The invention relates to a brake cylinder for motor vehicle brakes, in particular for brakes of commercial vehicles.
In order to generate the necessary great brake forces, commercial vehicles are equipped with brakes which are actuated by compressed air. What is known as the service brake is actuated here by way of compressed air cylinders, preferably in the form of diaphragm cylinders. In contrast, the parking brake cannot be actuated with the compressed air cylinder, as there is a legal requirement which states that parking brakes may only act mechanically, as only in this way is a constant brake force ensured over a practically unlimited time period. In the case of brakes which are loaded with compressed air, a drop in the brake force can namely occur as a result of leakages.
For this reason, the brakes of commercial vehicles are equipped with what are known as spring cylinders. Here, the brake actuating force is generated via a stressed spring. This, therefore, satisfies the requirement of a mechanically acting parking brake. As these spring cylinders have very high forces available as a rule, manual release in the service case is no longer appropriate. The release of the parking brake therefore takes place by means of compressed air. In order to make this possible, spring cylinders are equipped with a release piston. It is possible by way of this piston to compress the spring upon introduction of compressed air to such an extent that the brake is released. In order that the parking brake is not engaged during driving, the piston is loaded continuously with compressed air, with the result that the spring cannot relax and the parking brake therefore does not become active.
A parking brake is usually required only on one axle; this is the rear axle as a rule. The brakes on the rear axle are therefore equipped with what are known as combined cylinders. A combined cylinder comprises in practice two different cylinder types, namely a spring cylinder for the parking brake and a compressed air cylinder for the service brake. This is usually a diaphragm cylinder.
The two cylinders are usually arranged one behind another in the axial direction. The spring cylinder, which acts by way of its piston rod on the piston collar of the diaphragm cylinder and from this on the brake lever, is arranged behind the diaphragm part of the combined cylinder.
A disadvantage of this design is that a relatively large amount of installation space is required. It is, therefore, frequently very difficult to install these combined cylinders on front axles on account of the steering kinematics. The difficulties are increased still further as a result of the introduction of more complex chassis systems, such as individual wheel suspension systems. However, on account of the increasing requirements of the performance of the parking brake system, there is increasingly the desire to also install parking brakes on the front axle. However, this can frequently not be realized with the customary combined cylinders, which have been described above.
The present invention is based on the object of developing designs for combined cylinders which require considerably less installation space.
This and other objects are achieved by a brake cylinder for motor vehicle brakes, in particular for brakes of commercial vehicles, having a spring brake section, in particular for carrying out parking brake operations by means of a spring force accumulator spring, and a service brake section for carrying out service brake operations which are actuated by compressed air, the spring brake section and the service brake section being combined to form one structural unit, the brake cylinder being divided into two chambers by a piston, one of the two chambers serving as a pressure chamber for actuating the service brake, and the spring force accumulator spring of the spring section being arranged in a further chamber on the opposite side of the piston. A configuration is provided such that the spring force accumulator spring is releasable by ventilation of a second pressure chamber. The spring force accumulator spring acts on a further piston, which can be locked with respect to the piston by way of a locking mechanism, and can be released from the piston by release of the locking mechanism, and which is connected to a piston rod for actuating the vehicle brake directly or via further elements. The spring force accumulator spring acts between the two pistons after the release of the locking mechanism and displaces the two pistons relative to one another during a parking brake operation.
In contrast to the previously known embodiments, the present invention makes possible a structural design of the brake cylinder wherein the spring cylinder may be released by actuation of the cylinder for the service brake. The brake cylinder according to the invention could also be called a “short length combined” cylinder.
In one preferred embodiment, there is provision for the brake cylinder to be divided into two chambers by a piston, one of the two chambers serving as a pressure chamber for actuating the service brake, and the spring force accumulator spring of the spring section being arranged in the further chamber on the opposite side of the piston. However, this further chamber remains pressureless during actuation of the service brake and also during actuation of the parking brake. Furthermore, a restoring spring for the service brake is arranged in this further chamber on the opposite side of the piston. This design satisfies the requirements for a compact construction.
