DRUM BRAKE AND ELECTRIC ACTUATOR THEREFOR

Abstract: An actuator for a drum brake comprises a housing (1), a screw mechanism having at least one screw (3,6) and a nut (4, 7), which screw mechanism (2) is supported with respect to the housing (1), and drive means (3) which are drivingly connected to the screw mechanism for providing a linear movement of at least one of the screw and nut with respect to the housing upon driving the screw mechanism. The screw mechanism comprises two axially aligned screws which have opposite lead angles and two corresponding nuts, said screws each engaging a respective nut, the drive means being connected to both screws or to both nuts.

Description: Drum brake and electric actuator therefor The invention is related to an actuator for a drum brake, comprising a housing, a screw mechanism comprising at least one screw and a nut, which screw mechanism is supported with respect to the housing, and drive means which are drivingly connected to the screw mechanism for providing a linear movement of at least one of the screw and nut with respect to the housing upon driving the screw mechanism. Such actuator is known from US-A-5310026. It comprises a screw which is connected to the actuator housing by means of a thrust bearing. The associated nut is held slidably within the housing, and is driven linearly by rotating the screw. Upon actuation, a linear force is exerted through the nut and the housing. This housing should therefore be movable, so as to provide e.g. a balanced braking action in a drum brake. The object of the invention is to provide an improved actuator, in which the housing is not part of the force trajectory. This object is achieved in that the screw mechanism comprises two axially aligned screws which have opposite lead angles and two corresponding nuts, said screws each engaging a respective nut, the drive means being connected to both screws or to both nuts. The actuator according to the invention provides a driving action through the pair of nuts or screws, without exerting loads on the housing. As a result, the housing can be carried out as a relatively simple component, e.g. a cylindrical tube, the nuts having a corresponding cylindrical circumference and fitting slidably within the cylindrical tube. At their ends, the nuts may have an actuating member which can take several forms, dependant on the envisaged application of the actuator. At or near the ends of the nuts facing away from each other a seal, e.g. a sealing bellows is provided, which seals are connected to the respective ends of the cylindrical tube. The internally located screw thread of nuts and screws is thereby well protected against dirt, etcetera. This is particularly of importance in the case of ball or roller screws. The drive means are drivable through an opening in the housing, and can be carried out as a spline , e.g. a screw gear wheel. Furthermore, the drive means may comprise a motor, the stator of which engages the housing, and the rotor of which is drivingly connected to the spline. 2 According to a preferred embodiment, the motor is drivably connected to a drive gear wheel which meshes with the spline, said drive gear wheel being of greater width than the spline so as to allow axial displacements of the screws and spline while maintaing the mesh. As a result, the nuts which are driven towards or away from each other can adapt themselves freely to the surrounding structure through the actuating heads thereof. The maximum diameter of the spline is smaller than the diameter of the bore of the cylindrical tube, the nuts at their side facing the spline being recessed so as to allow displacement of said nuts along said drive gear wheel. The housing and the nuts can be held non-rotatably with respect to each other through a groove-pin connection. The invention is furthermore related to an arrangement for for a drum brake, comprising a backing plate to be connected to a vehicle wheel suspension, e.g. a steering knuckle, housing etc., a pair of brake shoes supported on said backing plate, a screw actuator engaging said brake shoes and a motor for driving said screw actuator so as to obtain a linear movement thereof for displacing the brake shoes outwardly and away from each other vice versa, said actuator comprising a housing and a screw mechanism which is supported with respect to the housing, and with at least one screw and a nut. Such arrangement is known from US-A-5310026 as well. According to the invention, the screw mechanism comprises two axially aligned screws which have opposite lead angles and two corresponding nuts, said screws each engaging a respective nut the drive means being connected to both screws or to both nuts. The actuating members may, according to a first possibility, be connected pivotably to a respective brake shoe. Alternatively, each actuating member may have an abutment surface, the brake shoes being held under pretension against a respective abutment surface by means of a spring. In the latter case, each nut is held non-rotatably in the cylindrical tube. In case of a pivotable connection between actuating members and brake shoes, no further precautions are necessary for preventing mutual rotations of the nuts with respect to the housing. The housing can be fixedly connected to the backing