Abstract:
A simply and compactly built drive system for a tube mill is provided whose force transmission is no longer threatened by flexures and other deformations of the grinding tube. A one-piece ring gear having a diameter less than or equal to the diameter of the grinding tube is attached to the small diameter end of a discharge cone of the grinding tube.

Description:
TECHNICAL FIELD 
     This invention relates to a drive system for a tube mill having a rotatably mounted grinding tube with a grinding stock feed and having a discharge cone in the shape of a truncated cone floatingly arranged outside the grinding tube mounting at the discharge end of the grinding tube. The discharge cone is closed at its small diameter by an end wall and has a plurality of discharge openings distributed about its circumference for the discharge of the grinding stock and of the mill gas, which discharge openings are enclosed by a stationary stock discharge housing. 
     BACKGROUND OF THE INVENTION 
     For the driving of rotary drums such as, for example, tube mills, it is known to mount on the outer circumference of the drum, in rotationally rigid fashion, a ring gear having a T-shaped profile in cross section and with teeth engaged by one or two drive pinions. A rotary drum drive of this type is shown in European patent number 0 175 109 published Jan. 30, 1990 for a Rotary Drum. Flexures of the drum can lead to drum curvatures, which also in combination with various thermal expansions, lead to relative displacements and/or inclinations of the tooth flanks with nonuniform load behavior over the tooth face width, which is detrimental in terms of force transmission, especially when the diameter of the ring gear is, as has been the case heretofore, larger than the diameter of the grinding tube. The large-diameter ring gears are usually assembled from two or more parts, so that the above-cited problems of the ring-gear drive can be further increased by pitch errors of the ring gear. 
     The so-called central drive shown in European patent number 0 184 326 published Jan. 18, 1989 avoids the conventional ring-gear drive in tube mills. A discharge cone in the shape of a truncated cone is floatingly arranged outside the grinding-tube mounting at the grinding-stock discharge end of the milling-grinding tube, which contains the grinding stock as well as the grinding bodies. The discharge cone is closed by an end wall at its small diameter, which end wall is engaged at the center by the drive shaft of the central drive. The discharge cone has openings distributed about the circumference for the discharge of the grinding stock and of the mill gas. The grinding-stock and mill-gas discharge openings are enclosed by a stationary stock discharge housing into which the ground stock falls downward and is discharged at the bottom while the mill gas, laden with fines, is withdrawn from the stationary stock discharge housing at the top. If this known tube mill were retrofitted with a ring-gear drive on account of necessity, the ring gear would be mounted on the outer circumference of the drum, as was heretofore usual, whereupon the above-described problems resulting from the large diameter of the ring gear would arise again. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to create for a tube mill a simply and compactly built drive system whose force transmission is no longer threatened by flexures and other deformations of the grinding tube. 
     SUMMARY OF THE INVENTION 
     In the drive system according to the invention, a drive gear is mounted on the truncated-cone-shaped discharge cone attached at the grinding-stock discharge end of the grinding tube, the drive gear having a diameter not greater than the diameter of the grinding tube. The torque transmitting drive wheel thus has a diameter maximally as large as the grinding-tube diameter, preferably smaller, so that the drive gear differs markedly in diameter from the large ring gears of the known ring-gear drives. In the drive system according to the invention, this comparatively small drive gear is flanged onto the small diameter of the discharge cone in simple fashion. According to an exemplary embodiment of the invention, the small-diameter drive gear is a ring gear which is distinguished first by light weight and comparatively low fabrication cost. The ring gear can be advantageously fabricated in one piece, so that pitch errors, such as occur in the usual large, multi-part ring gears, are avoided. Flexures and other deformations of the grinding tube scarcely manifest themselves on the small-diameter drive gear, so that the drive system according to the invention preserves the force transmission/torque transmission and uniformally distributes the load over the width of the tooth face. 
     The drive wheel flanged onto the small diameter of the discharge cone of the tube mill is preferably a one-piece ring gear into which at least one drive pinion comes into tooth profile engagement. In order to insure uniform load behavior over the tooth face width, it may be advantageous to make the drive pinion or drive pinions self-aligning in order to compensate for any possible alignment error of the ring gear and/or of the pinions. Furthermore, the ring gear can be kept narrow because the surface of the ring gear can be hardened, for example by induction hardening. 
     According to a further feature of the invention, the drive gear arranged on the small diameter of the discharge cone need not necessarily be made as a ring gear; the drive wheel can instead bear pole pieces and form the rotor of an electric motor, which rotor is surrounded in contact-free fashion by a stator of a gearless ring-motor drive. The ring motor that comes about in this way, because its rotor and stator are comparatively small in diameter, is correspondingly simple and economical to build. At the same time, the ring-motor drive has a continuously variable speed, and the speed is to be optimally adapted to the properties of the grinding stock as well as the mill-classifier grinding system in which the tube mill is employed. 
