Abstract:
A chain tension sensor for a chain conveyor, the conveyor including a frame and a chain having a plurality of flights. The tension sensor includes a reaction arm and a load sensing pin. The reaction arm includes a first end, a second end opposite the first end, and a load pad. The first end is pivotably coupled to the frame by a pivot pin defining a pivot axis. The load pad is adjacent the conveyor chain and positioned to contact flights passing the load pad. The flights contacting the load pad exert a force on the reaction in a direction that is perpendicular to the pivot axis. The load sensing pin is coupled to the reaction arm such that the load sensing pin senses the force that is exerted by the flights.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of prior-filed, co-pending U.S. application Ser. No. 13/297,067, filed Nov. 15, 2011, which is a continuation-in-part of prior-filed U.S. application Ser. No. 12/767,411, filed Apr. 26, 2010, now U.S. Pat. No. 8,061,510, issued Nov. 22, 2011, and also claims the benefit of prior-filed, co-pending U.S. Provisional Application No. 61/510,839, filed Jul. 22, 2011, the entire contents of all of which are hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    The present invention generally relates to mining equipment, and, in particular, to drive chain conveyors. Still more particularly, this application relates to a mechanism to sense the tension of a scraper chain of a chain conveyor. 
       BACKGROUND 
       [0003]    Conveyors, such as armored face conveyors, are part of an integrated longwall system that also comprises a coal-cutting machine and roof supports. As the longwall system removes mineral from the mineral block one strip (web) at a time, the load on the conveyor changes as the cutter moves along the conveyor. The conveyor progressively moves forward one web in order to reposition itself for the next cut. 
         [0004]    The mineral being mined is dragged along a top race of the conveyor by a continuous chain and flight bar assembly driven by sprockets at each end of the conveyor. More particularly, the conveyor typically includes a pair of spaced apart chains with the flight bars connecting the chains. At the delivery end, the mineral is discharged onto an adjacent conveyor while the continuous chain enters a bottom race where it proceeds to a return end, where a return end drum or sprocket reverses the direction of the chain. 
         [0005]    Conventional longwall conveyors typically either operate at a fixed overall length or may be fitted with a moveable end frame. The amount of slack in the chain is controlled by applying a pre-tension to the chain. The pre-tension prevents chain extension, reducing the amount of slack generated. 
         [0006]    An extendable end frame may be used to adjust the pre-tension by taking up increasing length of chain generated from inter-link wear and from stretching in the chain that occurs due to the load on the chain. The tension can be controlled by monitoring the amount of tension in the chain and adjusting the moveable end frame position with a feedback loop system. 
         [0007]    The operation of the longwall system involves frequent repositioning of the many parts that make up the conveying system. Keeping the equipment inline with the coal block is difficult, as no direct steering mechanism is available with these systems. The operators have to rely on their experience by adjusting the position of the conveyor relative to the coal block to counteract a tendency of the equipment to gradually creep sideways. This results in face creep, and often the only corrective action available to the operators is to angle the conveyor a few degrees off square to the coal block. This process is slow and requires considerable skill. The variations in load and the repositioning of the many parts of the conveying system result in changes in chain tensions. 
         [0008]    In certain operational situations, one of the chains of the chain and flight bar assembly may break on the top race. The unbroken chain can then enter the return race along with the broken chain. Lower tensions in the bottom race can be contained by the single chain, which continues to the return end and then over the return end sprocket. If the broken chain is not identified on the top race, the second chain will also fail, most likely when the broken portion of the chain approaches a discharge area. This additional failure can cause damage to related equipment. The failure is followed by prolonged down time to make a repair. Visual identification of the broken chain is possible, but is unlikely because the chain is covered with the mineral being conveyed. Additionally, on most installations, safety requirements prohibit operators from being adjacent the return end of the conveyor, which further reduces the opportunity for manual detection. 
         [0009]      FIG. 1 , which is taken from Bandy, U.S. Pat. No. 5,131,528, illustrates a prior art scraper chain conveyor.  FIG. 1  illustrates in simple form the various conveyor elements necessary for understanding of the conveyor equipment environment. The conveyor apparatus or assembly is shown generally by the character numeral  10  and includes a drive drum/sprocket  12  and an idler or guide drum/sprocket  14  separated by a span of a flexible conveyor  16 , illustrated partially in dashed line outline. As depicted, the conveyor  16  comprises dual conveyor chains  18  and a multiplicity of spaced flight bars  20  attached to the dual chains  18 . During operation of the conveyor assembly, the flight bars  20  push aggregate material, such as mined coal, along an underlying conveyor pan  21 . The conveyor assembly  10  is typically positioned juxtaposed to a mine wall where a seam of material is being mined for transporting the material to one end. The material is then transferred to an auxiliary conveyor for further disposition. 
