SPREADER UNIT FOR A MOTOR VEHICLE DRUM BRAKE

Abstract

A spreader unit for a drum brake is provided with a readjustment device which ensures that the clearance remains substantially constant even in the case of wear on the brake linings. The readjustment device is arranged between actuating pistons and changes the spacing of the actuating pistons in at least one readjustment step when an actuating travel of the first and second brake shoe holders has exceeded a travel setpoint value. A thermally actuable blocking device mechanically prevents the execution of the readjustment step by the readjustment device at a temperature above a temperature setpoint value

Claims

1-14. (canceled)

15. A spreader unit for a drum brake, comprising: a housing; a first and second brake shoe holders that are rotationally secured to the housing, and actuatable linearly along an axis, respectively away from one another in a spreading direction and respectively toward one another in a release direction; a first and a second brake shoe provided respectively with a brake lining; a readjustment device arranged between the actuating pistons, wherein the readjustment device changes the spacing of the actuating pistons in at least one readjustment step when an actuating travel of the first and second brake shoe holders has exceeded a travel setpoint value; and a thermally actuable blocking device mechanically prevents the execution of the readjustment step by the readjustment device at a temperature above a temperature setpoint value.

16. The spreader unit as claimed in claim 15, wherein the readjustment device further comprises a freewheel clutch having two claws, wherein the claws carry out a relative stroke during freewheeling and wherein one claw is rotationally secured to the first brake shoe holder and the other claw is connected to the second actuating piston via a threaded spindle, and wherein the blocking device is arranged operatively parallel to the clutch to keep the claws apart in the activated state.

17. The spreader unit as claimed in claim 16, wherein the claws have, on a respective one of their end faces, sawtooth ramps that extend in a circle and engage in one another, wherein the length of a ramp on the circular arc defines the predetermined step width and the height of the ramp defines a maximum claw stroke.

18. The spreader unit as claimed in claim 16, wherein the blocking device has a bimetallic element, and a blocking space is located between and is laterally delimited by the claws, wherein at least a portion of the bimetallic element passes into the blocking space at least when the maximum claw stroke has been reached.

19. The spreader unit as claimed in claim 18, wherein the bimetallic element has a spiral spring, the diameter of which changes depending on temperature and wherein an outer edge of the spiral spring passes into the blocking space upon temperature-dependent expansion.

20. The spreader unit as claimed in claim 18, wherein the bimetallic element has at least one arm extending from a center, which lengthens depending on temperature and wherein a distal end passes into the blocking space upon temperature-dependent lengthening of the arm.

21. The spreader unit as claimed in claim 20, wherein the bimetallic element has a plurality of arms.

22. The spreader unit as claimed in claim 21, wherein the plurality of arms are arranged in a star shape.

23. The spreader unit as claimed in claim 18, wherein the bimetallic element has two disk springs which are arranged in opposite directions, one respective side face of which assumes a convex or concave shape depending on temperature and at least the outer edges of which are located in the blocking space.

24. The spreader unit as claimed in claim 23, wherein at least two pairs of disks are provided.

25. The spreader unit as claimed in claim 24, wherein a spacer ring is located between the pairs of disks.

26. The spreader unit as claimed in claim 16, wherein the claw connected to the first brake shoe holder is a latching sleeve which is firmly connected to the first brake shoe holder, and the other claw coupled to the second actuating piston is a readjustment piston, wherein the latching sleeve is rotationally secured with respect to the second brake shoe holder, is axially movable with respect to the second actuating piston, and is elastically preloaded against the readjustment piston.

27. A spreader unit for a drum brake comprising: a housing; a first and second brake shoe holders that are rotationally secured to the housing, and actuatable linearly along an axis, respectively away from one another in a spreading direction and respectively toward one another in a release direction; a first and a second brake shoe provided respectively with a brake lining, having at least one rotation-translation converter mechanism; a readjustment device arranged between the actuating pistons, wherein the readjustment device changes the spacing of the actuating pistons in at least one readjustment step when an actuating travel of the first and second brake shoe holders has exceeded a travel setpoint value; and a thermally actuable blocking device which mechanically prevents the execution of the readjustment step by the readjustment device at a temperature above a temperature setpoint value.