plate. The drive means may comprise a magnet coupling for holding the braking torque under currentless condition (hand brake). The invention will now be described further with reference to a drum brake with an actuator according to the invention shown in the drawings. Figure 1 shows a view on the arrangement according to the invention including the actuator. Figure 2 shows a cross section to the arrangement according to figure 1 , including part of a brake drum. Figure 3 shows a first embodiment of the actuator according to the invention. Figure 4 and 5 show a second embodiment of the actuator. Figure 6 shows the actuator according to figures 4 and 5 in combination with the drive motor. The arrangement shown in figures 1 and 2 comprises a backing plate 19, which can be connected to a vehicle wheel suspension (not shown). Onto the backing plate, a pair of brake shoes 20, 21 is mounted. Said brake shoes 20, 21 are each supported on an anchor 28 of the backing plate 19. Through springs 26, 29, the brake shoes 20, 21 are constantly urged towards the anchor 28, as well as to the screw mechanism 2. Said screw mechanism 2, which is contained in a housing 1 which is fixedly connected to the backing plate 19 as well, comprises two axially aligned screws 3, 6 which have opposite lead angles, as well as two corresponding nuts 4, 7. The nuts 4, 7 are slidably, but non rotatably through spline/groove connection 27, within the housing 1. The screws 2, 3 and nuts 4, 7 engage each other through balls 30 (ball screws). The nuts 4, 7 each carry an actuating head 8, 9, against the surfaces 24, 25 of which the noses 31, 32 of the brake shoes 20, 21 are urged through spring 26. Between the screws 3, 6, a spline wheel 5 is connected. As shown in the enlarged detail of figure 6, this spline wheel 5 protrudes through an opening 12 in the housing 1. In this opening 12, a drive gear wheel 16 is accommodated, the width of which is greater than the width of the spline 5. In turn, the gear wheel 16 engages the pinion wheel 33 which is connected to the rotor 15 of the motor 13. This motor 13 is connected to the backing plate 19 as well, however to the side opposite the side where the actuator 1 , 2 is mounted. The drive gear wheel thus protrudes through the opening 34 in the backing plate 19, as well as through the opening 12 in the housing 1. Upon energising the stator 14 of the motor 13, the rotor 15, and thereby the pinion gear 33, 4 the drive gear wheel 16 and the spline 5 are set in rotation. Thus, the actuator and in particular the screw mechanism 2, provides the desired mutual displacement of the braking shoes 20, 21 towards or away from the brake drum 35 (shown in figure 2). In the embodiment of figures 1, 2 and 3, the nuts 4, 7 each carry an actuating member 8, 9 having a flat outer surface 24, 25. As mentioned, in that case it is necessary to prevent rotation of the nuts 4, 7 with respect to the housing 1 by means of the spline/groove connections 27. In the embodiment of figures 4, 5, and also of figure 6, the actuating members are carried out with flanges 36, 37 having eyes 22, 23. The corresponding ends 31, 32 of the brake shoes 20, 21 in that case carry eyes as well, in such a way that the pivotable connection is obtained by inserting pins through the respective eyes 22, 23, as well as the eyes in the ends 31, 32 (not shown) of the brake shoes 20, 21. In this embodiment, the nuts 4, 7 are held against rotation through this pivotable connection, so that no additional spline/groove connection is necessary here. By means of bellows 10, 11, the interior of the housing 1 is protected against dirt etcetera.

Claims: 5 Claims 1. Actuator for a drum brake, comprising a housing (1), a screw mechanism (2) comprising at least one screw (3) and a nut (4), which screw mechanism is supported with respect to the housing (1), and drive means (5) which are drivingly connected to the screw mechanism (2) for providing a linear movement of at least one of the screw (3) and nut (4) with respect to the housing (1) upon driving the screw mechanism (2), characterized in that the screw mechanism (2) comprises two axially aligned screws (3, 6) which have opposite lead angles and two corresponding nuts (4, 7), said screws (3, 6) each engaging a respective nut (4, 7), the drive means (5) being connected to both screws (3, 6) or to both nuts (4, 7). 2. Actuator according to claim 1, wherein the housing comprises a cylindrical tube (1), the nuts (4, 7) having a corresponding cylindrical circumference and fitting slidably within the cylindrical tube (1). 3. Actuator according to claim 2, wherein the nuts (4, 7) at their ends facing away from each other have an actuating member (8, 9). 4. Actuator according to claims 2 or 3, wherein at or near the ends of the nuts (4, 7) facing away from each other a seal, e.g. a sealing bellows (10, 1 1) is provided, which seals are connected to the respective ends of the cylindrical tube (1). 5. Actuator according to claims 2, 3 or 4, wherein the drive means comprise a disc (5) which is drivable through an opening (12) in the cylindrical tube (1). 6. Actuator according to claim 5, wherein the drive means comprise a spline (5), e.g. a screw gear wheel (5). 