Patent Grant
6938372
The present invention relates to a mine door assembly and more particularly to a pneumatically-powered mine door assembly having a hydraulic checking system.
Doors used in mines operate under conditions not usually encountered by typical doors. Mine doors have door leafs that tend to be heavy and dimensionally large and are thus subject to large forces due at least in part to ventilation air flow in the mine and consequent air pressure differentials on opposite sides of a door. A leaf can be as large as 10 feet wide and 20 feet high and sometimes even larger. It can weigh more than a thousand pounds when designed for pressure differentials of seven inches of water gauge and over two thousand pounds for a pressure differential of 20 inches of water gauge. Even small pressure differentials can create large forces on the leafs because of their relatively large surface areas. It is difficult to control door leaf movement because of these forces and because of the substantial inertia associated with the heavy door leafs. Thus, it is desirable for the opening and closing of mine doors to be powered by one or more actuators, such as pneumatic or hydraulic power cylinders. From a cost standpoint, pneumatic power cylinders are preferred over hydraulic power cylinders. It is also desirable to use pneumatic power rather than hydraulic power because compressed air that may already be available in relation to other mine operations can be used to power the door installation as well, thereby obviating the need to provide a separate power supply for the door installation.
Unfortunately, pneumatically-powered mine doors are vulnerable to door leaf runaway due to compressibility of the air in the pneumatic actuator. When the resistance to door movement is high, the pressure in the pneumatic actuator must build up sufficiently to overcome the resistance. If the resistance drops off while the pressure in the pneumatic actuator is still high, the door leaf can accelerate unexpectedly and swing with great speed. This is dangerous because a rapidly swinging door leaf could easily injure a person or damage machinery. At a minimum a runaway door leaf would cause undesirable wear or damage to the door installation. Furthermore, the mine environment creates conditions that favor door leaf runaway. For example, if the door leaf opens by swinging toward the high pressure side of the door, the initial resistance to opening the door will be much higher than the resistance after the door is opened a small amount and the air pressures on opposite sides of the door leaf begin to equalize. It is also possible that a door leaf will catch on part of the floor or ceiling due to the natural convergence of the floor and ceiling caused by the overburden. Similarly, rock or other debris could obstruct movement of a door leaf. As a result of these or similar obstructions, pressure could build up in the pneumatic actuator causing the door leaf to run away when the resistance drops after the leaf has overcome the obstruction.
One strategy that has been employed to partially obviate the problem with runaway door leafs is to arrange the door so the leafs open by swinging away from the high pressure side of the door. Alternatively, a bidirectional double door can be used wherein one leaf opens by swinging away from the high pressure and one leaf opens by swinging toward the high pressure. If at least one door leaf opens by swinging away from the high pressure side of the door installation, the pneumatic actuator does not need to build up as much pressure to initiate opening. Consequently, the runaway leaf problem is alleviated to some degree. However, this is not an entirely satisfactory resolution to the runaway leaf problem. The leafs are still susceptible to runaway caused by obstructions from the floor, ceiling, or debris. Moreover, the door installation does not seal well when there is a leaf that opens by swinging away from the high pressure side of the door because the force from the pressure differential tends to move the leaf toward the open position and tends to push any sealing flaps away from the surfaces against which they are intended to seal. A better seal can be obtained by having all the door leafs in a door installation open by swinging toward the high pressure side. This way the force from the pressure differential tends to tighten the seal by pressing the door leafs and sealing flaps tightly closed.
Thus, there is a need for a mine door installation powered by pneumatic cylinders that avoids the problem noted above.
An embodiment of the present invention is a pneumatically controlled mine door installation. The mine door installation has a frame installed in a mine passageway. At least one door leaf is mounted on the frame for swinging movement between open and closed positions. The door leaf has a first face that is subject to relatively higher air pressure and a second face that is subject to relatively lower air pressure when the door installation is closed. An extensible and retractable pneumatically-powered actuator is mounted with a first end connected to the door leaf and a second end connected to a pneumatic actuator anchor so that extension and retraction of the actuator causes the door leaf to swing back and forth between its open and closed positions. The door installation also has a hydraulic checking system for controlling the speed of the door leaf as it moves back and forth between open and closed positions.