28. The spreader unit of claim 27, wherein the at least one rotation-translation converter mechanism is at least one of: a ball-ramp apparatuses, wherein a first ball-ramp apparatus has a first actuating piston that is arranged so as to be rotatable about an axis and optionally a second ball-ramp apparatus has a second actuating piston that is arranged so as to be rotatable about an axis; at least one nut-spindle transmission; and a mixed configuration having features of the ball-ramp apparatuses and the at least one nut-spindle transmission.

29. The spreader unit of claim 27, wherein the spreader is electromechanically drivable with the aid of an electromechanical actuator comprising an electric motor.

30. The spreader unit of claim 29, wherein the spreader is equipped indirectly or directly with an electric load measurement device,

31. The spreader unit of claim 27, wherein the electric load measurement devices cooperates with the load measurement device for the purpose of applying the drum brake under electrical open-loop and/or closed-loop control.

32. The spreader unit of claim 29, wherein the at least one nut-spindle transmission has inserted rolling bodies,

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:

[0029] FIG. 1 shows a highly simplified sectional view of a drum brake of simplex design with a spreader unit;

[0030] FIG. 2 shows an embodiment of the spreader unit in axial section as yet without a blocking device;

[0031] FIGS. 3a, b, c show an enlarged illustration of the spreader unit with a first embodiment of the blocking device according to the invention with a bimetallic spiral spring in various stages of brake actuation;

[0032] FIGS. 4a, b show a star-shaped bimetallic element as a second embodiment of the blocking;

[0033] FIGS. 5a, b show a bimetallic element consisting of two disk springs as a third embodiment of the blocking device; and

[0034] FIG. 6 shows a bimetallic element consisting of two disk spring pairs as a fourth embodiment of the blocking device.

DETAILED DESCRIPTION

[0035] 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.

[0036] FIG. 1 shows, by way of example and in a highly simplified form, a generic drum brake 1 of simplex design. Other known designs of drum brake are likewise adaptable however. Two substantially circular-arc-shaped brake shoes 4, 4′, which are each supported in a rotatable manner at their first end 13, 13′, are each spread apart at their second end 12, 12′ by a spreader unit 2 and, in the process, are pressed against the radially inner wall 7 of a pot-shaped brake drum 6. The brake drum 6 is connected to a vehicle wheel (not shown) and rotates together with the latter with respect to the brake shoes 4, 4′. In the process, the brake linings 5, 5′ attached to the brake shoes 4, 4′ rub against the inner wall 7. Due to the wear on the brake linings 5, 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 linings 5, 5′ and the brake drum 6, the spreader unit 2 is arranged in the drum brake in such a way as to be mounted to float, preferably along the central axis A thereof.

[0037] FIG. 2 shows an embodiment of the spreader unit 2 in an unactuated initial state in axial section. The spreader unit 2 has two ball-ramp devices 3, 3′, which act on the brake shoes 4, 4′ via brake shoe holders 31, 31′. For this purpose, each brake shoe holder 31, 31′ has a transverse slot 36, 36′, in each of which a flat end 12, 12′ of a brake shoe 4, 4′ is arranged. The spreader unit 2 also has a substantially tubular housing 14 equipped with a mounting flange. The spreader unit 2 is driven by an electric-motor drive unit 8, indicated in FIG. 1, via the external toothing 10 of a driving sleeve 9, which, as a result, is rotated about the axis A and is supported in the housing 14 via rolling bearings 15, 15′ and is fixed axially by securing elements 16, 16′. Because the drum brake 1 according 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 sleeve 9.

[0038] In the embodiment shown, the rolling bearings 15, 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 apparatuses 3, 3′ on the brake shoe holders 31, 31′ and the brake shoes 4, 4′ in engagement therewith.

