Abstract:
The invention relates to a suction strand conveyor of a cigarette strand machine, which is guided around a contact wheel in an inlet area of a shaping arrangement. A plurality of suction air channels connect an inner bearing space of the contact wheel and an outer surface of the contact wheel with a suction chamber of the suction strand conveyor belt and provide a permanent removal of tobacco dust.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority is claimed with respect to application No. 199 09 216.8, filed in Germany on Mar. 4, 1999, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a device for assembling a continuous strand of tobacco, including a suction strand conveyor belt which is operationally connected with a suction chamber along an assembly track and which is conducted around a contact wheel that revolves along with the suction strand conveyor belt in the strand inlet area of a shaping arrangement that forms the strand of tobacco into a wrapped strand of cigarettes. 
     Individual and loosened tobacco fibers usually are sprinkled on a strand conveyor in the shape of an endless conveyor belt revolving in a tobacco channel to form a fiber strand for producing a strand of cigarettes. The strand is continuously transported out of the area of the strand formation and evened out before it is deposited on a strip of wrapping material, for example a strip of cigarette paper, which is moved into a shaping arrangement by means of a revolving shaping belt. The strip of wrapping material enters the inlet area of the shaping arrangement still flat and then is molded into a U-shape around the fiber strand during its passage through the shaping arrangement before it is placed completely around the strand and is closed around the strand, as a rule by gluing, along an overlapping seam to form a cylindrical wrapping. 
     The suction strand conveyor belt usually comprises an air-permeable textile tape on which tobacco particles can be caught and released again at another location. Because of the applied underpressure and/or because of the mechanical pressure in the course of the compression of the fiber strand, the correct transfer of all particles in the strand to the shaping arrangement is prevented. To assure that all particles in the strand, even those which have become stuck to the strand conveyor, are transferred to the shaping arrangement, the latter has a device for stripping off tobacco particles, a so-called scraper, at its inlet end. The scraper consists of a base body and has a scraper edge, which is positioned closely near the surface of the suction strand conveyor belt and scrapes the tobacco particles of the strand away from the belt. The transfer zone of the tobacco strand in which the fiber strand is transferred from the upper suction strand conveyor belt onto the lower cigarette paper strip, conveyed by the shaping belt into the shaper, should be made as short as possible, or respectively should extend as closely as possible next to the shaping belt. 
     To this end, good results have been achieved with the use of a contact wheel for reversing the suction strand conveyor belt. The contact wheel, which should be as small as possible, creates large centrifugal forces for the better removal of the tobacco strand and permits an optimal placement and design of a scraper assigned to the suction strand conveyor belt. However, a relatively tightly designed belt reversal is extremely sensitive to tobacco dust that is inevitably generated in this area. The tobacco dust acts destructively on the bearing of the contact wheel and is deposited on the circumferential surface of the wheel, so that the suction strand conveyor belt is lifted off and can be destroyed by the scraper. 
     An attempt to remove dust deposits from the circumferential surface of the contact wheel by placing the scraper against it has not lead to satisfactory results. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to increase the functional dependability of such a contact wheel as used for reversal of the suction strand conveyor belt, and thus optimize its operation in respect to improved strand formation, as well as to increase its service life. 
     The above and other objects are accomplished in accordance with the invention, wherein in the context of a device for assembling a continuous strand of tobacco as first describe above, the contact wheel is delimited by an air channel in a belt free area of a circumferential section thereof, and the air channel has a flow connection with an interior bearing space of the contact wheel, and another flow connection with the suction chamber. 
     As a result of the arrangement according to the invention, all inner and outer dust-catching surfaces of the contact wheel are evenly cleaned due to a permanently acting suction draft. In accordance with a further development, the inner surfaces are best reached by the suction draft in that the flow connection with the inner bearing space of the contact wheel is provided by air slits opening on both sides at the circumferential edge of the contact wheel, which are suitably designed in the shape of at least eight radial edge slits. 
     Shortly before turning around the contact wheel, the suction strand conveyor belt is usually guided along a fixed thrust travel block which, in accordance with a further development, is used to form the air channel by provision of a recess in the thrust travel block guiding the suction strand conveyor. 
     In accordance with a further development, the efficiency of the air channel is optimized in that it is laterally bordered by lateral walls, which are designed as knife strips and have openings which connect the air channel with the atmosphere. 
     Even more effective dust catching and dust aspiration is achieved in accordance with another aspect of the invention wherein the thrust travel block is provided underneath the openings of the lateral walls terminating in the air channel with a scraper-like particle retaining wedge, which engages a nip formed between the lower circumferential section of the contact wheel and the suction strand conveyor belt. 
     In accordance with a further aspect of the invention, a dust removal zone is achieved in that the air channel, which essentially runs concentrically in relation to the contact wheel, extends between the lower retaining wedge of the thrust travel block and an upper scraper, which is connected with the thrust travel block and placed against the contact wheel. 
     Dust removal from the inner bearing space of the contact wheel is made easier yet in that the front faces of the contact wheel have openings. 
     The advantage achieved by the invention lies in that, because of the permanent dust removal, or respectively freedom from dust, of the particularly sensitive functional areas, in particular of the bearings and the strand conveyor belt guide surface of the contact wheel, the operational dependability of the latter is maintained over a long time under full load conditions and, along with this, a dependable strand conveyor belt guidance and therefore an increase of the service life of the belt is also assured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in greater detail by means of an exemplary embodiment represented in the drawings. 
     FIG. 1 is a schematic side elevation, in partial section, showing portions of a strand conveying unit of a cigarette strand machine. 
     FIG. 2 is a cross section through the contact wheel in accordance with the invention of the strand conveying unit along the line  2 — 2  shown in FIG.  1 . 
