Device for metering a powder

The present invention relates to a device for metering a powder, having a device for the delivery of powder, a axially symmetrical metering element arranged underneath the delivery device, and a drive for the axially symmetrical metering element, wherein the axially metering symmetrical element has a surface profile extending in the circumferential direction, and is adapted in its configuration so that the powder is not compressed during movement about the axially symmetrical metering element.

FIELD OF THE INVENTION
 The present invention relates to a device for metering a powder, in
 particular for a dusting device for dusting printed sheets of paper,
 having a device for delivering the powder, an axially symmetrical metering
 element arranged underneath the delivery device, and a drive for the
 metering element.
 BACKGROUND OF THE INVENTION
 A print dusting installation in printing presses for dusting printed sheets
 with powder is known from German Patent DE 18 20 842 U1. This installation
 has a funnel tube for the powder, which terminates on a metering roller.
 The metering roller has metering chambers constituted by grooves extending
 in the longitudinal direction. An amount of powder is specifically
 transported out of the funnel via these metering chambers and conducted
 into an air conduit. There, the powder in the metering chambers is
 delivered.
 In connection with these metering chambers it is considered to be
 disadvantageous that the powder is held in the metering chambers by means
 of an interlock, so that the powder must be actively removed from the
 metering chambers. Since the metering chambers extend transversely with
 respect to the conveying direction, there is the danger that the powder is
 compressed in these metering chambers and cakes there.
 A roller is known from U.S. Pat. No. 4,867,063, which is provided with
 cells on its surface. Although it is possible to transport bulk material
 in a simple manner by means of this cell conveyor, when transporting
 powder the problem arises that the cells slowly become clogged, so that
 the transported volume is reduced over time. Thus, with such cell
 conveyors there is no assurance of a constant volume transport over an
 extended period of time.
 SUMMARY OF THE INVENTION
 The object of the present invention is therefore based on designing a
 device for metering a powder of the type mentioned at the outset in such a
 way, that the powder can be removed relatively simply from the metering
 element, and that there is no danger of the powder being compressed inside
 the metering chambers.
 In accordance with the present invention this object is attained by means
 of a device of the type mentioned at the outset in that the metering
 element has a surface profile extending in the circumferential direction.
 The fact, that the metering chambers extend in the circumferential
 direction of the metering element and therefore in the transport
 direction, prevents the powder from being compressed in the metering
 chambers. Moreover, the powder contained in the storage container is
 hardly affected, in particular no compressions or clumps are being
 created. Furthermore, the powder, which is located in the surface profile
 extending in the circumferential direction, can be delivered relatively
 easily out of this profile, because the powder lies relatively loosely in
 this profile.
 A further development provides that the surface profile is embodied in the
 form of circumferential channels, circumferential grooves, circumferential
 corrugations, or the like. A surface profile designed in this way has the
 considerable advantage that the surface of the roller receiving the powder
 is enlarged by the engraving, so that the adhesive forces by means of
 which the powder is maintained on the roller are considerably greater than
 those of metering elements with a smooth surface. In this case the powder
 adheres so well to the roller which, for example, has been provided with
 channels, that it does not fall out of the channels even under the action
 of gravity. The surface profile has a preferred depth between 0.2 mm and
 0.8 mm.
 In accordance with the present invention an increase in the adhesive force
 is achieved in that the surface of the metering element is made
 powder-friendly. Thus, the surface can be roughened, sand-blasted, etched,
 chromated or electrolytically provided with a matte finish. These surfaces
 receive the powder particularly well and optimally transport it in the
 direction of the delivery opening, so that it is assured that a constant
 amount of powder is always delivered.
 A further possibility of surface treatment provides that calcium carbonate
 or powder is glued to the surface of the metering element A roughening of
 the surface, which essentially corresponds to the grain size of the powder
 to be transported, is also achieved in this way.
 A preferred embodiment provides that the metering element is designed as a
 metering roller. However, it is also conceivable to design the metering
 element as a cone or a sphere with a horizontal shaft. The powder is
 applied to the metering element in the area of an upper section and is
 removed from the metering element in a lower section.
 Removal of the powder is assured by the present invention in that the
 metering element is provided with a device for loosening the powder from
 the metering element. This loosening device essentially operates in the
 circumferential direction or tangentially with respect to the
 circumferential direction. Here, the device can be a doctor blade, a brush
 or a blower nozzle. Such a device can enter into the surface profile of
 the roller, which extends in the circumferential direction, in a
 relatively simple manner and loosen the powder. This is not possible with
 cell conveyors of the above mentioned type since it is not possible, for
 example, to enter the individual cells with a doctor blade.