         [0010]    The drum/sprocket  12  is appropriately coupled to a conveyor drive motor  22 . Operation of motor  22  causes the sprocket intermeshing with the dual chains  18  to advance the conveyor  16 . A pair of sidewalls  24  forming a first portion of a “split frame” of conveyor assembly  10  serves to rotatably support the drum/sprocket  12 . The sidewalls  24  are illustrated as being telescopingly engaged with a second pair of sidewalls  26  forming a second portion of the frame and, collectively with sidewalls  24 , comprise the aforementioned split frame. The telescoping joint, indicated generally by character numeral  48 , permits the frame portions to be moved relative to one another. 
         [0011]    The idler drum/sprocket  14  is appropriately mounted for rotary movement between sidewalls  26 . Relative movement at the joint  48  between the adjacent sidewalls  24  and  26  causes the distance between the drum/sprockets  12  and  14  to vary accordingly. The dual conveyor chains  18  can be provided with increased or reduced tension depending upon the direction of adjusting movement of the supporting drum/sprockets with respect to each other. To provide this relative movement, assembly  10  has a tensioning means in the form of a pair of hydraulic cylinders  28 ,  30 . Each cylinder  28 ,  30  is mounted on and secured to an adjacent sidewall  26 . In other embodiments (not shown), only a single hydraulic cylinder can be used. The cylinders  28 ,  30  include respective pistons  32 ,  34 , each of which is operatively coupled to a sidewall  24  in any known and expedient manner. 
         [0012]    Movement of the pistons  32 ,  34  causes the first portion of the conveyor  16  represented by the side walls  24  to move longitudinally relative to the second portion and side walls  26 , thus relaxing or tensioning the chain  18 , as desired. Control of movement of pistons  32  and  34  is affected by a conventional hydraulic tensioning control circuitry, depicted generally by numeral  40  in  FIG. 1 . 
         [0013]    As stated above, a certain amount of tensioning of conveyor chain  18  is essential for the proper and efficient operation of the conveyor assembly  10 . Too little tension may cause the conveyor chain to ride up the teeth of the sprockets, and eventually become disengaged. Conversely, too much tension may cause the conveyor components to be over-stressed, increasing the risk of mechanical failure in the various parts of the conveyor apparatus. 
         [0014]      FIG. 2 , which is taken from Weigel et al., U.S. Pat. No. 7,117,989, illustrates a prior art mechanism for controlling the tension in a scraper chain in a conveyor.  FIG. 2  shows a tensionable return station  51 , which forms the auxiliary drive of a face conveyor and on which a spoked chain wheel  52  is located, which may be powered by drives (not shown). 
         [0015]    All channel sections  70  and machine frame  51  and, where applicable, any intermediate or transitional channels located between them, have a top race  54 A and a bottom race  54 B. In the top race  54 A the material to be conveyed (e.g. coal) is transported by means of scrapers  20  as far as the main drive, and in bottom race  54 B the scrapers run back to the auxiliary drive. The constantly changing load conditions in the top race  54 A cause the tension in the top race  54 A and bottom race  54 B of conveyor  16  to vary. 
         [0016]    In order to detect the tension of conveyor  16 , a sensor, indicated overall by  60 , is located on the frame of return station  51 , which forms the auxiliary drive. The sensor has a sliding body or sensor body  62  with a curved sliding surface  61 , which is coupled with a shaft  63  such that the sensor body  62  cannot be turned, said shaft reaching obliquely over the conveying trough and return trough for scraper conveyor  16  in top race  54  A of machine frame  51  of the chain conveyor. Shaft  63  is supported in bearing blocks  64 , one of which is indicated schematically at the rear side face of return station  51 . The weight of sensor body  62  causes its sliding surface  61  to be directly in contact with the upper face of a scraper  20  or with the upper face of vertical chain links  57  in the area of the measuring zone. At the same time, shaft  63 , supported in bearing blocks  64  such that it can swivel, forms a measuring shaft, and by means of shaft encoder  65  the relative position of measuring shaft  63  and thus also the relative position or swiveled position of sensor body  62  rigidly coupled with it may be detected and transmitted to the evaluation and control unit  72  via signal line  71 . Depending on the measurement signal of shaft encoder  65 , evaluation and control unit  72  then activates tensioning drive  55  of return station  51  via signal line  75 . 
         [0017]    In an extensive zone within top race  54 A of return station  51 , referred to below as the measurement zone, and extending between points  67  and  68  in the drawing marked with double arrows, scraper conveyor  16  has vertical play. In other words, between point  67  and point  68  along the track in top race  54 A, conveyor  16  can essentially move freely in a vertical direction, i.e. perpendicularly to the bottom of top race  73 ,  74 . 
