Abstract:
A thermoplastic endless belt has a smooth outer surface substantially free of discontinuities and an inner surface with a plurality of teeth at a given belt pitch. The teeth are adapted to engage a sprocket with circumferentially spaced sheaves at a sprocket pitch greater than the belt pitch. The belt is slightly stretchable so that the sprocket can drive the endless belt when engaging the teeth within a range of load on the belt. The belt is preferred for use in conveyors in food processing industries where the smooth outer surface can transport food items and is easier to clean and keep free of impurities.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Application No. 60/319,133 filed Mar. 5, 2002, International Application No. PCT/US03/03029 filed Feb. 3, 2003, and is a divisional of U.S. application Ser. No. 10/505,063 filed Aug. 18, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to endless belts for conveyors and, more particularly, to thermoplastic endless belts driven by sprockets.  
         [0004]     2. Description of the Related Art  
         [0005]     Low tension, direct drive conveyor belts are typically used in situations where hygiene and cleanliness are critically important. For example, in food processing plants such as those that process meat products for human consumption, low tension, direct drive belt conveyors are used to transport items. Sanitation is critically important and, therefore, the endless belts used in such conveyors are conventionally made of materials that can be hygienically cleaned, such as thermoplastics or stainless steel.  
         [0006]     Known belts are typically formed of interlocking links having teeth that are adapted to engage drive sprockets. One of the problems with such belts is that food particles can become lodged in the joints of the interconnecting links. Consequently, cleaning the belts can be difficult and may require removing the belt from the conveyor system for special cleaning operations.  
         [0007]     One solution to this problem is the use of flexible thermoplastic belts without interlocking links such as that disclosed in U.S. Pat. No. 5,697,491. Such belts having a smooth continuous surface (sometimes called “homogeneous belts”) are driven by V-guides wherein a radial groove in a drive pulley engages a longitudinal rib on the underside of the belt. One of the problems with such belts is that grease and oil from the food items can migrate to the groove or to the rib, which causes a loss of friction between the pulley and the belt. Consequently the driving force becomes unstable and unreliable. Moreover, such belts are under tension to ensure that the pulley imparts enough driving force. This tensioning raises other issues beyond slippage due to oils and contaminants. A thermoplastic belt under tension will stretch, which may require adjustment of the tension from time to time. In addition, there are additional costs associated with ensuring that the conveyor frame be sufficiently strong enough to handle the normal stresses of the pretensioned belt plus additional stresses caused by loading the belt.  
         [0008]     It is known to provide a drive sprocket or drum with transverse grooves that are complementary in shape to teeth on a flexible conveyor belt, as shown for example in U.S. Pat. No. 4,170,281. However, the belt is formed from interlocking links and the belt is still under tension. The problems associated with interlocking links and pretensioning remains.  
         [0009]     Another solution is disclosed in U.S. Pat. No. 5,911,307 where a timing belt is added to a homogeneous belt to engage a drive sprocket. As a result, reliance upon friction for motion is minimized, and the belt need not be under tension. There are some remaining problems, however. Assembling a timing belt to a homogeneous belt is costly and the bonding or adhering process is critical. Failure of the bond increases the risk of contamination and total belt failure.  
       SUMMARY OF THE INVENTION  
       [0010]     The invention solves these and other problems by starting with a conveyor belt segment having an outer surface substantially free of discontinuities and an inner surface with a plurality of teeth extending therefrom at a given belt pitch and opposite ends. The belt segment is stretchable through its length, and the teeth are integrally formed with the conveyor belt segment. In this manner, the belt segment can be made to a selected length and the ends spliced together to form an endless conveyor belt of predetermined size.  
         [0011]     Preferably, the belt segment is stretchable within a range of up to 3% of its total length. As well, the teeth can be formed of urethane while the rest of the belt is formed of copolyester. The teeth can also be treated with an anti-bacterial agent.  
         [0012]     Preferably, the teeth are continuously formed, sequentially, with the conveyor belt segment, as by extrusion. The teeth can extend from one edge of the belt to the other. As well, the belt segment can be unreinforced.  
