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BACKGROUND OF THE INVENTION 
     This invention relates generally to an overcast system for use at an intersection of passageways in a mine. The overcast system includes adjustable stairways positioned adjacent to an overcast structure allowing people to cross over the overcast for passage along a mine passageway. 
     Overcasts are used in mines at passageway intersections to prevent mixture of ventilation air flow in the intersecting passageways and to maintain airflow along the desired path within a mine. An overcast is a tunnel structure that forms a flow conduit connecting two sections of a first passageway at an intersection of the first passageway with a crossing or intersecting second passageway. The overcast includes a pair of generally parallel spaced apart upstanding side walls and a deck or roof extending between and secured to the side walls. The side walls and deck form a flow conduit (tunnel) communicating between the sections of the first passageway. End and wing panels are used in combination with the overcast to form an overcast system that substantially closes off flow communication between the first and second passageways at their intersection. The normal airflow pattern is to have inlet air flowing through the overcast and return air flowing over the overcast. However, under certain circumstances, the airflow could be reversed. Overcast systems are known in the art as exemplified in U.S. Pat. No. 5,466,187 the disclosure of which is incorporated herein by reference. An airflow segregating structure like an overcast may also be erected by first excavating the mine floor. The deck is positioned over at least a portion of the excavated area and could be closer to the original floor than a typical overcast. This type of structure is referred to as an undercast. An undercast can use any suitable deck support erected adjacent opposite sides of the excavation to support the deck. Such support can be erected in the excavation and/or erected to the side of the excavation on the original mine floor. Such support can be, e.g., a block wall or could also be length extensible steel panels like those used to form stoppings as are known in the art. As used herein, the overcasts and undercasts will be referred to as air crossings. 
     Air crossing systems are effective to segregate the airflows between the first and second passageways at the intersection. However, they limit the ability of miners to utilize the second passageway since a wall is created across the second passageway. Personnel have found ways to get around the air crossing systems including some ways that may not be safe or reliable. Many times personnel will maneuver through gaps in the walls of the air crossing or in the wing panels sealing the air crossing to the mine walls. Sometimes ramps are formed at the air crossing sidewalls with back fill. Ladders could likewise be used, but mine floors can be rough and uneven making use of a ladder difficult and possibly dangerous. None of the foregoing ways of circumventing an air crossing would be very effective in the event an injured miner needs to be evacuated from the mine or if an emergency evacuation is needed. 
     The use of permanent and semipermanent structures in mines presents difficulty because mines are not static in their size and shape. Common phenomenon in mine passageways is convergence and divergence of the mine walls, floor and roof. A mine passageway will change in size and shape, sometimes significantly, due to the weight of the overburden over the mine passageway. The walls, floor and/or the roof may converge inwardly into the passageway. Also, and less frequently, the walls, floor and/or roof may diverge making the passageway larger. Both convergence and divergence may occur in a passageway. As used herein, the phrase “dimensional instability” includes both convergence and divergence. 
     Dimensional instability can cause problems with any structure installed in a mine passageway. Structures may shift or try to change dimensions or become separated from a wall, floor, roof or one another. The position of a structure may also change. Dimensional instability presents potential structural integrity problems for any structure positioned in a mine particularly with the ability of a mine to apply tremendous forces to a structure. Thus, mine equipment has special design needs to be able to accommodate dimensional instability. For example, an opening or door in an air crossing may not be available or workable from time to time, because of structural movement or improper maintenance, as a means to cross from one side of an air crossing to the other side. Further, equipment such as conveyors may be positioned in the air crossing preventing a miner from traversing across the air crossing from side to side. High airflow rates and pressure differentials can also make doors difficult to use. In the event of an emergency, structures must work reliably to reduce risks by allowing personnel unimpeded and safe passage through a passageway. Likewise, in the performance of normal duties, structures should not be an impediment to miners. 
     Thus, there is a need for an air crossing system which controls airflow in a mine but which can be easily traversed by crossing over in one passageway and passing through in an intersecting passageway. The present invention utilizes a deck structure with an automatically adjustable stairway to form a walkway for conveniently and safely traversing from one location in a mine to another including the use of such structure as an air crossing or as a walkway to cross over mine equipment like large conveyor belts. 
