Abstract:
A conveyor includes an endless belt having teeth with drive faces raked away from the direction of travel. The conveyor also includes a drive pulley having sheaves with drive faces raked toward the direction of travel. The drive faces on the sheaves engage the drive faces on the teeth to move the belt and the raked angle tends to pull the belt inwardly. A stationary foot assists removing a driven tooth from its corresponding sheave.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 60/743,212, filed Feb. 2, 2006, which is incorporated herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to endless belts for conveyors and, more particularly, to a conveyor using raked teeth to drive an endless belt by a pulley. 
         [0004]    2. Description of the Related Art 
         [0005]    Conveyors with friction-driven flat belts are known systems for moving items from one location to another. A tensioned, endless belt extends between a drive pulley and a tail piece (typically a pulley or a fixed bar), whereby friction between the drive pulley and the belt enables transfer of torque from the former to the latter to thereby induce movement of the belt. Because tension on the belt is required to maintain the requisite friction for moving the belt, this type of conveyor does not perform well in environments where the tension and friction can be compromised. For example, the introduction of oil, grease, or other effluents from products carried on the belt can result in a loss of friction and thereby detrimentally affect the performance of the conveyor. 
         [0006]    Another type of conveyor comprises a direct or positive drive modular belt. In this type of conveyor, a modular belt formed of a plurality of interlocking links extends between a drive pulley and an idler pulley and comprises a plurality of teeth that engage corresponding sheaves on the drive pulley, or alternatively, teeth on the drive pulley engage the links or sheaves on the belt. Interaction between the teeth and sheaves transfers torque to the belt. As a result, the conveyor does not rely on friction for moving the belt, and friction reducing compounds do not affect performance in the manner described above for friction-driven belts. However, low tension, direct drive modular belts are difficult to clean and to maintain. They are also porous and therefore cannot readily carry products such as powders and the like. 
         [0007]    Conveyors with low friction, positive drive endless belts  100  having a flat surface  102  on one side and teeth  104  on the other side, as illustrated in  FIG. 1 , overcome the problems associated with the friction-driven flat belts and the modular belts. The seamless flat surface  102  is generally made of a thermoplastic material, non-porous and easy to clean, while the teeth  104  engage sheaves  106  on a drive pulley  108  to transfer torque to the belt  100  without requiring friction between the belt  100  and the drive pulley  108  or tension in the belt  100 . Such a conveyor is disclosed in U.S. Patent Application No. 60/593,493, which is incorporated herein by reference in its entirety. 
         [0008]    To account for belt stretching, it has been determined that the tooth pitch of the belt must be less than the sheave pitch of the drive pulley at less than maximum elongation of the belt. Also, the pulley pitch must equal the pitch of the belt at maximum elongation, give or take a fraction of a percent. Moreover, to ensure that the belt teeth are positioned to enter the pulley sheaves, the longitudinal width of each sheave in the pulley must exceed the belt tooth longitudinal width at least by the amount of distance generated by elongating the belt the maximum allowable amount over the span of the belt wrap. As a result of the pitch and width differences, the teeth and the sheaves will be longitudinally aligned as long as the elongation is at or below the maximum elongation. 
         [0009]    Due to the pitch difference between the belt and the pulley, only one belt tooth will be driven by a pulley sheave at any given moment. It has been found that this engaged tooth is always the tooth that is about to exit the pulley. For all subsequent belt teeth that enter the pulley sheaves at any given moment, there is a gap between the driving face of the belt tooth and the driving face of the pulley sheave, and that gap progressively increases in size for each successive tooth. Consequently, as the exiting tooth disengages from the drive pulley, there remains some amount of gap between the following belt tooth, i.e., the trailing tooth, and the face of its respective pulley sheave. At this time, the pulley continues to rotate relative to the belt without moving the belt, and the effective drive characteristics are lost until the driving face of the sheave abuts the driving face of the trailing tooth. In other words, the pulley rotates while the belt slips until a tooth engages again. Discounting any momentum of the belt and any friction between the belt and the pulley, the belt will effectively stop for a brief moment until the following sheave engages the trailing tooth, which thereby becomes the new “exit tooth”. 
