Patent Publication Number: US-11040855-B2

Title: Emergency braking system for mine shaft conveyance

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a national stage entry, under 35 U.S.C. Section 371, of International Application No. PCT/CA2017/050532, filed May 2, 2017, which claims the benefit of U.S. Provisional Patent Application No. 62/331,115 filed on May 3, 2016, the entire disclosure of each of which is hereby incorporated by reference hereinto. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to mine shaft conveyances, and more particularly to emergency braking systems for mine shaft conveyances. 
     BACKGROUND 
     In the mining industry, it is typical for an underground mine to be accessed from surface level via a vertical mine shaft using a mine shaft conveyance or “cage.” A mine cage may be considered as a form of elevator car. The cage may be made from metal and may have a substantially cuboid shape. The cage is typically suspended from a metal cable, which may be colloquially referred to as a “hoist rope” or simply as a “rope.” The rope is used to convey (raise and lower) the mine cage within the mine shaft. 
     Cages vary in size and weight. A small cage may weigh as little as 2,000 pounds, whereas a large cage may weigh as much as 80,000 pounds. The floor or “deck” of a cage may measure eight feet wide by twenty feet long, in one example embodiment. A cage may have a single deck or multiple decks stacked vertically for increased load capacity. 
     Cages commonly carry cargo, mining personnel, or both. The loads carried by a cage may vary from trip to trip. For example, on some occasions, a cage may convey 170 people at once. Estimating 200 pounds per person, this represents a cargo of approximately 34,000 pounds. On other occasions, the cage may be occupied only by a single person, e.g. the cage operator or “cage tender,” who may weigh only 200 pounds or thereabouts. On still other occasions, the cage may be heavily loaded with cargo, which may weigh tens of tons. 
     A mine cage is conveyed up and down a mine shaft along vertical guide members or rails referred to as shaft guides. Shaft guides are typically attached to opposing faces of a mine shaft, on opposite sides of a cage. A shaft guide may have a rectangular cross-section and may be made from wood or from steel. In the latter case, the steel may be tubular. The cage may have rollers or other guide means for tracking the shaft guides during ascent or descent. 
     If a mine cage rope severs, the cage can go into freefall. Given typical mine shaft depths, which are currently in the range of 5,000 to 8,500 feet and are increasing, a cage freefall may have catastrophic results. Even when a cable is not severed, a cage may be subject to conditions, such as “slack rope” conditions (e.g. resulting from cage hang-ups in the mine shaft), resulting in a sudden drop (when the hang-up resolves) followed by a sudden deceleration (when the rope slack is taken up). Such a sudden deceleration may impart significant forces (e.g. multiple Gs) upon the cage. As with mine cage freefall, these forces may damage cargo and may be harmful or fatal for human occupants. At least for that reason, cage freefall, and slack rope or “overspeed” conditions are generally undesirable. 
     Some mine cages employ emergency arrest mechanisms designed to decelerate or stop the cage when a freefall or overspeed condition occurs. Such emergency arrest mechanisms have historically employed safety dogs. A safety dog is a spring-loaded mechanism which is mounted onto a mine cage. During normal mine cage operation, the safety dog is retracted and the mine cage is raised or lowered freely. In an emergency freefall condition, the safety dog deploys, causing a downwardly inclined, chisel-like tooth to engage and dig into the adjacent mine shaft guide. 
     An emergency braking system typically incorporates two safety dogs per mine shaft guide. Safety dogs rely on excavation of shaft guide material, e.g. digging a furrow into the shaft guide, in order to decelerate the mine cage. Thus safety dogs are primarily or exclusively used with wooden shaft guides. 
     Safety dogs may be considered disadvantageous for various reasons. 
     Firstly, safety dogs are not well-suited for use with steel mine shaft guides, which are too hard for the tooth/teeth of a conventional safety dog to dig into. Thus, use of safety dogs may force a mine operator to use wooden shaft guides. Yet wooden shaft guides may be considered inferior to steel shaft guides for various reasons, such as inconsistent material uniformity (e.g. due to knots in wood), inferior material strength relative to steel, and difficulty of acquisition/purchase of suitable wooden shaft guides. 
     Secondly, safety dogs damage wooden shaft guides when deployed. Wooden shaft guides also tend to degrade or lose structural integrity over time. Ultimately, wooden shaft guides may need to be replaced, which is costly and results in mine shaft downtime. 
     Thirdly, a mine cage whose movement is arrested by safety dogs may not experience a smooth deceleration but rather may experience a series of jolts which may be harmful for cargo and unpleasant for, or harmful to, human occupants. For example, jolting deceleration may occur when the tooth or teeth of a safety dog cause(s) a length of wood comprising the shaft guide to splinter or split vertically. When that occurs, as the mine cage decelerates, the safety dog tooth or teeth may periodically enter the free space of the vertical split, which offers no resistance and thus no braking force. In that case, the mine cage may experience a moment of acceleration until the safety dog once again digs into wood. Another possible consequence of a splitting shaft guide is the application of a significant and possibly damaging lateral load onto an opposing mine shaft guide. This may occur when one safety dog has caused its shaft guide to split while an opposing safety dog is braking effectively. The uneven braking forces on opposite sides of the mine cage may cause the cage to abruptly tilt away from vertical, or to swing, within the mine shaft. Inconsistencies in wooden shaft guides (e.g. varying moisture content, cracks, knots) may similarly result in inconsistent mine cage deceleration. 
     An alternative emergency arrest mechanism to the safety dog is the Blair hoist. A Blair hoist uses two hoist ropes to raise and lower an elevator car. Both ropes share in carrying the rope end load. The theory behind Blair hoists is that the likelihood of dual rope failure is extremely low. As such, Blair hoists may employ no other emergency arrest mechanisms. Put another way, the low probability of complete severance of both ropes may be considered to obviate the need for “on-board” safety arrest mechanisms. 
