An electromechanically actuatable brake, in which a brake lining is pressed by an electric motor against a brake disk via a threaded roller drive. To enable releasing the brake in the event of a malfunction, a nut of the threaded roller drive is braced axially via a self-locking-free spindle, which is blocked against relative rotation by a permanent magnet brake, the permanent magnet brake is releasable by supplying current to the permanent magnet brake. When the permanent magnet brake is releasable by having current supplied to it, the brake lining pressed against the brake disk sets the spindle, forming the bracing means, into rotation and presses the entire threaded roller drive away from the brake disk and as a result the brake is released.

BACKGROUND OF THE INVENTION
 The invention relates to an electromechanically actuatable brake.
 From British Patent GB 2 190 441, one such electromechanically actuatable
 brake is known with a two-part actuating device, namely a first part for
 overcoming the air clearance and a second part for pressing a brake lining
 against a brake body. Both parts of the actuating device have a separate
 spindle drive, drivable by its own electric actuating motor. The two parts
 of the actuating device can be driven simultaneously or successively to
 execute a brake actuation, by putting the two motors into operation. Both
 parts of the actuating device are joined together by a blocking bracing
 means embodied as a pair of levers. The pair of levers is pivotably
 supported about a stationary shaft. The first part of the actuating device
 engages the pair of levers with slight spacing from the shaft, while the
 second part of the actuating device engages the pair of levers with major
 spacing from the shaft. Because of this arrangement, the first part of the
 actuating device, which acts directly on the brake lining, is capable of
 rapidly overcoming the air clearance; via the pair of levers and the first
 part of the actuating device, the second part of the actuating device can
 generate a high contact-pressure force on the brake lining. Both motors of
 the actuating device can be equipped with a brake. Thus, unintended
 adjustment of the brake during braking operations can be avoided with a
 constant braking force while the motors are turned off.
 The known brake is intended particularly for use in railroad vehicles.
 There the requisite installation space for the two parts of the actuating
 device and the relatively large-volume for the pair of levers is available
 in the known brake device. But because of its weight and volume, the brake
 would be unsuited to disposition in the bowl of a wheel rim of a road
 vehicle. Moreover, because of the two motors, the brake is expensive and
 requires increased expense for control. Thus, the bracing means is blocked
 only when, in successive drive of the two parts of the actuating device,
 the part serving to press the brake lining is not driven, or in other
 words only whenever only the air clearance is overcome. If both parts of
 the actuating device are driven simultaneously, conversely, the bracing
 means is not blocked. The bracing means can be released by swiveling the
 pair of levers about the fixed shaft. Pivoting the pair of levers is done
 by driving the motor of the part of the actuating device used to press the
 brake lining. If one or both motors of the actuating device fail, problems
 can arise in releasing the actuating brake.
 From GB 2 190 441, a brake is also known for pressing a brake lining
 against a rotating brake body (brake disk, brake disk or the like), which
 has an actuating device with a threaded roller drive, which is drivable by
 an electric motor. Both tightening and releasing the known brake are
 accomplished with the electric motor. To prevent residual braking moments
 caused by hysteresis of the actuating unit from acting on the brake body
 in the event of a malfunction, such as failure of an electronic control
 system of the brake during braking, a preferably spiral restoring spring
 is provided in one embodiment of the known brake; this spring engages the
 actuating unit and drives the actuating unit, together with the electric
 motor, to rotate in the release direction so that the brake lining is
 lifted from the brake body.
 This brake has the disadvantage that when brake pressure is built up, the
 force of the restoring spring must additionally be overcome, and the
 electric motor must therefore be dimensioned correspondingly larger and
 supplied with a higher current. In addition, there is a dynamic loss, and
 a loss of efficiency.
 Another disadvantage is that to keep a built-up brake force constant, the
 electric motor must be supplied with such a high current that it keeps the
 brake lining pressed against the brake body with a constant contact
 pressure, counter to the force of the restoring spring, which entails
 thermal problems. Another factor is that the known brake cannot be used as
 a parking brake, because it releases when it has no current. Another
 disadvantage is that the brake cannot be released if the threaded roller
 drive is jammed.
