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FIELD OF THE INVENTION 
   The subject matter of the present invention relates to a plow guide assembly for a chain-pulled plow. The plow guide assembly includes one or more plow guide troughs having multiple parts and being connectable to one another by coupling elements so as to be tension-proof and angularly movable. Each plow guide trough has a center piece with superposed chain guide channels. Two end pieces are joined to corresponding sides of the center piece. The end piece is preferably a single-piece casting with superposed chain channel sections that are separated by a cast sliding spacer piece. 
   BACKGROUND OF THE INVENTION 
   In underground mining operations, it is known to use plow systems which comprise a shiftable chain scraper conveyor which can be used both as a face conveyor and as a gate conveyor. On the working face side, plow guide troughs are mounted on the conveyor line pans of the chain scraper conveyor, on which plow guide troughs the plow is guided. The plow guide troughs have chain channels for guiding the endless plow chain disposed on them so as to be able to pull the plow along the wall face. In modern high-performance plow systems, to which the present invention is preferably addressed, the plow on the plow guide is forcibly guided. In the prior art, many measures have already been proposed to reinforce the conveyor line pans and the plow guide troughs so as to keep up with the ever-increasing rated capacities of the chain scraper conveyors and the plow drives in high-performance plow systems. Thus, it is known that the plow guides can be disposed on the goaf side and that the plow can be designed as a sword plow, with the sword plow extending with a sword under the chain scraper conveyor (DE 36 27 733) or reaching with a sword between the top strand and the bottom strand. 
   Although satisfactory, there remains a need for a plow guide with plow guide troughs on conveyor line pans suitable for use with a high performance plow system, such as the Gleithobel® plow system. Furthermore, there is a need for a plow guide that has a sufficiently high tool life, and which can be economically manufactured. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a plow guide assembly and plow guide troughs particularly adapted for high-performance plow systems such as Gleithobel® plows (available from DBT GmbH) which are disposed along the working face side of conveyor line pans of a chain scraper conveyor. The present invention uses the plow guide troughs according to German Patent DE 32 44 038 A1 as a starting point. In the generic plow guide troughs, the entire conveyor line pan, including the plow guide trough, is designed to comprise three parts, i.e., a center piece and two end pieces, so as to reinforce the individual plow guide troughs and to increase the tool life of the plow system. Preferably, at least the end pieces of the plow guide troughs on the working face side are made of single-piece highly refractory castings which, in the form of welded pieces, have been welded onto the conveyor line pan and onto the center piece. In the generic plow guide troughs and conveyor line pans, both the plow guide troughs on the working face side and the conveyor bottom and the spill plate side on the goaf side are designed in three parts. As a result, the time required to mount a generic plow guide trough or a conveyor line pan is comparatively high. The coupling elements in the generic plow guide troughs are bolts which have to be inserted into bolt holes in the spill plate pieces and in the sliding spacer pieces in the end pieces of the plow guide troughs. 
   The present invention provides a plow guide with plow guide troughs for mounting on conveyor line pans of a high-performance Gleithobel plow system. The plow guide has a sufficiently high tool life even if the rated capacities of the plow system are high. And, the plow guide can be manufactured economically. 
   According to the present invention, the end pieces of the plow guide have holding pockets which have been joined by casting and which are open on the working face side. High-duty toggles can be inserted into the holding pockets, requiring only minor assembly time. Consequently, in the plow guides according to the present invention, toggle connections form the coupling elements for connecting adjoining plow guide troughs in a tension-free manner, and the three-part design of the plow guide troughs with a center piece and two end pieces is retained. The toggle connections can be inserted into holding pockets that are integrally joined to the end pieces by casting and which do not require any after treatment. To reduce costs, the center pieces can be mass-produced while the end pieces can be produced to accommodate different widths, different coupling elements, or similar requirements. By using toggle connections instead of bolt connections for the generic conveyor line pans, it is possible to considerably increase the rated capacity of the plow system, while at the same time avoiding the risk of rupturing the coupling elements. 