     The Blair hoist wraps both ropes onto a drum at the same time typical to single rope drum hoisting. This mode of operation separates Blair hoisting from the more conventional multi-rope Koepe (friction) hoist in that lifting force is not transferred to the hoist rope through frictional contact. The primary advantage of Blair hoists over friction hoists is that the former, unlike the latter, does not require any balancing “tail ropes” to be suspended from the underside of the shaft conveyance to balance the suspended loads on either side of the hoist. Such suspended tail ropes may be considered to undesirably limit the useful hoisting depth of friction hoist systems to approximately 5000 feet, due to entanglement of the tail ropes induced by the Coriolis effect of the Earth&#39;s rotation. 
     A possible disadvantage of Blair hoists is their installation and operational cost. The complexity of Blair hoist systems may require or warrant an increased maintenance staff size, significant infrastructure provisions and high energy usage to operate. Installation costs alone may increase the hoist plant cost by ten million dollars relative to equivalent installations using single rope hoisting technology. 
     A further alternative emergency arrest mechanism to the safety dog and the Blair hoist is the mechanical gripping wedge, a mechanism commonly used on industrial cargo elevators. A mechanical gripping wedge is an inverted wedge that is deployed in the event of elevator car freefall, which causes instantaneous capture of the elevator car. Mechanical gripping wedges have gradually been accepted into the mining industry in view of a belief that rope severance generally occurs when the elevator car is ascending in the hoist way. In such scenarios, energy transfer upon instantaneous capture of the elevator car does not have a significant downward velocity component, and G-forces on any occupants within the car tends to be negligible. 
     However, it is also possible for a rope to sever while the car is descending. In that case, mechanical gripping wedges would be poorly suited for safely arresting a mine cage. This is in view of the significant G forces that would likely be imparted upon the downwardly falling elevator car upon its instantaneous capture by the mechanical gripping wedge, which may damage cargo and may result in injury or fatality to human occupants. 
     SUMMARY 
     In one aspect of the present disclosure, there is provided a clamp for installation at a right angle junction of a roof and an adjacent wall of a substantially cuboid-shaped mine shaft conveyance as part of an emergency braking system of the mine shaft conveyance, the clamp comprising: a pair of L-shaped brackets, in like orientation and occupying parallel planes, spaced apart in fixed relation to one another; and a pair of opposing brakes disposed on corresponding respective legs of the pair of L-shaped brackets, the pair of opposing brakes for clamping a mine shaft guide between the brakes for emergency braking; wherein the corresponding legs of the pair of L-shaped brackets on which the pair of opposing brakes is disposed are attached to a first plate defining a vertical mounting face of the clamp and wherein the remaining two legs of the pair of L-shaped brackets are attached to a second plate defining a horizontal mounting face of the clamp; and wherein the vertical mounting face meets the horizontal mounting face at a right angle to facilitate mounting of the clamp to the mine shaft conveyance at the right angle junction of the roof and the wall of the mine shaft conveyance through attachment of the horizontal mounting face to the roof of the mine shaft conveyance and attachment of the vertical mounting face to the wall of the mine shaft conveyance. 
     In another aspect of the present disclosure, there is provided an emergency braking system for a mine shaft conveyance, the system comprising: a brake; a control system for, upon detection of a mine shaft conveyance freefall or overspeed condition, incrementally engaging the brake at an incremental brake engagement rate; and a load cell, coupled to the control system, for sensing a load of the mine shaft conveyance, wherein the control system is operable to dynamically set the incremental brake engagement rate based, at least in part, upon the load of the mine shaft conveyance as sensed by the load cell. 
     In yet another aspect of the present disclosure, there is provided a method of activating an emergency brake of a mine shaft conveyance, the method comprising: sensing a load of the mine shaft conveyance; based on the sensed load of the mine shaft conveyance, dynamically determining a rate at which an emergency brake shall be incrementally engaged; and upon detecting a freefall or overspeed condition of the mine shaft conveyance, incrementally engaging the emergency brake at the dynamically determined rate. 
     In a further aspect of the present disclosure, there is provided a method of installing an emergency braking system onto a substantially cuboid-shaped mine shaft conveyance, the method comprising: positioning a clamp at a right angle junction of a roof and an adjacent wall of the mine shaft conveyance, the clamp including: a pair of L-shaped brackets, in like orientation and occupying parallel planes, spaced apart in fixed relation to one another; and a pair of opposing brakes disposed on corresponding respective legs of the pair of L-shaped brackets, the pair of opposing brakes for clamping a mine shaft guide between the brakes for emergency braking; wherein the corresponding legs of the pair of L-shaped brackets on which the pair of opposing brakes is disposed are attached to a first plate defining a vertical mounting face of the clamp and wherein the remaining two legs of the pair of L-shaped brackets are attached to a second plate defining a horizontal mounting face of the clamp, the vertical mounting face meeting the horizontal mounting face at a right angle; attaching the vertical mounting face to the wall of the mine shaft conveyance; and attaching the horizontal mounting face to the roof of the mine shaft conveyance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the figures which illustrate example embodiments, 
         FIG. 1  is a top perspective view of an upper portion of a mine elevator car equipped with an emergency braking system; 
         FIGS. 2, 3, 4 and 5  are a perspective view, side elevation view, top plan view, and front elevation view, respectively, of a clamp component of the emergency braking system of  FIG. 1 ; 
         FIGS. 6 and 7  are cross-sectional views taken at lines  6  and  7 , respectively, of the clamp of  FIG. 3 ; 
         FIG. 8  is a cross-sectional view of the brake shoe components of the clamp of  FIGS. 6 and 7 ; 
         FIG. 9  is a simplified schematic block diagram of select electrical and hydraulic components of the emergency braking system of  FIG. 1 ; 
         FIGS. 10 and 11  are perspective and side elevation views, respectively, of a drawbar component of the emergency braking system of  FIG. 1  in a fully triggered state; 
         FIGS. 12, 13 and 14  are simplified schematic views of the drawbar of  FIGS. 10 and 11  in an untriggered or normal use state, a first triggered state, and a second triggered state respectively; 
         FIG. 15  is a schematic side view of the clamp of  FIG. 2  attached to an elevator car adjacent to a mine shaft guide; 
         FIGS. 16 and 17  are schematic side views of a hypothetical alterative clamp attached to an elevator car adjacent to a mine shaft guide at two different points in time; 
         FIG. 18  is a simplified schematic block diagram of select electrical and hydraulic components of an alternative embodiment of emergency braking system; and 
         FIGS. 19, 20 and 21  are a perspective, side elevation, and front elevation view, respectively, of an alternative spring-loaded drawbar forming part of an alternative emergency braking system. 