     SUMMARY OF THE INVENTION 
     Among the several objects and features of the present invention may be noted the provision of a deck structure that can be used as a walkway that will accommodate dimensional instability of the mine passageway; the provision of a deck structure that permits safe and convenient traversing from one location in a mine to another; the provision of such a deck structure with an automatically adjustable stairway system that will accommodate dimensional instability; the provision of such a deck structure as an air crossing system that provides for reliable traversing thereof through both the first and second passageways the provision of a such a stairway that can be used at various angles of inclination up to vertical for traversing the air crossing; the provision of such a stairway that utilizes the air crossing for orienting the steps of the stairway generally horizontally; the provision of such a stairway that can be freestanding and positioned adjacent a deck of the air crossing; the provision of such a stairway that is height adjustable for use with a variety of different height air crossings; the provision of such a stairway that can be used either free standing or secured to an air crossing; the provision of an air crossing system utilizing one or more automatically adjustable stairways to provide a means to traverse an air crossing; and the provision of a stairway that accommodates uneven floors, different height air crossings, changes in height of the air crossing and is durable in operation in a mine environment. 
     The present invention involves the provision of a stairway for a mine air crossing installed in a mine passageway. The stairway comprises a pair of elongate generally parallel supports at opposite sides of the stairway. Each support includes upper and lower parallel stringers disposed one above the other at a respective side of the stairway. A plurality of generally horizontal steps extend between the supports, each step having a front edge, a rear edge, and opposite ends positioned adjacent respective supports. Two horizontal pivot connections are provided between each end of each step and the upper and lower stringers of a respective support to form parallelograms at opposite sides of the stairway whereby the steps remain generally horizontal regardless of the angle of inclination of the stairway and regardless of a mine convergence or divergence causing a longitudinal shift of an upper stringer relative to a lower stringer. Handrails are mounted on the steps at opposite sides of the stairway. Each handrail has a plurality of generally vertical posts each having a lower end attached to a respective step and an upper end, and a hand bar having generally horizontal pivot connections with the vertical posts adjacent their upper ends. The hand bars are removable from respective posts so that when the stairway is moved to a position in which said supports are vertical, said vertical posts of the handrails are positioned to be grasped by a person climbing the stairway. 
     The present invention also involves the provision a stairway for a mine air crossing installed in a mine passageway. The stairway comprises a pair of elongate generally parallel supports at opposite sides of the stairway. Each support comprises upper and lower parallel stringers disposed one above the other at a respective side of the stairway, the supports having opposing inside faces defining an interior space therebetween. A plurality of generally horizontal steps extend between the inside faces of the supports, each step having a front edge, a rear edge, and opposite ends positioned adjacent respective supports. Two horizontal pivot connections are provided between each end of each step and the upper and lower stringers of a respective support to form parallelograms at opposite sides of the stairway whereby said steps remain generally horizontal regardless of the angle of inclination of the stairway and regardless of a mine convergence or divergence causing a longitudinal shift of an upper stringer relative to a lower stringer. The horizontal pivot connections comprise knock-down connections configured for easy assembly and disassembly of the stairway, said knock-down pivot connections comprise a plurality of holes in the stringers and a plurality of pivot members removably attachable to respective steps and stringers. The pivot connections are securable from a position outside the interior space to facilitate assembly and disassembly of said stairway in a mine. 
     The present invention also provides a stairway for a mine air crossing installed in a mine passageway. The stairway includes a pair of elongate generally parallel supports at opposite sides of the stairway, each support comprising upper and lower generally parallel stringers disposed one above the other at a respective side of the stairway. A plurality of generally horizontal steps extend between the supports, each step having a front edge, a rear edge, and opposite ends positioned adjacent respective supports. Two generally horizontal pivot connections are provided between each end of each step and the upper and lower stringers of a respective support to form parallelograms at opposite sides of the stairway whereby said steps remain generally horizontal regardless of the angle of inclination of the stairway and regardless of a mine convergence or divergence causing a longitudinal shift of an upper stringer relative to a lower stringer. A generally horizontal landing is attached to the supports at upper ends thereof. Means is provided for fastening the landing to an upper surface of the air crossing thereby to secure the stairway to the air crossing and set the angle of inclination of the steps. 