   To make the end pieces of the plow guide troughs more wear resistant, the end pieces can have a preferably circumferentially closed upper chain channel section. The upper chain channel section of one of the lateral pieces can optionally have an overlapping projection and the upper chain channel section of the other end piece can utilize a mating recess for engaging the overlapping projection. Overlapping the end pieces of adjoining plow guide troughs and conveyor line pans minimizes wear due to fine coal abrasion which can be caused by angle formations between the individual conveyor line pans. Therefore, the tool life of the plow guide and thus the tool life of the entire plow system is increased. The overlapping projection and the mating recess may be designed in the shape of a ring and preferably in the shape of a centering ring and a mating centering recess so as to seal off the upper chain guide channel for the return strand of the plow chain. The preferably circular inside cross section of the upper chain channel section further reduces the susceptibility of the end pieces to wear. In a preferred modification, no transverse forces are absorbed or transmitted with the centering ring and the centering recesses. To implement this, the centering ring can conically taper towards its front side and/or can have a chamfering on its front side while the centering recess conically tapers with increasing depth and/or is countersunk on the bottom. It is also useful if one of the end pieces has an overlapping tongue above its upper chain guide channel section or in the region abutting the upper chain guide channel section and if the other end piece has a matingly overlapping slot with which the overlapping tongue mates so as to keep the overlapping parts on the chain guide channels free from the forces acting in the transverse direction. 
   To further minimize the seepage and deposition of fine coal dust in the upper chain guide channel, the center pieces of the plow guide can be made of a single-piece casting with a circumferentially continuous upper chain guide channel. For the purpose of inspection and simple assembly of the return strand of the plow chain, the center pieces of at least some plow guides can be made of a casting base with an access opening to the upper chain channel section. The access opening can be closed with a detachably attached hood. In the closed position, the outer surface of the hood preferably is aligned with the outer surface of all end pieces as well as the outer surfaces of the center pieces that are made of a single-piece casting. To facilitate the accessibility to the pulling strand of the plow chain, it is especially useful if the upper chain channel section in the casting base, in which the access opening is disposed, has a bottom opening leading to the lower chain channel section, which bottom opening can be filled with an insert piece, preferably an insert screen with suitable openings for allowing fine coal to pass through. Using the insert screen can ensure that during operation of the plow system, fine coal from the otherwise closed upper chain guide channel is discharged into the lower chain guide channel. The detachable insert piece replaces the sliding spacer piece (not present in the casting base) in the region of the bottom opening, with its upper surface being suitably designed in the shape of a rounded groove. It is especially preferred if the insert piece can be inserted and locked into the bottom opening without fastening elements or screws which require the use of a tool. Thus, the insert piece can have lugs disposed on the front and rear long side, and the casting base can have pocket inserts for the lugs, which pocket inserts have edges that are open toward the bottom opening. The insert piece which is inserted with its lugs into the appropriately designed pocket slots is subsequently affixed so as to face downwardly and in the direction of travel of the plow chain. The insert piece preferably has a step disposed on the front long side. A forwardly projecting lug is formed in the center of the step. The hood has a minimum of one longitudinal crosspiece disposed on the lower surface of the hood. The crosspiece, in the assembled state, abuts the step so as to be able to affix the insert piece only by inserting the hood into the bottom opening. It is useful for the rear surface of the hood to be designed as a preferably rounded groove section for bounding the upper chain guide channel. 
   The hood is locked to the casting bases preferably by one or more self-locking screws which have plate-shaped caps with rectangular locking attachments and which can be inserted into indentations on the upper surface of the hood, which recesses have a rectangular cross section. The locking screws preferably have a bolt screw disposed on the underside of the cap and can be screwed with a nut to the casting base from the goaf-side rear. To avoid an unintentional detachment of the hood as a result of vibrations, the hood can have a mounting hole for a damping element disposed between two indentations. 
   In a particularly preferred embodiment, the three-part plow guide trough itself forms the working face side of a conveyor line pan, with a single-piece intermediate metal sheet which extends continuously through the center piece and both end pieces or a single-piece continuous conveyor bottom being joined by welding to the goaf-side rear of the plow guide trough. This configuration with an intermediate metal sheet offers the advantage that the top strand of the chain scraper conveyor for each conveyor line pan can be designed as a two-way trough. To make mounting the intermediate metal sheet easier, the rear surfaces of the center piece and the end pieces can have projecting cast-on attachments, to which the intermediate metal sheet can be joined by welding. It is also to be preferred if the bottom of the bottom strand for the line pan is an angled bottom metal sheet which is joined by welding and which extends over the entire length of the center piece and the two end pieces. To further reduce the cost, the lateral profiles for guiding the scrapers in the bottom strand of the chain scraper conveyor do not have specific design features. 