         [0018]    In the embodiment shown, the scraper chain is running with optimum tension, i.e. some chain links in the measuring zone are slightly lifted away from the bottom of top race  74 . When the chain is dangling, on the other hand, chain links  57 ,  58  and scrapers  59  within the area of the measuring zone and in the area of the machine frame are in contact at every point with the bottom of top race  73  or  74  of return station  51 , and sensor body  62  is at its largest downwards deflection. This state is detected by evaluation and control device  72  and tensioning drive  55  is extended. If the tension of scraper conveyor  16  increases, vertical and horizontal chain links  57 ,  58  together with scrapers  59  of scraper conveyor  16  may move even higher in the measuring zone, due to the absence of restrictive guidance and the existing vertical play ( 67  or  68 ), which causes sensor body  62  to be swiveled clockwise and this deflection to be detected by shaft encoder  65  and transmitted to evaluation and control device  72  as a measurement signal. If the chain reaches a preset tension corresponding to that of a tight chain, this is detected directly by shaft encoder  65  as a result of the greater deflection of sensor body  62 , and evaluation and control device  72  then activates tensioning drive  55 , in some cases via a closed-loop control algorithm, through signal line  75  such that tensioning cylinder  56  is retracted in order to reduce the tension in scraper conveyor  16 . 
         [0019]    Other mechanisms for monitoring chain tension include those shown in U.S. Pat. No. 5,505,293 and in U.S. Pat. No. 4,657,131. 
         [0020]    In some existing constructions, load sensing pads are positioned in a wear strip of a top flange in the moveable end frame. However, this positioning exposes the pads to overheating resulting from friction. These load pads are also subjected to the full impact forces generated from each flight member passing the load pad. In addition, in such constructions, the chain typically needs to be set at the highest load to accurately measure the amount of slack generated as the chain is run, and setting the tension at the highest loading increases inter-link wear, thereby reducing the life of the chain. 
       SUMMARY 
       [0021]    This disclosure takes as its starting point the typical longwall conveyor described above in which the delivery end is fixed and the return end has a telescopic sliding frame. An object of this disclosure may be to provide a device for detecting and adjusting the tension of the scraper chain, which determines the tension reliably and simply. Another object of this disclosure may be to provide such a device that reliably senses chain tension while at the same time not adversely affecting the chain path. 
         [0022]    This disclosure may also provide a means of identifying broken chain as it leaves the return sprocket and enters the top race of the conveyor. When detected, the chain can be stopped automatically by the armored face conveyor control system, to avoid the potential for further damage, and warn the operators that repair of the chain is required. 
         [0023]    Another object of this disclosure may be to provide sliding frames at both ends of the conveyor to allow the conveyor ends to be independently adjusted to each end of the coal block, while maintaining good chain tension and control. 
         [0024]    Providing the delivery and return end frames with a telescopic section addresses the problem of face creep by allowing the operator to quickly adjust the position of both ends of the conveyor, thus offsetting the effects of face creep. This may be important on conventional end discharge conveyor systems, where the correct relationship between the longwall discharge conveyor and an auxiliary cross conveyor (beam stage loader) must be maintained. This problem presents an increasing challenge where there are two longwall conveyors operating side by side, which is often the case with sub-level caving or longwall to coal caving. 
         [0025]    In one independent embodiment, a spring assembly is provided for a sensor assembly in an endless conveyor. The conveyor includes a frame, at least one chain, and a plurality of flights coupled to the chain. The sensor assembly includes a moveable arm and a sensor. The spring assembly may generally include a pin passing through the arm, a nut for securing the arm relative to the pin, and a spring element for applying a pre-load force on the arm, and the spring element may be adjustable to change the pre-load force. 
         [0026]    In another independent embodiment, a chain tension sensor is provided for a chain conveyor having a frame and a chain having a plurality of flights. The tension sensor includes a reaction arm pivotably coupled to the frame, flights contacting the reaction arm exerting a force on the reaction arm in a first direction; a load sensing pin coupled to the reaction arm and operable to sense the force exerted on the reaction arm; and a spring assembly coupled between the frame and the reaction arm to bias the reaction arm away from the frame. 
         [0027]    Independent aspects of the invention will become apparent by consideration of the detailed description, claims and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a plan view of a prior art delivery discharge end scraper chain conveyor arrangement. 
           [0029]      FIG. 2  is a schematic view of a prior art tension sensor for detecting and tensioning a scraper chain. 
           [0030]      FIG. 3  is a plan view of an improved tension sensor. 
           [0031]      FIG. 4  is a perspective view of an alternate embodiment of the tension sensor shown in  FIG. 3 . 
           [0032]      FIG. 5  is a perspective view of the tension sensor shown in  FIG. 4 , as mounted at the return end of a conveyor. 
           [0033]      FIG. 6  is a perspective view of a load cell used in the tension sensor of  FIGS. 4 and 5 . 
           [0034]      FIG. 7  is a schematic top view of the chain, two tension sensors and a tension control. 
           [0035]      FIG. 8  is a top view of a conveyor and a secondary or auxiliary conveyor. 