 Another disadvantage is that an air clearance, that is, a spacing between
 the brake lining and the brake body when the brake is released, increases
 with increasing wear of the brake linings. As a result, on the one hand a
 positioning travel of the brake lining until the brake lining contacts the
 brake body becomes greater and accordingly it takes longer until the brake
 grabs. The dynamic loss is additionally increased. Furthermore, the force
 necessary to overcome the force of the restoring spring increases, because
 the restoring spring is deformed more markedly. The energy that has to be
 brought to bear by the electric motor of the actuating device to deform
 the restoring spring is equivalent to the product of the deformation
 travel and deformation force; thus as the air clearance increases, this
 energy increases at least quadratically, which quite severely worsens the
 efficiency of the brake when the brake linings become worn.
 ADVANTAGES OF THE INVENTION
 The electromechanically actuatable brake of the invention as defined by the
 characteristics of claim 1 has a bracing means that is releasable by
 rotation and is blocked against rotation by an anti-jam device. For
 instance, the bracing means can be embodied on the order of a bayonet
 mount, which can be released by turning the mount by a small angle and
 displacing the mount axially away from the brake body. Another option is a
 bracing means that has a screw thread, which can be moved away from the
 brake body by turning the bracing means and is blocked against rotation by
 the anti-jam device. Thus, a screw thread that extends over less than one
 full revolution can be adequate.
 The actuating device is braced against the bracing means when the brake is
 actuated, or in other words for pressing the brake lining against the
 brake body. In braking and when the brake is released, the bracing means
 acts as a fixed abutment for the actuating device but otherwise has no
 function. The brake is actuated and released with the actuating device.
 Thus the bracing means affects neither the actuating device itself nor its
 efficiency.
 If in a malfunction, for instance a failure of the current supply to the
 electric motor or a failure of its electronic control system, or if a
 spindle drive of the actuating device jams or becomes hard to move, the
 brake cannot be released with the actuating device, then the blocking of
 the bracing means is released, making the bracing means freely rotatable.
 The rotation of the bracing means can be done by a reaction force, with
 which the brake lining, pressed against the brake body, acts on the
 actuating device, if the bracing means is embodied as a self-locking
 device. The rotation of the bracing means can also be done by means of a
 prestressed spring element, which rotates the bracing means in the release
 direction when the anti-jam device is released. With the release, the
 bracing means moves away from the brake body, or if the bracing means is
 embodied like a bayonet mount, the bracing means is displaceable freely
 away from the brake body, so that the actuating device together with the
 brake lining is also released from the brake body; in other words, the
 bracing means acting as an abutment for the actuating device is removed or
 at least its distance from the brake body is increased. The invention has
 the advantage that its bracing means neither affects the function of the
 actuating device nor worsens its efficiency.
 By the provision of a prestressed spring element, which rotates the bracing
 means when the anti-jam device is released and thus releases the bracing
 means, the brake linings are lifted from the brake body. A residual
 braking moment is prevented from acting in the event of a malfunction, and
 the complete release of the brake is assured. This improves the
 proformance of a vehicle equipped with the brake of the invention during a
 malfunction. The brake of the invention can also be used as a parking
 brake, which maintains braking moment without being supplied with current.
 The brake of the invention has the advantage of optimally utilizing the
 energy stored in the brake because of the structural design of its
 releasable bracing means. The screw thread thereof is designed for optimal
 reverse torque; that is, the tightening force of the brake, in the event
 of a malfunction, is automatically reduced to zero when the brake is
 released. As a result, fewer structural components for releasing the brake
 in a malfunction are needed, which has advantages with respect to
 installation space, weight, costs, and power electronics.
 Advantageous features and refinements of the invention defined are the
 subject of the invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT
 The brake of the invention shown in the drawing is embodied as a disk brake
 10 with a so-called floating or sliding caliper 12, in which two brake
 linings 14 are disposed, one on each side of a brake disk 16 rotatable
 between them.
 As the actuating device, the disk brake 10 has a spindle drive,
 specifically, because of its good efficiency, a threaded roller drive 18.