7. Actuator according to claim 6, wherein the drive means comprise a motor (13), the stator (14) of which engages the housing (1), and the rotor (15) of which is drivingly connected to the spline (5). 8. Actuator according to claim 7, wherein the rotor (15) is drivably connected to a drive (16) gear wheel which meshes with the spline (5), said drive gear wheel (16) being of greater width than the spline (5) so as to allow axial displacements of the screws (3; 6) and spline (5) wheel while maintaing the mesh. 9. Actuator according to claim 8, wherein the maximum diameter of the spline (5) is smaller than the diameter of the bore (17) of the cylindrical tube (1), and the nuts (4, 7) at their side facing the spline (5) being recessed (18) so as to allow displacement of said nuts (4, 7) along said drive gear wheel (5). 10. Actuator according to any of the preceding claims, wherein the screw mechanism (2) comprises ball or roller screws. 1 1. Actuator according to any of claims 1-10, wherein the housing ( 1 ) and a nut (4) are held non-rotatably with respect to each other through a groove-pin connection (27). 12. Actuator according to any of claims 1-11, wherein an encoder sensor is provided for measuring rotations and/or linear displacements. 13. Actuator according to any of the preceding claims, wherein at least one component is obtained by means of hard-turning. 14. Actuator according to any of the preceding claims, wherein at least one component is obtained by means of powder metallurgy. 15. Arrangement for a drum brake, comprising a backing plate (19) to be connected to a vehicle wheel suspension, e.g. a steering knuckle, housing etc., a pair of brake shoes (20, 21) supported on said backing plate (19), a screw actuator engaging said brake shoes and a motor (13) for driving said screw actuator (1, 2) so as to obtain a linear movement thereof for displacing the brake shoes (20, 21) outwardly and away from each other, said actuator comprising a housing (1) and a screw mechanism (2) which is supported with respect to the housing (1), and with at least one screw (3, 6) and a nut (4, 7), characterized in that the screw mechanism (2) comprises two axially 7 aligned screws (3, 6) which have opposite lead angles and two corresponding nuts (4, 7), said screws (3, 6) each engaging a respective nut (4, 7) the drive means (5) being connected to both screws (3, 6) or to both nuts (4, 7). 16. Arrangement according to claim 15, wherein the housing comprises a cylindrical tube (1), the nuts (4, 7) having a corresponding cylindrical circumference and fitting slidably within the cylindrical tube (1). 17. Arrangement according to claim 16, wherein the nuts (4, 7) at their ends facing away from each other have an actuating member (8, 9). 18. Arrangement according to claim 17, wherein each actuating member (8, 9) is connected pivotably (22, 23) to a respective brake shoe (20, 21). 19. Arrangement according to claim 17, wherein each actuating member (8, 9) has an abutment surface (24, 25), the brake shoes being held under pretension against a respective abutment surface (24, 25) by means of a spring (26). 20. Arrangement according to claim 19, wherein each nut (4, 7) is held non-rotatably in the cylindrical tube (1). 21. Arrangement according to claim 20, wherein at or near the ends of the nuts (4, 7) facing away from each other a seal, e.g. a sealing bellows (10, 1 1) is provided which seals are connected to the respective ends of the cylindrical tube (1). 22. Arrangement according to claim 21, wherein the drive means comprise a spline (5) which is drivable through an opening (12) in the cylindrical tube (1). 23. Arrangement according to claim 22, wherein the drive means comprise a screw gear wheel (5). 24. Arrangement according to claim 23, wherein the drive means comprise a motor 8 (13), the stator (14) of which engages the housing (1), and the rotor (15) of which being drivingly connected to the screw gear wheel (5). 25. Arrangement according to claim 24, wherein the rotor (15) is drivable connected to a drive (16) gear wheel which meshes with the screw gear wheel (5), said drive gear wheel (16) being of greater width than the screw gear wheel (5) so as to allow axial displacements of the screws (3; 6) and screw gear (5) wheel while maintaing the mesh between those gear wheels (5, 16). 26. Arrangement according to claim 25, wherein the maximum diameter of the screw gear wheel (5) is smaller than the diameter of the bore (17) of the cylindrical tube (1), and the nuts (4, 7) at their side facing the drive gear wheel (5) being recessed (18) so as to allow displacement of said nuts (4, 7) along said drive gear wheel (5). 27. Arrangement according to any of claims 15-26 wherein the screw mechanism (2) comprises ball and/or roller screws. 28. Arrangement according to any of claims 15-27, wherein the housing (1) is connected to the backing plate (19). 29. Arrangement according to any of claims 15-28, wherein the drive comprise a magnet coupling for holding the braking torque under currentless condition (hand brake).

Inventors: KAPAAN HENDRIKUS JAN, ZWARTS JACOBUS, KAPAAN, HENDRIKUS, JAN, ZWARTS, JACOBUS
Applicants: KAPAAN HENDRIKUS JAN, SKF ENG & RES CENTRE BV, ZWARTS JACOBUS, KAPAAN, HENDRIKUS, JAN, SKF ENGINEERING & RESEARCH CENTRE B.V., ZWARTS, JACOBUS
Application year: 2000
Main classification: F16D65
IPC codes: F16D51, F16D65, F16D66