           [0036]      FIG. 9  is a side view of the conveyor and auxiliary conveyor shown in  FIG. 8 . 
           [0037]      FIG. 10  is a top view of a double conveyor system. 
           [0038]      FIG. 11  is a perspective view of an end frame of a chain conveyor. 
           [0039]      FIG. 12  is an enlarged view of the end frame of the chain conveyor of  FIG. 11 . 
           [0040]      FIG. 13  is a perspective view of a sensor assembly. 
           [0041]      FIG. 14  is an assembly view of the sensor assembly shown in  FIG. 13 . 
           [0042]      FIG. 15  is cross-sectional view of the sensor assembly shown in  FIG. 13  taken along line  15 - 15 . 
           [0043]      FIG. 16  is an enlarged cross-sectional view of the sensor assembly shown in  FIG. 15 . 
           [0044]      FIG. 17  is an enlarged cross-sectional view of the sensor assembly shown in  FIG. 15 . 
           [0045]      FIG. 18  is an exploded view of a spring assembly. 
           [0046]      FIG. 19  is a cross-sectional view of the sensor assembly shown in  FIG. 18 . 
           [0047]      FIG. 20  is an enlarged cross-sectional view of a sensor assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    Before any independent embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other independent embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Use of “including” and “comprising” and variations thereof as used herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Use of “consisting of” and variations thereof as used herein is meant to encompass only the items listed thereafter and equivalents thereof. Further, it is to be understood that such terms as “forward”, “rearward”, “left”, “right”, “upward” and “downward”, etc., are words of convenience and are not to be construed as limiting terms. 
         [0049]      FIG. 3  illustrates an improved version of the tension sensing means  60  shown in  FIG. 2 . Conventionally, to allow for optimum use of the length of the tailgate or return end or station  51 , a wear strip  101  is installed to guide the conveyor  16  down to the track or race  54  A level. The tensioning means, or tension sensor  104 , of  FIG. 3 , comprises a wear strip  101  including a wear plate  108  that contacts the top surface of the conveyor  16 . 
         [0050]    The wear plate  108  is supported by a wear strip support  112 , and the wear plate  108  is connected to the wear strip support  112  by a pin  116  at one end and a load-sensing pin  120  at the other end. The wear plate  108  engages the top surface of the conveyor  16 , and changes the path or trajectory of the movement of the conveyor  16 . This contact and change in direction of the conveyor  16  causes a force to be applied on the wear plate  108 . The load-sensing pin  120  that connects the wear plate  108  to the wear strip support  112  senses this force. The output from the load-sensing pin  120  is then be used to determine the tension of the conveyor  16 , and to adjust the tension, as needed, using any conventional chain tensioning system, such as the joint  48  and pistons  32  and  34  and circuitry of  FIG. 1 . 
         [0051]    An alternate and preferred embodiment  124  of the tension sensor is illustrated in  FIG. 4 . In  FIG. 4 , a load cell  128  is located between a wear plate  132  and a wear strip support  136 . The load cell  128 , which is illustrated in  FIG. 6 , is a cylinder including a plurality of spaced apart passageways  130  through the cylinder. Within the passageways are load sensors (not shown), which measure the compression force on the load cell  128 . By placing the load cell  128  between the wear plate  132  and the wear strip support  136 , the load cell  128  responds to the force applied to the wear plate  132  by the conveyor  16 . In order to provide redundancy, as shown in the preferred embodiment illustrated in  FIG. 4 , two spaced apart load cells  128  are placed between the wear plate  132  and the wear strip support  136 . More particularly, the wear strip support  136  includes a cavity  138  that receives the load cells  128 , and the wear plate  132  is connected to the wear strip support  136  by means of a screw  140 . 
         [0052]      FIG. 5  illustrates a perspective view of the load sensor  124  mounted on the conveyor apparatus  10  at the return end  51 . As shown, the cavity  138  receiving the load cells  128  can be formed by a plate  142  secured to the wear strip support  36 . This provides ready access to the load cells  128  from adjacent the conveyor apparatus  10 , without the need for significant disassembly of conveyor parts. This thus permits ready access and repair of the tension sensor  124 , when the need arises. 
         [0053]    The disclosure also illustrates, in  FIG. 7 , the providing of two such tension sensors on such a conveyor apparatus  10 . More particularly, in this embodiment, the conveyor  16  includes the two spaced apart chains  18 , and the plurality of flights or flight bars  20  that are connected and spaced apart but between the two chains  18 . Each conveyor flight  20  has a first end and a second end. Each flight bar end is spaced apart from its respective adjacent chain. A tension sensor, such as the tension sensor illustrated in  FIGS. 2 ,  3  and  4  above, is provided in a respective wear strip for each one of the two conveyor chains  18 . Each tension sensor  124  is electrically connected via a line  154  to a comparator  158 . 