 The drive for the metering element preferably is a step motor. In this
 connection it is provided in a further development that the drive, and in
 particular the entire device, is resiliently seated. The device is made to
 vibrate by the step motor, so that the flow of the powder is aided by
 this. Because of this it is possible to omit stirrers or the like.
 In order to prevent the powder from flowing out of the storage container,
 the metering element has an outlet opening which, viewed in the direction
 of rotation, is offset by more than 180.degree. with respect to the inlet
 opening. In this way the powder is prevented from directly entering the
 inlet opening via the surface profile of the metering element to the
 outlet opening, and from being mixed there in an uncontrolled manner with
 the air flow. The non-conveying area of the metering element is sealed,
 for example by means of a doctor blade.
 In connection with a further development it is provided that the metering
 zone is laterally bordered by labyrinth seals. These labyrinth seals can
 be emptied via a drain. Labyrinth seals have the essential advantage that
 they can be produced relatively simply and do not require maintenance.
 Furthermore, a slide ring seal is provided between the drive and the
 metering element, which protects the drive against penetration by powder.
 Further advantages, characteristics and details of the present invention
 ensue from the following description, in which a particularly preferred
 exemplary embodiment is described in detail, making reference to the
 drawings. Here, the characteristics represented in the drawings and
 mentioned in the specification as well as in the claims can be important
 for the present invention, either respectively by themselves or in any
 arbitrary combination.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 In FIG. 1 is a metering element is identified by the reference numeral 1,
 which has a funnel used as a storage container 2, in which powder 3 is
 stored. The storage container 2 has an outlet opening 4, which is seated
 on a roller housing 5. A metering roller 6 is arranged in this roller
 housing 5 and is seated, rotatable around a horizontal shaft 7. This
 metering roller 6 has an essentially cylindrical shape and is located,
 almost free of play, in an appropriate bore in the roller housing 5. The
 outlet opening 4 terminates in an inlet opening 8 provided in the upper
 area of the roller housing 5, through which the powder 3 stored in the
 storage container 2 can enter the roller housing 5.
 In FIG. 2 it can be seen that the inlet opening 8 terminates directly on
 the surface 9 of the roller 6. The surface 9 of the metering roller 6 is
 provided with a multitude of channels, which are axially symmetrical in
 the circumferential direction and extend past the inlet opening 8 on both
 sides of the metering roller 6. No powder gets into the channels 10 in
 this area, so that the grooves act as a labyrinth seal 14 there. The
 powder is transported by means of the channels 10 in the direction toward
 an outlet opening 11, which is not located directly underneath the inlet
 opening 8, but is offset by more than 180.degree. in the direction of
 rotation (arrow 12) with respect to the inlet opening 8. A doctor blade 13
 is furthermore located in this outlet opening 11, which enters into the
 channels 10 of the metering roller 6 and by means of which the powder 3 in
 the channels 10 is lifted out, so that it can fall over the outlet opening
 11 downward out of the roller housing 5 into a catch funnel. The doctor
 blade 13 assures that the channels 10 are completely emptied and are again
 available for receiving powder 3 at the inlet opening 8.
 It can additionally be seen in FIG. 2 that two bores 16 are provided on the
 underside 15 of the roller housing 5 in the area of the front ends of the
 metering roller 6, through which powder which enters the labyrinth seal 14
 can be removed into the catch funnel, so that it cannot reach the bearings
 of the shaft 7.
 It can furthermore be seen in FIG. 2, that the metering roller 6 is
 overhung, wherein a step motor 19 used as the drive 18 is located on the
 one side of a holding bracket 17, and on the other side of the holding
 bracket 17 the roller housing 5 with the metering roller 6, which has been
 placed on the shaft of the step motor 19. The overhung position of the
 metering roller 6 allows a rapid and uncomplicated exchange for
 maintenance and/or repair purposes. The holding bracket 17 is resiliently
 fastened by means of bolts 27 and rubber buffers 28 on a frame 29.
 The transported volume is adjusted by means of the step motor 19, in
 particular by its number of revolutions. The clock time is matched by
 means of the frequency of the step motor 19 to the clock speed of the
 paper sheets.