     A stairway for a mine air crossing installed in a mine passageway is provided. The stairway includes a pair of elongate generally parallel supports at opposite sides of the stairway, each support comprising upper and lower parallel stringers disposed one above the other at a respective side of the stairway. A plurality of generally horizontal steps extend between the supports, each step having a front edge, a rear edge, and opposite ends positioned adjacent respective supports. Two generally horizontal pivot connections between each end of each step and the upper and lower stringers of a respective support to form parallelograms at opposite sides of the stairway whereby the steps remain generally horizontal regardless of the angle of inclination of the stairway. A pair of variable length columns are provided, each mounted on a said step adjacent upper ends of the supports, said columns having lower ends for resting on a mine floor to at least partially support the steps and supports. 
     The present invention involves an air crossing system for a mine. The air crossing system includes a pair of generally vertical sidewalls with upper edges, the sidewalls are in spaced apart relation. A deck extends between the sidewalls and is secured thereto adjacent the upper edges of the sidewalls. A stairway is positioned adjacent to at least one of the sidewalls and includes a pair of elongate generally parallel supports at opposite sides of the stairway, each support comprising upper and lower stringers disposed one above the other at a respective side of the stairway. A plurality of generally horizontal steps extend between the supports, each step having a front edge, a rear edge, and opposite ends positioned adjacent respective supports. Two pivot connections are provided between each end of each step and the upper and lower stringers of a respective support form parallelograms at opposite sides of the stairway whereby the steps remain generally horizontal regardless of the angle of inclination of the stairway. 
     The present invention also involves a stairway for use with an elevated deck in a mine to form a walkway extending over an area to be traversed. The stairway includes a pair of elongate supports at opposite sides of the stairway, each support comprising upper and lower stringers disposed one above the other at a respective side of the stairway. A plurality of generally horizontal steps extend between the supports, each step having a front edge, a rear edge, and opposite ends positioned adjacent respective supports. Two generally horizontal pivot connections are provided between each end of each step and the upper and lower stringers of a respective support to form parallelograms at opposite sides of the stairway whereby the steps remain in a generally horizontal orientation regardless of the angle of inclination of the stairway. Retaining means is secured to the stairway adjacent to an upper step of the stairway for retaining the steps in a fixed angular position relative to the deck. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an air crossing system including an air crossing positioned in intersecting passageways in a mine; 
     FIG. 2 is a plan view of the air crossing system of FIG. 1; 
     FIG. 3 is an enlarged perspective view of a stairway as shown in FIG. 1 showing the stairway in a normally inclined orientation; 
     FIG. 4 is an enlarged perspective view of a stairway in a generally vertical orientation for use; 
     FIG. 5 is an enlarged fragmentary perspective view of the stairway showing details of the mounting of steps and handrail to a step support; 
     FIG. 6 is an enlarged fragmentary bottom view of a step showing detail of the pivotal step mount arrangement of the structure of FIG. 5; 
     FIG. 7 is an enlarged fragmentary view of an alternative pivotal mount arrangement for mounting steps to the step supports; 
     FIG. 8 is an enlarged fragmentary perspective view of the stairway showing details of a pivotal hand bar mount; 
     FIG. 9 is an enlarged fragmentary perspective view of the stairway showing details of an alternative handrail mount; 
     FIG. 10 is a side elevation view of a modified form of stairway utilizing adjustable columns for support; 
     FIG. 11 is an enlarged fragmentary side view of an alternate mounting arrangement for the stairway; 
     FIG. 12 is an enlarged fragmentary plan view of another alternate mounting arrangement for the stairway; 
     FIG. 13 is a schematic illustration of a stairway mounted on an air crossing showing the effect of the floor converging on the inclination of the stairway; and 
     FIG. 14 is an enlarged fragmentary side view of an alternate mount for the support columns. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawing (FIGS. 1,  2 ) a mine, designated generally as  10 , and as shown includes at least two passageways  12 ,  14  crossing at an intersection  18 . The passageways  12 ,  14  have sections  12 A,  12 B and  14 A,  14 B respectively on opposite sides of the intersection  18 . A deck structure system which is shown as an air crossing system designated generally as  20 , is positioned in the mine  10  in the intersection  18 . The air crossing system  20  includes a deck structure which in one embodiment, and as shown is an air crossing  22 . The air crossing  22  is illustrated as an overcast and includes a pair of adjustable stairways, each designated generally  24 , mounted on opposite sides of the air crossing  22 . The air crossing system  20  maintains the airflow streams in the passageways  12 ,  14  substantially separate at the intersection  18  to maintain the air flowing in the desired flow pattern in the mine. The present invention will be described as an overcast, however it is envisioned that the deck structure could be utilized as an undercast. Further, if regulation of air flow is not a requirement, the deck structure can be utilized as an elevated walkway to traverse over an area in the mine from location to another. For example, the deck structure could be used as a walkway or crossing to move from one side of machinery or equipment, like a conveyor belt, to the other side. 