[0039] Each ball-ramp device 3, 3′ mainly comprises a spreader piston 22, 23, an actuating piston 17, 18 that is rotatable about the axis A with respect to the spreader piston 22, 23, and a multiplicity of balls 34. At their respectively facing end faces, the first actuating piston 17 and the first spreader piston 22, and the second actuating piston 18 and the second spreader piston 23, respectively, each have an identical number of depressions 32, 33 on the same circumferential circle, said depressions 32, 33 being arranged in a manner distributed regularly in the circumferential direction. The depressions 32 in the actuating pistons 17, 18 are each formed—in a manner flattening out gradually—in a first circumferential direction, and the depressions 33 in the spreader pistons 22, 23 correspond to the depressions 32 but are flattened out in a second, opposite circumferential direction. A ball 34 is arranged between each depression 32 and 33. As a result of the rotation of the actuating pistons 17, 18 with respect to the spreader piston 22, 23 in an actuating direction, the balls 34 roll into the flattened-out regions of the depressions 32, 33 and push the piston pairs apart (and vice versa).

[0040] The two spreader pistons 22, 23 are secured against rotation in relation to the housing 14 and 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 holder 31, 31′ on the brake shoe 4, 4′ via the transverse slot 36, 36′. In this case, each brake shoe holder 31, 31′ is connected in a manner secured against rotation to the respective associated spreader piston 22, 23 via an outer sleeve 35, 35′ bent inward at the edges. In this case, each spreader piston 22, 23 is arranged in a manner spaced apart axially from the associated brake shoe holder 31, 31′ within the outer sleeve 35, 35′, with in each case a spring element 37, 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 element 37 in order to rule out damage to the brake.

[0041] The transmission of the rotary movement required for the ball-ramp apparatuses 3, 3′ is performed via a first actuating piston 17 and a second actuating piston 18. For the purpose of uniform contact between the brake linings 5, 5′ and the inner wall 7, the two actuating pistons 17, 18 are arranged in the driving sleeve 9 in an axially floating manner and in a manner secured against rotation by means of axial guide.

[0042] A readjustment device 26 mainly comprises a readjustment piston 19, which is in the same force transmission path as the first actuating piston 17, and a latching sleeve 27, which is in engagement with the readjustment piston 19 and is arranged in a manner secured against rotation with respect to the second spreader piston 28 but in a manner which allows axial movement to a limited extent and under elastic preload against the readjustment piston.

[0043] A separate readjustment piston 19 is screwed into a threaded hole 20 in the first actuating piston 17 as far as a stop position by means of a threaded portion 21 and thus lies in the force transmission path between the first actuating piston 17 and the second actuating piston 18. At its end face facing the second actuating piston 18, the readjustment piston 19 has an axially projecting annular first sawtooth ramp 38 having a plurality of tooth-shaped ramps or notches inclined in one circumferential direction.

[0044] The latching sleeve 27 is arranged in a manner substantially integrated into the second actuating piston 18. It has an axially projecting annular second sawtooth ramp 39, which corresponds to the first sawtooth ramp 38 and which likewise consists of a plurality of tooth-shaped ramps or notches inclined in an opposite circumferential direction.

[0045] The latching sleeve 27 is mounted via a central hole on a stem 28 of the second spreader piston 23 in a manner which allows axial movement and in a manner secured against rotation with respect to the housing 14. In the embodiment shown, a transverse pin 29 is provided in the stem 28 to prevent rotation, the latching sleeve 27 being supported on said pin 29 in both circumferential directions via an axially projecting slotted collar 42.

[0046] The latching sleeve 27 is pressed permanently with a slight force onto the sawtooth ramp 38 of the readjustment piston 19 by a compression spring 30 supported on the second actuating piston 18. A sliding disk 43 reduces the friction and the rotation of the compression spring 30 when the second actuating piston 18 is rotated.

[0047] In the embodiment shown, the compression spring 30 is 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.

