BACKGROUND OF THE INVENTION The improvement contemplated here is related especially to a pneumatic disc (30) brake as described in US Patent RE 35,055 reissued October 10,1995 based on US Patent 5,205, 380 to Jacques Paquet and Claude Rancourt. This patent includes a positive mechanical brake of the type known as a fail-safe or parking brake in a pneumatic pressure type brake system. Thus in such fail-safe brake systems mechanical brakes are preloaded with springs and a pressurized bladder will overcome the preloaded mechanical brake so that the parking brake will not be applied to the discs when the air pressure in the bladder is great enough to overcome the preloaded springs. If there is a pressure drop in the air bladder the parking brake will be applied.

SUMMARY OF THE INVENTION An aim of the present invention is to provide an improved fail-safe braking system including a device for disengaging the preloaded disc brakes, even though the pressurized air system might be down, in order to allow for towing of the vehicle.

Thus, a fail-safe braking system in accordance with the present invention comprises a disc brake assembly for a vehicle wheel on a vehicle, wherein the wheel includes a hub journalled to an axle on the vehicle. The disc brake assembly is concentric with the axle and includes a housing mounted to the vehicle, a radial disc within the housing and means mounting the disc to the wheel. The disc has at least a first radial planar friction surface and a first brake shoe provided adjacent the first planar friction surface movable axially towards

and away from the first friction surface of the disc for friction engagement therewith and release thereof. Means are provided for restraining the first brake shoe from rotating with the disc. An intermediate housing wall member extends parallel to the radial disc and is located such that the first brake shoe moves axially between the intermediate housing wall and the radial disc. A movable abutment means is mounted for axial movement within the housing between the intermediate wall and a housing wall remote from the first brake shoe such that the intermediate wall is between the movable abutment means and the first brake shoe. Spring means extend between the housing wall and the abutment means.

Push rods extend between the spring means and the first brake shoe such that in a parking brake mode the spring means urges against the push rods to press the first brake shoe against the first friction surface of the disc. A first fluid bladder is provided between the intermediate wall and the abutment means whereby the first fluid bladder, when expanded, forces the abutment means to overcome the spring means to release the first brake shoe from the first friction surface of the disc. The spring means includes a plurality of spring assemblies spaced apart circumferentially on the housing wall and levers are associated with each spring assembly whereby each lever has a first pivot on the intermediate housing wall and a second pivot on the abutment means and a spring assembly engagement portion on the lever between the first and second pivot points such that the lever will cause the respective spring assembly to be compressed when the first bladder is inflated, thereby retracting the push rods in order to release the first brake shoe from the first friction surface.

BRIEF DESCRIPTION OF THE DRAWINGS: Figure 1 is a perspective view of a disc brake assembly in accordance with the present invention taken from one end thereof; Figure 2A is a fragmentary exploded view of the disc brake assembly in accordance with Figure 1 ; Figure 2B is an enlarged fragmentary exploded view based on Figure 2A;

Figure 3 is a perspective view. of the disc brake assembly taken from the other end thereof ; Figure 4 is a perspective partially exploded view of the disc brake assembly shown is Figures 1 and 3; Figure 5 is a perspective exploded view of a detail of the brake assembly shown in Figure 4; Figure 6 is a further perspective view of the detail shown in Figure 5 but from a different angle; Figure 7 is an enlarged perspective view of the detail shown in Figures 4 and 5 but assembled; Figure 8 is a perspective exploded view of a further detail of the invention; Figure 9 is a fragmentary axial cross-section taken of the detail shown in Figures 6 to 8; Figure 10 is a fragmentary enlarged axial cross-section similar to Figure 9 but showing the detail in a different operative position.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a disc brake assembly (10) in accordance with one possible embodiment of the present invention. This disc brake assembly (10) is somehow similar to the ones shown and described in U. S. Patents Nos. 5,205, 380 (RE 35,055) and 5,330, 034, which are hereby incorporated by reference.

However, the disc brake assembly (10) of the present invention is designed with some new features, including a new parking brake.

The disc brake assembly (10) is designed to be used on a large truck or bus with a pneumatic braking system. Slightly different versions can be designed for other kinds of vehicles, including vehicles with a hydraulic braking system such as for aircrafts : In accordance with the present invention, and as best shown in FIGS. 1,2A and 2B, the disc brake assembly (10) comprises a side flange (12) bolted to a central hub (14). This side flange (12) supports a number of mounting bolts (18) provided to secure the corresponding wheel or wheels on the vehicle

(not shown). The disc brake assembly (10) also comprises an external cover (20) bolted to a housing (22). A rotor (30), also referred to as an annular rotor disc (30), is slidably mounted on the hub (14) between the external cover (20). The rotor (30) is essentially a radial disc (30) or, preferably, a pair of disc (30) s between which ventilation channels are provided. Rotational engagement between the hub (14) and the rotor (30) is made possible through a set of axial splines (14a) provided on the hub (14) and corresponding inner teeth (30a) provided on the interior of the rotor (30).

The disc brake assembly (10) comprises rotating parts and fixed parts.