     
    
    
     DETAILED DESCRIPTION 
     In this document, the term “exemplary” should be understood to mean “an example of” and not necessarily to mean that the example is preferable or optimal in some way. 
     Referring to  FIG. 1 , an upper portion of an exemplary mine elevator car  100  (a form of mine shaft conveyance) equipped with an emergency braking system is illustrated in top perspective view. The elevator car  100  (also referred to herein as a “mine cage”) has a generally cuboid shape, a rectangular roof  102 , a rear wall  104  and a truncated front wall  106 . The truncated front wall  106  may constitute a header for a door (not expressly depicted) used for ingress to and egress from the elevator car  100 . 
     A slot  108  in the roof accommodates a tang (not illustrated in  FIG. 1 ) to which a hoist cable (also not depicted in  FIG. 1 ), for raising and lowering the elevator car  100 , is attached. The tang is attached to the elevator car  100  car by way of a spring-loaded drawbar, described below. The drawbar incorporates a triggering mechanism for activating the emergency braking system. The triggering mechanism and the emergency braking system will both be described in more detail below. 
     The elevator car  100  travels along a plurality of shaft guides  110 ,  112 ,  114  and  116 , which are depicted in  FIG. 1  using dashed lines. Each of the shaft guides is a vertical member or rail having a rectangular cross-section and is affixed to the mine shaft walls. 
     In this embodiment, there are two shaft guides  110 ,  112  on the rear side of the elevator car  100  and two shaft guides  114 ,  116  on the front side of the elevator car  100 . The four shaft guides are at or near the corners of the rectangular elevator car roof  102 . The number of mine shaft guides, their shape, and their placement relative to the corners of the elevator car roof  102  may vary in alternative embodiments. 
     Four guide roller assemblies  120 ,  122 ,  124  and  126  are mounted atop the roof  102  at the location of shaft guides  110 ,  112 ,  114  and  116  respectively. The guide roller assemblies facilitate low friction guided movement of the elevator car  100  up or down the shaft guides within the mine shaft. 
     Four clamps  130 ,  132 ,  134  and  136  are also mounted atop the elevator car  100  at the location of the shaft guides  110 ,  112 ,  114  and  116  respectively. The clamps are components of the emergency braking system. Each of the  130 ,  132 ,  134  and  136  is designed to clamp onto a respective shaft guide when the elevator car  100  enters a freefall or overspeed condition. The clamps are mounted to the elevator car  100  at a right angle junction of the elevator car roof  102  and the adjacent wall  104  or  106 . This is described in more detail below. 
       FIGS. 2-7  provide various views a single example clamp  200 . It will be appreciated that the clamp depicted in these figures is shown in a disengaged condition, i.e. as it appears when emergency braking is not being performed. It will further be appreciated that each of clamps  130 ,  132 ,  134  and  136  of  FIG. 1  is an instance of the example clamp  200  shown in  FIGS. 2-7 . 
     Referring initially to  FIGS. 2-5 , the exemplary clamp  200  is shown in perspective view, side elevation view, top plan view, and front elevation view, respectively. The clamp  200  has a right angle or “L-shaped” profile that is characterized by a horizontal portion  202  and a vertical portion  204  (see e.g.  FIG. 3 ). The L-shaped profile allows the clamp to be attached at the right-angle junction of the elevator car roof and an elevator car wall. In particular, the horizontal portion  202  of the clamp  200  is for attachment to the elevator car roof  102 , and the vertical portion  204  of the clamp  200  is for attachment to the elevator car wall  104 . This design may enhance the ability of the clamp  200  to withstand significant G forces during emergency braking with minimal damage or wear, as will be described. 
     The example clamp  200  incorporates a pair of L-shaped brackets  210 ,  212  (see e.g.  FIG. 2 ) in like orientation, spaced apart in fixed relation to one another, occupying parallel planes (i.e. the opposing faces of the brackets are parallel). The pair of L-shaped brackets  210 ,  210  is thus defined by a pair of corresponding, parallel vertically oriented legs  214 ,  216  and a pair of corresponding, parallel horizontally oriented legs  218 ,  220  (see e.g.  FIG. 2 ). 
     The example clamp  200  further includes a horizontal bracket plate  222  and a vertical bracket plate  224 , which meet at a right angle  226  (see e.g.  FIGS. 2 and 3 ). The horizontal bracket plate  222  defines a horizontal mounting face  232  ( FIG. 3 ) for attachment to the elevator car roof  102  via a plurality of attachment points. In this embodiment, the attachment points are holes  223  defined in the plate  222  for receiving bolts or other fasteners (see e.g.  FIGS. 2 and 4 ). Similarly, the vertical bracket plate  224  defines a vertical mounting face  234  ( FIG. 3 ) for attachment to the elevator car wall  104  via a plurality of attachment points, which in this embodiment are holes  225  defined in the plate  224  for receiving bolts or other fasteners (see e.g.  FIGS. 2 and 5 ). The plates  222 ,  224  are not necessarily of the same thickness. 
     A pair of parallel upstanding stabilizing ribs or plates  236 ,  238  extends transversely between the L-shaped brackets  210 ,  212  atop horizontal bracket plate  222  (see  FIGS. 2 and 4 ). The ribs  236 ,  238  contribute to the structural integrity of the clamp  200 . 
     The pair of L-shaped brackets  210 ,  212 , the horizontal bracket plate  222 , the vertical bracket plate  224 , and the stabilizing ribs  236 ,  238  may all be made from the same material, e.g. a metal such as aluminum, and may be welded together for example. 
     In the present embodiment, each of L-shaped brackets  210 ,  212  is at least six times thicker than a thickest one of the horizontal bracket plate  222  and vertical bracket plate  224 . Moreover, each of the stabilizing ribs  236 ,  238  is half as thick as the thinnest one of plates  222  and  224 . These relative thicknesses may strike a favorable compromise between maximizing clamp strength while minimizing clamp weight. 