Another embodiment of a pneumatically-powered mine door installation for operation in a mine with an air ventilation system comprises a mine door frame installed in a mine passageway. First and second door leafs are mounted on opposite sides of the mine door frame for swinging movement between open and closed positions. Each door leaf has a first face that is subject to relatively higher air pressure and a second face that is subject to relatively lower air pressure when the first and second door leafs are in their closed positions. Each door leaf also has an extensible and retractable pneumatically-powered actuator mounted with a first end connected to the respective door leaf and a second end connected to a pneumatic actuator anchor so that extension and retraction of the actuator causes the respective door leaf to swing back and forth between its open and closed positions. The door installation also has a hydraulic checking system for controlling the speed of the first and second door leafs as they swing back and forth between their open and closed positions.
A control system for operating a pneumatically-powered door installation according to the present invention comprises a moveable control valve for selectively supplying air power to one or more actuators to cause swinging movement of one or more door leafs in a door installation in a mine passageway. The moveable control valve is biased toward a first position in which air power is not supplied to the one or more pneumatic actuators and moveable to a second position in which air power is supplied to the one or more pneumatic actuators. A second valve is operable to selectively open and close an air supply line between the control valve and a source of compressed air. The control valve is moved to its second position by the compressed air when the air supply line is open. The control system further comprises a calibrated vent for venting the air supply line between the control valve and the second valve.
Another embodiment of a control system for operating a pneumatically-powered mine door installation according to the present invention comprises a moveable control valve for selectively supplying air power to one or more actuators to open one or more door leafs in a door installation. The control system also comprises one or more operating valves operable to open and close an air supply line between the control valve and a source of compressed air. When the air supply line is open, the control valve is moved to a position supplying air power to said one or more actuators to open the one or more door leafs. The control system further comprises a limit valve which is also operable to open and close said air supply line between the control valve and the source of compressed air. The limit valve is operably linked to a second door installation whereby the air supply line is closed when the second door installation is open.
One embodiment of a pneumatically-powered door assembly comprises a door frame. At least one door leaf is mounted on the door frame for swinging movement between open and closed positions for opening and closing the door assembly. An extensible and retractable pneumatically-powered actuator is mounted with a first end connected to the at least one door leaf and a second end connected to a pneumatic actuator anchor so that extension and retraction of the actuator causes the door leaf to swing back and forth between its open and closed positions. The assembly includes a hydraulic checking system for controlling the speed of the at least one door leaf as it moves between its open and closed positions. The hydraulic checking system comprises a housing containing hydraulic fluid. A piston in the housing separates the interior of the housing into a blind end fluid chamber containing a first volume of hydraulic fluid and a rod end fluid chamber containing a second volume of hydraulic fluid. A rod extends through an opening in the housing. One end of the rod is connected to the piston in the housing and the other end of the rod is exterior of the housing. A hydraulic circuit provides fluid connection between the first and second fluid chambers. The hydraulic circuit has one or more fluid flow restrictions for providing resistance to flow of hydraulic fluid through the hydraulic circuit. The piston and rod are slidable relative to the housing to allow extension and retraction of the rod with respect to the housing. The rod is connected to either the door leaf or a hydraulic checking system anchor and the housing is connected to the other of the door leaf and the hydraulic checking system anchor so that swinging movement of the door leaf causes the rod to extend or retract with respect to the housing. Extension or retraction of the rod requires hydraulic fluid to flow through the one or more fluid flow restrictions. The stroke of the rod as the at least one door leaf moves from its closed to its open position is less than the stroke of the pneumatic actuator as the at least one door leaf moves from its closed position to its open position.
Another embodiment of a pneumatically-powered door assembly comprises a door frame. First and second door leafs are mounted on opposite sides of the door frame for swinging movement between open and closed positions. Por each of the first and second door leafs an extensible and retractable pneumatically-powered actuator is mounted with a first end connected to the respective door leaf and a second end connected to a pneumatic actuator anchor so that extension and retraction of the actuator causes the respective door leaf to swing back and forth between its open and closed positions. A hydraulic checking system controls the speed of the first and second door leafs as they swing between their open and closed positions. The hydraulic checking system comprises first and second housings containing hydraulic fluid. A first piston is located within the first housing and a second piston is located within the second housing, each of the first and second pistons separate the interior of the respective housing into a first fluid chamber containing a first volume of hydraulic fluid and a second fluid chamber containing a second volume of hydraulic fluid. A hydraulic circuit provides fluid connection between the first and second fluid chambers of each housing. The hydraulic circuit has one or more fluid flow restrictions for providing resistance to flow of hydraulic fluid through the hydraulic circuit. A first rod extends through an opening in the first housing and has one end connected to the first piston and the other end on the exterior of the first housing. A second rod extends through an opening in the second housing and has one end connected to the second piston and the other end on the exterior of the second housing. Each piston and rod is slidable relative to the respective housing to allow extension and retraction of the rod with respect to the housing. The extension and retraction of either rod requires hydraulic fluid to flow through the one or more fluid flow restrictions. Each rod is connected to either the respective door leaf or a hydraulic checking system anchor. Each housing is connected to the other of the respective door leaf or the hydraulic checking system anchor so that swinging movement of the first and second door leafs causes the respective rod to extend or retract with respect to its housing. For each door leaf the stroke of the respective rod as the door leaf moves from its closed to its open position is less than the stroke of the respective pneumatic actuator as the door leaf moves from its closed position to its open position.