         [0054]    In the preferred embodiment, as illustrated in  FIG. 7 , the part of the conveyor that contacts the tension sensor  124  is the end or tip of the flight bar  20 . In other embodiments, not shown, a tension sensor  124  can be placed above each of the chains, instead of the flight tips. The tip of the flight bar  20  will only contact the wear strip intermittently. As a result, the tension sensor  124  will only produce intermittent signals. 
         [0055]    To eliminate transient load spikes and to allow for the odd missing flight bar  20 , the tension sensor  124  collects a rolling average reading over a number of flight bars. As each flight bar tip passes along the load sensor, even at a constant chain tension, the signal varies due to the changing geometry of the system. The tension sensor  124  records the peak signal value as each flight bar  20  passes over the wear plate  132 . If the rolling average peak reading is too low, then the tension means opens the joint  48  to stretch the chain, or vice versa. The tension means is initialized by establishing a required peak signal value by stopping the conveyor with a flight bar under the sensor, fitting a temporary load transducer to the chain itself, and then moving the joint  48  to tension the static chain. When the chain is at the required tension, the tension sensor  124  stores the signal, and it is this signal value that the tension sensor  124  maintains while the conveyor is running. 
         [0056]    The above overview is a simplified version of the sensor signal management system, and applies to steady chain load increase or decrease during the coal cutting cycle. The tension sensor  124  must also deal with special events such as starting a full conveyor or the rapid unloading of a conveyor, like when the shearer stops cutting. Collecting a rolling average signal cannot respond quickly enough to deal with these events, so advance action must be taken. For example, the sprocket is extended to significantly stretch the chain before loaded conveyor startup to prevent generation of slack chain. 
         [0057]    In the event of a chain break, the tension in the two chains  18  will be different. The outputs of the tension sensors  124  are compared by a comparing means, comparator  158 , and in the event of a significant difference, the operation of the conveying apparatus  10  can be stopped so the broken chain can be repaired. In the preferred embodiment, the tension sensors  124  are provided adjacent the top race of the return end of the conveyor apparatus. If additional sensors or sensing of the tension at other locations in the conveying apparatus is desired, other tension sensors  124 , in other locations, can be used. The use of the two tension sensors  124  is also beneficial, for the output from the tension sensors  124  can be averaged to produce a more accurate indication of overall conveyor tension. The comparator  158  forms a part of the chain tensioning system such as the joint  48  and pistons  32  and  34  and circuitry of  FIG. 1 . 
         [0058]    As illustrated in  FIG. 8 , an auxiliary or secondary conveyor  200  is located at one end of a conveyor apparatus  210 . The material on the conveyor  16  leaves the conveyor and is transferred to the auxiliary conveyor  200 . During operation of the conveyor apparatus  210 , the location of the conveyor apparatus  210  may move relative to the location of the auxiliary conveyor  200 . Currently, operators need to make various adjustments in order to try to accommodate such movement. This can result in difficulty maintaining conveyor operation. 
         [0059]    In order to accommodate some movement of the conveyor apparatus  210  relative to the auxiliary conveyor  200 , the conveyor apparatus frame accommodates sliding movement at both ends. At one end, the sliding movement adjusts the tension of the conveyor  16 , and sliding movement at the other end accommodates movement of the conveyor apparatus  210  relative to the auxiliary conveyor  200 . If the conveyor apparatus  210  moves relative to the auxiliary conveyor  200 , an operator can move the sliding end of the conveyor  210  adjacent the auxiliary conveyor  200 . Movement of the sliding end of the conveyor  210  can also be occasioned by the use of tensioning means, as described hereinafter, as used on the tensioning end  51  of the conveyor  16 . Only in this instance, the movement is not intended to affect the tension of the conveyor  16 , but the location of the end of the conveyor apparatus  210  relative to the auxiliary conveyor  200 . When movement at this end of the conveyor occurs, the chain tension does change, so the other end of the conveyor apparatus  210  is adjusted by the automatic tensioning means to return the conveyor  16  back to the appropriate tension. Movement of the sliding end of the conveyor  210  adjacent to the auxiliary conveyor  200  must overcome the maximum working chain tensions (which are at there highest as these top chains reach this frame; plus significant sliding friction due to the typical large size and weight of the Main gate equipment. 
         [0060]    More particularly, a driven drum/sprocket  312  is appropriately coupled to a conveyor drive motor  322 . Operation of motor  322  causes the sprocket intermeshing with the dual chains  18  to advance the conveyor  16 . More particularly, as illustrated in  FIGS. 8 and 11 , in addition to the hydraulic pistons  32  and  34  spanning the joint  48  at the return end  51 , a pair of sidewalls  324  forming a first portion of a “split frame” of the main gate end of the conveyor apparatus serves to rotatably support the drum/sprocket  312 . The sidewalls  324  are illustrated as being telescopingly engaged with a second pair of sidewalls  326  forming a second portion of the frame and, which collectively with sidewalls  324 , comprise the aforementioned split frame. The telescoping joint, indicated generally by character numeral  348 , permits the frame portions to be moved relative to one another. 