[0048] When the two actuating pistons 17 and 18 are driven by the driving sleeve 9, the readjustment piston 19 is rotated relative to the latching sleeve 27, which is immovable in the circumferential direction. In the process, the ramps of the readjustment piston 19 run up onto the ramps of the latching sleeve 27 during each actuation or braking operation and push them slightly into the second actuating piston 18, counter to the spring action of the compression spring 30. With increasing wear on the brake linings 5, 5′, the required stroke and thus the angle of rotation of the driving sleeve 9 and of the readjustment piston 19 coupled thereto increases. As soon as the angle exceeds the circumferential length of a ramp of the sawtooth ramps 38, 39, the axial toothing between the latching sleeve 27 and the readjustment piston 19 jumps into the next notch. If the brake is then released after a braking operation, the first actuating piston 17 rotates back into the unactuated initial position together with the driving sleeve 9. However, the readjustment piston 19 is hindered from rotating back by the rotationally secured latching sleeve 27 and, as a result, is unscrewed from the first actuating piston 17 by the corresponding amount. Thus, when the drum brake 1 is released, the brake is readjusted incrementally in a fully automatically travel-controlled manner.

[0049] In the following text, embodiments of the blocking device which is used with the spreader unit according to FIG. 2 are described.

[0050] FIGS. 3a, b show, in this regard, a detail of the region of the claw clutch, consisting of the latching sleeve and the readjustment piston 19. A blocking space 50 is located in between. To this end, an annular depression 51 is located in the end face of the readjustment piston 19 and a circular depression 52 that is coaxial therewith is located in the latching sleeve 27. In this case, the outside diameter of the annular depression 51 is greater than that of the circular depression 52. The sawtooth ramps 38, 39 follow radially on the outside of the depressions. The bottoms of the depressions 51, 52 are parallel to one another and opposite one another. The blocking space 50 is delimited on one side by the bottom of the annular depression 51 and on the other side by the opposite sawtooth ramp 39 on the latching sleeve 27.

[0051] A spiral spring 53 made of a bimetal is located within the annular depression 51. The depth of the annular depression corresponds to the width of the spiral spring 53. One side of the spiral spring 53 lies on the bottom of the annular depression 51 such that its other side reaches as far as the tips of the sawtooth ramp 39 on the latching sleeve 27. In this case, the spiral spring 53 can be fastened to a central peg in the annular depression 51.

[0052] FIG. 3a shows an unactuated brake in the cold state. The two sawtooth ramps 38, 39 engage in one another such that the spiral spring 53 projects into the circular depression 52. This is possible since its diameter in the cold state is smaller than the diameter of the circular depression 52. As long as the brake does not heat up excessively during a braking operation, the diameter of the spiral spring 53 remains smaller than the diameter of the circular depression 52 such that, as shown in FIG. 3b, it does not fill the annular depression 51 as far as the outside diameter thereof, although the two sawtooth ramps 38, 39 are 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 with FIG. 2.

[0053] The situation in which the brake heats up significantly is different. This is illustrated in FIG. 3c. When the two sawtooth ramps 38, 39 are not in engagement, i.e. have carried out the maximum claw stroke, the annular spring expands into the blocking space 50 between the sawtooth ramp 39 on the latching sleeve 27 and the edge of the bottom of the annular depression 51. Thus, even when they have rotated through a step width with respect to one another, the sawtooth ramps 38, 39 cannot pass into engagement in a new latching step. Rather, when the brake is released, the sawtooth ramps 38, 39 will initially take up their old angular position again and come into engagement there again later when the spiral spring 53 has cooled down and contracted again.

[0054] 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.

[0055] Instead of a spiral spring, it is also possible to use a star-shaped element according to FIGS. 4a, b, which has a center 55 from which, for example, four arms 56 exhibiting bimetal extend. In the cold state, the arms 56 are shortened (FIG. 4a) and in the heated state they are stretched (FIG. 4b) such that their distal ends are located between the latching sleeve 27 and the readjustment piston in the blocking space 50 such that the sawtooth ramps 38, 39 cannot take up a rotated latching position.

[0056] FIG. 5a shows an embodiment of the bimetallic element in the form of two coaxially arranged disk springs 60, 61 made 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 space 50 are thus located close together and cannot fill the width of the blocking space 50. 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 space 50 and so the sawtooth ramps 38, 39 cannot take up a rotated latching position, thereby preventing readjustment.

[0057] 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 springs 60, 61, it is possible—as shown in FIG. 6—for two (or more) disk spring pairs 65 to be placed in succession, wherein a spacer ring 66 is located in each case in between.

[0058] 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.