         [0013]     In another aspect of the invention, a conveyor includes a drive sprocket having a plurality of sheaves spaced from each other at a given sprocket pitch, an idler spaced from the sprocket; and an endless, unreinforced, substantially homogeneous thermoplastic belt, elastically stretchable through its length, and wrapped around the sprocket and the idler. The belt has a plurality of inwardly facing teeth, each tooth sized to be loosely received in a sprocket sheave. Further, the belt has a belt pitch less than the sprocket pitch below the maximum elasticity of the belt. The belt teeth are integrally formed with the remainder of the belt. There is minimal friction between the belt and the sprocket, so that when the sprocket is caused to rotate, the belt will be driven by the engagement of at least one tooth in a sheave. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     In the drawings:  
         [0015]      FIG. 1  is a perspective side view of a belt according to the invention installed between two sprockets;  
         [0016]      FIG. 2  is an enlarged view in elevation of a portion of  FIG. 1 ;  
         [0017]      FIG. 3  is a view similar to  FIG. 2  with the belt under load;  
         [0018]      FIG. 4  is a side elevational view of a portion of the belt, partially in cross-section, shown in  FIGS. 1-3 ;  
         [0019]      FIG. 5  is a perspective side view of a belt according to the invention installed between two sprockets of a different configuration than  FIG. 1 ;  
         [0020]      FIG. 6  is an enlarged view of a portion of  FIG. 5 ; and  
         [0021]      FIG. 7  is diagrammatic view illustrating a method of making a belt according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     An endless belt  100  according to the invention is seen in  FIG. 1  in a typical installation between two sprockets  102  and  103 . The sprockets  102 ,  103  are conventional and they can be any of a number of different forms and sizes. One sprocket  102  is a drive sprocket, the other  103  being an idler or slave sprocket. In this configuration, the upper span  105  of the belt will travel in the direction of arrow  107 . Each sprocket  102  or  103  has a number of transverse grooves or sheaves  104  spaced around its circumference. The belt  100  has a plurality of teeth  106  equidistantly spaced from each other on the inside surface  108  of the belt. The teeth  106  engage the sheaves  104  of each sprocket. It will be understood that any given installation may have one, two, or more sprockets for a single endless belt. Also, for any given installation, one or more of the sprockets may be a drive sprocket, as for example sprocket  102 , with the other sprocket or sprockets being idler sprockets. It will be understood that there need only be enough tension on the endless belt  100  to keep at least one of its teeth  106  engaged in a sheave  104  of the drive sprocket  102 .  
         [0023]     The belt  100  has an outside surface  110  that is fairly smooth and free of discontinuities. Preferably, the belt  100  is made of a thermoplastic material such as Pebax® resin, polyester or polyurethane. The outside surface  110  on the upper span  105  is the carrying surface for transport of items. Because it is smooth and free of discontinuities, there is no place for particles or contaminants to lodge. Moreover, the belt  100  can be cleaned in situ, without the need to remove it from the installation.  
         [0024]     Greater detail about the structure of the belt  100  is shown in  FIGS. 2 and 3 . A combination of material selection and construction enables the belt to stretch within a range of 0-3% of its total length under a load ranging from 0-100 pounds per inch of width (nearly 18 Kg per cm). The belt  100  has a pitch  112  defined as the distance between the centerlines of adjacent teeth  106 . The belt pitch  112  is measured along a belt pitch line  114 , which corresponds to the neutral bending axis of the belt. As the belt  100  bends around the sprocket  102 , the neutral bending axis is that imaginary plane on one side of which the belt material is under compression and on the other side of which the belt material is under tension.  
         [0025]     Similarly, the sprocket pitch  116  is the arc length between the centerlines of adjacent sheaves  104 , measured along the sprocket&#39;s pitch circle  118 . The sprocket pitch circle  118  in this case corresponds to the belt pitch line  114  as the belt  100  moves around the sprocket  102 . In other words, the sprocket pitch circle  118  will have the same radius as the belt pitch line  114  as the belt goes around the sprocket. For a thermoplastic belt, the area of greatest stress on the belt  100  occurs at zone  120 , and the area of least stress occurs at zone  122  just as the belt is released from the drive sprocket  102 .  
         [0026]     Looking now also at  FIG. 4 , it can be seen that each tooth  106  extends upwardly from a surrounding inner surface  124  of the belt  100 . Preferably, the belt  100  has a thickness between adjacent teeth  106  in a range of approximately 3 to 4 mm, although it will be understood that the actual thickness will depend upon a given application. The invention is not limited to a particular thickness of the belt. For this embodiment, each tooth  106  stands approximately 4 mm above the inner surface  124 . Each tooth  106  has tapered sidewalls  126 , each sidewall preferably tapering from the inner surface  124  at an angle of about 72.5 degrees. Each tooth will thus have a width at its base wider than that at the tip. Each sheave  104  on the sprocket  102  is wider than the corresponding tooth  106  on the belt  100 , preferably on the order of 11 to 15 percent, and is tapered with drive walls  128 . Thus, for example, if the widest part of the tooth  106  is 8 mm, the widest part of a sheave  104  might be 11.25 mm, permitting a belt stretch of up to 3% where the teeth  106  can still be received in the sheaves  104 . If the widest part of a sheave is 10 mm, the widest part of a tooth can be 7 mm in order to permit belt stretch of up to 3%. It is recognized that the belt will not stretch evenly, i.e., it will stretch more between the teeth than including the teeth. This is because the belt is thinner between the teeth than at the teeth. Nevertheless, stretch here is measured overall, regardless of which portion of the belt is actually stretching under load. Also, the depth of each sheave  104  is greater than the height of each tooth  106  wherein each drive wall  128  is longer than a corresponding sidewall  126  so that the tooth will not bottom out in the sheave.  