         [0061]    Relative movement at the joint  348  between the adjacent sidewalls  324  and  326  thus causes the distance span between the drum/sprockets  312  and  14  to vary accordingly. The conveyor  16  can be provided with increased or reduced tension depending upon the direction of adjusting movement of the supporting drum/sprockets with respect to each other. To provide this relative movement, the conveyor assembly  310  has a pair of hydraulic cylinders  328  and  330 , each mounted on and secured to an adjacent sidewall  326 . The cylinders have respective pistons  332  and  334 , each of which is operatively coupled to a sidewall  324  in any known and expedient manner. 
         [0062]    The location of the conveyor apparatus relative to the auxiliary conveyor is further illustrated in  FIG. 9 . If desired, in lieu of operator correction of the location of the conveyor apparatus, the conveyor apparatus can be physically connected by a bar  352  to the auxiliary conveyor. In this instance, tension is maintained at this end of the conveyor by some tensioning means, such as the tensioning means previously described. But in order to accommodate some movement in the event the auxiliary conveyor and main conveyor change location, either a hydraulic accumulator (now shown), or some relief valve (now shown) must be provided in the hydraulic tensioning means in order to allow for the movement of this sliding end of the conveyor apparatus  210 . When this end of the conveyor apparatus  210  adjusts by movement of the auxiliary conveyor  200 , then tension is corrected, as described before, by the return end  51 . 
         [0063]    The problem of conveyor apparatus movement relative to the auxiliary conveyor is especially relevant where a pair of conveyor apparatus is used. As illustrated in  FIGS. 10A and 10B , it is known to use one conveyor adjacent to a coal face, and a second conveyor apparatus behind the roof supports to collect coal that falls from the longwall roof as the longwall advances. In this instance, the double sliding frame ends would be used with both conveyor apparatus. 
         [0064]    Additionally the frame-sliding  48  and  348  can be adjusted to correctly align the conveyor end with both edges of the coal block, moving both the return end frame and delivery end frame at the same time to maintain correct chain tension during this adjustment. This would not be a normal requirement or mode of operation as the position of the Return End Frame to coal block is less critical in most cases. 
         [0065]    This aspect of the disclosure thus has the following benefits. Manual or automatic control of the delivery end frame sliding module makes fine adjustments for optimum discharge of material from the extendable longwall armored face conveyor to the cross beam stage loader conveyor. 
         [0066]    Since the changes in the overall length of the conveyor, as a result of adjusting the delivery end sliding frame module will change the chain tension, adjustments must be in small increments and effected slowly to give the automatic chain tensioning system time to react. At all times it is the automatic chain tensioning system that controls and maintains correct chain tension, not the adjustment of the delivery end frame module. 
         [0067]    In another embodiment, a sensor assembly  510  for detecting tension in a chain  514  is provided. This embodiment is shown in  FIGS. 11-17 , and all reference numbers begin at  500 . 
         [0068]      FIGS. 11-12  illustrate a portion of a longwall conveyor  522  including a return end  526  ( FIG. 11 ), a conveying element or chain  514  that travels between the return end  526  and a delivery end (not shown), and the sensor assembly  510  proximate the return end  526 . The return end  526  includes a frame  538 , an idler or take-up shaft  542  mounted on the frame  538 , and at least one hydraulic actuator (not shown). The frame  538  moves with respect to the delivery end, between an inner retracted position and an outer extended position through the extension and retraction of the hydraulic actuator. The chain  514  passes around the take-up shaft  542  to travel in a continuous loop between the delivery end and the return end  526 . The chain  514  includes a plurality of flight members  550  mounted on the chain  514  and spaced apart by a first distance in a direction of travel  554  of the chain  514 . 
         [0069]    As shown in  FIGS. 13-16 , the sensor assembly  510  is positioned adjacent a wear strip  562  of a flange portion  566  of the frame  538  and includes a reaction arm  570 , a main support hinge pin  574 , a reaction bracket  578  ( FIGS. 14-16 ), a load sensing pin  582  ( FIGS. 14-16 ), and a spring assembly  586 . 
         [0070]    The reaction arm  570  has a first end  590 , a shoulder  594 , a second end  598  ( FIG. 14 ), and a load pad  602 . The first end  590  is rotatably coupled to a secondary support plate  606  of the frame  538  by the main support hinge pin  574 . The shoulder  594  is positioned proximate the first end  590 . The second end  598  includes a hole  622  ( FIGS. 14 and 15 ) extending from the second end  598  partially through the reaction arm  570  in a longitudinal direction. The load pad  602  is positioned intermediate the first end  590  and the second end  598 . As shown in  FIG. 12 , the load pad  602  is positioned parallel to the wear strip  562  to contact the flight members  550  passing the wear strip  562 , causing the reaction arm  570  to rotate about the hinge pin  574 . The load pad  602  also provides a continuous guide surface to guide the flight members  550  as the flight members  550  travel around the take-up shaft  542 . 
