The invention concerns a parking brake, especially for an automatic transmission for a motor vehicle or for an automatized, manual, motor vehicle shift.
Normally, with motor vehicles automatic transmissions possess a mechanically acting parking brake apparatus, in which, for example, when in park, a blocking pawl drops into a tooth gap of a parking brake gear. The gear is connected to the output shaft of the transmission and thus to an axle of the vehicle. Mechanical and electrohydraulic systems are known to be used as operative connectors between a parking brake and its regulatory apparatus in the interior of the motor vehicle. Mechanical operative connectors, for instance, comprise encased push/pull cables between a selector lever affixed to the body, an electro-hydraulic transmission control placed in the automatic transmission as well as to the parking brake activation apparatus located within the transmission housing. A purely electrical activation connection between the automatic transmission regulating apparatus in the interior of the vehicle and the electrohydraulic transmission control demands, as a condition, the conversion of the electrical signal into the mechanical movement of a pawl to block/unblock the parking brake. Essential advantages of the purely electrically executed actuation between the operative apparatus of the automatic transmission and the electrohydraulic transmission control are found in an acoustic decoupling of the operative apparatus from the input drive string of the motor vehicle and the free choice of the location of the regulating apparatus in the interior of the vehicle.
DE 43 22 523 A1, for instance, describes an electrohydraulic control device for an automatic transmission of a motor vehicle, with an electrically based operative connection between the regulatory apparatus in the interior of the vehicle for the automatic transmission and the electrohydraulic transmission control, wherein the shift stages D, R, and N are effected by the application of pressure to individual shifting members, while the stage P automatically engages upon loss of pressure in all connected shifting members.
This arrangement leads to the fact, that not only in stage P, but also in all shifting stages of the gear-shift lever, upon loss of pressure, the parking brake is actuated. The engagement of the parking brake is executed, in this case, by a spring accumulator, its release being carried out, in contrast, hydraulically. The supply of the necessary pressure for applying the said brake is effected by means of a pressurized oil circulation system, which is inherent in the automatic transmission. In order to be able to release the engaged parking brake in case of failure of the electrical voltage supply, or of a defect in the electronic control device of the automatic transmission, or yet a failure in the transmission oil supply, additionally an emergency disengagement arrangement is provided, by means of which the parking brake can be mechanically released.
DE 41 27 991 C2, for example, likewise discloses a parking brake system for a motor vehicle automatic transmission, with an electrically based operative connection between the regulatory apparatus of the automatic transmission, which is in the vehicle interior and the shiftable parking brake mechanism, wherein the parking brake is engaged by means of a mechanical spring accumulator and released by means of an electrohydraulic transmission control functioning at an elevated pressure. For this purpose, the activation rod, by which the parking brake is caused to engage and/or release, simultaneously serves as a piston in a single action cylinder-piston combination. For the release of the parking brake and for its retention in a released condition, the cylinder is subjected to pressure by a solenoid valve admitting pressurized oil. The hydraulic pressure is, advantageously, supplied directly from the pressurized oil circulation of the automatic transmission, which also furnishes the hydraulic transmission control. In the selected gear shift position P, the solenoid valve is in the closed position and the cylinder is freed of pressure. In the non-pressurized stance, the cylinder pushes the parking brake into the engaged position. The force for this action of the cylinder is supplied by the spring force of the spring accumulator. In one embodiment, the proposal is to provide an elevated pressure reservoir tank with a check valve, in order to hold the parking brake disengaged for a specified time if the motor stalls and pump pressure fails.
Finally, the generic DE 198 34 156 A1 discloses a parking brake system for an automatic transmission in a motor vehicle, wherein the parking brake pawl, as a result of sliding induced by a spring accumulator, falls into its restraining place and can be freed by means of hydraulic pressure, which is supplied by the oil circulatory system of the automatic transmission. At the same time the spring accumulator is pretensioned. For the prevention of an involuntary engagement of the parking brake upon loss of pressure, a hydraulic parking brake activation device provides a mechanical detent by one or more balls. The ball detent action is activated by an electromagnet, in which an activating rod is placed in a linearly sliding motion as a result of energization of the electromagnet. When this occurs, the balls are brought into the locking position by the activating rod which has a diameter equivalent to that of the ball. As long as the electromagnet is energized, just so long is the parking brake held in its non-braking position. A blocking mechanism which can carry out this operation requires, counter to the available opposing force of the spring accumulator, only a relatively small activation force to effect a linear motion in the activation rod which must be brought away from the electromagnet. The holding force of the ball detents, which likewise must be brought away from the electromagnet, is very small. The design criteria for the electro magnet are, the required pulling force for the activation of the braking mechanism, the path of the activation and the necessary holding power. In the case of a locking mechanism in accord with DE 198 34 156 A1, the activation path measures about 3 mm. Since it is known that the attraction force of an electro magnet decreases rapidly as separation therefrom increases, the result is that the electromagnet, with a relatively large activation path of the locking mechanism, must have correspondingly large dimensions. As a result of this, mounting space problems can arise and an unwanted large power must be provided to energize the electromagnet, which must be supplied, as a rule, by the electronic transmission control.
