CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of U.S. patent application Ser. No. 09/188,918 filed Nov. 9, 1998, now U.S. Pat. No. 6,164,871. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to mine ventilation and, more particularly, to mine doors and mine stoppings. 
     So-called “stoppings” are widely used in mines to stop off the flow of air in passageways in the mines, a “stopping” generally being a masonry (e.g., concrete block) or metal wall installed at the entrance of a passageway to block flow of air therethrough. Such stoppings are typically provided with a doorway or opening and a door therein for occasional access to the blocked-off passageway. This arrangement is satisfactory under usual conditions when the stopping should prevent the passage of air through the passageway. In some conditions, however, high pressure concussive air is forced against the stopping and door. This may occur, for instance, in longwall mining where a large roof can collapse very rapidly and displace a large amount of air, thereby creating a shock wave of high pressure concussive air. Likewise, a mine blast will also force high pressure concussive air against the stopping and the door. In conventional stopping arrangements, the concussive air cannot escape through the stopping without causing significant damage to the stopping, the door or both. 
     Moreover, prior art stoppings are constructed generally perpendicular to the direction of air flow through the passageway. This construction is not optimal for decreasing the stress on the stopping caused by the concussive air. 
     SUMMARY OF THE INVENTION 
     Among the several objects and features of the present invention may be noted the provision of an improved mine stopping having a door which seals an opening in the stopping; the provision of such a stopping wherein the door can swing open in either of two directions to avoid damage to the stopping due, for example, to concussive air generated by longwall mining or by a mine blast; and the provision of such a stopping which is effective in controlling mine ventilation. 
     Further among the several objects and features of the present invention may be noted the provision of a mine stopping which may be installed in a mine passageway so as to reduce dynamic air pressure exerted on the stopping; the provision of such a mine stopping which is less likely to be damaged by concussive air; and the provision of such a mine stopping which is effective in controlling mine ventilation and which is economical to manufacture. 
     Briefly, this invention is directed to a mine stopping comprising a wall extending across a passageway in a mine to close the passageway. The wall has an opening therein. A door is hinged adjacent the opening for swinging between a closed position in which the door closes the opening to inhibit the passage of air therethrough, a first open position in which the door is swung in one direction away from the closed position, and a second open position in which the door is swung in an opposite direction away from the closed position. The door is movable to either of the first and second open positions when the door is subjected to substantial concussive air pressure thereby to permit concussive air to pass through the opening in both directions. 
     In another aspect of the invention, a swinging door system for closing an opening in a mine stopping comprises a door hinged adjacent the opening for swinging between a closed position in which the door closes the opening to inhibit the passage of air therethrough, a first open position in which the door is swung in one direction away from the closed position, and a second open position in which the door is swung in an opposite direction away from the closed position. The door is movable to either of the first and second open positions when subjected to substantial concussive air pressure thereby to permit concussive air to pass through the opening in both directions. 
     In yet another aspect of the present invention, a mine stopping is installed in a mine passageway having air flowing through the passageway in a first direction. The mine stopping comprises a rigid wall extending across the passageway to close it, and an opening in the wall to permit the passage of air therethrough for regulating the airflow. The wall extends between the opening and one side of the passageway at an oblique angle with respect to the direction of air flow through the passageway whereby air flowing through the passageway strikes the wall at the oblique angle thereby to reduce the dynamic air pressure exerted on the wall. 
     Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevation of a mine stopping of the present invention having a swinging door mounted thereon; 
     FIG. 2 is a horizontal cross section taken in the plane of line  2 — 2  of FIG. 1; 
     FIG. 3 is an enlarged front elevation of the door; 
     FIG. 4 is an enlarged vertical cross section taken in the plane of line  4 — 4  of FIG. 3; 
     FIG. 5 is a side elevation showing the door in a first open position with respect to a door frame, the door being shown in phantom lines in a second open position; 
     FIG. 6 is an enlarged front elevation of the swinging door with a locking bar mounted thereon; 
     FIG. 7 is an enlarged vertical cross section taken in the plane of line  7 — 7  of FIG. 6; 
     FIG. 8 is a horizontal cross section like FIG. 2 showing a second embodiment of the invention; and 
     FIG. 9 is a horizontal cross section like FIG. 2 showing a third embodiment of the invention; 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, FIGS. 1-5 show a sloped mine stopping, generally designated  10 , having a rigid wall  12  extending across a passageway  14  in a mine  15  to close the passageway, an opening  16  in the wall, and a swinging door  18  for closing the opening. The opening  16  is preferably defined by a rectangular door frame, generally designated  20 . The door  18  is hinged adjacent the opening  16 , the door preferably being hinged on the door frame  20 . However, it is contemplated that the door may be hinged directly on the wall  12 . The door frame  20  has opposing top and bottom horizontal frame members, designated  22  and  23 , respectively, and two opposing vertical right and left side frame members, designated  24  and  25 , respectively. The horizontal and vertical frame members  22 - 25  may be formed from channel bar stock, for example. The door  18  includes a main panel portion  28 , preferably formed from steel plate and reinforcing box beams  29  extending along its bottom and vertical sides. The door also includes an upper portion  30  preferably formed by a separate steel strip secured, such as by welding, to the main portion. The upper portion  30  is shaped to form a sleeve  32  extending parallel to the top edge of the door for receiving a horizontal hinge pin  34  which is secured to the top frame member  22 . The door  18  is swingable on the hinge pin  34  relative to the frame  20  between a closed position (FIG. 4) engaging surfaces of the frame members  22 - 25  to inhibit the flow of air through the opening, a first open position in which the door is swung in one direction away from its closed position (FIG.  5 ), and a second open position in which the door is swung in an opposite direction away from its closed position (shown in phantom lines in FIG.  5 ). It is to be understood that other hinge configurations for the door  18  may be used within the scope of this invention. For instance, while the door  18  is preferably hinged along its top edge, it will be understood that the door may be hinged at its bottom or at one side. 
     Preferably, a seal  36 , such as the rubber “D” seal shown in FIG. 4, is attached to the reinforcing box beams  29  at the periphery of the door  18  for sealingly engaging the bottom frame member  23  and the right and left side frame members  24 ,  25  of the frame  20  to seal against the passage of air through the opening  16  when the door  18  is in its closed position. A skirt  38  is attached to the main panel portion  28  at the top edge of the door  18  and extends substantially parallel to the top frame member  22  along substantially its full length. The skirt  38  is preferably made of neoprene and is attached by a retainer in the form of a steel channel  40  secured by screws  41  extending through the channel, skirt and into the door  18 . As shown in FIG. 4, the skirt  38  is shaped to engage the top frame member  22  when the door  14  is in its closed position to further seal the opening  16 . Preferably, a right corner piece  42  and a left corner piece  43  are mounted on adjacent corners of the door  18  for frictional, sealing engagement with the frame  20 . As shown, the right and left corner pieces  42 ,  43  are attached, as by screws  45 , near the bottom corners of the door to engage the right and left frame members  24 ,  25 , respectively, and the bottom frame member  23 . The right and left corner pieces  42 ,  43  help to seal the opening  16  at the bottom corners where the seal  36  may leave gaps. The right and left corner pieces  42 ,  43  are preferably relatively stiff and are made of thick rubber or other suitable material. The seal  36  and the corner pieces  42 ,  43  frictionally engage the frame members  22 - 25  to hold the door  18  closed under most circumstances. When the door  18  is subjected to substantial air pressure, such as concussive air from a mine blast or longwalling cave-in, the air pressure will overcome the friction between the seal  36  and corner pieces  42 ,  43  and the frame members  22 - 25  to force the door to an open position. Moreover, the air pressure may force the door  18  to swing in either direction to its first or second open position. The ability of the door  18  to swing in both directions is advantageous. Whenever a large, abrupt air displacement occurs, such as occurs in longwall mining or in a mine blast, there is an initial expansion or push of concussive air in a direction away from the displacement, followed by a pulling of air in the opposite direction toward the displacement as the low pressure on the displacement side of the stopping  10  is equalized with the higher pressure on the other side of the stopping. The construction of the swinging door  18 , as described above, allows the air flow caused by the displacement to pass through the opening  16  in both directions. 
     Referring to FIGS. 6 and 7, the mine stopping  10  preferably includes a mechanism for maintaining the door  18  in its closed position. In this embodiment, the mechanism includes an elongate locking bar  50  releasably mounted on the door  18  in a locking position in which the bar extends laterally beyond the door for engagement with the door frame  20  to prevent the door from moving toward either of its first or second open positions. As shown in FIG. 5, the bar  50  is secured to both the right and left frame members  24 ,  25 , although securement to only one frame member is contemplated. Preferably, the locking bar  50  is removably mounted on the door  18  by a plurality of generally U-shaped wire ties  52  (also referred to as twist clamps), each tie having a hook  54  at each end engageable with a slot of a bracket  55  affixed to the door, and a central portion  56  adapted to be twisted so as to deform the wire tie around the bar to hold the bar in engagement with the door. The bar  50  is similarly attached to the right and left frame members  24 ,  25  by wire ties  52  engageable with slots (not shown) in the frame members, the arrangement operable to prevent the door from swinging in either direction to an open position. The bar  50  is easily removable from its locking position simply by removing the wire ties  52 . It is to be understood that other mechanisms for maintaining the door closed may be used within the scope of this invention. 
