 18 between the screw 3 and the motor housing 7 for realisation of the ZPD function.

List of reference numerals 1 nut 2 rotor 3 screw 4 torque support bearing 6 stator 7 motor housing 8 bearing linear actuator 12 electric motor 14 joint 15 base 16 rubber plug 18 opposite end piston 21 hydraulic pressure chamber 22 valve arrangement 23 roller bearing 24 ball rod part 26 channel

Claims (10)

1. Patent claims 1. Linear actuator (10) for a hydraulic braking system, comprising an electric motor (12) with a stator (6) and a rotor (2), further comprising a motor housing (7), further comprising a nut (1) and screw (3) which are built rotatable with respect to each other, and with a piston (20) axially moveable into a hydraulic pressure chamber (21), characterized in that said nut (1) is fixed to said rotor (2) and said screw (3) is fixed to said piston (20).
2. 2. Linear actuator (10) according to claim 1, whereby the axial length of the nut (1) is larger than the axial length of the screw (3).
3. 3. Linear actuator (10) according to claim 2, whereby the axial length of said nut (1) is larger than the axial length of said screw (3) by a factor between 2 and 4, especially 3.
4. 4. Linear actuator (10) according to one of the claims 1 to 3, whereby said screw (3) is arranged rotatable within said nut (1) by means of a roller bearing (23).
5. 5. Linear actuator (10) according to claim 4, whereby the rolling elements of said roller bearing (23) are built as balls (24).
6. 6. Linear actuator (10) according to claim 5, whereby said screw (3) comprises a rod part (25) with, especially four, longitudinal channels (26).
7. 7. Linear actuator (10) according to one of the previous claims, whereby a joint (14) is mounted at the base (15) of said screw (3).
8. 8. Linear actuator (10) according to one of the previous claims, whereby a plug (16) is arranged in said motor housing (7) between said screw (3) and said motor housing (7).
9. 9. Linear actuator (10) according to claim 8, whereby the plug is made of rubber.
10. 10. Braking system for a vehicle, comprising a linear actuator according to one of the previous claims.