   To operate a plow system with the plow guides and conveyor line pans according to the present invention, it is especially useful if ribs are welded on underneath the bottom metal sheet, both on the working face side and on the goaf side, with the underside of the ribs on the goaf side preferably having a curved design so as to serve as elastic arches which make it easier for the plow guide to climb when the plow system advances. The toggle connections preferably comprise toggles with toggle heads on both toggle ends, which toggles can be prevented from falling out or being detached by providing suitable retention elements in the pocket recesses that open toward the outer surfaces of the plow guide troughs. In particular, the toggle heads of the toggles can have tongues disposed on them. The tongues on one end mate with mating slots in the pocket recesses, thus making it possible for a retention element to be disposed only on one end of the toggles. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and embodiments of the plow guide according to the present invention follow from the claims and the following description of examples of conveyor line pans and troughs of a plow system which are diagrammatically represented in the drawings. In the drawings: 
       FIG. 1  shows a partially broken top view of a preferred embodiment of two plow guide troughs and conveyor line pans of a plow system in accordance with the present invention. 
       FIG. 2  shows the plow guide troughs of the plow system according to  FIG. 1 , with a view of the working face side of the plow system. 
       FIG. 3  shows a partially broken view of the front side of the preferred embodiment plow guide troughs according to the present invention. 
       FIG. 4  shows a sectional view along line IV—IV seen in  FIG. 1 . 
       FIG. 5  shows a perspective view of a preferred embodiment hood which is used in some of the plow guide troughs according to the present invention. 
       FIG. 6  shows a plan view of a preferred embodiment insert screen which can be inserted into the center piece in plow guide troughs having an opening. 
       FIG. 7  shows a sectional view along line VII—VII seen in  FIG. 6 . 
       FIG. 8  shows a lateral view of a detail of the locking screw used to affix the hood. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2  show two conveyor line pans  2 , 3  for a plow system. The conveyor line pans are arranged side by side and are connected to one another so as to be tension-proof but angularly movable, with, in  FIG. 1 , the lower side of conveyor line pans  2 , 3 . The conveyor line pans  2 , 3  face the working face side comprising plow guide troughs, referenced overall by reference numerals  20  and  50 , on which a Gleithobel plow system (not shown in detail) is forcibly guided. Except for the differently designed plow guide troughs  20 , 50  on the working face side, conveyor line pans  2 , 3  are of identical design, as can be especially well seen in  FIGS. 2 and 3 . As shown in  FIG. 3 , each conveyor line pan  2 , 3  has a profiled side wall  4  on the goaf side, with attaching parts  7  projecting upwardly. The side walls  4  are connected via an angled bottom strand metal sheet  5  and an intermediate bottom  6  to plow guide troughs  20  and  50  to form a flexurally rigid structure. Intermediate bottom  6  bounds a top strand vat-holding space for holding a top strand vat, referenced overall by reference numeral  8 , which top strand vat is designed as a working trough and can be removed by lifting it upwardly from conveyor line pans  2 , 3 . Bottom strand metal sheet  5  and the inside surfaces  9  of the lateral profiles  4  and  11  of plow guide troughs  20 , 50  bound a bottom strand  12  for the return strand of a chain scraper conveyor (not shown in detail). Below the bottom metal sheet  5  of the bottom strand, ribs  13  are welded on the goaf side. The ribs, on their lower surface  13 ′, are arched in a bow-like fashion so that ribs  13  not only ensure a reinforcement between bottom metal sheet  5  and lateral profile  4  on the goaf side, but also an elastic support of the plow system on the goaf side. On the working face side, additional ribs  14  are welded on below plow guide troughs  20 , 50  and angled bottom metal sheet  5 . To make it easier to join bottom metal sheet  5  of the bottom strand and intermediate bottom  6  by welding, both lateral profiles  4  and plow guide troughs  20 , 50  have attachment points  15 , 16  or steps disposed on their inside surfaces. 