         [0010]    Some slip between the belt and the pulley is what enables a direct drive application to work. This temporary disengagement of belt teeth from pulley sheaves causes the average belt speed to be less than the average pulley speed. In fact, the average belt speed is less than the pulley speed by the percentage of elongation that is still available in the belt (maximum elongation—current elongation). Because of this necessary slip, any friction between the pulley and the belt will compromise the benefits of direct drive. Friction between the belt and the pulley will retard slippage and can cause the trailing tooth to miss the pulley sheave altogether. To avoid such friction, the belt and the pulley can be made of, or coated with anti-friction materials, the pulley can be designed such that the belt and pulley have reduced contact area between the sheaves, and the belt is preferably maintained under low tension. 
         [0011]    Also, to ensure that the engaged (driven) tooth stays engaged until the appropriate time to exit the sheave, a position limiter is used adjacent the belt. 
         [0012]    Long belt runs in conveyor applications such as those moving coal, ore, or gravel typically require heavy, reinforced belts to minimize stretching and large drive motors to move the heavy belt as well as the load on the belt. The foregoing thermoplastic, low friction, direct driven belt has not been shown to be conducive to long belt runs. And the complexity of requiring position limiters to keep the drive tooth engaged with the pulley would only serve to complicate such a system in the harsh environment of moving coal, ore, or gravel. 
       SUMMARY OF THE INVENTION 
       [0013]    According to the invention, a low friction, direct drive conveyor overcomes the limitations of the prior art with an endless thermoplastic belt, and at least one drive pulley in contact with the endless thermoplastic belt for driving the belt in a drive direction. Either the drive pulley or the belt has sheaves in a surface, and the other has teeth on a surface. Each tooth and each sheave has a drive face extending at an acute angle from the surface, in the drive direction from the pulley surface and away from the drive direction from the belt surface. A foot is disposed adjacent the drive pulley at an exit point of a tooth from a sheave to urge the teeth from the sheaves as they pass the foot. In this way, the drive face extending from the drive pulley surface will grab the drive face extending from the belt surface to urge the belt against the drive pulley and move the belt in the drive direction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the drawings: 
           [0015]      FIG. 1  is a side view of a prior art conveyor. 
           [0016]      FIG. 2  is an enlarged view in elevation of a portion of one embodiment of a conveyor according to the invention; 
           [0017]      FIG. 3  is an enlarged portion of the belt of  FIG. 2 ; 
           [0018]      FIG. 4  is an enlarged portion of the drive pulley of  FIG. 2 ; and 
           [0019]      FIG. 5  is an elevational view of a portion of a second embodiment of a conveyor according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    One embodiment of a conveyor according to the invention can be seen in  FIGS. 2-4 . The conveyor  200  comprises an endless belt  202  with an outer smooth, generally continuous carrying surface  204  on one side of the belt, and a plurality of teeth  206  evenly spaced from each other on an opposite side of the belt. The belt  202  is preferably made of a thermoplastic material and may be reinforced. An inner surface  208  extends between adjacent teeth, generally parallel to the carrying surface. The belt  202  wraps partly around a pulley  210  having a plurality of transverse grooves or sheaves  212  equally spaced from each other about the periphery of the pulley. Each tooth  206  has a drive face  214  and each sheave has a drive face  216 . As the pulley  210  rotates in a drive direction denoted by arrow A, each tooth  206  is drawn into a corresponding sheave  212  with the drive face  214  of the tooth facing the drive face  216  of the sheave as the belt  202  wraps around the pulley  210 . The pitch of the belt teeth  206  is less than the pitch of the pulley sheaves  212  along a coincident are C at a given radius from the center of the pulley, and the width of each sheave is greater than the width of each tooth. Thus, as the belt  202  begins to wrap around the pulley  210 , the entering tooth  206 ′ to enter its corresponding sheave  212 ′ will have its drive face  214  spaced from and not engaged with the drive face  216  of the sheave. This condition prevails until it reaches the position of the last tooth  206 ″, which engages the drive face  216  of the corresponding sheave  212 ″ and which engagement pulls the belt  202  in a drive direction denoted by arrow B. 