In another embodiment a pneumatically-powered door assembly comprises a door frame. At least one door leaf is mounted on the door frame for swinging movement between open and closed positions for opening and closing the door assembly. An extensible and retractable pneumatically-powered actuator is mounted with a first end connected to the at least one door leaf and a second end connected to a pneumatic actuator anchor so that extension and retraction of the actuator causes the door leaf to swing back and forth between its open and closed positions. A hydraulic checking system for controls the speed of the at least one door leaf. The hydraulic checking system comprises a housing containing hydraulic fluid. A piston in the housing separates the interior of the housing into a blind end fluid chamber containing a first volume of hydraulic fluid and a rod end fluid chamber containing a second volume of hydraulic fluid. A rod extends through an opening in the housing. One end of the rod is connected to the piston in the housing and the other end of the rod is exterior of the housing. A hydraulic circuit provides fluid connection between the first and second fluid chambers. The hydraulic circuit has one or more fluid flow restrictions for providing resistance to flow of hydraulic fluid through the hydraulic circuit. The piston and rod are slidable relative to the housing to allow extension and retraction of the rod with respect to the housing. The rod is connected to either the door leaf or a hydraulic checking system anchor and the housing is connected to the other of the door leaf and the hydraulic checking system anchor so that swinging movement of the door leaf causes the rod to extend or retract with respect to the housing. Extension or retraction of the rod requires hydraulic fluid to flow through said one or more fluid flow restrictions. The location of the hydraulic checking anchor is selected so the ratio of the angular velocity of the door leaf to the rate at which the rod moves with respect to the hydraulic housing generally decreases as the door leaf moves along a substantial portion of the path from its closed position to its open position. The pneumatic actuator is mounted so the ratio of the angular velocity of the at least one door leaf to the rate at which the first end of the pneumatic actuator moves with respect to the second end of the pneumatic actuator generally increases as the door leaf moves along a substantial portion of the path from its closed position to its open position.
Other advantages and features of the present invention will be in part apparent and in part pointed out hereinafter.
Corresponding parts have corresponding reference characters throughout the drawings.
The technology of the present invention can be applied to both single-leaf door installations and double-leaf door installations.. After the construction and operation of a single-leaf door installation has been described, a detailed description of the construction and operation of a double-leaf door installation of the present invention will be provided. A detailed description of control systems that are particularly suited for operation of door installations of the present invention will also be provided.