   The present invention relates in particular to the multi-part structure of plow guide troughs  20 , 50  which bound conveyor line pans  2 , 3  on the working face side. As can be especially well seen in  FIGS. 1 and 2 , both the right-hand plow guide trough  20  and the left-hand plow guide trough  50  comprise three parts that are affixed, and preferably welded together. Both plow guide troughs  20 , 50  have a long center piece  21 , 51 , to which, on both ends, end pieces  22 , 23  and  52 , 53  are secured and preferably welded on. The end pieces are made of castings with integrally cast-on toggle holding pockets  24 , 25  and  54 , 55 . Plow guide troughs  20  and  50  differ only as to the design of their center piece  21  and  51 , respectively, while end pieces  22  and  52  and  23  and  53 , respectively, which are made of castings, are identically designed. End pieces  22  and  52  have toggle holding pocket halves  24  and  54  with undercuttings on the butt joints. In the undercuttings, the left (as seen in the figures) head  18  of toggle  1  can be form-fittingly anchored with its associated toggle head tongue  18 ′ without the need of other retention elements. The respectively other toggle holding pocket half  25  and  55  comprises a pocket extension  26  and  56  for a detachable toggle retention element  19  which extends over the tongue on the other toggle head. 
   As is shown in  FIGS. 1 ,  3  and  4  by way of an example, both the end pieces  22 , 52 , 23 , 53  and the center pieces  21 , 51  of plow guide troughs  20  and  50  have an upper chain channel  10 A,  10 C and  10 D for the return strand of the plow traction chain and a lower chain channel  10 B for the pulling strand of the plow chain. The lower chain channel  10 B is open toward the face so that a plow (not shown in detail) can be forcibly guided on slide rail  70  of plow guide troughs  20 , 50 —which slide rail bounds the lower chain channel  10 B on the face side—by means of arm attachments which engage in the lower chain channel  10 B where they are connected to the pulling strand of the plow chain. At the same time, the Gleithobel runs on slideway  71  which on all plow guide troughs  20 , 50  projects beyond slide rail  70  on the face side. 
     FIG. 3  also clearly shows that the upper chain guide channel  10 A has a circular cross section and is circumferentially closed. As  FIGS. 1 through 3  also show, end pieces  22 , 52  of plow guide troughs  20  and  50  have centering pins  72  which project beyond the front sides of end pieces  22 , 52  and which can engage in mating centering recesses on end pieces  23  and  53  of the plow guide troughs. End pieces  22  and  52  each have a sliding spacer piece such as centering rings  27  and  57 , respectively. The centering ring projects beyond the front side and tapers conically toward its tip and mates with a correspondingly designed centering recess  28  in end pieces  23  and  53 . The centering recess tapers conically as its depth increases. The conically tapering centering ring  27 , 57  has a chamfering defined on its front end. The conically tapering centering recess  28  is matingly countersunk. Between centering ring  57  and centering recess  28 , sealing surfaces form. The sealing surfaces prevent coal dust from entering or exiting along the joint between the conveyor line pans. Centering rings  27  and  57  form an extension of the upper chain channel section  10 A that projects beyond the associated end piece  22  and  52 , respectively, so that when angles form along the joint between two neighboring plow guide troughs  20 , 50  in the region of the upper chain channel  10 A, it is not possible for fine coal to enter between plow guide troughs  20  and  50 . Transverse forces between conveyor line pans  2 , 3  and plow guide troughs  20 , 50  are preferably not transmitted by way of mating centering rings  27 , 57  and centering recesses  28  but exclusively by way of centering pins  72  and the mating recesses and by way of overlapping tongues  74  and overlapping slots  75  above the upper chain guide channel  10 A. 
   As  FIG. 1  indicates, the two conveyor line pans  2 , 3  differ only as to the construction and design of the center pieces  21  and  51  for plow guide trough  20  and  50 . Center piece  21  is a casting with a cross section that is constant throughout its length, defines a circumferentially closed upper chain guide channel  10 C, with conical or chamfered circular expansions  29 A and  29 B being disposed along the transition region from the upper chain channel sections  10 A of the side pieces  22 , 23  to the chain guide channel  10 C in the center piece  21 . The expansions ensure that along the welded joint between end pieces  22 , 23  and center piece  21 , the plow chain can travel without touching abutted edges even in the presence of mounting inaccuracies. 
   Center pieces  51  differ from center pieces  21  in that center piece  51  has an access opening  58  in the front section  58 A—facing the working face side—of a casting base  59 , which front section forms a loading ramp. Via access opening  58 , the return strand of the plow chain, which return strand is installed in the upper chain guide channel  10 D,  10 A, can be accessed. The opening  58  is closed by means of a hood  60  which is anchored in opening  58  by means of two locking screws  80 . As seen in  FIGS. 1 and 4 , access opening  58  also leads to an access to the lower chain guide channel  10 B since, in addition to access opening  58  in the loading ramp, casting base  59  also has a bottom opening  78  in the transition region from upper chain channel  10 D to lower chain channel  10 B. Bottom opening  78  is closed by means of an insert piece  90  which is designed as an insert screen. 