 A spindle drive has the advantage that an always-constant air clearance
 can be established between the brake linings 14 and the brake disk 16,
 regardless of wear of the brake linings 14. This has the advantage that a
 positioning distance and a positioning time until the brake linings 14,
 upon actuation of the disk brake 10, come to contact the brake disk 16 do
 not get any longer even when the brake linings 14 become worn.
 The threaded roller drive 18 includes a nut 20, which coaxially surrounds a
 spindle 22. In a cylindrical interstice between the nut 20 and the spindle
 22, threaded rollers 24 are distributed over the circumference, whose
 threads engage both the nut thread and the spindle thread. When the nut 20
 is driven to rotate, the rollers 24 orbit the spindle 22 like the planet
 wheels of a planetary gear and cause an axial motion of the spindle 22.
 One of the two brake linings 14 is disposed, in a manner fixed against
 relative rotation, on one face end of the spindle 22 and can be pressed
 against one side of the brake disk 16 in order to bring a braking force to
 bear by driving the nut 20 to rotate. The other one of the two brake
 linings 14 is pressed in the process, in a known manner, against the other
 side of the brake disk 16 by the reaction force of the brake caliper 12.
 For releasing the brake 10, the nut 20 of the threaded roller drive 18 is
 driven to rotate in the opposite direction.
 For its rotational drive, the nut 20 has a polygonal profile in its outer
 circumference, onto which a polygonal cuff 26 whose inside has a
 complementary polygonal profile is thrust. By way of example, the
 polygonal profile can have a square cross section, whose corners are
 intersected by a concentric circle, as known from DIN 32712. A polygonal
 profile is especially suitable for connecting the nut 20 of the threaded
 roller drive 18 to the polygonal cuff 26 in a manner fixed against
 relative rotation; these elements are intended to be longitudinally
 displaceable relative to one another when loaded with a torque. The two
 polygonal profiles have a clearance fit from one another; that is, the nut
 20 is axially displaceable relative to the polygonal cuff 26 and is
 connected to the polygonal cuff in a manner fixed against relative
 rotation. The polygonal cuff 26 is supported rotatably in the floating
 caliper 12 by two ball bearings 28.
 Permanent magnets 34 are inserted into an outer circumference of the
 polygonal cuff 26; the polygonal cuff 26 forms a rotor of an electric
 actuating motor of the brake 10 of the invention. A stator that has
 lamination packets 32 and stator windings on the permanent magnets 34
 surrounds the polygonal cuff 26 forming the rotor. The stator 32, 34 is
 inserted solidly into the floating caliper 12. By turning on the actuating
 motor 26, 30, 32, 34, the nut 20 of the threaded roller drive 18, which is
 connected in a manner fixed against relative rotation to the polygonal
 cuff 26 forming the rotor of the actuating motor, is set into rotation,
 and the spindle 26 is axially displaced thereby and the brake 10 of the
 invention is actuated or released, depending on the direction of the
 rotation, as described above.
 In the axial direction, the nut 20 of the spindle drive 18 is braced via a
 needle bearing 36 against a disk-like flange hub 38, which is pressed in a
 manner that transmits both force and moment onto a tang 40 that is
 integral with a second spindle 42. The second spindle 42 is screwed into a
 female thread of a cap 44 secured onto the floating caliper 12; the cap
 forms the spindle nut for the second spindle 42. This spindle and spindle
 nut arrangement 42, 44 forms a bracing means, which is releasable by
 rotation, for the nut 20 of the threaded roller drive 18 of the actuating
 device. The bracing means 38, 42, 44 is free of anti-jamming because of
 the pitch of its screw thread and the diameter of its spindle 42.