     FIG. 3 is a cutaway, perspective view of the contact wheel shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 , there is shown a shaping arrangement inlet of a cigarette strand machine, including a delivery end of a suction strand conveyor belt  1  embodied as an air-permeable endless conveyor belt, a portion of a shaping arrangement  2  with a shaping bed  3  indicated by a dash-dotted line, and an inlet finger  4 . The suction strand conveyor belt  1  revolves in the direction of the arrow  6  in a tobacco channel  7  and conveys a fiber strand of tobacco fibers (not represented) from a strand assembly zone (also not represented) for delivery to shaping arrangement  2 . The fiber strand is conveyed in the tobacco channel hanging from suction strand conveyor belt  1 , where it is maintained against the suction strand conveyor belt by means of a suction draft from a suction chamber  8 , which acts through suction strand conveyor belt  1  and encloses the suction strand conveyor belt. 
     In accordance with FIG. 2, shaping bed  3  has a strand guidance surface  9 , which forms a strand channel that is curved in cross section and has a radius of curvature continuously reduced in the running direction of the strand from an inlet cone  11  to the shaping element. The shape of the strand channel at its terminal end is in the desired strand shape (not further shown). A shaping belt  13  is conducted through inlet cone  11  along strand guidance surface  9  and into the strand channel, not further represented, of shaping arrangement  2  in the direction of the arrow  12 , and carries a wrapping material strip  14  through shaping arrangement  2 . The technology of these machines is sufficiently known, so that a more detailed representation and description is not required here. 
     The suction strand conveyor belt  1  converges in the area of inlet cone  11  toward strand guidance surface  9  of shaping bed  3  and places the tobacco strand, which was sprinkled on in a strand assembly zone, not represented, and evened out in a customary manner, on the surface of wrapping material strip  14  being moved through shaping arrangement  2 . In the course of this process, the tobacco strand is greatly compressed by the action of the suction strand conveyor belt  1  during its passage through inlet cone  11  and is deformed in accordance with the cross-sectional change of the inlet cone in the conveying direction  12  of the shaping belt  13  such that it can enter the strand channel, which is partially enclosed by strand guidance surface  9  of shaping bed  3 . The inlet finger  4  has a scraper  16  arranged as a further strand guidance mechanism in the area of the strand guidance surface  9  adjacent to shaping bed  3  and which constitutes, together with the strand guidance surface  9  of shaping bed  3 , a strand guidance, which further compresses and shapes the fiber strand. 
     On its front, scraper  16  is fastened on inlet finger  4  of shaping arrangement  2 . The underside of scraper  16  is embodied as a strand guidance surface  17  and, in the installed position of the scraper, is located opposite the strand guidance surface  9  of shaping bed  3 , so that both strand guidance surfaces  9  and  17  define a strand guidance channel. In this case, strand guidance surface  17  has a concave curve and is aligned with an adjoining strand guidance surface  18  formed on the underside of inlet finger  4 . This design of the strand guidance surfaces provides the deformation of the incoming tobacco strand into a cylindrical fiber strand in a known manner, while at the same time wrapping the fiber strand in wrapping material strip  14 . 
     On its delivery end in the area of the shaping inlet, suction strand conveyor belt  1  is guided around a contact wheel  19 , which is caused to revolve by, and thus rotates with, suction strand conveyor belt  1 . The entire delivery end of the suction strand conveyor belt and contact wheel  19  are covered by lateral walls embodied as knife strips  21 , which extend on the underside as far as along strand guidance surface  9  in shaping bed  3 . A so-called thrust travel block  22  is inserted between the two knife strips  21  for guiding suction strand conveyor belt  1 . Thrust travel block  22  includes a scraper-like retaining wedge  23  placed against contact wheel  19  at a distance within a range of one tenth of a millimeter in the lower nip formed between contact wheel  19  and suction strand conveyor belt  1 . On its top, thrust travel block  22  is equipped with a scraper  24  placed against the circumferential surface of contact wheel  19 . Thrust travel block  22  is provided with a recess in the form of an air channel  26  above retaining wedge  23 , which by an approximately concentric course in relation to contact wheel  19  defines a belt free circumferential section (i.e. free of the suction strand conveyor belt). Air channel  26  extends between the upper and lower course of the suction strand conveyor belt  1  between scraper  24  and retaining wedge  23 . Air channel  26  has a flow connection at one end with suction chamber  8  and a flow connection at the other end with inner bearing space  27  of contact wheel  19 . In this regard, contact wheel  19  is provided with a total of eight radial air slits  28 , four on each side of contact wheel  19 , opening at the circumferential edge thereof, so that a flow connection is provided between inner bearing space  27  of contact wheel  19  and suction chamber  8  via the passage of air slits  28  when they are released by suction strand conveyor  1 . To increase the efficiency of the suction flow, openings  29  are provided in the front face  30  of contact wheel  19 , and openings  31  coupled to the atmosphere are provided in each knife strip  21 , in an area of the lower end of air channel  26 . Dust particles not caught and removed by scraper  24 , are suctioned off the circumferential surface of contact wheel  19  by a permanent draft created by the suction air flow from contact wheel  19  to suction chamber  8 . As a result, tobacco dust cannot collect between the circumferential surface of contact wheel  19  and suction strand conveyor belt  1  which could result in lifting the suction strand conveyor belt off the surface of contact wheel  19 , so that damage of the suction strand conveyor belt  1  by scraper  17  is prevented. At the same time, dust deposits in bearing space  27  of contact wheel  19  are prevented by the passage of the air channel through air slits  28 , so that bearing damage is prevented over a long period of time. 
     The invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art, that changes and modifications may be made without departing from the invention in its broader aspects, and the invention, therefore, as defined in the appended claims, is intended to cover all such changes and modifications that fall within the true spirit of the invention.