The brake cylinder is also equipped with a second pressure chamber. The design is selected in such a way that the spring force accumulator spring can be released by ventilation, that is to say by the dissipation of pressure in this second pressure chamber. The structurally simple solution results if the spring force accumulator spring acts on a further piston which can be locked with respect to the piston for the service brake by means of a locking mechanism and can be released from said piston by release of the locking mechanism, and which is connected to a piston rod for actuating the vehicle brake directly or via further elements, the spring force accumulator spring acting between the two pistons after the release of the locking mechanism and displacing said two pistons relative to one another during a parking brake operation.
There is provision here, during the parking brake operation, for the piston which is loaded by the spring force accumulator spring to be capable of displacement relative to the piston for the service brake operation. As has already been mentioned, it is particularly advantageous that, in order to release the piston which is loaded by the spring force accumulator spring, first of all the piston for the service brake operation can be moved in the direction of the piston for the parking brake, that the locking mechanism can be closed subsequently, and that subsequently both pistons can be moved into the release position by way of the restoring spring by pressure relief of the piston for the service brake operation. Accordingly, the piston for the service brake operation has a dual function, namely the actuation of the brake during driving and the restoring of the spring-loaded piston into the release position. The locking mechanism is closed by the application of pressure to the piston which is loaded by the spring force accumulator spring, at the very latest after the release position has been reached. This expediently takes place when the piston for the service brake operation is moved into the position for driving the piston for the parking brake operation.
If, however, only the piston for the parking brake operation is to be moved, first of all the locking mechanism has to be released. This expediently takes place by pressure relief of the piston for the parking brake operation.
In a structurally simple manner, the locking mechanism includes two conical faces which make contact with one another in the locked position. Here, one conical face is arranged on the piston for the service brake operation and the other conical face is arranged on a switching cylinder which is operatively connected to the second pressure chamber. The conical faces are formed in such a way that the conical face, which is assigned to the piston for the service brake operation, increases in the moving direction of the two pistons. The cone angle is relatively small, with the result that the conical faces can become clamped. The switching cylinder is expediently configured in two pieces and includes a switching ring which lies facing the piston for the service brake operation and a switching cup which lies facing away from the piston for the service brake operation, the switching cup containing the second pressure chamber for actuating the locking mechanism.
In order to release the locking mechanism, there is provision for an annular chamber to be provided between the piston for the parking brake operation and the switching ring, in which annular chamber a compression spring is inserted which is supported at one end on an inwardly oriented collar of the piston and at the other hand on an outwardly oriented collar of the switching ring. As soon as the second pressure chamber is ventilated, that is to say becomes pressureless, the spring becomes active and releases the locking mechanism, or the two conical faces move out of contact. In order that the second pressure chamber is completely sealed off, there is provision for the switching ring and the switching cup to overlap, and for a seal to be introduced into this region.
There is provision according to a second embodiment for the piston, which is loaded by the spring force accumulator spring, to be in engagement with a threaded spindle which is operatively connected to a brake in such a way that, in the release state of the brake cylinder, the spindle is secured against rotation by the applied brake force, and that the threaded spindle is capable of rotation as a result of the reduction or cancellation of the brake force, with the result that the piston can be moved in the longitudinal direction of the spindle as a result of the relief of the spring force accumulator spring.
In this embodiment, the force which acts from the spring force accumulator spring can be regulated.
There is also provision in this embodiment for the threaded spindle not to be self-locking, and for it to be possible for the frictional force of the brake to be regulated by use of a control piston, which can be loaded with pressure. The pitches of the threads of the threaded spindle are accordingly very high, with the result that the adjusting speed of the spring-loaded piston is correspondingly high.
In order to transfer the brake cylinder from the parking brake operation into the release position, there is provision for it to be possible for the threaded spindle to be driven rotationally. As a result, the spring-loaded piston which is in engagement with the threaded spindle is moved along the spindle in the direction of the piston for the parking brake operation. As in the embodiments which were described first of all, the service brake operation takes place by the application of pressure to the corresponding piston.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.