Single-Leaf Door Installation
An exemplary single-leaf door installation of the present invention, generally designated 1, is shown in
When the door leaf 3 is in its closed position the entire perimeter of the door leaf 3 is adjacent the door frame 5, thereby forming an obstruction to airflow through the mine passageway 27. One or more conventional sealing flaps. (not shown) may be provided along the perimeter of the door leaf 3 to further restrict airflow through the door installation 1. Due to operation of the mine ventilation system (not shown), one side 9 of the door installation 1 is typically subjected to a relatively higher air pressure than the other side 11 of the door installation 1. Because the high pressure face 15 of the door leaf 3 is under more pressure that the low pressure face 17, a net force is exerted on the door leaf 3. Even a modest pressure differential can generate a large force because of the large surface area of the door leafs 3. In the embodiment shown in
The door installation 1 comprises an extensible and retractable pneumatically-powered actuator 19 for providing powered opening and closing of the door leaf 3. In the embodiment shown in
As is well known, compressed air can be used to drive extension or retraction of the rod 23 of the pneumatic power cylinder 19. The pneumatic cylinder 19 may be connected to the door leaf 3 so that extension of the rod 23 causes swinging movement of the door leaf 3 toward its open position and retraction of the rod 23 causes swinging movement of the door leaf 3 toward its closed position. For example, as shown in
As shown in
A hydraulic checking system 37 is used to control movement of the door leaf. In one embodiment, the hydraulic checking system 37 comprises a double-acting hydraulic checking cylinder 39. One preferred hydraulic checking cylinder 39 is commercially available as model number JK21487 from Jack Kennedy Metal Products and Buildings, Inc. of Taylorville, Ill. One with ordinary skill in the art could identify other suitable hydraulic checking cylinders for use in the hydraulic checking system. Those familiar with hydraulic systems will understand that the hydraulic cylinder is a very common form of hydraulic device, but that the cylindrical shape is not essential to operation of the device. Thus, the term hydraulic checking cylinder is intended to include any hydraulic device that operates in substantially the same way as the hydraulic checking cylinder in
A suitable hydraulic checking cylinder comprises a housing 41 containing hydraulic fluid 43, as depicted schematically in
A hydraulic circuit provides fluid connection between the blind end fluid chamber 49 and the rod end fluid chamber 51. The hydraulic circuit has at least one flow restriction that limits the flow of hydraulic fluid through the hydraulic circuit. For example, a flow restriction may comprise an adjustable needle valve. Those having ordinary skill in the art will recognize that ball valves, globe valves, gate valves, spool valves, and many other types of valves could be used for the flow restriction without departing from the scope of this invention. One exemplary hydraulic circuit 65 suitable for use in a single-leaf door installation 1 of the present invention is shown schematically in FIG. 4. Two fluid pathways 67, 69 provide fluid connection between the two fluid chambers 49, 51. The first fluid pathway 67, indicated by the solid-tailed arrows in
For reasons that will become clear after operation of the door installation 1 is described below, the hydraulic checking circuit 65 of
Operation of Single-Leaf Door Installation
Because mine door installations are normally kept closed (as shown in FIG. 2), the basic operation of the door installation 1 begins when the control system is triggered to direct extension of the pneumatic actuator 19. Extension of the pneumatic actuator 19 causes the door leaf 3 to swing toward its open position (shown in FIGS. 1 and 3), which requires extension of the rod 53 in the hydraulic checking cylinder 39. This, in turn requires the piston 45 and rod 53 to slide within the housing along the sliding axis 55, thereby changing the volumes of the two fluid chambers 49, 51. To accommodate the changing volumes in the two fluid chambers 49, 51, hydraulic fluid 43 must flow through the flow restriction in the hydraulic circuit. Similarly, when the control system is triggered to direct retraction of the pneumatic actuator 19, the door leaf 3 swings closed which causes the rod 53 in the hydraulic checking cylinder 39 to retract. This also requires hydraulic fluid 43 to flow through the flow restriction to accommodate the changing volumes of the two fluid chambers 49, 51. Because the flow of hydraulic fluid 43 through the hydraulic circuit is limited by the flow restriction, the rate of extension and retraction of the rod 53 in the hydraulic checking cylinder 39 is also limited. Accordingly, the hydraulic checking system 37 prevents the door leaf 3 from swinging too rapidly notwithstanding any drop off in the external resistance to movement of the door leaf 3.
