Abstract:
A skew detection and correction system, and method of correcting skew, in a conveyor system having an endless web that is propelled in a longitudinal direction, includes providing a skew detector and determining if at least some of the cross members of the web are non-perpendicular to the longitudinal direction and providing a differential lubricant application. The lubricant applicator is responsive to the skew detector and applies a relatively greater amount of lubricant to one lateral side of the web than to the other side of the web. This causes the side of the web receiving less lubricant to stretch more than the other side to reduce the difference in the amount of stretch.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. provisional patent application Ser. No. 60/747,953, filed on May 23, 2006, the disclosure of which is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention pertains to conveyor systems having an endless web that is defined by laterally extending parallel cross members that are connected at opposite ends, such as by chains, and in particular to a method of correcting skew in the cross members resulting from unequal expansion of the chains on opposite sides of the web. The invention is particularly adapted for use with article sorters, but may be applied to other such conveyor systems. 
     An article sorter, such as the RS200 positive displacement sorter marketed by Dematic Corporation of Grand Rapids, Mich., has an endless web that is made up elongated parallel slats that are interconnected by a matched pair of chains. The web is supported by carrier wheels attached to the sides of the chains. Because of the nature of a positive displacement sorter, wear on the chains tends to not be the same. Therefore, one chain tends to stretch more than the other chain. This can cause the slats to become skewed. Skew is where the slats, which are normally perpendicular to the direction of movement of the web, become non-perpendicular to such direction. Skew causes the carrier wheels to be at an angle to the direction of movement of the web. This causes an increase in chain drag resulting in additional energy required to propel the web as well as extra wear on the sorter. Also, excessive skew of the slats can cause problems with tracking articles on the sorter and proper diverting of the articles to the desired lanes for sortation. 
     Uneven elongation of the chains leads to one of the chains requiring replacement prematurely. As the chains are a matched set, uneven elongation of either chain leads to both of the chains being replaced prematurely. This is very expensive because it essentially requires an almost complete disassembly of the web. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a technique for determining that one of the two chains supporting the cross members of a conveyor has become more elongated, or stretched, than the other chain and to take corrective action to make the stretch of the chains more equal. 
     A skew detection and correction system, and method of correcting skew, in a conveyor system having an endless web that is propelled in a longitudinal direction, according to an aspect of the invention, includes providing a skew detector and determining if at least some of the cross members of the web are non-perpendicular to the longitudinal direction and providing a differential lubricant application. The lubricant applicator is responsive to the skew detector and applies a relatively greater amount of lubricant to one lateral side of the web than to the other side of the web. This causes the side of the web receiving less lubricant to stretch more than the other side to reduce the difference in the amount of stretch. 
     The lubricant applicator may be an aerosol applicator. The lubricant applicator may apply lubricant according to a duty cycle and may apply a relatively greater amount of lubricant to one lateral side of the web than to the other side by varying the duty cycle for each of the lateral sides of the web. 
     The skew detector may be made up of a plurality of sensors and a control. The sensors sense longitudinal position of opposite ends of at least two spaced apart cross members. The control determines skew as a function of the longitudinal positions of the opposite sides. The control may determine diagonal distances between opposite ends of the two spaced apart cross members and determine skew from differences in the diagonal distances. 
     Opposite lateral sides of the cross members may be interconnected by first and second endless chains and the web may be supported by first and second sets of spaced apart wheels. The lubricant applicator may apply a relatively greater amount of lubricant to one of the endless chains than to the other and/or may apply a relatively greater amount of lubricant to one of the sets of wheels than to the other. 