     The clamp  200  further includes a pair of opposing brakes  240 ,  242  for clamping a mine shaft guide therebetween (see e.g.  FIGS. 2 to 5 ). The brakes  240 ,  242  are disposed on the vertical portion  204  of the clamp body. In this embodiment, each brake  240 ,  242  is disposed on a vertically oriented leg  214 ,  216  of one of the L-shaped brackets  210 ,  212 , respectively. The pair of brakes  240 ,  242  is disposed mostly below the horizontal portion  202  of the clamp body in this embodiment (see e.g.  FIGS. 2 and 3 ). 
     The brakes  240 ,  242 , which are hydraulic brakes in this embodiment, are oriented horizontally to facilitate clamping of a vertical mine shaft guide disposed between the brakes. As such, the hydraulic cylinder  250 ,  252  of each respective brake  240 ,  242  is mounted horizontally onto the vertical portion  204  of the clamp  200  (see e.g.  FIG. 2 ). 
     The various components comprising brakes  240 ,  242  are shown in greater detail in the cross-sectional views of  FIGS. 6 and 7 , which are taken at lines  6  and  7 , respectively, of  FIG. 3 . As illustrated, each brake  240 ,  242  is made up of multiple components generally classifiable into two subsets: fixed components and moving components. 
     Fixed components are components of a brake  240  or  242  that do not move relative to the body of clamp  200  when the brake is engaged and disengaged. The fixed components of brakes  240 ,  242  include hydraulic cylinders  250 ,  252 , clamp plates  254 ,  256  and cover plates  258 ,  260 , respectively. 
     Moving components are components of a brake  240  or  242  that move relative to the body of clamp  200  as the brake is engaged and disengaged. The moving components of brake  240 , which move (translate horizontally in  FIGS. 6 and 7 ) as a unit referred to as brake shoe  241 , include piston  262 , bolt  264 , cylinder collar  266 , wear shoe mount plate  268 , wear shoe  270  and alignment pins  272 . Similarly, the moving components of brake  242 , which also move (translate in an opposite direction to the opposing brake shoe  241 ) as a unit referred to as brake shoe  243 , include piston  282 , bolt  284 , cylinder collar  286 , wear shoe mount plate  288 , wear shoe  290  and alignment pins  292 . 
     The alignment pins  272 ,  292  may alternatively be referred to as guide pins or guide dowels. A pair of alignment pins  272 ,  292  flanks each piston  262 ,  282  respectively. Each of the alignment pins  272  is received in a respective guide hole through clamp plate  254 . Similarly, each of the alignment pins  292  is received in a respective guide hole through clamp plate  256 . The guide holes may be carefully machined so as to be transverse (perpendicular) to their respective clamp plates  254 ,  256  and to precisely accommodate alignment pins  272 ,  292 , within narrow tolerances. This may promote reliable extension and retraction of each brake shoe  241 ,  243  by movement of the single respective piston  262 ,  282  driving each brake shoe. 
     For example, the linear or dimensional tolerance of the alignment pins  272 ,  292  with respect to their guide holes (e.g. the difference between the outer diameter of each pin and the inner diameter of its respective hole) may be in the range of several thousandths of an inch. The geometric tolerance of each alignment pin with respect to the mount plate  268 ,  288  from which it extends may be in the range of one half to one ten-thousandth of an inch, to ensure that the pin extends precisely perpendicularly from the mount plate and precisely aligned with its respective hole in the adjacent clamp plate. 
     If the tolerances were too wide, there may be an unacceptably high risk of binding of the brake shoes  241 ,  243 . This is in view of the single cylinder  250 ,  252  driving each respective brake shoe  241 ,  243 . In particular, if the cylinder that drives a brake shoe should become even slightly misaligned above or below horizontal, the respective piston could be driven on a slight angle, which could in turn result in binding of the alignment pins within their horizontal guide holes. Use of tight tolerances discourages this from happening while allowing only a single (sole) centrally disposed cylinder of the brake to be used to engage the brake. This may advantageously limit clamp weight and complexity. As such, the design of clamp  200  may be considered to represent a good compromise between limiting clamp weight and ensuring reliable clamp operability. 
     The above-described single cylinder design is in comparison to a hypothetical brake design that uses, for each brake, a pair of cylinders (one at the location of each of the pair of alignment pins shown in  FIGS. 6 and 7 ) and a single central alignment pin (at the central location of the piston shown in  FIGS. 6 and 7 ). Such a hypothetical design may be considered less risky, i.e. reliable even with wider linear and geometric tolerances of the retaining pin and its associated hole, since any binding of the central alignment pin may be resolved by the pistons in turn “walking” or wobbling the alignment pin and brake shoe out into a deployed state. However, the hypothetical two-cylinder brake would come at the cost of significantly more weight than a one-cylinder brake design as shown in  FIGS. 6 and 7 . 
     As should now be apparent from  FIGS. 6 and 7  and the foregoing description, each of the brakes  240 ,  242  comprises a respective brake shoe  241 ,  243 . Brake shoe  241  includes a wear shoe mount plate  268 , a sole piston  262  extending orthogonally and centrally from a back face of the wear shoe mount plate  268 , and a pair of alignment pins  272  flanking the piston  262  and extending orthogonally from the back face of the wear shoe mount plate  268 . Similarly, brake shoe  243  includes a wear shoe mount plate  288 , a sole piston  282  extending orthogonally and centrally from a back face of the wear shoe mount plate  288 , and a pair of alignment pins  292  flanking the piston  282  and extending orthogonally from the back face of the wear shoe mount plate  288 . The clamp body of clamp  200  comprises a respective guide hole for slidably receiving each of the alignment pins  272 ,  292 . In some embodiments, the geometric tolerance of each of the alignment pins with respect to its respective guide hole may be approximately one-half of to one ten thousandth of an inch (i.e. one twenty thousandth to one ten thousandth of an inch), and the linear tolerance of each of the alignment pins with respect to its respective guide hole may be approximately several thousandths of an inch. 
       FIG. 8  is a cross-sectional view of only the brake shoes  241 ,  243  of brakes  240 ,  242  respectively. As illustrated, the brake shoes  241 ,  243  are horizontally translatable between a disengaged position (shown in  FIG. 8 ), in which the wear shoe  270 ,  290  of each respective brake shoe  241 ,  243  is retracted away from a shaft guide  294  disposed between the brakes  240 ,  242 , and an engaged position in which the wear shoe  270 ,  290  of each respective brake shoe  241 ,  243  is advanced inwardly until it engages (is pressed firmly against) a respective side of the shaft guide  294 . 