     The air crossing  22 , in the preferred embodiment, extends between the passageway sections  12 A,  12 B and forms a flow conduit for conducting air from passageway section  12 A to section  12 B. The air crossing  22  substantially prevents the air flowing therethrough from flowing into the passageway sections  14 A,  14 B at the intersection  18  and likewise prevents air from flowing from the passageway sections  14 A,  14 B to the passageway sections  12 A,  12 B at the intersection. It is to be noted however, that the passageway sections  12 A,  12 B can be in flow communication with the passageway sections  14 A,  14 B elsewhere in the mine  10 . The normal airflow pattern is such that inlet air flows through the air crossing  22  and passageway  12  and return air flows over the air crossing  22  and through the passageway  14 . However, the air flow pattern could be the reverse as described above. 
     As seen in FIGS. 1,  2 , the air crossing  22  includes a pair of spaced apart side supports such as sidewalls  30  which are erected in the mine  10  in the intersection  18 . The sidewalls  30  extend between the mine sections  12 A,  12 B and across the ends of the mine sections  14 A,  14 B at the intersection  18 . The sidewalls  30  are generally vertical having bottom edges  31  in engagement with a mine floor  32 . The air crossing  22  also includes a top deck or roof  34  extending between and secured to the sidewalls  30  at upper edges  33  of the sidewalls  30 . Preferably, the deck  34  and sidewalls  30  form a deck structure and are preferably comprised of a plurality of side-by-side, length adjustable panels  35  secured together as is known in the art and as described in U.S. Pat. No. 5,466,187. In a preferred embodiment, the deck  34  is generally flat and generally horizontal while the walls  30  are generally flat and generally vertical. The present invention also envisions air crossings  22  of different shapes and configurations. For example, the air crossing  22  could have an arched deck  34 , or could be in the form of a squash culvert or pipe. If an air crossing  22  does not have a flat deck, a platform (not shown) could be erected on the air crossing for each stairway  24  to provide a relatively flat surface to affix the stairway to. The mine  10  includes a roof  36  defining the top of the passageways  12 A,  12 B,  14 A,  14 B and the intersection  18 . To provide clearance for traversing the deck  34  of the air crossing  22 , a mine roof section  36 A above the intersection  18  can be excavated elevating the mine roof section above the deck  34  for clearance therebetween. The deck  34  and stairways  24  form an elevated walkway. As discussed above, if the deck structure is to be used as an elevated walkway to traverse equipment, machinery or other obstacles, it can be erected anywhere in a mine and need not be located at an intersection of passageways. 
     As also seen in FIGS. 1,  2  the air crossing system  20  includes end and wing panels  37 ,  38  respectively, extending between the air crossing  22 , in particular, the sidewalls  30  and deck  34 , and mine walls  40 ,  42 , roof  36  and floor  32  at each end of the air crossing. The end and wing panels  37 ,  38 , in cooperation with the deck  34  and walls  30 , close the remaining flow paths between the passageways  12 ,  14  around the air crossing  22  to complete the air flow separation at the intersection  18 . Alternatively, the air crossing  22  can be longer than the intersection  18  thereby having end portions extending into the passageway sections  12 A,  12 B. By proper sizing of the walls  30  and deck  34 , the end and wing panels  37 ,  38  need not be used. The mine walls  40 ,  42  are oftentimes more stable farther from the intersection  18  and therefore do not necessarily require the automatic size adjustability of the end and wing panels  37 ,  38 . 