         [0071]    The hinge pin  574  is mounted to the secondary support plate  606  of the frame  538  and is positioned substantially transverse to the direction of travel  554  of the chain  514 . The hinge pin  574  restricts the motion of the reaction arm  570  in every direction except rotation (see arrow  630 ) about the hinge pin  574 . 
         [0072]    As shown in  FIGS. 14-16 , the reaction bracket  578  is mounted to the secondary support plate  606  of the frame  538  and includes a slot  638 . The reaction bracket  578  is configured to fit within the second end  598  of the reaction arm  570  such that the slot  638  is aligned with the hole  622  extending through the reaction arm  570 . The load sensing pin  582  is positioned in the slot  638  of the reaction bracket  578  and within the hole  622  of the reaction arm  570 . The load sensing pin  582  is therefore positioned substantially perpendicular to the hinge pin  574 . The load sensing pin  582  is attached to a sensing cable  650  ( FIGS. 15 and 16 ). 
         [0073]    As shown in  FIG. 17 , the shoulder  594  includes a head side  662 , a spring side  666 , and a bore  668  extending between the head side  662  and the spring side  666  through the reaction arm  570  in a direction tangential to a direction of rotation  630  of the reaction arm  570  (i.e., perpendicular to the hinge pin  574 ). Referring to  FIGS. 17 and 18 , the spring assembly  586  includes a pin or bolt  670 , a nut  672 , a plurality of spring washers  674 , and a retaining washer  678 . The bolt  670  is coupled to the wear strip  562  and passes through the shoulder bore  668 . The bolt  670  includes a smooth portion  680 , a shoulder  682 , and a threaded portion  684  for threadingly engaging the nut  672 , which is tightened to secure the shoulder  594  with respect to the bolt  670 . 
         [0074]    The spring washers  674  are positioned around the bolt  670  adjacent the spring side  666 , between the shoulder  594  and the wear strip  562 . In the embodiment shown in  FIG. 19 , the bolt  670  includes a cavity recess  686  to reduce the material contact between the wear strip  562  and the bolt  670 , thereby reducing the amount of heat transfer from the wear strip  562  to the bolt  670 . The retaining washer  678  is positioned between the spring side  666  of the shoulder  594  and the spring washers  674 . The retaining washer  678  is screwed onto the bolt  670  past the threaded portion  684  of the bolt  670 , effectively “capturing” the spring washers  674  around the smooth portion  680 . Each spring washer  674  has a generally frusto-conical shape that creates a spring force as the spring washer  674  is compressed. The compression of the spring washers  674  therefore applies a pre-loaded force to the reaction arm  570 , biasing the reaction arm  570  away from the frame  538 . The retaining washer  678  centers the top-most spring washers  674  with respect to the bolt  670 . 
         [0075]    In the embodiment shown in  FIG. 17 , the nut  672  is capped in order to prevent the nut  672  from being tightened against the shoulder  594 . This maintains a clearance between the nut  672  and the reaction arm  570 , allowing the pre-load force of the spring washers  674  to be applied on the load pin  582 . In another embodiment (see  FIGS. 18-20 ), the nut  672  is open allowing the nut  672  to be tightened against the shoulder  594  ( FIG. 20 ). As the nut  672  is tightened, the retaining washer  678  compresses each spring washer  674 , and the reaction arm shoulder  594  is secured against the retaining washer  678 . Tightening the nut  672  causes the retaining washer  678  to draw closer to the bolt shoulder  682  ( FIG. 19 ). Once the retaining washer  678  contacts the bolt shoulder  682 , the nut  672  cannot be tightened any further. In this way, the bolt shoulder  682  provides mechanical lock-out, preventing over-compression of the spring washers  674 . 
         [0076]    The spring washers  674  may be stacked in a number of configurations in order to obtain the desired pre-load force on the reaction arm  570 . For instance, the spring washers  674  may be stacked in alternating sets such that the “peaks” of two washers  674  are against each other, and the “peaks” of the adjacent washers  674  are inverted with respect to the first two (see  FIG. 19 ). The desired configuration can be accomplished using fewer or more washers  674  in each set. Alternatively, all of the washers  674  can be aligned in one direction. In another alternative, a single spring washer  674  may be used. In still other constructions, a different type or shape of spring may be used. 
         [0077]    A plurality of shims  690  (see  FIG. 18 ) may be added to the area between the retaining washer  678  and the cavity recess  686  in order to account for the build-up of tolerances in the bolted joint and/or to apply additional compressive force on the spring washer(s)  674 . 