 The brake 10 of the invention has an anti-jam device 46, which blocks the
 bracing means 38, 42, 44 against rotating. As the anti-jam device, by way
 of example an electromagnetic spring pressure brake which is preferably
 closed when without current, or an indexable trip-free mechanism can be
 used. In the exemplary embodiment shown, the anti-jam device 46 includes a
 permanent magnet brake 48 with an armature disk 50, which is disposed on a
 side of the flange hub 38 remote from the threaded roller drive 18. Via
 bolts 52 inserted into the flange hub 38, the flange hub 38 and the
 armature disk 50 are joined together in a manner fixed against relative
 rotation. An annular permanent magnet 54, which is mounted solidly on an
 inside of the cap 44, attracts the armature disk 50 against a friction
 lining 56 in the form of an annular disk, which is inserted into the face
 end of the permanent magnet 54 toward the armature disk 50. In this way,
 the flange hub 38 forming the bracing means for the nut 20 of the threaded
 roller drive 18, is blocked against rotation; it forms an abutment against
 which the nut 20 of the threaded roller drive 18 is braced when the brake
 lining 14 is pressed against the brake disk 16.
 An electromagnet 58 is inserted into the annular permanent magnet 54 of the
 permanent magnet brake 48. By being supplied with current, the
 electromagnet 58 builds up a magnetic field that is oriented counter to
 the magnetic field of the permanent magnet 54 and virtually cancels the
 latter magnetic field out. A prestressed diaphragm spring 59 in the form
 of an annular disk, placed between the flange hub 38 and the armature disk
 50, by its tensile force lifts the armature disk 50 away from the friction
 lining 56, so that the permanent magnet brake 48 is released. The armature
 disk 50 and together with it, the flange hub 38 forming the bracing means
 become freely rotatable; that is, the blocking of the bracing means is
 undone. The electromagnet 58 is connected to a power supply that is
 independent of the power supply to the actuating motor 26, 30, 32, 34, so
 that release of the brake 10 will be assured even if the power supply of
 the actuating motor 26, 30, 32, 34 fails.
 When the brake 10 is actuated, a contact pressure, with which the brake
 lining 14 mounted on the spindle 22 of the threaded roller drive 18 is
 pressed against the brake disk 16, exerts a reaction force on the nut 20
 of the threaded roller drive 18 in the opposite direction, that is, away
 from the brake disk 16. When the permanent magnet brake 48 is released,
 this reaction force causes a rotation of the flange hub 38 together with
 the second spindle 42, which is pressed onto the flange hub 38 and is
 screwed, free of jamming, into the cap 44 of the floating caliper 12. The
 effect of this rotation is that the second spindle 42, and together with
 the second spindle the flange hub 38, as well as the nut 20 together with
 the threaded rollers 24 and the spindle 22 of the threaded roller drive 18
 all move axially away from the brake disk 16, until the brake linings 14
 are either free or contact the brake disk 16 with an only slight contact
 pressure which is not enough to move the second spindle 42 farther. This
 slight contact pressure brings about a slight and acceptable residual
 braking moment of the brake 10 of the invention. Thus even in the event of
 a malfunction, that is, failure of its actuating motor 26, 30, 32, 34 or
 jamming of the threaded roller drive 18, the brake 10 of the invention can
 be released by releasing the permanent magnet brake 48.
 In order to lift the brake linings 14 from the brake disk 16 in the event
 of a malfunction so that no residual braking moment is operative at the
 brake 10, the brake may have a prestressed spring element, in the form of
 a leaf-spiral spring 60 or torsion spring, which surrounds the second
 spindle 42 and engages it and braces it against the cap 44. This
 prestressed spiral spring 48, after the release of the permanent magnet
 brake 48, rotates the second spindle 42 in such a way that the second
 spindle 42 moves together with the flange hub 38 away from the brake disk
 16. Via the nut 20, the threaded rollers 24 and the spindle 22 of the
 threaded roller drive 18, the brake lining 14 is lifted away from the
 brake disk 16. In this embodiment of the invention, the second spindle 42
 need not be free of jamming. The prestressed spring element furnishes the
 entire reverse torque for the releasable bracing means 38, 42. It
 compensates for fluctuations in reverse torque caused by fluctuations in
 the coefficient of friction, temperature factors, etc. Furthermore, it
 minimizes the reverse torque that holds the permanent magnet brake 48
 closed, or in other words has a reinforcing effect.
 The foregoing relates to a preferred exemplary embodiment of the invention,
 it being understood that other variants and embodiments thereof are
 possible within the spirit and scope of the invention, the latter being
 defined by the appended claims.