The rotating parts rotate with reference to the fixed parts. The fixed parts are those that are directly or indirectly rigidly attached to the axle of the vehicle. One of the rotating parts of the disc brake assembly (10) is the hub (14) on which the rotor (30) is mounted. Non-rotating parts include sets of brake pads. One set of brake pads (40) is attached to the external cover (20). The second set of brake pads (50) is attached to a slidable plate. referred to as the inner carrier (52). An annular air bellows (80) is located between the intermediate housing wall (23) of the housing (22). The air bellows (80) is in fluid communication with the pneumatic braking system of the vehicle. In some vehicles, a hydraulic actuator would be used instead and connected to a corresponding hydraulic braking system.

When braking, the air bellows (80) is filled with pressurized air to expand the bellows or bladder (80) to thereby generate the braking force. This force urges a piston plate (81) to slide away from the intermediate housing wall (23). The movement of the piston plate (81) causes the inner carrier (52) to move as well, thereby urging the second set of braking pads (50) against the corresponding side surface of the rotor (30). The rotor (30) then moves axially against the first set of brake pads (40). The mechanical energy of the rotation is transformed into heat due to the friction between both side surfaces of the rotor (30) and the brake pads (40,50). The braking action stops when the pressure in the air bellows (80) falls under a given value. Return springs or other devices (not shown) are used to move the inner carrier (52) and the piston plate (81) back to their original position.

FIGS. 2A and 2B show an exploded view of the disc brake assembly (10). They illustrate the various components described herein. They also show other components, including a wear compensation assembly (90) and its corresponding plate (92). This plate (92) is set between the piston plate (81) and the inner carrier (52). It should be noted that in FIG. 2A, the piston plate (81) and the wear compensation plate (92) are in engagement with one another. They are shown detached from one another in FIG. 2B.

All vehicles are required to have a parking brake. Some vehicles comprise a mechanical system, activated for instance using a lever or a cable, in order to lock the brakes in a full braking position. In the case of the present invention, the disc brake assembly (10) is designed to set itself in a full braking mode when the pressure in the pneumatic air supply becomes zero. The same could have been achieved using a hydraulic system. Most legislations require that a vehicle with a pneumatic braking system be provided with a safety feature in case of a failure thereof. Accordingly, if the pressure of the pneumatic air supply is lost for any reason, the disc brake assembly (10) must go to a full braking mode.

A truck that loses its source of pressurized air would be prevented from moving.

To achieve this goal, the disc brake assembly (10) is provided with a plurality of small spring assemblies actuators (100) located on the housing wall (114). Each spring assembly is made up of a pair of disc springs (102), also called "Belleville springs", mounted with interference on push rods (104) Preferably, a plurality of Belleville springs (102) are stacked together to increase the return force. Each stack urges a free end of each of the push rods (104) against a corresponding flange (106) radially projecting from the piston plate (81). During the parking brake mode, these push rods (104) apply a strong axial force on the piston plate (81) and maintain the disc brake assembly (10) in a full braking mode.

Each stack of Belleville springs (102) is maintained in a"disabled" position when the vehicle is in a travel mode, which happens when the pneumatic system of the vehicle functions normally and when the driver has released the manual parking brake.

FIG. 4 shows that on the other side of the intermediate housing wall (23) there is provided an additional annular air bellows (110) and an abutment

ring (112) to be mounted therein. An annular housing wall (114) is then attached to the housing (22) by means of bolts (116). As can be seen, five sets of braking actuators (100) are used in the illustrated embodiment. Five casings (120) are thus provided to enclose the stacks of Belleville springs (102) and their push rods (104) (104). Each casing (120) is closed by a corresponding cover (122) bolted thereon.

Each cover (122) is used to retain the outer periphery of the corresponding disc springs (102).

FIGS. 5 and 6 show a partially exploded view of one of the actuators (100).

FIG. 7 shows the braking actuator (100) in place. These drawings show that the braking actuator (100) comprises a lever (130). Stub shafts (130a) of the lever (130) are pivoted in the abutment ring (112) about a first pivot, mounted for axial movement next to the additional annular air bellows (110).

Notches (112a) are provided on the back of the abutment ring (112) to receive the stub shafts (130a), as best shown in FIG. 8. The other end of the lever (130) is pivoted about a shaft mounted on the intermediate wall at 130b.

FIGS. 9 and 10 show the operation of the braking actuators (100). In FIG. 9, the illustrated braking actuator (100) is in the parking mode. The Belleville springs (102) urge the push rod (104) forward. As can be appreciated, the combined force from all brake actuators (100) will create a relatively large force. The bellows (110) is shown collapsed so, that the abutment ring is in its furthest position from the spring actuators (100).

In FIG. 10, the additional annular air bellows (110) is pressurized, thereby expanding the bladder. It will remain constantly pressurized until the parking brake is activated once again or if the air pressure supply is depressurized.

When pressurized, the additional annular pressure bellows (110) pushes on the abutment ring (112) and thus the levers (130), which then pivot counter clockwise around the second pivot point (130b) on the intermediate wall (23). This causes the Belleville springs (102) to be compressed, thereby retracting the push rod (104).

It should be noted that in FIG. 10, only the abutment ring (112) is shown with the bellows (110) in a pressurized state.

A manual brake release is provided at each braking actuator (100).

This allows the parking brake to be released without air pressure. For instance, if the vehicle breaks down, one must be still able to tow it away. To do so, each manual brake release of the illustrated embodiment comprises a release screw (140) designed to be rotated on itself in order to axially move a follower (142).

Moving the follower (142) will move the corresponding lever (130) to the appropriate location in order to disengage the braking actuator (100).