     Each of the wear shoes  270 ,  290  has a respective flat face  271 ,  291  that is oriented substantially vertically, i.e. substantially parallel to the vertical shaft guide  294  against which the wear shoes  270 ,  290  will be pressed when the brakes are engaged (see e.g.  FIG. 8 ). Each of the flat faces  271 ,  291  accordingly occupies a plane that is perpendicular to both of the horizontal mounting face  232  and the vertical mounting face  234  of the clamp  200  (see  FIG. 3 ). 
     Referring again to  FIG. 8 , each wear shoe  270 ,  290  has tapered ends  273 ,  293  respectively. The tapered ends  273 ,  293  allow the wear shoes to serve as guide shoes when the brake is not engaged. In other words, should the wear shoes  270 ,  290  inadvertently buffet the shaft guides during normal elevator car ascent or descent while the brakes are disengaged, the wear shoes will not present any notable obstruction but rather will behave as a guide wear shoe. The tapered ends may also limit damage to the wear shoe in the event that the wear shoe encounters an offset at a shaft guide splice joint, i.e. a slight misalignment of the adjoining vertical shaft guide sections (which sections may be misaligned by up to ¼). Without the taper, a blunt wear shoe edge that strikes the guide offset could result in serious damage to the wear shoe (e.g. peening of the edge). That in turn could interfere with the proper application of suitable clamping forces when the emergency brakes are engaged. In view of the tapered ends  273 ,  293 , the exemplary wear shoes  270 ,  290  of the present embodiment have a generally trapezoidal longitudinal cross-sectional shape. 
     The emergency braking system  400  of the elevator car  100  is depicted schematically in  FIG. 9 . In particular,  FIG. 9  is an example, simplified schematic block diagram of select electrical and hydraulic components of the emergency braking system  400 . The components illustrated in  FIG. 9  are associated with engaging a single one of the hydraulic brakes comprising a single clamp  200  ( FIG. 2 ). Additional, analogous components, which are omitted from  FIG. 9  for clarity, may be used for the other clamps. 
       FIG. 9  adopts the following conventions: boxes represent discrete electrical or hydraulic components; standard weight arrows between boxes represent electrical connections between components; and bold arrows between boxes represent hydraulic connections between components. The directionality of each arrow in  FIG. 9  represents a direction of flow of the electrical signal or hydraulic fluid, respectively. 
     As illustrated, the components of emergency braking system  400  include a trigger  404 , a controller  406 , a pump  408 , an accumulator  410 , a valve  412 , and a hydraulic cylinder  414  of an emergency brake. The system  400  may include additional components that are omitted from  FIG. 9  for clarity and brevity. Although not depicted in  FIG. 1 , all of these elements of the emergency braking system  400  may be carried by the elevator car  100  (e.g. the components may sit atop elevator car roof  102 ). 
     The trigger  404  is a device that activates when the elevator car  100  enters a freefall or overspeed condition. The trigger may for example be an electrical switch, such as a rocker switch, toggle switch, proximity switch, or optical switch. The trigger  404  may for example be associated with a spring-loaded drawbar which activates the trigger  404  upon severance of a hoist rope. An example spring-loaded drawbar having one example type of trigger is described below. 
     The controller  406  is programmable logic controller (PLC) or similar controller that is responsible for sending appropriate control signals to a valve  412  (described below) for causing hydraulic fluid to flow for engaging the emergency brakes in the event of a freefall or overspeed condition of the elevator car  100 . The controller  406  detects the freefall or overspeed emergency condition of the elevator car  100  by way of a signal from trigger  404 . The PLC may be a commercially available PLC product, such as an Allen-Bradley™ PLC product for example. The PLC may be programmed to operate as described herein using ladder logic software. Use of PLC technology may be motivated by a desire to operate the emergency brake circuit efficiently and reliably. An alternative embodiment could have a “hard-wired” system that uses relay contactors to control the sequence logic. 
     Pump  408  is a pump for generating hydraulic pressure for powering hydraulic systems of emergency braking system  400 . The pump  408  may be periodically activated by way of a “low-pressure” setting from an accumulator pressure switch. For example, as accumulator pressure reaches the low pressure setting, the pressure switch contacts may close and the hydraulic pump may be started. Once the accumulator pressure reaches a high pressure setting in this same switch, the contacts may open and the hydraulic pump may be shut off. In this way, hydraulic fluid in an accumulator  410 , described below, may be pressurized. In the present embodiment, the pump  408  performs this pressurization in a “closed loop” fashion. In this context, “closed loop” refers a closed system in which hydraulic fluid is pressurized without introduction of ambient air. This is done to shield the system  400  from introduction of dirt or contaminants and to reduce or eliminate a risk of hydraulic fluid frothing, either of which may compromise proper operation of hydraulic components such as hydraulic valves or hydraulic brakes. The pump may be an electric pump, such as a standard gear pump manufactured by Parker Fluidpower™ being driven by a 1.5 hp-24 vDC electric motor. 
     Accumulator  410  is a vessel for storing pressurized hydraulic fluid that has been pressurized by pump  408  for use in quickly activating the hydraulic brakes in a freefall or overspeed elevator car condition. Accumulator  410  may for example be a commercially available Parker Fluidpower™ product, such as a bladder type accumulator having a one-gallon capacity. 
     Valve  412  is an electrically actuated hydraulic valve. The valve  412  is capable of opening or closing at a variety of different rates based on a received electrical control signal from controller  406 . The valve  412  may actually comprise two subcomponent valves that cooperate to achieve that result, namely a hydraulic “dump” valve and a pilot pressure isolation valve. In some embodiments, a two-valve arrangement may be better suited than a single valve for ensuring proper valve control in view of the possibly extremely high pressure of hydraulic fluid within system  400 . In some embodiments, the valve  412  may for example be, or may include, a directional hydraulic valve comprising a spool that is actuated by a solenoid or other actuator. 