     As illustrated in FIG. 3, the stairway  24  includes a tread section  51  with a lower end  52  adapted to rest on the mine floor  32  and an upper end  54  adapted to be supported in an elevated position adjacent the deck  34 . The tread section  51  is automatically adjustable to accommodate different heights of air crossings  22 . The tread section  51  includes a pair of spaced apart side supports  56  with a plurality of steps or treads  58  pivotally mounted thereon and extending between the supports  56 . The steps  58 , FIGS. 5,  6 , are generally Z shaped in transverse cross section, each comprising relatively wide tread panels having a top surface  59 , a bottom surface  60 , a front (nose) edge  61 , a back edge  62  and opposite ends  63 L,  63 R. A front flange  65  depends from the panel  64  adjacent the front edge  61 , and a back flange  66  extends up from the panel  64  adjacent the back edge  62 . Hand holes  72  can be provided through the front flanges to facilitate use of the stairway. It is preferred that the top step  58  not have a back flange  66  to eliminate a tripping hazard. The steps  58 , in use, are generally horizontal end-to-end and front to back. In a preferred embodiment, for a tread panel  64  width having a back-to-front width of about 9 ½ inches, the top surface  59  will incline slightly downwardly from the back edge  62  to the front edge  61  a distance of up to about ⅜″ and preferably in the range of about ¼″ through about ⅜″ or at an angle in the range of about 1° through about 3°. It has been found that such an incline makes the stairway easier to traverse by reducing tripping on the front edge  61  and also improves water run off from the steps  58 . The steps  58  are preferably made of steel. 
     The steps  58  are pivotally mounted on the supports  56  and form a parallelogram with each support  56  allowing movement of the steps in unison while being retained in a preferred generally parallel relationship. As seen in FIG. 3, each support  56  includes a pair of spaced apart elongate stringers  70  disposed one generally above the other in a generally vertical plane. The stringers  70  are also preferably generally parallel. The pairs of stringers  70  have inside faces  69  defining an interior space  71  therebetween in which the steps  58  are positioned, opposite ends  63 L,  63 R of the steps being positioned adjacent respective stringers  70 . The stringers  70  are structural members such as square tubes made of steel. The lower end  52  of the stairway  24  can be provided with height adjustable pads  73 , FIG. 3, secured to the stringers  70  to compensate for an uneven mine floor  32 . Shims (not shown)could be used instead of the pads  73  to compensate for such unevenness. Wheels or skid plates (not shown) could also be used in place of the pads  73  to facilitate movement of the lower end  52  along the mine floor  32 . 
     The steps  58  are pivotally mounted on the stringers  70  preferably utilizing two generally horizontal pivoting connections on each end  63 L,  63 R of each step  58 . It is preferred that the pivotal mounting be of a knock down type to provide easy assembly and disassembly of the stairway  24  in a mine  10 . FIGS. 5,  6  show one form of pivotal mounting. Channel members  76 , such as angles, are affixed to the bottom surfaces  60 , as by welding. As shown, two pairs of channels  76  are secured to each step  58 . One pair of channels  76  is positioned adjacent each step end  63 L,  63 R with one of the two channels in a pair being positioned adjacent the front edge  61  and the other being positioned adjacent the back edge  62 . The channels  76  adjacent the front edge have aligned bores  78  to define a first pivot axis extending lengthwise of the step  58  and the channels  76  adjacent the back edge have aligned bores  78  defining a second pivot axis generally parallel to the first pivot axis. 
     The stringers  70  are perforated with apertures  80  extending between opposite sides of the stringers  70 . The apertures  80  in each stringer  70  are spaced apart and positioned in a row. The rows of apertures  80  in the upper and lower stringers  70  forming a support  56  are generally parallel. The apertures  80  permit the installation of elongate pivot rods  82  from a position outside of the interior space  71  of the stairway  24  for removable attachment to the steps  58  and stringers  70 . The rods  82  are pushed through respective aligned apertures  80  and bores  78 , thereby pivotally mounting the steps  58  to the stringers  70 . Hitch pins  83  are installed in end portions of the rods  82  to secure the rods within the bores  78  and apertures  80 . The steps  58  and the upper and lower stringers  70  of each support  56  thus form parallelograms on each side of the stairway  24  whereby the steps  58  move in unison in response to movement of one or more steps or stringers and maintain the angular relationship between themselves. The rods  82  form two generally horizontal pivot connections between each end  63 L,  63 R of each step  58  and the upper and lower stringers  70  of a respective support  56 . Specifically, the steps  58  remain generally horizontal regardless of the angle of inclination of the stairway  24  and regardless of a mine convergence or divergence causing a longitudinal shift of an upper stringer relative to a lower stringer. 