In the hydraulic checking circuit 65 shown in
The total volume of hydraulic fluid filling the interior of the housing 41 varies as the rod 53 extends and retracts because as the rod 53 retracts it occupies more volume in the housing 41. A reservoir is required to hold at least the volume of hydraulic fluid 43 displaced by the rod 53 when it is fully retracted. For example in the hydraulic checking circuit 65 shown schematically in
The pressurized hydraulic fluid reservoir 79 performs another important function. Conventional rod seal packings (not shown) used in hydraulic cylinders are directional seals designed to prevent hydraulic fluid from leaking out of the housing when there is a high internal pressure. The seals are not suitable for keeping air from leaking into the hydraulic cylinder when there is negative internal pressure, such as might occur in the rod end fluid chamber 51 when the rod 53 is forced to retract into the housing 41. Failure to address this problem makes the hydraulic checking system 37 susceptible to entrainment of air and other contaminants which would interfere with proper functioning of the hydraulic checking cylinder 39. The hydraulic fluid reservoir 79 of the checking circuit 65 shown in
Mounting Alternatives
Depending on the specific objectives of the particular door installation, it may be advantageous to vary the locations at which the pneumatic actuator 19 and the hydraulic checking cylinder 39 are connected to the door leaf 3 and to vary the locations of the respective anchors 33, 59 for the pneumatic actuator 19 and the hydraulic checking cylinder 39. Both the pneumatic actuator 19 and the hydraulic checking cylinder 39 operate by applying a force to the door leaf 3. The pneumatic actuator 19 provides a driving force that acts along a line of action 95 between the connections 29, 31 of the pneumatic actuator 19 to the door leaf 3 and to the pneumatic cylinder anchor 33. The hydraulic checking cylinder 39 provides a checking force which acts along a line of action 97 between the connections 57, 61 of the hydraulic checking cylinder 39 to the door leaf 3 and to the hydraulic checking cylinder anchor 59. As the door leaf 3 moves back and forth between its open and closed positions, the angles between the lines of action 95, 97 for the forces and the plane of the door leaf 3 will vary. This will affect the mechanical advantage of the pneumatic actuator 19 and hydraulic checking cylinder 39. The rate of extension or retraction of the pneumatic actuator 19 and the hydraulic checking cylinder 39 as a function of the angular velocity of the door leaf 3 will vary depending on the angular position of the door leaf 3. By selecting appropriate locations for the anchors 33, 59 and connections 29, 57 to the door leaf 3, one can optimize the power of the pneumatic actuator 19 when it is most needed or optimize the checking action of the hydraulic checking system 37 when it is most needed.
For example, in the exemplary embodiment shown in
In contrast, in the embodiment shown
An additional advantage of anchoring the hydraulic checking cylinder 39 to the column 13 on the side of the door frame 5 is that there is no need to construct a separate linkage to anchor the hydraulic checking cylinder 39, which cuts the manufacturing expense. It is also advantageous to design the door installation 1 so that the hydraulic checking cylinder 29 has a shorter operating stroke than the pneumatic actuator 19. As the stroke of the hydraulic checking cylinder 39 becomes longer, the columnar stresses on the rod 53 increase. To account for the increased columnar stresses a disproportionately heavy and more expensive hydraulic checking cylinder is required. Shorter strokes do increase the operating pressure in the hydraulic checking system 37. However, hydraulic systems can tolerate much higher operating pressures than pneumatic systems so having the stroke for the hydraulic checking cylinder 39 shorter than the stroke of the pneumatic actuator 19 is acceptable. In the embodiment shown in
The alternative configuration shown in
Double-Leaf Door Installation
Most powered mine door installations need to allow passage of heavy machinery and vehicles used in mining. Thus, mine door installations usually have two door leafs to provide a wider passageway through the door. In the following description of double-leaf door installations, a part will be given the same reference number used in the description of the single-leaf embodiments if there is no substantial difference between the part used for the single-leaf embodiments and the double-leaf embodiments. As shown in
Referring to
The hydraulic checking circuit 113 for the double-leaf door installation 101 operates much like the hydraulic circuit 65 for the single-leaf door installation 1 in that hydraulic fluid 43 flows along a first fluid pathway 131 when the door leafs 3 open and a second fluid pathway 133 when the door leafs 3 close. When the door leafs 3 are opening, hydraulic fluid 43 generally flows along the opening pathway 131 (indicated by the arrows with broken-line tails in FIGS. 10 and 11), which runs from the rod end fluid chambers 51 of the hydraulic checking cylinders 39, then through the opening speed needle valve 123 and opening path check valve 125, then through the check valves 137 of the sequence flow control valves 121 to the blind end fluid chambers 49 of the hydraulic checking cylinders 39. When the door leafs 3 are closing, hydraulic fluid 43 generally flows along the closing pathway 133 (indicated by the arrows with solid line tails on FIGS. 10 and 11), which runs from the blind end fluid chambers 49 through the adjustable needle valves 135 in the closing sequence flow control valves 121, then through the adjustable closing speed needle valve 127 and closing pathway check valve 129, and then to the rod end fluid chambers 51 of the hydraulic checking cylinders 39. The opening path check valve 125 prevents fluid 43 from flowing through the opening speed adjustable needle valve 123 during closing, and the closing path check valve 129 prevents fluid 43 from flowing through the closing speed adjustable needle valve 127 during opening.