     The emergency braking system  400  may also include a battery  170  (not expressly depicted in  FIG. 9 ). The battery  170 , which may sit atop the elevator car roof  102  e.g. as shown in  FIG. 1 , may power electrical components of system  400 , including the pump(s), valve(s), and control system on the elevator car  100 . 
     As noted above, the elevator car  100  of  FIG. 1  is suspended from a hoist rope by way of a spring-loaded drawbar, which is attached to the roof  102  of the elevator car  100 . An example spring-loaded drawbar  300  is illustrated in  FIGS. 10-14 . In particular,  FIGS. 10 and 11  illustrate the example drawbar  300  in a fully triggered state, i.e. as it would appear some time after a hoist cable has been severed, in perspective and side elevation views respectively. In contrast,  FIGS. 12-14  are simplified schematic views of the drawbar  300  in three respective states: an untriggered or normal use state; a first triggered state; and a second triggered state. The second triggered state of  FIG. 14  corresponds to the fully triggered state depicted in  FIGS. 10 and 11 . 
     Referring to  FIGS. 10 and 11 , it can be seen that the drawbar  300  includes an upstanding tang  302  with a hole  304  at its distal end. The hole  304  is for attachment of a hoist cable. The tang  302  passes slidably or freely through a slot  306  in a horizontal plate  308  which may be attached to, or may form part of, the roof  102  of the elevator car  100  of  FIG. 1 . 
     A proximal (lower) end of tang  302  is fixedly attached to a base  310 . Four upstanding posts  312  are also fixedly attached to the base  306  at their lowermost ends. The posts  312  flank the lower end of tang  302  on opposite sides, two per side. Each post  312  passes slidably or freely through a respective hole in plate  308  and has a limit  314  defined at its distal (uppermost) end. In the present embodiment, each limit  314  takes the form of a cap. 
     A coil spring  316  surrounds each of the posts  312 . Each spring  316  is disposed or sandwiched between the underside of plate  308  and the top of base  310 . The springs  316  thus collectively bias, with a biasing force B, the underside of limits  314  against the upper surface of the plate  308 . As such, the limits  314  individually and collectively define a stop for limiting downward movement of the post  312  (and thus tang  302 ) relative to plate  308  (and thus elevator car roof  102 ). When the tang  302  is at this limit of movement (as in  FIGS. 10 and 11 ), the drawbar  300  is considered to be in a “fully triggered” condition, i.e. as it may appear once the drawbar  300  reaches a steady state after a hoist cable has severed. 
     It will be appreciated that the springs  316  individually or collectively constitute a form of biasing element and that other forms of biasing elements, such as leaf springs, could be used in alternative embodiments. 
     As perhaps best see in  FIG. 11 , one of the limits  314  on one side of tang  302  (on the right hand side of  FIG. 11 ) defines or fixedly attaches a first emergency braking trigger activator  320  or simply “first trigger activator  320 .” In the present embodiment, the first trigger activator  320  takes the form of a wing or ramp which flares or widens upwardly. The trigger  320  is designed to come into contact with and activate a toggle switch  340  (see  FIG. 12 ) when the elevator car  100  enters a freefall or overspeed condition, in order to engage the emergency braking system. 
     Referring to  FIGS. 10 and 11 , another one of the caps  314 , on the other side of tang  302  (on the left hand side of  FIG. 11 ), defines or fixedly attaches a second emergency braking trigger activator  322  or simply “second trigger activator  322 .” In the present embodiment, the second trigger activator  322  takes the form an upstanding metal tab. The metal tab is designed to come into proximity with, and to thereby trigger, a proximity switch  342  (see  FIG. 12 ), also when the elevator car  100  enters a freefall or overspeed condition. 
     In the present embodiment, the proximity switch  342  acts as a failsafe or backup switch for engaging the emergency braking system in the event that the toggle switch  340  fails. As such, the toggle switch  340  and the proximity switch  342  may be referred to as the primary and secondary braking activation switches, respectively. In this example, the primary and secondary braking activation switches collectively comprise the trigger  404  of  FIG. 9 . 
     In normal (i.e. non-freefall and non-overspeed) mine shaft elevator operating conditions, the elevator car  100  will be suspended from a hoist cable  330  by way of the tang  302  of drawbar  300  (see  FIG. 12 ). Because the springs  316  support the plate  308  from underneath, and because plate  308  is attached to, or forms part of, the elevator car roof  102 , the weight of the elevator car  100 , and any cargo (human or otherwise), will be borne by the springs  316 . The spring constant of the springs  316  is typically chosen so that the springs  316  compress at least partially under this weight, even when the elevator car  100  is empty. As a result, the distal ends of posts  312 , and the majority of tang  302 , will protrude upwardly through the plate  308  during normal operation. The first and second triggers  320 ,  322  at the distal ends of posts  312  will accordingly be well clear of their respective switches  340 ,  342  (see  FIG. 12 ). As such, the emergency braking system will remain disengaged during normal elevator operation. 
     As the elevator car  100  is raised and lowered within the mine shaft by the hoist cable  330 , the springs  316  may be compressed to the level that the shoulders on the lower end of the the tang  302  (which form part of base  310 ) contact the rest plate on the elevator car frame. The springs  316  are chosen so that, during such normal operation, the triggers  320 ,  322  will not contact their respective switches  340 ,  342  despite the fact that the springs  316  are compressed and thereby store energy. 
     In operation, in the event that the hoist cable  330  severs, e.g. as depicted in  FIG. 13 , then the elevator will enter a freefall condition. In that condition, the tang  302  will no longer be pulled upwardly by the cable  330 . As a result, the tang  302  will suddenly be driven downwardly by the opposing biasing force B of the rebounding springs  316 . 
     Before the tang  302  reaches the limit of its downward travel relative to plate  308  (as collectively defined by limits  314 ), the first trigger activator  320  will strike the roller arm  344  of the toggle switch  340  (see  FIG. 13 ). This will cause the toggle switch  340  to close electrically. The closure of switch  340  is considered as a tripping of trigger  404  ( FIG. 9 ). The tripping of trigger  404  is detected by controller  406  ( FIG. 9 ) which, in response, activates the emergency braking system  400 . 