     As seen in FIG. 3, a landing  85  is provided for facilitating transition from the top step  58  to the deck  34 . In the illustrated structure, the landing  85  extends rearward from the back of the top step  58  and has an upper surface  86  that is coextensive with the top surface  59  and preferably generally parallel to the top surface  59 . It is preferred that the landing  85  and top step  58  be an integral structure. A particularly important feature of the present invention is the use of the landing  85  to orient the position of the steps  58  generally horizontally. The landing  85  fixes the positions of the steps  58  so they are preferably inclined slightly downwardly from back edge  62  to front edge  61 , as described above, and are generally horizontal end-to-end. The landing  85  is preferably secured on the top surface of the deck  34  with mechanical fasteners  88  such as bolts. In the erection of the air crosssing  22 , the deck  34  can be fairly accurately positioned in a generally horizontal plane. The deck  34  will remain generally in its original generally horizontal orientation after installation. This allows for the angle of the top surfaces  59  of the steps  58  to be fixed and maintained without frequent adjustment to compensate for dimensional instability. By fixing the orientation of the landing  85  and top step  58 , the orientation of the remaining steps  58  is fixed, regardless of the angle of inclination of the stringers  70  or stairway  24 . The angular orientation of the steps  58  is thus not dependent on the condition or orientation of the mine floor  32  adjacent the lower end  52  of the stairway  25  nor is further excavation required at the lower end  52  to achieve a desired angular orientation of the steps. Alternatively, if the deck  34  of the air crossing is not as close to horizontal as desired or shifts, shims or the like (not shown) can be used between the landing  85  and deck  34  to position or reposition the landing as desired. 
     In an alternative embodiment, as seen in FIG. 7, the steps  58  can be pivoted to the stringers  70  via threaded mechanical fasteners such as hex head bolts  90 . In this embodiment, internally threaded members  91  are secured to the bottom surfaces  60  of the tread panels  64 , similar to the channels  76  described above. The bolts  90  are installed through the apertures  80  in the stringers  70  from the outboard side to the inboard side (or interior  71 ) and threadably engaged with the members  91  to pivotally mount the steps  58  on the stringers  70 . It is preferred that the bolts  90  be limited in axial movement within the members  91  to prevent binding against the stringers  70 . One means of preventing binding is to have blind bottoms in the internally threaded bores of the members  91 . The blind bottoms limit axial movement of the bolts  90  preventing their binding against the stringers  70  when tightened. Another means of preventing binding is to have the non-threaded sections of the bolts  90  sufficiently long that when the non-threaded portion engages the members  91  further axial movement is limited, preventing the bolts from binding against the stringers  70 . By tightening the bolts  90  against the blind bottoms or the non-threaded portion, the bolts are frictionally retained to prevent loosening. 
     As best seen in FIGS. 3,  5 , the stairway  24  includes a pair of spaced apart handrails, each designated generally as  94 . The handrails  94  are mounted for pivotal movement with the steps  58  relative to the stringers  70 . As shown, each handrail  94  comprises a series of generally vertical posts  96  removably mounted on brackets  97  which are secured to the steps  58  inboard of and adjacent opposite ends  63 L,  63 R thereof. The brackets  97  are affixed, as by welding, to the tread panels  64  and extend outward or forward from the front edges  61  of the tread panels. The brackets  97  include tubular sockets  98  with upwardly opening generally vertical bores  95 . The posts  96  have lower ends retained in the sockets  98  with T-handled set screws  99 . The sockets  98  and posts  96  are positioned forward of the front edges  61  of the steps  58 . By being positioned forward of the front edges  61 , the posts  96  do not interfere with step movement even when the angle of inclination of the stairway  24  is vertical (FIG.  4 ). Further, the posts  96  are rigidly mounted on the steps  58  and are generally perpendicular to the top surfaces  59  whereby the posts  96  remain generally vertical regardless of the angle of inclination of the stringers  70 . Each handrail  94  includes a hand bar or railing  100  pivotally mounted on the posts  96  by means of generally horizontal pivots  101  (FIG.  8 ), such as bolts and nuts. Preferably the hand bars  100  are positioned adjacent upper ends of respective posts  96 . If desired, the hand bars  100  may be removed when the stairway  24  is in a generally vertical orientation allowing a stairway user to grasp the posts  96  while climbing the stairway, much like a ladder. The pivots  101  and post mountings to the steps are preferably of a knock down type to permit easy assembly and disassembly of the stairway in a mine  10 . 