     These and other objects, advantages and features of this invention will become apparent upon review of the following specification in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of a conveyor system according to the invention; 
         FIG. 2  is an enlarged portion of the conveyor system illustrated at II in  FIG. 1 ; 
         FIG. 3  is the same view as  FIG. 1  illustrating skew of a cross member; 
         FIG. 4  is a side elevation of a conveyor system illustrating an alternative embodiment thereof; 
         FIG. 5  is a block diagram of a skew detector system; 
         FIG. 6  is an illustration of measurement of cross members that do not exhibit skew; 
         FIG. 7  is the same view as  FIG. 6  with cross members that exhibit skew; 
         FIG. 8  is a table illustrating determination of amount of skew; 
         FIG. 9  is a perspective view of a skew detector assembly according to an alternative embodiment of the invention; 
         FIG. 10  is a side elevation of the skew detector assembly in  FIG. 9 ; 
         FIG. 11  is the same view as  FIG. 5  of the skew detector assembly of  FIGS. 9 and 10 ; 
         FIG. 12  is the same view as  FIG. 6  using the skew detector system of  FIGS. 9 through 11 ; 
         FIG. 13  is the same view as  FIG. 7  using the skew detector system of  FIGS. 9 through 11 ; 
         FIG. 14  is an end elevation of a manifold assembly portion of a differential lubricant applicator; 
         FIGS. 15   a  and  15   b  are respective left and right side chain nozzle assemblies of the differential lubricant applicator; 
         FIGS. 16   a  and  16   b  are respective left and right side wheel lubricant nozzle assemblies of the differential lubricant applicator; 
         FIG. 17  is a side elevation of a chain nozzle assembly lubricating a chain; 
         FIG. 18  is a top plan view of a wheel lubricant nozzle assembly lubricating a wheel; 
         FIG. 19  is a top plan view of an article sorter frame illustrating placement of chain and wheel lubricant nozzle assemblies; and 
         FIG. 20  is a diagram illustrating use of duty cycle variation for differential application of lubricant. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and the illustrative embodiments depicted therein, a conveyor system  25  includes an endless web  26  made up of a plurality of cross members  28  that are interconnected at opposite ends by attaching the cross members, using fasteners  31  and special adapters  33 , to left and right chains  30  ( FIGS. 1-3 ). The endless web travels in a longitudinal direction, which is illustrated by the arrow in  FIG. 1 . In the illustrative embodiment, conveyor system  25  is a positive displacement shoe and slat sorter of the type disclosed in commonly assigned U.S. Pat. Nos. 4,738,347; 5,127,510; 5,165,515; 5,732,814; 5,927,465; 6,041,909; 6,513,642; 6,814,216; 6,860,383; and 6,935,483, the disclosures of which are hereby collectively incorporated herein by reference. 
     In the embodiment illustrated in  FIGS. 1 through 3 , cross members  28  are closely spaced slats having generally planar upper surfaces thereby defining a flat-top conveying surface. A plurality of pusher shoes  32  is provided. Each pusher shoe selectively travels laterally along one of the cross members in order to laterally displace an article traveling on the conveying surface. This causes the article to be diverted to one of several spurs (not shown) in order to sort the articles according to some scheme. In an alternative conveyor system  125 , cross members  128  are in the form of rounded tubular members and pusher shoes  132  travel along a plurality of cross-members ( FIG. 4 ). 
     As can be seen by reference to  FIG. 3 , conveyor system  25  is subject to a condition, known as “slat skew,” in which cross members  28  become non-perpendicular to the longitudinal direction of propulsion of web  26 , which is indicated by the arrows in  FIG. 3 . The amount of slat skew is indicated by the angle α, but also can be expressed as the difference between the position of one end of a cross member  28  from the other end of the cross member. Skew can be both plus or minus depending upon which lateral end of the slat lags the other end. Slat skew is caused when chains  30  stretch unevenly from one side of the web to the other side. As the skew on the slats increases, carrier wheels  35  will be angled to the direction of flow which pushes the web into the side wear strip (not shown) and causes an increase in chain drag. Increases in chain drag require more horsepower to pull the chain which increases the amount of electrical energy consumed. Excess slat skew may also cause article tracking and divert problems. 
     In order to overcome slat skew, a skew detector  34  is provided ( FIG. 5 ). Skew detector  34  includes laterally spaced apart sensors  36   a ,  36   b , which are positioned to identify laterally spaced apart triggers  46   a ,  46   b  on one of the slats. In the illustrative embodiment, triggers  46   a ,  46   b  are magnetic targets and sensors  36   a ,  36   b  are magnetic proximity sensors which detect the magnetic target. A pair of targets  46   a ,  46   b  are mounted to each of a “start” cross member  128  and an “end” cross member (not shown). The “start” and “end” cross members are spaced apart in web  26  by an amount that, in the illustrated embodiments, is 100 feet. While the slat spacing is not critical, it should be less than half of the web length. Sensors  36   a ,  36   b  are connected via conductors  40   a ,  40   b  with inputs  41   a ,  41   b  of a high-speed counter circuit  38 . Counter circuit  38  is an input card of a control system  37 , such as a programmable logic controller of the type known in the art. In the illustrative embodiment, control system  37  is a Momentum™ programmable logic controller (PLC) marketed by Modicon. A clock signal, which is 1 millisecond in the illustrated embodiments, is received on an input  42  of counter circuit  38 . In operation, counter circuit  38  counts the number of clock pulses  42  beginning with the detection of the respective targets  46   a ,  46   b  on the “start” cross member  128  and ending with detection of the corresponding triggers  46   a ,  46   b  on the “end” cross member (not shown). In this manner, counter circuit  38  counts the amount of time between passage of each lateral end of the “start” cross member to the corresponding lateral end of the “end” cross member. Counter circuit  38  also counts the number of clock pulses along the diagonals between a start trigger  46   a  and end trigger  46   b  and between a start trigger  46   b  and end trigger  46   a . The diagonal measurements are designated H 1  and H 2 . The magnetic targets on the start and end slats may be distinguishable, such as by coding, or the like, but this is not always necessary. Skew detector  34  is positioned at an upstream end of web  26 , such as where articles are placed onto the web. The reason is that web  26  is typically driven at a downstream end where all articles have been discharged from the web by a pair of sprockets driving the corresponding chains  30 . The drive sprockets (not shown) are fixed to a motor-driven shaft and are thereby rotated in unison. Therefore, any skew in the cross members is not present at the discharge end, namely, the downstream end, of web  26 . Idler sprockets (not shown) at the article input upstream portion of web  26  are independent freely rotatable. Therefore, slat skew is at a maximum at the article input end, namely, the upstream end, of web  26 . 