     Referring again to FIGS. 1 and 2, the mine stopping  10  includes the rigid wall  12  extending across the passageway  14 . The wall  12  preferably includes a first wall section  62  extending between the opening  16  and one side of the passageway, and a second wall section  63  extending between the opening and the opposite side of the passageway. Each wall section is generally vertical and is generally planar. At least one of the two wall sections  62 ,  63  extends at an oblique angle A (see FIG. 2) with respect to the direction of air flow through the passageway  14 . In the preferred embodiment, both sections  62 ,  63  extend at an oblique angle with respect to the direction of flow. It should be apparent that the direction of air flow is generally parallel to the walls of the passageway  14  adjacent the stopping  10 . Conventional prior stoppings are installed perpendicular to the flow of air, which causes the stopping to experience the full amount of velocity pressure caused by the flow. (Note that static pressure is neglected in this discussion.) In the present invention, the angling of the wall  12  reduces the velocity pressure exerted against the stopping by a factor of the sine of the angle between the direction of flow and the wall. In the prior stoppings mentioned above, the angle between the direction of flow and the stopping is generally 90°. Since the sine of 90° is 1, the velocity pressure has its full impact on the stopping. In the present invention, as an example, the wall  12  may be constructed such that the angle A between the direction of flow and the wall is, for instance, 45°. Since the sine of 45° is 0.707, the velocity pressure impact on the door would be only 0.707 times the full velocity pressure. 
     The wall  12  may be constructed of a plurality of elongate extensible panels extending vertically in side-by-side relation from a floor to a roof of the passageway  14 . The panels are preferably positioned substantially in a plane at an oblique angle with respect to the direction of air flow through the passageway  14 . Installation of such panels is described in U.S. Pat. No. Re. 32,675, which is incorporated herein by reference, and suitable panels are available from Jack Kennedy Metal Products, Taylorville, Ill. The wall may also be constructed of masonry blocks or other similar materials. 
     It is to be understood that the stopping  10  with angled wall sections  62 ,  63  may be constructed without a door or other structure to close the opening  16 . It should also be understood that the opening  16  may be positioned immediately adjacent the passageway  14 , i.e., the wall  12  may consist of only one section extending from one side of the passageway to an opening at the other side of the passageway, the opening being defined in part by the passageway and in part by the stopping  10 . 
     FIG. 8 shows a second embodiment wherein the wall sections  162 ,  163  are oriented more than ninety degrees relative to the direction of air flow shown. However, as noted above, the direction of air flow in a mine passageway  14  is reversible. Thus, the angled wall sections may be angled more than ninety degrees relative to one direction of air flow, and may be angled at less than ninety degrees to flow in the opposite direction, but preferably the wall sections will generally be oblique to the direction of air flow. Moreover, as shown in FIG. 8, the wall sections  162 ,  163  may be angled at a shallower angle than in the first embodiment, such as about 30 degrees. The wall sections may also be angled at angles greater than those shown. The exact angle of the wall may depend upon such factors as the expected direction of air flow, or the length of the mine passageway in which the stopping is installed. The preferred angle for most stoppings is in the range of 40-50 degrees. 
     The sloped stopping  10  is advantageous as described above because it reduces the velocity pressure against the stopping. It is further advantageous in that it directs the air flow more satisfactorily toward the doorway or opening  16  in the stopping. This is accomplished because the sloped stopping acts as a funnel to direct the air flow toward the opening. This feature helps to improve the ventilation of the mine in that it allows air to flow more rapidly and efficiently through the stopping. 
     Referring to FIG. 9, in a less preferred embodiment, the swinging door and frame  20  of the first embodiment is shown installed in a conventional stopping  212  having walls installed perpendicular to the direction of air flow. Thus, the swinging door of this invention may be used advantageously in any mine stopping, and is not limited to use in the angled stopping of this invention. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     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.
A mine stopping installed in a mine passageway having air flowing through the passageway in a first direction includes a rigid wall extending across the passageway to close the passageway and an opening in the wall to permit the passage of air therethrough. The wall extends between the opening and one side of the passageway at an oblique angle with respect to the direction of air flow through the passageway whereby air flowing through the passageway strikes the wall at the oblique angle thereby to reduce the dynamic air pressure exerted on said wall.