   The design of casting base  59 , hood  60 , locking screws  80  and insert screen  90  will be explained below in greater detail with reference to  FIGS. 4 through 8 . On front side  61 , hood  60  which is perspectively shown in  FIG. 5  has a curved design which corresponds to the curvature of front side  58 A of center piece  51 , which front side serves as a loading ramp, while rear side  62  of the hood is circularly curved so as to complement the circular shape of the upper chain channel  10 D in center piece  51 . On the lower side, hood  60  has two longitudinal crosspieces  63 , 64 , the distance between which is greater than the depth of slide rail  70  ( FIG. 4 ). The anterior longitudinal crosspiece  63  abuts a longitudinal ledge  76  disposed on the upper side of slide rail  70 . As a result, the hood  60  in opening  57  is locked against horizontal movements perpendicular to the direction of chain travel. The rear longitudinal ledge  64  rests on lug  91  which faces the working face side and which is located in the center of insert screen  90 . As a result, the insert screen  90  is affixed on the working face side in opening  78 . As especially well seen in  FIG. 7 , lug  91  is an extension of step  99  on the front side  98  of insert screen  90  and is set off against upper side  92  of insert screen  90 , which upper side is curved in a groove-like manner. 
   In the upward direction, rear side  62  of hood  60  ends in a rounded lug  65  which extends along the entire length of hood  60  and above which stop ledge  66  for detachably locking hood  60  in opening  58  is disposed. Stop ledge  66  has two rectangular indentations  67  disposed in it, which indentations mate with one locking lug  83  each. The locking lug  83  has a rectangular cross section and projects laterally beyond screw bolt  82 , of caps  81  of locking screws  80  ( FIG. 8 ). Locking lugs  83  are tightened against the front side of casting base  59  by use of nuts  84 . The nuts  84 , from the goaf side, are screwed onto screw bolts  82  that pass through the holes in casting base  59  in the region of indentations  67 , with the bottom sections  68  of indentations  67  being clamped between locking nuts  83  on caps  81  of locking screws  80  and the front side of casting base  59  so as to anchor hood  60  in its upper locking ledge  66  as well. To protect locking screws  80  and the hood fastener, casting base  59  ends in rail  77 , the cross section of which has the shape of a T. At the midpoint between the two indentations  67 , hood  60  also has a mounting hole  69  for a damping element (not shown in detail). 
   Insert screen  90  is shown in detail in  FIGS. 6 and 7  and can be detachably anchored without additional screw fasteners in bottom opening  78  of casting base  59 . In addition to step  99  and central lug  91  which are pushed against the downwardly tapering bounding wall of the bottom opening by one of the longitudinal ledges of the hood, insert piece  90  has two additional lugs  95  disposed on its goaf-side rear side  94 . The lugs, on the upper end of the groove-shaped upper side  92 , project backwardly. Furthermore, insert screen  90  has longitudinal slots  96  which form open passages for discharging fine coal dust from the upper chain guide channel. And in lug  91 , the insert screen has opening  97  for engaging a tool for mounting and dismounting insert screen  90 . 
     FIG. 1  shows that insert screen  90  with its goaf-side lugs  95  is seated in open-edged pocket slots  79 , with lugs  95  being engaged in pocket slots  79  in such a way that in the mounted state, insert screen  90  is secured on the goaf side against vertical movements and axial movements in the direction of travel of the plow chain. 
   The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Summary:
The subject matter of the present invention relates to a plow guide for a chain-pulled plow for use in underground mining operations, with plow guide troughs  20  and  50  having multiple parts and being connectable to one another by means of coupling elements so as to be tension-proof and angularly movable. Each plow guide trough has a center piece  21  and  51  with superposed chain guide channels  10 C,  10 B,  10 D, to both sides of which center piece an end piece,  22,23  and  52,53  is joined by welding, which end piece is a single-piece casting with superposed chain channel sections that are separated by a sliding spacer piece that is cast on. To make available a high-performance Gleithobel system with a high tool life, cast-on holding pockets  24,25  and  54, 55 —which are open on the working face side—for toggle connections with toggles  1  as coupling organs are disposed on end pieces  22,23  and  52, 53 . (FIG.  2 )