Patent ID: 12234875

DETAILED DESCRIPTION

Because the fundamental manner of operation both of a drum brake and of a rotation-translation converter device (for example a ball-ramp device) are sufficiently well known they will not be discussed in further detail below.

FIG.1shows, by way of example and in a highly simplified form, a generic drum brake1of simplex design. Other known designs of drum brake are likewise adaptable however. Two substantially circular-arc-shaped brake shoes4,4′, which are each supported in a rotatable manner at their first end13,13′, are each spread apart at their second end12,12′ by a spreader unit2and, in the process, are pressed against the radially inner wall7of a pot-shaped brake drum6. The brake drum6is connected to a vehicle wheel (not shown) and rotates together with the latter with respect to the brake shoes4,4′. In the process, the brake linings5,5′ attached to the brake shoes4,4′ rub against the inner wall7. Due to the wear on the brake linings5,5′, the spreading travel required for a braking process increases continuously up to a defined wear limit. In order to compensate for any asymmetric, nonuniform contact between the linings5,5′ and the brake drum6, the spreader unit2is arranged in the drum brake in such a way as to be mounted to float, preferably along the central axis A thereof.

FIG.2shows an embodiment of the spreader unit2in an unactuated initial state in axial section. The spreader unit2has two ball-ramp devices3,3′, which act on the brake shoes4,4′ via brake shoe holders31,31′. For this purpose, each brake shoe holder31,31′ has a transverse slot36,36′, in each of which a flat end12,12′ of a brake shoe4,4′ is arranged. The spreader unit2also has a substantially tubular housing14equipped with a mounting flange. The spreader unit2is driven by an electric-motor drive unit8, indicated inFIG.1, via the external toothing10of a driving sleeve9, which, as a result, is rotated about the axis A and is supported in the housing14via rolling bearings15,15′ and is fixed axially by securing elements16,16′. Because the drum brake1according to the invention is provided as a service brake, all the braking processes and not just a static, constant application of force during parking, for instance, are initiated via the driving sleeve9.

In the embodiment shown, the rolling bearings15,15′ are in the form of needle bearings, but other types of rolling bearing are likewise permissible within the invention. The braking process is performed via the ball-ramp apparatuses3,3′ on the brake shoe holders31,31′ and the brake shoes4,4′ in engagement therewith.

Each ball-ramp device3,3′ mainly comprises a spreader piston22,23, an actuating piston17,18that is rotatable about the axis A with respect to the spreader piston22,23, and a multiplicity of balls34. At their respectively facing end faces, the first actuating piston17and the first spreader piston22, and the second actuating piston18and the second spreader piston23, respectively, each have an identical number of depressions32,33on the same circumferential circle, said depressions32,33being arranged in a manner distributed regularly in the circumferential direction. The depressions32in the actuating pistons17,18are each formed—in a manner flattening out gradually—in a first circumferential direction, and the depressions33in the spreader pistons22,23correspond to the depressions32but are flattened out in a second, opposite circumferential direction. A ball34is arranged between each depression32and33. As a result of the rotation of the actuating pistons17,18with respect to the spreader piston22,23in an actuating direction, the balls34roll into the flattened-out regions of the depressions32,33and push the piston pairs apart (and vice versa).

The two spreader pistons22,23are secured against rotation in relation to the housing14and are thus movable only linearly along the axis A. In the embodiment shown, the security against rotation is ensured by the support of the respective brake shoe holder31,31′ on the brake shoe4,4′ via the transverse slot36,36′. In this case, each brake shoe holder31,31′ is connected in a manner secured against rotation to the respective associated spreader piston22,23via an outer sleeve35,35′ bent inward at the edges. In this case, each spreader piston22,23is arranged in a manner spaced apart axially from the associated brake shoe holder31,31′ within the outer sleeve35,35′, with in each case a spring element37,37′ preloaded in between. As a result, a limited linear movement in the axial direction is made possible and this ensures that, as the drum shrinks, the increase in clamping force is limited by the compression of the spring element37in order to rule out damage to the brake.

The transmission of the rotary movement required for the ball-ramp apparatuses3,3′ is performed via a first actuating piston17and a second actuating piston18. For the purpose of uniform contact between the brake linings5,5′ and the inner wall7, the two actuating pistons17,18are arranged in the driving sleeve9in an axially floating manner and in a manner secured against rotation by means of axial guide.

A readjustment device26mainly comprises a readjustment piston19, which is in the same force transmission path as the first actuating piston17, and a latching sleeve27, which is in engagement with the readjustment piston19and is arranged in a manner secured against rotation with respect to the second spreader piston28but in a manner which allows axial movement to a limited extent and under elastic preload against the readjustment piston.

A separate readjustment piston19is screwed into a threaded hole20in the first actuating piston17as far as a stop position by means of a threaded portion21and thus lies in the force transmission path between the first actuating piston17and the second actuating piston18. At its end face facing the second actuating piston18, the readjustment piston19has an axially projecting annular first sawtooth ramp38having a plurality of tooth-shaped ramps or notches inclined in one circumferential direction.

The latching sleeve27is arranged in a manner substantially integrated into the second actuating piston18. It has an axially projecting annular second sawtooth ramp39, which corresponds to the first sawtooth ramp38and which likewise consists of a plurality of tooth-shaped ramps or notches inclined in an opposite circumferential direction.