     The manner in which slat skew detector  34  can measure skew in web  26  is illustrated in  FIGS. 6 and 7 . It can be seen that when there is no slat skew, the diagonal measurements H 1  and H 2  between the lateral ends of the “start” and “end” cross members are equal. Thus, when the number of pulses measured for H 1  equals the number of pulses for H 2  within a given tolerance, it is determined that there is no corrective action required. When the number of pulses for H 1  exceeds those for H 2 , or vice versa, skew can be determined using the table in  FIG. 8 . Angle α is the angle of skew. “Skew B” is the amount of lag distance of one lateral end of the cross member with respect to the other lateral end of that cross member in the direction of web movement. Pulse differences can be either positive or negative. If positive, the amount of skew is as illustrated in  FIG. 7 . If negative, the skew is the opposite with the right lateral end of the cross members lagging behind the left lateral end of the cross members, as illustrated in  FIG. 7 . 
     An alternative skew detector  134  includes a detector assembly  44  that is positioned below cross members  28  ( FIGS. 9 and 10 ). Detector assembly  44  includes four detectors  136   a ,  136   b ,  136   c ,  136   d , two of which are oriented toward each lateral end of the cross member. In the illustrative embodiment, sensors  136   a - 136   d  are magnetic sensors. Magnets  146   a  and  146   b  are positioned on a “start” cross member  228  in a position that they will be detected by sensors  136   a  and  136   b . Two magnets  146   c  and  146   d  are positioned on “end” cross member  328  at a position that they will be detected by sensors  136   c ,  136   d . Thus, skew detector  134  is similar to skew detector  34  except that separate sensors are used to detect the “start” cross member targets and the “end” cross member targets. Sensors  136   a ,  136   d  are connected to gate channel  41   a . Sensors  136   c  and  136   b  are connected to gate channel  41   b . Placement of magnets  146   a - 146   d  is illustrated in  FIG. 11 . 
     Determination of skew with skew detector  134  is similar to that for skew detector  34 . There is a slight variation in measurement of diagonals H 1  and H 2  because the “end” triggers  146   c ,  146   d  are closer together than the “start” triggers  146   a ,  146   b . However, any difference is minuscule over the 100-foot measurement range and is found to not appreciably affect the measurement. 
     Once the amount and direction of skew is determined, indicating that one of the chains  30  is stretched more than the other chain  30 , a differential lubricant applicator  50  applies a lesser amount of lubricant to the less-stretched chain than to the more-stretched chain. This results in the less-stretched chain stretching more relative to the greater stretched chain, thereby resulting in the chains becoming more equal in stretch, thereby reducing the slat skew. While it may seem counterproductive to withhold lubricant from one of the two chains, it should be understood that it is the differential in the amount of stretch of the chain that results in premature wear to the chains, not the stretch of the chains, per se. 
     Differential lubricant applicator  50  includes a manifold assembly  52  ( FIG. 14 ). Manifold assembly  52  includes “side A” manifolds  54  and “side B” manifolds  56 . Side A manifolds  54  are connected to conveyor system  25  in order to lubricate the chain of the takeaway side of the web, namely, the side to which articles are diverted. Side B manifolds  56  are connected to conveyor system  25  in order to lubricate the chain on the other side of the web. Manifold assembly  52  includes chain manifolds  58  responsible for lubricating the chains on opposite sides of the web. Manifold assembly  52  further includes wheel manifolds  60   a ,  60   b  which are responsible for lubricating the wheels and axles at two locations on each side of the web. Manifold assembly  52  includes an air regulator  62  and a control panel  63 . Each manifold includes inverter air valve  64 , a nozzle air valve  66  and an injector block  68 . Each lubricant manifold delivers an aerosol of oil and air mixture to the conveyor system in a manner that will be apparent to the skilled artisan. In the illustrative embodiment, differential lubricant applicator  50  is marketed by Orsco under Model No. VSR-0038-6-8 or equivalent lubrication system. 
