OPERATING UNIT FOR AN ELECTRIC PARKING BRAKE

Abstract

An operating unit for a parking brake of motor vehicles including an operatively connected unit having of a drive spindle and nut, which forms an axially longitudinally adjustable element of a linear drive. The drive spindle has a drive side for connection to an electric drive and a substantially cylindrical spindle portion having an external thread. The nut has a sleeve-shaped central body having an internal thread and a head portion designed to act as a pressure piston on a brake element of the parking brake. The internal and external threads intermesh in the operative connection and define a common axial axis of rotation. The thread is a symmetrical thread, and the flank angles, relative to a radial reference plane perpendicular to the axis of rotation, have angle values which are substantially equal in magnitude.

Claims

1. An operating unit for a parking brake of motor vehicles, the operating unit comprising: an operatively connected unit including a drive spindle and a nut which acts as an axially length-adjustable element of a linear drive; the drive spindle has a drive side for connection to an electric drive and a substantially cylindrical spindle portion with an external thread; the nut has a sleeve-shaped central body with an internal thread and a head portion formed in order to act as a pressure piston on a brake element of the parking brake; the internal and external threads intermesh as an operative connection and have a common axial axis of rotation; the threads are symmetrical threads and flank angles of the threads relative to a radial reference plane perpendicular to the axis of rotation, have angle values which are substantially equal in magnitude.

2. The operating unit as claimed in claim 1, wherein the drive spindle is formed as a cold formed metal molded part.

3. The operating unit as claimed in claim 1, wherein the nut is formed as a cold formed metal molded part.

4. The operating unit as claimed in claim 1, wherein the flank angles are selected to be from 12° to 18°.

5. A linear drive for a parking brake of motor vehicles comprising the operating unit as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 shows an operating unit consisting of nut and spindle in longitudinal section.

[0021] FIG. 2 and FIG. 3 show a thread portion in section transverse to the thread.

[0022] FIG. 2 schematically represents an asymmetrical thread, FIG. 3 a symmetrical thread.

[0023] FIG. 4 shows a cut-out from an asymmetrical thread with a thread in engagement.

[0024] FIG. 5 shows a schematic diagram of a thread tooth with closing fold in cross-section.

DETAILED DESCRIPTION

[0025] FIG. 1 shows an operating unit 10 as a system consisting of nut 20 and drive spindle 11 in operatively connected engagement. Such an assembly can be used as a linear drive element in an electric parking system, is per se, however, also suitable for technically equivalent tasks such as closing or locking systems in which, in the end position, a constant axial force acts on the assembly.

[0026] Drive spindle 11 can be divided substantially into two sub-portions. On one hand, drive side 12 which can enclose a bearing 14. The transmission of force is performed here from a drive motor. This transmission can be performed, as known in the prior art, from a drive motor via a clutch element directly or indirectly via a transmission, toothed belt or another suitable means. The other lower portion is actual spindle thread portion 13. It is technically embodied as a substantially rod-shaped structural element with an external thread 15. This drive spindle 11 is shown in operative connection with a nut 20. The latter can be divided into a head portion 22 and a sleeve-shaped central body 21 which bears an internal thread 23. In the installed state, drive spindle 11 is normally mounted to be stationary, but rotationally driven in a brake system. Nut 20 is mounted axially displaceable relative to drive spindle 11. Head portion 22 can be formed as a part of a pressure piston which acts on a brake element such as e.g. a brake shoe. Nut 20 and drive spindle 11 possess in the operatively connected state a common central axis A which forms a central longitudinal axis for the thread.

[0027] Marked portion X corresponds (by way of example) to FIG. 4. The thread shown is purely schematic.

[0028] FIGS. 2 and 3 show a partial section through a spindle or nut along the central axis with several threads. FIG. 2 shows an asymmetrical thread with thread flanks of varying gradients, FIG. 3 shows a symmetrical thread. In FIG. 2, a radial plane R runs through the point of intersection of the elongation of the thread flanks on one hand and stands on the other hand perpendicular on central longitudinal axis A (not shown here). Both angles θ and 6 are selected to be of different magnitudes, in the example shown, 0<S.

[0029] A.sub.as schematically indicates that surface which serves during operation as a contact surface for the mating flank. It is apparent that plotted force F.sub.a hits the contact surface at an angle 90°—α. The steeper the flank, the smaller β is and accordingly the smaller force component F.sub.r which acts in the radial direction is. How steep the second flank can be selected in the case of given angle β depends among other things on the pitch of the thread.

[0030] FIG. 3, in an analogous manner to FIG. 2, shows the ratios in an idealized symmetrical thread with substantially identical flank angles α, α′. It is apparent on the basis of the force parallelogram why a symmetrical thread structure was not favored earlier for the use described—the surface pressure calculated with F.sub.s is higher in the case of identical F.sub.a in FIG. 3 than in FIG. 2. A.sub.sy schematically designates the contact surface with the mating flank in interactive connection.

[0031] FIG. 4 corresponds approximately to cut-out X in FIG. 1, but a detail 70 with an asymmetrical thread is shown. Thread portion 71 should be assigned by way of example to the nut, (lower) thread portion 80 to the drive spindle. Spaces 73, 74, 83, 84 shown in FIG. 4 arise because thread base and thread tip of a transmission enable restricted play of nut and spindle and also act during operation as a lubricant depot. A thread tooth 75 of the nut contacts a thread tooth 85 of the drive spindle in a restricted area which is marked as flank surface 81 or 82 and corresponds to A.sub.as in FIG. 2. It is apparent in the region of space 83, in the thread base, that the notching effect at transition 86 of the steeper flank into the thread base is greater than in the case of transition 87. Arrow F.sub.a indicates the axial force action which presses the thread flanks onto one another as shown.

[0032] FIG. 5 shows, in a schematic section, a thread tip 90 with two flanks 94 and 95 of different gradients of an asymmetrical thread with a closing fold 92.

[0033] The features of the invention disclosed in the above description, in the drawings and in the claims can be significant both individually and also in any desired combination to achieve the invention.