         [0027]     The belt pitch  112  when the belt  100  is at rest is less than the sprocket pitch  116 . Thus, as illustrated in  FIG. 2 , a belt  100  under no load will effectively be driven in the stress zone  120  by the drive wall  128  of a single sheave  104  acting against sidewall  126  of a single tooth  106  received within the sheave. On the other hand, as illustrated in  FIG. 3 , a belt  100  under load P tends to stretch so that the belt pitch  112 , especially within the stress zone  120 , more nearly equals the sprocket pitch  116 . In this case, more teeth  106  are engaged by corresponding sheaves  104 . Under maximum stress, the belt pitch  112  will equal the sprocket pitch  116  as the belt  100  is pulled around the sprocket  102 . In the embodiment illustrated in  FIG. 3  wherein the sprocket  102  has ten sheaves  104 , the maximum number of sheaves that will drive a corresponding tooth, at least in whole or in part, is six. Since the teeth  106  are smaller than the sheaves  104 , and since the belt pitch  112  is smaller than the sprocket pitch  116  (below maximum load), the elasticity of the belt will permit anywhere from one to six teeth on the belt to be driven by the sprocket  102 . The higher the load on the belt, the more teeth will be engaged by the sprocket.  
         [0028]     Looking now at  FIGS. 5 and 6 , it can be seen that a belt  150  according to the invention can be used with existing sprockets. Here, the belt  150  is shown installed on sprockets  152  and  153  of the type that can be obtained from Intralox, Inc. Sprocket  152  may be a drive sprocket, in which case sprocket  153  will be the slave sprocket. Each sprocket  152 ,  153  has a number of transverse grooves or sheaves  154  adapted to receive teeth  156  in driving engagement as described above. The relative dimensions of components in this embodiment will be as described above for the embodiment of  FIGS. 2 and 3 . In addition, there may be tracking tabs  158  adapted to engage corresponding slots  160  in the belt  150  to maintain proper alignment of the belt.  
         [0029]     Preferably, the teeth  106 ,  156  will be integral with the belt  100 ,  150 . A method of making an endless belt according to the invention is shown in  FIG. 7 . An appropriate thermoplastic material such as polyester or polyurethane is placed in an extruder  170 . Using conventional extrusion methods, a flat ribbon  172  of thermoplastic material is extruded from the extrusion nozzle  174 . While the flat ribbon  172  is still soft and malleable, it is passed over a profile drum  176 . The profile drum  176  has on its outer surface a plurality of grooves  178  corresponding in shape to the teeth  106 ,  156  to be formed. A compression drum or belt  180  presses against the flat ribbon  172 , compressing the ribbon, and forcing a portion of the flow into the grooves  178 . Upon exiting the profile drum  176 , the formed ribbon  182  passes through a cooling station  184  where it hardens. The temperature of the flat ribbon  172 , the pressure on the ribbon  172  between the compression drum  180  and the profile drum  176 , and the cooling time are all established and determined to anticipate the final thickness of the belt, the dimensions of each tooth, and the belt pitch. In this respect, it will be understood that there is a predetermined amount of shrinkage of the formed ribbon  182  as it cools. Upon cooling, the formed belt is collected and stored until it is needed to form an endless belt.  
         [0030]     Another method of manufacturing the belt is to start with a homogeneous belt of approximately 7 to 8 mm in thickness, and machine away material between adjacent teeth  106 ,  156  to a depth of about 3 to 4 mm. This method necessarily generates scrap.  
         [0031]     Another method of manufacturing the belt is to start with a homogeneous belt approximately 3-4 mm in thickness, and injection mold teeth at an appropriate belt pitch onto one surface of the belt. In this method, the teeth can be different material. For example, the belt can be formed of polyester such as COPE, with the teeth being formed of a urethane.  
         [0032]     Another method of manufacturing the belt is to start with a homogeneous belt approximately 3-4 mm in thickness, and friction mold teeth at an appropriate belt pitch onto one surface of the belt. In this method, the teeth can be different material. For example, the belt can be formed of polyester such as COPE, with the teeth being formed of a urethane. It has been found that applying a 160 Hz orbital motion of a polyester tooth on a polyester belt for three seconds creates enough softening for the tooth to bond to the belt.  
         [0033]     It will be understood that certain variations and modifications can be made without departing from the scope of the invention. For example, the length of each tooth need not extend to the edge of the belt as illustrated. A narrow rank of teeth may be sufficient. Further, a narrow belt having a rank of teeth can be preformed and adhered or bonded to an inner surface of a larger belt. Moreover, if desired, coloring pigments and/or antibacterial agents can be added to the thermoplastic prior to extrusion.  
         [0034]     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 will permit.