     An alternate mount for the posts  96  is shown in FIG.  9 . This post mount may be used when the stairway  24  will not be used in a vertical or near vertical orientation. Sockets  102  are secured to the steps  58 , as by welding, and can be positioned on the tread panels  64  at locations similar to sockets  98 , except they are not forward of the front edges  61 . The sockets  102  have generally vertical post receiving bores  103  that are generally perpendicular to the top surfaces  59 . The posts  96  are retained in the sockets  102  by T-handled set screws  99 . It is desired to be able to use the stairway  24  at a variety of angles of inclination as measured from horizontal preferably in the range of about 30° to about 90° (generally vertical). It is envisioned, that the stairway could also be used at angles of inclination from generally vertical to generally horizontal. By having the width of the steps from front to back appropriately sized to prevent interference therebetween, and by having the back flange  66  downturned instead of upturned to eliminate a tripping hazard, the tread section  51  could be positioned where the stringers  70  are close to horizontal. The steps  58  would then form a substantially continuous ramp. A generally vertical orientation is seen in FIG.  4 . The above described handrails  94  are constructed so as to not interfere with movement of the stringers  70  to the desired angle of inclination. Further, the hand rails  94  are mounted inboard of the stringers  70  in the interior  71  to help reduce the possibility of a miner or other personnel from inadvertently stepping on the top stringers  70  and slipping or losing balance. 
     FIG. 10 shows an alternate embodiment in which the elevated or top end  54  of the stairway  24  is supported by telescoping support means that permits height adjustment of the upper end of the stairway. As shown, the telescoping support means includes a pair of spaced apart telescoping columns  104  having variable lengths. The columns  104  have lower sections  109  telescopically fitted with upper sections  110  to provide for height (length) adjustment. As shown, an upper section  110  is preferably mounted within a lower section  109  and axially movable therein. The lengths of the columns  104  are fixed with T-handled set screws  111  mounted on the lower sections  109 . The upper ends  113  of the columns  104  are rigidly secured to the stairway  24 , adjacent the upper end  54 , at opposite ends of the top step  58 . Preferably the upper ends  113  have anchor plates  114  secured thereto, as by welding, which are secured to the bottom surface  60  of the top step  58  as by mechanical fasteners  115  such as bolts and nuts to provide knock down capability for ease of assembly and disassembly. FIG.  14  shows an alternate means of securing the columns  104  to the stairway. As shown in FIG. 14, sleeves  112  are secured to the bottom of the steps  58  as by welding and open downwardly. The upper ends  113  of the columns  104  are secured in the sleeves with T-handled set screws  111 . Fixing of the orientation of the columns  104  fixes the orientation of the steps  58 . It is preferred that the columns  104  be generally perpendicular to the top step  58  whereby when the columns  104  are generally vertical, the steps  58  are generally horizontal. The height of the columns  104  and the length of the stringers  70  will determine the angle of inclination of the stairway  24 . The columns  104  are preferably secured to the mine floor  32 . As shown in FIG. 10, anchor plates  116  are secured to the lower ends  114  of the columns  104 , as by welding. Mechanical fasteners  117 , such as anchor bolts, secure the anchor plates  112  and hence the columns  104  to the floor  32 . 
     A stairway  24  using columns  104  for support can be free standing or secured to the air crossing  22  for additional stability. A landing  120  can also be included to provide a smooth transition between the top step  58  and deck  34 . The landing  120  can be hingedly mounted to the top step  58  as at  121  to allow relative pivotal movement between the deck  34  and top step  58  about a generally horizontal axis. The use of a column supported stairway  24  would be advantageous when the deck  34  is not a relatively flat deck, e.g., a squash culvert as discussed above. The hinged landing  120  could be made sufficiently long, front to back, to provide the necessary walkway from the top step  58  to the deck  34 . The landing  120 , by being hinged, would provide a smooth transition from the top step  58  to the deck  34  and accommodate different heights between the top step and deck. 