The adjustable needle valves 123, 127, 135 allow great latitude in adjusting the opening and closing speeds of the door leafs 3. The opening speed needle valve 123 can be adjusted to vary the opening speed of the door leafs 3 independent of the closing speed. Similarly, the closing speed needle valve 127 can be adjusted to vary the closing speed of the door leafs 3 independent of the opening speed. The settings of the closing sequence needle valves 135 can be adjusted to vary the rate at which one of the two door leafs 3 closes without affecting the rate at which the other of the two door leafs 3 closes. This feature allows coordinated setting of the closing sequence needle valves 135 to insure that the door leafs 3 close in a desired sequence. For example, if one of the two door leafs 3 has an astragal sealing flap (not shown) that closes against the other of the two door leafs 3, the closing sequence needle valves 135 can be set so the door leafs close in the required sequence. Because the adjustable needle valves 123, 127, 135 allow great variability in the resistance from the hydraulic checking system 117, a door installation having the hydraulic checking circuit 113 of
The pressurized reservoir 79 performs similarly to the pressurized reservoir 79 in the single-leaf door installation 1. When the rods 53 are retracting a volume of hydraulic fluid 43 corresponding to the displacement of the rods 53 flows to the pressurized reservoir 79. Conversely, the pressurized reservoir 79 releases enough fluid 43 to fill the hydraulic checking cylinders 39 when the rods 53 extend. Also, whenever the pressure in the rod end fluid chambers 51 drops below the pressure in the pressurized reservoir 79 the check valve 89 in the reservoir flow control valve 87 opens to prevent the pressure in the rod end fluid chambers 51 from dropping below ambient air pressure. Furthermore, the adjustable needle valve 91 in the reservoir flow control valve 87 prevents fluid 43 exiting the rod end fluid chambers 51 during extension of the rods 53 from entering the pressurized reservoir 79 rather than flowing through the opening speed needle valve 123. Thus, the setting for the adjustable needle valve 91 in the reservoir flow control valve 87 is preferably set to be slightly more restrictive that the setting of the opening speed adjustable needle valve 123.
Control System
Control for the double-leaf door installation 101 may be provided either through conventional controls or through one of the control systems illustrated schematically in
The control system further comprises a mechanism that selectively shifts the spool in the four-way valve 169. Preferably the control valve 169 is biased to its neutral position by springs 179 or the like. When the spool 177 of the four-way valve 169 in
In practice, long pneumatic hoses 195 would be used for the air supply lines 159, 171 between the operating valves 187, 189 and the four-way valve 169. This is because the operating valves 187, 189 may be one hundred feet or more from the door installation 101 so that opening and closing of the door 101 can be controlled at a location that is conducive to pulling long trains of vehicles through the door installation. Thus, a person at the front of a long line of vehicles does not have to backtrack to the door 101 to close it after the last vehicle passes through. Furthermore, it is desirable for the pressure in the pneumatic circuit 115 to be quite high to provide quick response to the operating valves 187, 189. Unfortunately, the combination of high pressure and long pneumatic hoses means there is a delay from the time the operating valves 187, 189 are released to the time the pressure inside the hoses 195 equilibrates, which can create a delay from the time the operating valves 187, 189 are released and the time the door leafs 3 stop moving. Thus, even if the door leafs 3 need to be stopped in an emergency, for example, they will continue to move for a period of time after the operating valves 187, 189 are released. To reduce the response time, a calibrated vent 197 may be provided adjacent each spool-shifting piston 183, 185. The calibrated vents 197 are small enough that they are easily overcome by the pressurized air source 157. However, when the operating valves 187, 189 are released, the calibrated vents 197 quickly vent the pressurized air right at the four-way valve 169 which dramatically decreases the response time of the door leafs 3 to release of the operating valves 187, 189. It is preferable to have the vents 197 as close to the four-way valve 169 as possible to quickly reduce the air pressure acting on the four-way valve. For example, a vent 197 may be formed by inserting a pipe tee at one end of the four-way valve 169. The pipe tee may have a hole drilled through a plug that is screwed into one leg of the tee to form the calibrated vent 197.
It is also contemplated that two door installations 101 of the present invention may be used together in tandem to create an air lock 201 as shown in FIG. 12. It is a current legal requirement in coal mines, for example, to use air locks in which at least one door installation is closed at any given time. Thus, the pneumatic control circuit 115 shown in
The control systems 151, 153 shown in
When introducing elements of the present invention or the preferred embodiment 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 they may be additional elements other than the listed elements.
As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
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