Thus, the present invention has the purpose of improving an electrohydraulically controlled parking brake, in accord with the generic state of the technology, in regard to a minimizing of the necessarily consumed electrical capacity and a reduction of the mounted space of the parking brake activation apparatus.
In accord with the invention, this purpose is achieved with a parking brake, which possesses the features of the principal claim. Further advantageous embodiments of the invention are evident in the subordinate claims.
The invention proceeds from the concept of a known parking brake system, in which a parking brake locking pawl for the blocking of a transmission is hydraulically disengaged, wherein, the required hydraulic energy for this action is advantageously supplied by the electrohydraulic control unit of the transmission. An advantageous activation element of the releasing arrangement of the parking brake is a hydraulically actuated cylinder, which is in mechanical connection with the parking pawl. Simultaneously, with the withdrawal of the parking pawl from its engagement, a spring accumulator is pretensioned, by means of the spring force of which, the parking brake, upon a depressured positioning of the hydraulic release element, engages. The now withdrawn parking brake lockup pawl is, in a known manner, held in its withdrawn position by a mechanical latch arrangement actuated by the electromagnet, which is energized in this situation.
In accord with the invention, the proposal is now made for mechanical activation of the locking of the parking brake blocking system to effect the holding of the parking brake in its non-braked condition. The locking, in this plan, is to be actuated by the blocking apparatus of the parking brake (the pawl) by the release of the same from its blocking position. An energized electromagnet holds the locking apparatus in its locking position for the retention of the parking brake in the non-braking condition. The blocking apparatus, especially the pawl which blocks the parking brake, also possesses a mechanical operative-connection to the locking apparatus, for instance, by means of a known interconnection disk of the parking brake system.
In the case of a released parking brake, the principle of the mechanical restraint of the parking brake blocking apparatus, in connection with the electromagnet for the holding of locking apparatus in its locking position, so acts in a known manner, that the electromagnet does not need to bring a full counter-force against overwhelming forces from the spring accumulator intended for the parking brake block, but need only create the restraining forces of the blocking element. The invented mechanical tension of the locking apparatus so acts, that the electromagnet does not have to generate any actuation forces, and accordingly, has only a purely holding function. In this way, the electromagnet need be designed to act constructively only on the holding forces. Consequently, the electrical capacity data and the installation size of said electromagnet is small.
In one embodiment of the invention, the proposal is made, to activate the locking apparatus simultaneously by hydraulic and electromagnetic means for the holding the parking brake in the non-braking state. Using as a starting point, the generic state of the technology disclosed by DE 198 34 156 A1, there becomes available in this way, a mechanical support for the pull of the electromagnet magnets, wherein the mechanical forces to be exerted for the activation of the locking apparatus are reduced in proportion to the path of the activation, in order to reduce the electromagnetic force. Correspondingly, the required electrical capacity as well as the installation space of the electrical equipment become less for the energization of the electromagnet.
A preferred, known principle of the mechanical locking of the parking brake has been disclosed by DE 198 34 156 A1. This design calls for a detent to be established by means of balls, for which a specially designed activation rod is linearly displaced by an energized electromagnet to provide actuation and holding functions. In this arrangement, the balls, by means of a conically shaped section of said rod, fix the said rod in a set position. In this locked position, the balls create a form-fit between, first, the actuating element of the parking brake apparatus, the actuating element possibly being the piston of a hydraulically or pneumatically operating cylinder and, second, an immovable part of the spring accumulator.
In this manner the activating element of the parking brake/release apparatus is mechanically blocked, as long as the actuating rod of the locking apparatus is held in its fixed position by the electromagnet.