         [0021]    A tooth  206  can be seen more clearly in  FIG. 3  having a height that is preferably less than the depth of a sheave  212 . The tooth drive face  214  is disposed on a trailing side of the tooth  206 , relative to the drive direction B. As well, the tooth drive face  214  is raked. In other words, the tooth drive face  214  extends at an acute angle α from an imaginary plane  220  that is perpendicular to the carrying surface  204  and to the inner surface  208 . Moreover, the tooth drive face  214  extends away from the drive direction B. 
         [0022]    Similarly, a sheave  212  can be seen more clearly in  FIG. 4 . The sheave drive face  216  is disposed on a trailing side of the sheave  212 , relative to the drive direction A of the pulley  210 , and extends at an acute angle α from an imaginary plane  220  that is perpendicular to the circumferential edge of the pulley  210 . In this case, the drive face  216  extends toward the drive direction A. 
         [0023]    As a consequence, when the sheave drive face  216  engages the tooth drive face  214  of the last tooth  216 ″, their respective orientations tend to cause the sheave  212  to pull the corresponding tooth  206  inwardly toward the center of the pulley  210 . In a situation where the belt  202  effectively a wraps around the pulley  210  as in  FIG. 2 , it may be necessary to assist removal of the driven tooth  206  from the corresponding sheave  212  when the tooth is due to exit the sheave at an exit point  226 . A stationary foot  222  is mounted adjacent to the pulley  210  at the exit point  226  in a position where the tooth  206  will contact the foot. The foot  222  will have a bearing surface  224 , at least a portion of which is, positioned tangent to an imaginary circle formed by the bottom of the teeth  206  as the teeth wrap around the pulley  210 . As the drive face  216  of the sheave  212  in the pulley continues to urge the belt  202  in the drive direction A, the bearing surface  224  of the foot  222  will bar the tooth  206  from continuing to rotate with the pulley, and instead urge the tooth out of the sheave by forcing the tooth drive face  214  to slide relative to the sheave drive face  216  against the tendency to pull the tooth  206  inwardly toward the center of the pulley. Because the foot  222  is stationary, each succeeding driven tooth  206  will likewise be urged out of its corresponding sheave  212 . 
         [0024]    They angle α is preferably about 2° or 3°, although it may vary from application to application and from load to load. Typically it will be in a range from 1° to 5°. The angle should be able to provide 1 or 2 pounds of force in order to draw the corresponding tooth inwardly of the sheave toward the center of the pulley. 
         [0025]    This tendency of the raked tooth and sheave arrangement to grab and pull the belt toward the pulley means that the belt need not wrap very much around the pulley, if at all, thereby further minimizing the possibility of friction and enhancing the direct drive features. It also means that the arrangement is well-suited for other applications for elongated conveyors such as the embodiment shown in  FIG. 5 . This is a type of application that might be used for carrying a constant load such as coal, gravel, or ore. An endless thermoplastic belt  300  that may be reinforced extends over a plurality of drive pulleys  302 , each having an identical configuration to the others, separated from each other across a span  303  and similar to that illustrated in  FIG. 2 . Each pulley  302  may have more or less sheaves  304 . The belt  300  has a plurality of teeth  306  on one side in the manner shown in  FIG. 2 , and as the belt passes over a pulley, the teeth  306  are driven by corresponding sheaves  304  in the same manner explained above in a direction denoted by the arrow C. Preferably, each pulley  302  is a drive pulley in that it separately drives a portion of the belt that it contacts. In this way, the pulleys  302  can pull the belt  300  over a long distance. Optionally, each pulley  302  can be fitted with a foot (not shown) in order to assist disengaging the teeth  306  from the corresponding sheaves  304 . 
         [0026]    Consequently, a long conveyance can be driven by a plurality of smaller motors instead of one large motor. In addition, the belt can be lighter in weight and even stretchable instead of the conventional heavy belts currently in use. It will be apparent that the belt  300  need only be strong enough to support and pull the length of span  303  between adjacent pulleys  302 . 
         [0027]    While the invention has been specifically described in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and the scope of the appended claims should be construed as broadly as the prior art limitation, and the scope of the appended claims should be construed as broadly as the prior art will permit. For example the grooves or sheaves can be on the belt and the teeth can be on the pulley. As well, the leading edges of the sheaves and teeth can be any shape, and need not be raked at the same angle as the drive faces.