         [0078]    During operation, the load pad  602  of the reaction arm  570  contacts the flight members  550  of the chain  514  as the flight members  550  pass between the return end  526  and the delivery end. In this manner, the load pad  602  is subjected to the vertical component of the chain tension. Contact with the flight members  550  causes the reaction arm  570  to rotate about the hinge pin  574 . 
         [0079]    Referring to  FIG. 15 , as the reaction arm  570  rotates in the direction of rotation  630 , the second end  598  deflects upwardly, exerting an upward force on the load sensing pin  582 . The reaction bracket  578  resists this deflection, exerting a downward force on the load sensing pin  582 , thereby creating a shear load condition on the pin  582 . The load sensing pin  582  senses the magnitude of the shear force and/or the strain and transmits a signal indicative of the force or strain through the sensing cable  650  to a chain controller (not shown). The chain controller then uses this information to determine the tension in the chain  514  and to calculate the necessary change in position of the return end frame  538  in order to maintain the desired tension in the chain  514 . 
         [0080]    The chain controller may be a component of a system for automatically controlling the conveyor  10 , such as that described and illustrated in U.S. Provisional Patent Application No. 61/510,850, filed Jul. 22, 2011, the entire contents of which are included in parent U.S. Provisional Application No. 61/510,839, or in U.S. patent application Ser. No. 13/553,215, filed Jul. 19, 2012, entitled Systems And Methods For Controlling A Conveyor In A Mining System, (Attorney Docket No. 051077-9174-01), the entire contents of both of which are also hereby incorporated by reference. 
         [0081]    The biasing force of the spring assembly  586  provides a pre-load force that can be calibrated. Instead of calibrating the tension to the maximum load the chain  514  may experience during operation (e.g., in one embodiment, approximately five tons; in other embodiments, this maximum load may be greater than or less than this value), the positive pre-load permits the chain tension to be set to a lesser load. This may reduce inter-link chain wear and sprocket wear and, ultimately, increase the life of the chain  514 . In addition, the tolerance “stack-up” of the spring washers  674  provides a wide range of configurations and pre-load characteristics for the reaction arm  570 . In one example, a pre-load in the range of 200 to 400 lbs. may provide improved results for even very high material loads. 
         [0082]    In one embodiment, the pre-load acts on the reaction arm  570  in a “positive” direction (i.e., substantially parallel to the direction of the force exerted on the reaction arm  570  by the flight members  550 ). The positive base load may facilitate accurate measurement in strain gauge sensors, enhancing accuracy of the system. In addition, the positive pre-load may also reduce the occurrence of negative outputs, which can falsely trigger system alerts. 
         [0083]    Due to the perpendicular orientation of the load sensing pin  582  with respect to the hinge pin  574 , the load sensing pin  582  only senses the vertical component (e.g., the rotation of the reaction arm  570  about the hinge pin  574 ) of the force exerted on the reaction arm  570 . This effectively isolates the load sensing pin  582  from impacts to the load pad  602  of the reaction arm  570 , resulting in improved reliability and a more accurate electrical signal. 
         [0084]    Also, in one embodiment, the load pad  602  has a length that is a significant proportion of the distance between the flight members  550 . In one embodiment, the load pad  602  has a length in a range between approximately 60% and approximately 70% of the distance between the flight members  550 . This significant length provides a smaller gap between the moment when one flight member  550  contacts the load pad  602  and the moment when a second flight member  550  contacts the load pad  602 , reducing the oscillation of the load pad  602  (and therefore the load sensing pin  582 ) between a loaded position and an unloaded position. This aids the load sensing pin  582  in generating a smooth, level signal. 
         [0085]    Spurious loading arising from the impact of the flight members  550  with the load pad  602  is absorbed by the main support hinge pin  574 , which is positioned at a right angle to both the direction of travel  554  of the chain  514  and the flight members  550 . In addition, the load sensing pin  582  is not directly in contact with the wear strip  562 , reducing the impact loading and insulating the load sensing pin  582  from heat caused by the friction contact of the flight members  550  sliding against the underside of the wear strip  562 . 
         [0086]    In an alternative independent embodiment, the conveyor  522  may include a plurality of load sensor assemblies  510 . For example, the conveyor  522  may include a sensor assembly  510  mounted on each side of the chain  514 , with each sensor  510  measuring the tension in the associated chain  514  independently and permitting the operator to detect breakage in either chain  514 . Because the chains  514  are connected to one another by the flight members  550 , some amount of the tension load in the chains  514  will be shared in the event that a chain  514  breaks. 
         [0087]    While the described location of the sensor assembly  510  is beneficial because the sensor assembly  510  is subjected to less direct impact loads, in an alternative embodiment, the sensor assemblies  510  may be spaced along the length of and on either side of the conveyor  522 . 
         [0088]    Thus, the invention may generally provide, among other things, a chain tension sensor.