     More specifically, controller  406  sends appropriate control signals to valve  412  to cause it to open at a particular rate. In some embodiments, this rate may be a predetermined rate that has been predetermined to cause the emergency brakes to activate acceptably quickly for the application in question. For example, in some embodiments in which human occupants are to be carried by the elevator car  100 , “acceptably quickly” may mean a rate that results in a deceleration force of 32.2 ft/sec/sec (1 G) upon the elevator car  100  when the car carrying its maximum safe weight capacity. The appropriate rate for opening valve  412  to achieve this result may for example be empirically determined. 
     In some embodiments, opening valve  412  may be a multi-step process. For example, first, a hydraulic “dump” valve may be opened, causing a spool within the valve to shift. The shifting of the spool in that valve may permit pilot pressure isolation valves of accumulator  410  ( FIG. 9 ) to drain. This may in turn cause the isolation valve spool to shift, which may permit high-pressure hydraulic fluid to leave the accumulator  410  and flow into the hydraulic cylinder  414  ( FIG. 9 ). This process may be used to cause hydraulic fluid to flow into the hydraulic cylinders  250 ,  252  ( FIGS. 6 and 7 ) on each clamp  130 ,  132 ,  134  and  136  ( FIG. 1 ). 
     Pressurizing the hydraulic cylinders  250 ,  252  in turn causes the pistons  262 ,  282  to quickly move towards one another ( FIG. 8 ) until wear shoes  270 ,  290  engage opposing surfaces of the shaft guide  294 . The friction of this engagement dissipates kinetic energy as heat, eventually bringing the elevator car  100  to a stop. 
     Once the tang  302  reaches the absolute limit of its downward travel relative to plate  308  (see  FIG. 14 ), the second trigger  322  will be positioned proximately to the secondary proximity switch  342 . This will cause the proximity switch  342  to close electrically. The closure of switch  342  will activate the emergency braking system  400 , as described above, in the event that closure of toggle switch  340  has failed to do so. This is done for redundancy and robustness. It is not absolutely required to have such a redundant switch in alternative embodiments. 
     As alluded to above, the L-shaped profile of the clamp  200  may enhance the ability of the clamp  200  to withstand significant G forces during emergency braking with minimal equipment damage or wear. Referring to  FIG. 15 , there is depicted a schematic side view of an L-shaped clamp  200  attached to an elevator car  100  adjacent to a mine shaft guide  294 . As described above, the example clamp  200  is attached at a right angle junction of the roof  102  and wall  104  of the elevator car  100 . 
     In particular, as shown in  FIG. 15 , the horizontal portion  202  of the clamp is attached to the elevator car roof  102  at a plurality of attachment points, and the vertical portion  204  of the clamp is attached to elevator car wall  104  at a plurality of attachment points. The attachment points may be designed to accommodate fasteners, such as bolts. Four example fasteners  400 ,  402 ,  404  and  406  are depicted in  FIG. 15  for the sake of illustration. It will be appreciated that a different number or type of fasteners may actually be used, or that attachment may be performed at multiple points without fasteners (e.g. via welding). 
     When the emergency brakes of clamp  200  are applied to the shaft guide  294  while the elevator car  100  is in a freefall or overspeed condition, the deceleration will impart a sudden upward force F upon the clamp  200 . As shown in  FIG. 15 , this force will be applied upwardly largely in line with the vertical portion  204  of the clamp  200 . As such, the force F will be a shear force relative to the vertical portion  204  of the clamp and relative to fasteners  400  and  402 . Although some portion of force F may also act as a tension force upon the horizontal portion  202  and fasteners  404  and  406 , that portion will not be the entirety of force F. This design may accordingly result in less wear upon the clamp  200  or fasteners  400 ,  402 ,  404  and  406  over time than other designs. 
     For example, a hypothetical alternative clamp design is depicted in  FIGS. 16 and 17 . As shown in those figures, the alternative clamp  500  has simple cuboid shape. The body of the clamp  500  is designed for attachment to the roof  102  of the elevator car  100  using example fasteners  502 ,  504 . A distal portion  501  of the clamp  500 , housing one or more brakes (not depicted), extends from or overhangs an edge of roof  102  so as to position the brake(s) adjacent to mine shaft guide  394 . 
     Should the brake(s) of hypothetical clamp  500  be applied in a freefall or overspeed condition, the deceleration would impart a sudden upward force F 1  upon the overhanging distal portion of the clamp  500 . This force F 1  would act largely or fully as a tensile force, or upward prying force, upon the body of clamp  500  and fasteners  502 ,  504 . Moreover, in view of the distance D between the point at which the force F 1  is applied and the first fastener  502 , the tensile force F 2  experienced at fastener  502  may be magnified relative to F 1 , due to the lever principle of physics, e.g. if the rightmost edge of the clamp body acts as a fulcrum. 
     Over time, repeated applications of this magnified tensile force F 2  upon fastener  502  may cause the fastener to weaken or fail. This may in turn cause the clamp  500  to become loose, with a gap  506  possibly forming between the elevator car  100  and the clamp  500  (see  FIG. 17 ). The looseness of the hypothetical clamp  500  may worsen over time and may eventually necessitate clamp reattachment or replacement, which would involve undesirable elevator downtime and may increase costs. 
     The disclosure above describes how the emergency braking system  400  is triggered when an elevator car enters a freefall condition upon the severing of the hoist rope. It will be appreciated that the emergency braking system  400  could be triggered in the same way should the elevator car enter an overspeed condition not involving severing of the rope, e.g. upon the hang-up and subsequent limited-distance drop of the elevator car  100  within the mine shaft during descent. 
     Various alternative embodiments are possible. For example, some embodiments of emergency braking system may be designed to incrementally activate the emergency brakes at different rates based upon the load currently being borne by the elevator car. This may be done with a view to stopping the elevator car without subjecting it to unacceptably high or unsafe G forces regardless of whether it is heavily loaded or lightly loaded. Such an alternative embodiment is depicted in  FIG. 18 . 
       FIG. 18  is a simplified schematic diagram of an emergency braking system  600 . In particular, the components illustrated in  FIG. 18  are associated with engaging a single one of the hydraulic brakes comprising a single clamp  200  ( FIG. 2 ). Additional, analogous components, which are omitted from  FIG. 18  for clarity, may be used for the other clamps. 