The latching sleeve27is mounted via a central hole on a stem28of the second spreader piston23in a manner which allows axial movement and in a manner secured against rotation with respect to the housing14. In the embodiment shown, a transverse pin29is provided in the stem28to prevent rotation, the latching sleeve27being supported on said pin29in both circumferential directions via an axially projecting slotted collar42.

The latching sleeve27is pressed permanently with a slight force onto the sawtooth ramp38of the readjustment piston19by a compression spring30supported on the second actuating piston18. A sliding disk43reduces the friction and the rotation of the compression spring30when the second actuating piston18is rotated.

In the embodiment shown, the compression spring30is in the form of a wave spring. Further embodiments, for example a spiral spring or a disk spring assembly, are likewise permissible within the invention.

When the two actuating pistons17and18are driven by the driving sleeve9, the readjustment piston19is rotated relative to the latching sleeve27, which is immovable in the circumferential direction. In the process, the ramps of the readjustment piston19run up onto the ramps of the latching sleeve27during each actuation or braking operation and push them slightly into the second actuating piston18, counter to the spring action of the compression spring30. With increasing wear on the brake linings5,5′, the required stroke and thus the angle of rotation of the driving sleeve9and of the readjustment piston19coupled thereto increases. As soon as the angle exceeds the circumferential length of a ramp of the sawtooth ramps38,39, the axial toothing between the latching sleeve27and the readjustment piston19jumps into the next notch. If the brake is then released after a braking operation, the first actuating piston17rotates back into the unactuated initial position together with the driving sleeve9. However, the readjustment piston19is hindered from rotating back by the rotationally secured latching sleeve27and, as a result, is unscrewed from the first actuating piston17by the corresponding amount. Thus, when the drum brake1is released, the brake is readjusted incrementally in a fully automatically travel-controlled manner.

In the following text, embodiments of the blocking device which is used with the spreader unit according toFIG.2are described.

FIGS.3a, bshow, in this regard, a detail of the region of the claw clutch, consisting of the latching sleeve and the readjustment piston19. A blocking space50is located in between. To this end, an annular depression51is located in the end face of the readjustment piston19and a circular depression52that is coaxial therewith is located in the latching sleeve27. In this case, the outside diameter of the annular depression51is greater than that of the circular depression52. The sawtooth ramps38,39follow radially on the outside of the depressions. The bottoms of the depressions51,52are parallel to one another and opposite one another. The blocking space50is delimited on one side by the bottom of the annular depression51and on the other side by the opposite sawtooth ramp39on the latching sleeve27.

A spiral spring53made of a bimetal is located within the annular depression51. The depth of the annular depression corresponds to the width of the spiral spring53. One side of the spiral spring53lies on the bottom of the annular depression51such that its other side reaches as far as the tips of the sawtooth ramp39on the latching sleeve27. In this case, the spiral spring53can be fastened to a central peg in the annular depression51.

FIG.3ashows an unactuated brake in the cold state. The two sawtooth ramps38,39engage in one another such that the spiral spring53projects into the circular depression52. This is possible since its diameter in the cold state is smaller than the diameter of the circular depression52. As long as the brake does not heat up excessively during a braking operation, the diameter of the spiral spring53remains smaller than the diameter of the circular depression52such that, as shown inFIG.3b, it does not fill the annular depression51as far as the outside diameter thereof, although the two sawtooth ramps38,39are not in engagement and drop back into the next latching step upon further rotation in order that a wear-related correction of the clearance can subsequently take place, as is described in connection withFIG.2.

The situation in which the brake heats up significantly is different. This is illustrated inFIG.3c. When the two sawtooth ramps38,39are not in engagement, i.e. have carried out the maximum claw stroke, the annular spring expands into the blocking space50between the sawtooth ramp39on the latching sleeve27and the edge of the bottom of the annular depression51. Thus, even when they have rotated through a step width with respect to one another, the sawtooth ramps38,39cannot pass into engagement in a new latching step. Rather, when the brake is released, the sawtooth ramps38,39will initially take up their old angular position again and come into engagement there again later when the spiral spring53has cooled down and contracted again.

Since, however, not every braking operation is associated with increased development of heat, during one of the subsequent braking operations, readjustment will not be impeded, such that the lining wear that has accrued thus far can be compensated.

Instead of a spiral spring, it is also possible to use a star-shaped element according toFIGS.4a, b, which has a center55from which, for example, four arms56exhibiting bimetal extend. In the cold state, the arms56are shortened (FIG.4a) and in the heated state they are stretched (FIG.4b) such that their distal ends are located between the latching sleeve27and the readjustment piston in the blocking space50such that the sawtooth ramps38,39cannot take up a rotated latching position.

FIG.5ashows an embodiment of the bimetallic element in the form of two coaxially arranged disk springs60,61made of bimetal, which are arranged in a coaxial manner such that their mutually facing faces are concave in the cold state. Their edges in the blocking space50are thus located close together and cannot fill the width of the blocking space50. When the brake heats up (FIG.5b), the disk springs flap over such that their concave sides are now located on the outside and their edges are spaced apart. As a result, they fill the width of the blocking space50and so the sawtooth ramps38,39cannot take up a rotated latching position, thereby preventing readjustment.

In order to achieve a sufficiently great difference between the edge width in the cold state and in the heated state even in the case of small flapping angles of the disk springs60,61, it is possible—as shown inFIG.6—for two (or more) disk spring pairs65to be placed in succession, wherein a spacer ring66is located in each case in between.

The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the scope of the following claims.