     FIGS. 11,  12  illustrate alternative embodiments of the present invention having the columns  104  mounted to the stairway  24  on opposite sides thereof outboard of the interior  71 . i.e., laterally outside the planes defined by the sets of stringers  70  and adjacent the upper end  54 . FIG. 12 shows one of the columns  104  mounted on one side of the stairway  24 . The other column  104  is similarly mounted to the stringers  70  on the other side of the stairway  24 . An anchor plate  114 , and hence the respective column  104  is secured to a bracket  130  that is preferably L-shaped having two flanges  132 ,  134 . As shown, the anchor plate  114  is secured to the flange  132  as with mechanical fasteners  136  such as bolts and nuts. The flange  134  is pivotally mounted on a support  56  extending between the respective upper and lower stringers  70 . Preferably the bracket  130  is mounted on the pivot rods  82  that pivotally mount the top step  58  to the stringers  70  by having the rods  82  extend through apertures (not shown) in the flange  134 . Thus, the bracket  130  moves with the steps  58  as part of the parallelogram. Fixing the orientation of the flange  132  with the column  104  fixes the angle of the steps  58 . 
     FIG. 12 shows a second form of outboard mounting for the columns  104 . As shown, the landing  85  is adjacent the upper end  54  and extends laterally outward of each of the supports  56 . The anchor plates  114  are rigidly affixed to the bottom surface of the landing  85  as with mechanical fasteners  140  such as bolts and nuts. Sleeves  112  could alternately be secured to the bottom of the landing  85  similar to their mounting on the steps  58  as seen in FIG.  14 . T-handled set screws  111  can be used to affix the columns  104  to the landing  85 . Preferably, the columns  104  are generally perpendicular to the steps  58  and landing  85 . Fixing the orientation of the columns  104  fixes the orientation of the steps  58 . When the columns  104  are generally vertical, the steps  58  are generally horizontal. 
     It will be apparent from the foregoing that the present invention provides an improved air crossing system  20  for use in a mine. The air crossing system  20  provides a pair of stairways  24  that automatically adjust to the mine floor  32  during installation of the stairway, keeping the steps  58  generally horizontal and automatically adjust for dimensional instability. FIG. 13 schematically illustrates movement of the stairway to a smaller angle of incline (the stairway shown in broken lines) due to the floor converging into the mine passageway  14 . The stairways  24  are adapted for use at various angles of inclination without modification or remounting making them versatile in installation and use. Should the mine converge or diverge, the stairway  24  will automatically adjust to compensate for the dimensional instability while keeping the steps  58  generally horizontal. Should convergence or divergence become severe enough to affect the usability of the stairways  24 , they can be easily adjusted to reorient the steps  58  to a generally horizontal orientation by remounting the landing  85  using shims or the like for adjustment or by adjusting the length of the columns  104 . The knock-down structure permits easy assembly and disassembly in the mine. The handrails  94  are structured to permit their use at the various angles of inclination and do not interfere with movement of the stairway  24 . The handbars  100  will remain generally parallel to the stringers  70  at the various angles of stairway inclination. 
     FIG. 13 also illustrates an additional embodiment of the present invention. The stairway  24  is also adapted to be used as a walkway between two mine floors, an upper mine floor  32 U and a lower mine floor  32 L. A portion of the mine floor  32 U adjacent the edge  145  of the upper floor  32 U can serve as a deck  146  and have the landing  85  secured thereto such as with anchor bolts  147 . The deck  146  could be excavated to make it generally horizontal to fix the orientation of the steps  58  generally horizontal. The angle of landing  85  and the steps  58  could also be adjusted by the use of shims (not shown) between the landing  85  and deck  146 . It is also envisioned that columns  104  could be used to support the stairway as shown in FIG.  10 . 
     The stairway  24  can have different constructions. For example, the steps  58  could be channel shaped having two depending flanges instead of one depending flange  65  and one upstanding flange  66 . The columns  104  could be screwjacks like the ones used in basements to support floor beams. Other forms of the generally horizontal step pivots could also be used. For example, spring loaded pins could be mounted on the steps with the pins being biased to outwardly extending positions and retained in place with hitch pins or the like. The steps  58  could be inclined end-to-end, while still being generally horizontal, to permit water drainage therefrom. 
     When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Summary:
An overcast system for a mine is disclosed. The system includes and overcast positioned at an intersection of mine passageways and operable to direct airflow through the passageways in a desired flow pattern at the intersection. Stairways are positioned adjacent opposite sides of the overcast providing a means for people to traverse between the opposite sides of the overcast. The stairways are self adjusting to accommodate overcasts of different heights while providing generally horizontal steps. The stairways are constructed with knockdown assembly means to provide for easy assembly and disassembly in the mine.