     Chain manifolds  58  deliver the lubricant aerosol to left and right chain lubricant nozzle assemblies  70   a ,  70   b  ( 15   a ,  15   b ). Each chain lubricant nozzle assembly includes a pair of nozzle tips  72   a ,  72   b  which lubricate the chain links on opposite sides of the respective chain. The nozzle assembly includes a mounting bracket  74  for mounting to a convenient portion of the conveyor system  25  and a body  76   a ,  76   b  for connecting with tubing (not shown) leading back to chain manifold  58  and for conveying the aerosol to nozzle tip  72   a ,  72   b . In the illustrative embodiment, each nozzle tip  72   a ,  72   b  is approximately three inches in length having a zero-degree spray pattern which produces a vertical cone, as is best illustrated in  FIG. 17 . 
     Wheel manifolds  60   a ,  60   b  lead to a wheel/axle lubricant nozzle assembly  78   a ,  78   b  ( FIGS. 16   a ,  16   b ). Each wheel/axle lubricant nozzle assembly includes a horizontally oriented nozzle tip  80   a ,  80   b  extending from a body  84   a ,  84   b . Each body  84   a ,  84   b  connects with tubing (not shown) leading to the respective wheel manifold  60   a ,  60   b  and provides the lubricant aerosol to the corresponding nozzle tip  80   a ,  80   b . A mounting bracket  82   a ,  82   b  mounts the body and, hence, the nozzle tip to the conveyor system at a convenient location. The purpose of the wheel/axle lubricant nozzle assembly is to apply lubricant to the circumference of a carrier wheel  35  and the axle face of each carrier plate  88  ( FIG. 18 ). Two wheel/axle lubricant nozzle assemblies are provided on each side of conveyor system  25 . Nozzle tip  88   a ,  88   b  in the illustrative embodiment is approximately three inches in length and has a 45 degree spray pattern which produces a cone of aerosol at an approximate 45 degree angle. 
     Positioning of nozzle assemblies  70   a ,  70   b ,  78   a ,  78   b  with respect to the frame of conveyor system  25  is illustrated in  FIG. 19 . The wheel/axle nozzle assemblies  78   a ,  78   b  are mounted at both the charge and discharge ends of the conveyor system as illustrated respectively to the left and right ends of frame assembly  27  in  FIG. 19 . The chain lubricant nozzle assemblies  70   a ,  70   b  are mounted at the discharge end, or downstream end, of conveyor system  25 , to the right as illustrated in  FIG. 19 . The chain lubricant nozzle assemblies are positioned inside the catenary bed. However, it should be understood that the positioning of the nozzle assemblies is for illustration purposes only and other locations may be apparent to the skilled artisan. 
     In order to differentially lubricate chains  30 , differential lubricant applicator  50  causes lubricant to be applied during an “on” period and not applied during an “off” period thereby establishing an application duty cycle. If it is desirable to apply more lubricant to a particular chain, the duty cycle is increased by decreasing the off time of the nozzle assembly. If it is desired to apply less lubricant to a particular chain, the duty cycle is decreased by increasing the off time of the respective nozzle assembly. One scheme for carrying out such differential lubrication is illustrated in  FIG. 20 .  FIG. 20  illustrates six columns illustrating the off cycles for the two chain lubricant nozzle assemblies  70   a ,  70   b  and the four wheel/axle lubricant nozzles  78   a ,  78   b . The corresponding “on” time is about one (1) second. The horizontal rows correspond to the amount of skew that is measured by skew detector  34 ,  134 . As previously set forth, skew can be negative or positive values depending upon which lateral end of the cross members lags the other lateral end. It can be seen that for small amounts of skew, only slight variations in duty cycle are provided for opposite sides of the web. Thus, for small amounts of skew in the cross members, only a slightly greater amount of lubricant is applied to the stretched chain than to the opposite chain. However, for large amounts of skew in the cross members, almost all of the lubricant is applied to the stretched chain and little to the unstretched chain. 
     Variations may be made in the illustrated embodiments without departing from the scope of the invention. For example, although the differential lubricant applicator is illustrated as an aerosol applicator, mechanical applicators or drip lubricant applicators may be utilized. Also, although the skew detection is illustrated via an automated system, manual measurements of skew may also be utilized especially where the conveyor system is shut down for certain periods of the day, thus enabling skew to be manually measured. Although the invention was illustrated with a sortation conveyor, the conveyor system may, alternatively, be a slat conveyor, a traveling walkway, a baggage carrousel, or the like. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.