     The ring gear drive, the ring gear and the at least one driving pinion, are enclosed by a stationary ring-gear housing. Only a single ring seal is required between the lateral wall of the ring-gear housing facing toward the discharge cone and toward the small outer diameter of the discharge cone, because the other lateral wall of the ring-gear housing is completely closed. By virtue of the good sealability of the ring-gear housing, oil leaks are minimized in the case of circulating oil lubrication of the ring-gear drive, and even oil-splash lubrication would be possible. 
     In the drive system according to the invention, the flange arranged on the small diameter of the discharge cone can be made as a multifunction flange, in that the mill drive according to the invention can be retrofitted with a central drive, which then engages on the small diameter of the discharge cone or on its end wall. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     An exemplary embodiment of the invention is illustrated in the accompanying Drawing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The Drawing is a section view of the left end of a tube mill with a mill tube or grinding tube  10  mounted rotatably on its circumference for rotation about a horizontal axis. The stock for grinding, grinding bodies such as for example steel balls, and mill air, which can also include a hot gas stream in the case of combined milling and drying of moist stocks, enter from the right. Adjacent to the horizontal grinding tube  10  at the grinding-stock discharge end is a floatingly arranged discharge cone  11  in the shape of a truncated cone, via which the tube mill is driven in the manner according to the invention, as will be described in what follows. 
     A plurality of openings  12  for the discharge of the ground stock as well as the mill air are arranged in distributed fashion about the circumference of discharge cone  11 . These discharge openings  12  are enclosed by a stationary stock discharge housing  13 , in which the ground stock falls downward, is discharged at the bottom and is conveyed, usually by a bucket conveyor, to a downstream classifier of a closed-circuit grinding system, while the mill gas, laden with fines, is withdrawn from the stationary stock discharge housing  13  at the top and likewise led to the classifier, the oversize material or grit of which is recirculated to the grinding-stock feed end of the tube mill. 
     Adjacent to the small diameter of truncated-cone-shaped discharge cone  11  is a cylindrical intermediate section, which is closed at its end by an end wall  14 . A comparatively small-cross-section, T-flange-shaped, one-piece ring gear  15  is flanged, via its ring-shaped web, onto this end wall or onto the small diameter left end of discharge cone  11 , the ring gear  15  having an outside diameter that is smaller than the diameter of grinding tube  10 . The comparatively small, one-piece drive ring gear  15 , which can also be fabricated economically as a welded construction, is in tooth profile engagement with at least one drive pinion  16 , which is advantageously self-aligning and is driven at a speed of, for example,  12  to  18  revolutions per minutes by a main drive, not illustrated, attached at the left side, with a two-speed reducing gearbox  17 . Reference numeral  18  identifies an auxiliary drive. 
     In the simply and compactly constructed drive system according to the inventions, flexures as well as other deformations of the rotary grinding tube  10  have little effect on the position of ring gear  15 , so that a uniform load is distributed over the width of the tooth face, especially if ring gear  15  is narrow in construction. 
     The ring gear  15  and the at least one drive pinion  16  are enclosed by a stationary ring-gear housing  19 , which completely encloses the end of the tube mill with its left lateral wall and has only a single ring seal  20  protruding between the lateral wall  21  of the ring-gear housing facing toward discharge cone  11  and facing the small outside diameter of the discharge cone. In this way, possible oil leaks are minimized in the case of circulating oil lubrication of the ring-gear drive, and it is even possible to employ oil-splash lubrication. 
     The illustrated construction is particularly well suited for use in a rotary tube mill having the grinding tube  10  rotatably supported on its circumference for rotation about a substantially horizontal axis. The discharge cone  11  is in the shape of a truncated cone, having a large diameter end rigidly secured to the discharge end of the grinding tube  10  and a small diameter end closed by an end wall  14  perpendicular to the axis of rotation of the grinding tube  10 . The discharge cone  11  is supported only by the connection of its large diameter end to the grinding tube  10 . The end wall  14  of the discharge cone  11  provides a convenient torque transmitting member to which the large diameter ring gear  15  is attached. The ground stock and the mill air are discharged radially through a plurality of circumferentially spaced radial openings  12  in the truncated cone, rather than axially through the small end of the discharge cone  11 . A radially outward facing cylindrical surface on the small end of the truncated cone provides a convenient sealing surface for a ring seal  20  between the discharge cone  11  and the laterally inner wall  21  of a stationary ring gear housing  19 . By placing the discharge openings  12  in the discharge cone  11  and attaching the drive gear  15  on the small end of the cone, only one seal is required for sealing the ring gear housing  19  relative to the rotating discharge cone  11 . The illustrated construction permits the ring gear  15  to have a diameter that is smaller than the diameter of the grinding tube  10 . Placing the ring gear  15  on the end of the discharge cone isolates the ring gear  15  from the effect of flexures and distortions of the grinding tube  10 , which occur during operation of the tube mill, and the tooth loading is more uniform over the width of the gear teeth, thereby providing a longer service life for the ring gear ( 15 ). Besides tube mills, the drive system may have applications in other rotary drums, for example drum dryers or the like.