       FIG. 18  adopts the same conventions as  FIG. 9 , described above. As illustrated in  FIG. 18 , the emergency braking system  600  include a trigger  604 , a controller  606 , a pump  608 , an accumulator  610 , a valve  612 , and a hydraulic cylinder  614  of an emergency brake. Each of these components serves essentially the same function, and has the same general interrelationships with other system components, as the correspondingly named components of  FIG. 9 , and thus will not be described anew. 
     The emergency braking system  600  of  FIG. 18  includes an additional component not depicted in the emergency braking system  400  of  FIG. 9 , namely load cell  602 . The load cell  602  is a component that periodically senses a load of the elevator car and sends an electrical signal corresponding to the sensed load to the controller  604 . 
     An example of a spring-loaded drawbar  700  which incorporates a load cell  602  is illustrated in  FIGS. 19-21 , in perspective, side elevation, and front elevation view respectively. The drawbar  700  is of a similar design to the drawbar  300  of  FIGS. 10 and 11 , including a tang  702  with hole  704  for a cable, a base  710 , and springs  716 . These components serve similar purposes to the components of drawbar  300  of the same name, described above. 
     One additional component of drawbar  700 , which does not have a counterpart in drawbar  300  described above, is load cell  602 . Load cell  602  is a sensor (or, in this example, multiple sensors) that generates signals indicative of a load of the elevator car. The load cells  602  may be sandwiched between a plate  708  and the flanges of a head channel  601  for example. 
     Referring to  FIG. 18 , the controller  606  periodically receives a sensed load signal from the load cell  602  (e.g. when the elevator car is stationary and the emergency brakes are disengaged) and stores a value in a memory (not expressly depicted) indicative of the sensed load. This value allows the controller  606  to dynamically determine the rate at which to incrementally engage the emergency brakes (i.e. to dynamically set the incremental brake engagement rate) based, at least in part, upon the load of the elevator car as sensed by the load cell. In particular, the controller  606  ( FIG. 18 ) may be operable to dynamically set the incremental brake engagement rate to be faster for a heavier sensed load of the elevator car than for a lighter sensed load of the elevator car. For example, the controller  606  may be operable to dynamically set the incremental brake engagement rate to be proportional to a magnitude of the sensed load of the elevator car. 
     For example, in the example drawbar  700  of  FIGS. 19-21 , the load cells  602  may generate signals collectively indicative of the load of the elevator car based on strain from applied force (sensed weight). A summation module in the controller  606  may sum the individual load cell signals to provide an indication of the total elevator car weight with cargo. This value may be compared to preset ranges of values having associated preset values representing an appropriate rate at which to dynamically engage the brakes (or, more specifically to this embodiment, to dynamically open the valve  612  of  FIG. 18 ). The volume and speed of hydraulic oil release can thus be controlled to provide an incremental brake engagement rate that is tailored to the load being decelerated. 
     Other variations are possible. For example, the example clamp  200  of  FIG. 2  has an L-shaped profile for attaching the clamp at junction of a cage wall and a cage roof. It is possible that alternative embodiments of clamp could have an L-shaped profile for attaching the clamp at a junction of a cage wall and a cage floor or deck. In that case, the horizontal portion of the clamp body may be for attachment to the elevator car floor or deck. A pair of opposing brakes may be disposed on the vertical portion of the clamp body so as to be disposed mostly or entirely above the horizontal portion of the clamp body. In this case, the cage structure would go into a compression mode during a capture event, i.e. during emergency deceleration. This may be advantageous when carrying extremely heavy payloads on the cage floor or deck. 
     The trigger used to trigger the emergency braking system need not necessarily be a rocker switch or a proximity switch and need not utilize redundant switches. 
     It is not absolutely required for the brakes to be hydraulic brakes as disclosed above in every embodiment. For example, in alternative embodiments, the brake shoes could be spring-applied through the use of Belleville spring stacks positioned immediately behind the brake shoe with the brake shoe being held in a disengaged position by hydraulic pressure. To engage the brakes, the hydraulic force may be removed, thereby allowing the spring stacks to extend. 
     The emergency braking systems, clamps, and methods described above may be used with virtually any type of mine shaft conveyance, including elevator cars for carrying cargo (possibly referred to as “skips”), elevator cars for carrying human occupants, or elevator cars for carrying both cargo and human occupants. 
     The following clauses describe additional aspects of the present disclosure. 
     Clause 1. An emergency braking system for a mine shaft conveyance, the system comprising: a brake; a control system for, upon detection of a mine shaft conveyance freefall or overspeed condition, incrementally engaging the brake at an incremental brake engagement rate; and a load cell, coupled to the control system, for sensing a load of the mine shaft conveyance, wherein the control system is operable to dynamically set the incremental brake engagement rate based, at least in part, upon the load of the mine shaft conveyance as sensed by the load cell. 
     Clause 2. The emergency braking system of clause 1 wherein the control system is operable to dynamically set the incremental brake engagement rate to be faster for a heavier sensed load of the mine shaft conveyance than for a lighter sensed load of the mine shaft conveyance. 
     Clause 3. The emergency braking system of clause 1 wherein the control system is operable to dynamically set the incremental brake engagement rate to be proportional to a magnitude of the sensed load of the mine shaft conveyance. 
     Clause 4. A method of activating an emergency brake of a mine shaft conveyance, the method comprising: sensing a load of the mine shaft conveyance; based on the sensed load of the mine shaft conveyance, dynamically determining a rate at which an emergency brake shall be incrementally engaged; and upon detecting a freefall or overspeed condition of the mine shaft conveyance, incrementally engaging the emergency brake at the dynamically determined rate 
     Clause 5. The method of clause 4 wherein the dynamic determining sets the rate at which the emergency brake shall be incrementally engaged to be slower for a lighter sensed load of the mine shaft conveyance than for a heavier sensed load of the mine shaft conveyance. 
     Clause 6. The method of clause 4 wherein the dynamic determining sets the rate at which the emergency brake shall be incrementally engaged proportionally to the sensed load of the mine shaft conveyance. 
     Other modifications may be made within the scope of the following claims.