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 pulley with circumferentially spaced sheaves at a pulley pitch greater than the belt pitch. The belt is slightly stretchable so that the pulley can drive the endless belt when engaging the teeth within a range of load on the belt. Means are provided to minimize friction between the belt and the drive pulley. Also, a position limiter ensures that the driven tooth stays engaged optimally with the drive sheave.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/814,342, filed Jul. 19, 2007, which is a National Phase Patent Application of PCT/US2006/002013, filed Jan. 19, 2006, which claims the benefit of U.S. Provisional Patent Application No. 60/593,493, filed Jan. 19, 2005, all of which are incorporated herein in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to endless belts for conveyors and, more particularly, to thermoplastic, toothed endless belts driven by pulleys. 
         [0004]    2. Description of the Related Art 
         [0005]    Low tension, direct drive conveyor belts are often 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. 
         [0006]    It is known to use thermoplastic belts with a smooth continuous surface on one side and teeth on the other side adapted to engage grooves or sheaves in a drive pulley, as shown for example in U.S. Pat. No. 5,911,307. But such a thermoplastic belt has characteristics of both a flat, stretchable belt that might be typically driven by a friction pulley, and a toothed belt driven by a drive pulley. These characteristics reflect the two basic ways that a drive pulley can transmit torque to the belt. In a flat belt, torque is transmitted to the belt through friction between the drive pulley surface and the adjacent surface of the belt. The effectiveness of this type of drive is a function of belt tension (both initial pretension and the tension generated due to the product load) and the coefficient of friction of the material of the belt surface and the material of the pulley surface. A friction driven flat belt is subject to contaminants that can affect the coefficient of friction. Moreover, elongated belts typically stretch over time and under load and such stretching can affect its tension. A thermoplastic belt in particular can stretch 3% of its length or more. 
         [0007]    For these reasons, direct drive belts are preferred in such facilities as food handling operations. In an ideal toothed belt, torque is transmitted to the belt through the contact of a face of a tooth or recess on the pulley to a face of a tooth or recess on the belt. But the use of a thermoplastic toothed belt as a direct drive belt with a pulley introduces problems, primarily because of the elasticity of the belt. 
         [0008]    Because a thermoplastic belt stretches under load, the belt teeth may not always mate with the pulley recesses or sheaves as the belt wraps around the pulley. Prior solutions have determined that the tooth pitch of the belt must be less than the 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 width of each sheave in the pulley must exceed the belt tooth width at least by the amount of distance generated by elongating the belt the maximum allowable amount over the span of the belt wrap. 
         [0009]    Yet problems remain in ensuring that the belt teeth stay engaged with the pulley sheaves over the full range of belt elongation and load in the field. Due to the necessary 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 engage the pulley sheaves at any given moment, there is a gap between the face of the belt tooth and the face of the pulley sheave, and that gap progressively increases in size for each successive tooth. The size of these gaps are a function of belt tension, in that each respective gap is largest when the belt has minimum tension and smallest when the belt is at maximum tension. If the belt tension exceeds a predetermined maximum, the entry tooth will no longer sit properly in the pulley sheave and effective drive characteristics will be lost. In other words, the pulley may rotate while the belt slips until a tooth engages again. 
         [0010]    It can be seen that as the exiting tooth disengages from the drive pulley there remains some amount of gap between the following belt tooth and the face of its respective pulley sheave. Therefore, 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 re-engages the new “exit tooth”. For this brief moment no torque is transmitted from the pulley to the belt and thus the belt speed is temporally retarded. 
         [0011]    This motion causes a slight amount of vibration and noise in the system. Vibration increases in frequency as pulley tooth pitch is reduced and/or pulley rotation speed is increased. It may be nearly undetectable in belt applications with a small tooth pitch and a large amount of mass for damping, such as when large product loads approach a predetermined maximum for belt elongation. But for many applications, particularly where loads are light and/or belt speed is slower, the resultant vibration and noise may be unacceptable. 
         [0012]    Nevertheless 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 (max elongation—current elongation). Because of this necessary slip, any characteristics of a flat belt drive will compromise the benefits of direct drive, e.g. friction. Friction between the belt and the pulley will retard slippage and may cause the trailing tooth to miss the pulley sheave altogether. 
         [0013]    Another problem occurs when the belt is under virtually no tension. In some application such as a horizontally positioned conveyor, the weight of the lower span of the belt tends to pull the teeth at the exit point out of the respective pulley sheave. The critical area of belt wrap around the pulley is the short distance between the exit point and one pulley sheave pitch back. If the belt tooth remains engaged through this arc then proper drive will be achieved, but if not, belt teeth will “pop” and the driving dynamics will become uncontrolled. 
       SUMMARY OF THE INVENTION 
       [0014]    The invention solves the aforementioned problems by providing a direct drive conveyor comprising an endless thermoplastic belt and at least one drive pulley. Either the belt or the pulley has teeth and the other has recesses adapted to receive the teeth as the belt wraps around the pulley to an exit point. A position limiter is disposed against the belt near the exit point. In one embodiment, the endless thermoplastic belt has teeth and the drive pulley has recesses. 
         [0015]    The position limiter can be located adjacent to the exit point or prior to the exit point or after the exit point. Preferably, the position limiter is a roller, but it can be an arm bearing against the belt, a scraper bar, or a shoe. As well, the position limiter can be no wider than the drive pulley. 
         [0016]    In another aspect of the invention, means are provided to minimize friction between the belt and the drive pulley. The friction minimizing means can be a friction reducing coating the belt or the drive pulley or both. As well, the friction minimizing means can be minimal contact surfaces between the belt and the drive pulley. 
         [0017]    In another aspect of the invention, the conveyor includes an idler spaced from the drive pulley. The idler is preferably a pulley with teeth or recesses as the case may be, having a pitch between equal to or less than the pitch of the belt without tension. The idler can be a pulley having a driven surface on teeth or in recesses, where each driven surface has a wall with portions at different angles. As well, the idler can be a stationary disk that bears against the belt. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    In the drawings: 
           [0019]      FIG. 1  is a perspective side view of a prior art belt installed between two pulleys; 
           [0020]      FIG. 2  is an enlarged view in elevation of a portion of  FIG. 1 ; 
           [0021]      FIG. 3A  is a view similar to  FIG. 2  showing a conveyor according to the invention; 
           [0022]      FIG. 3B  is a view similar to  FIG. 3  showing another aspect of a conveyor according to the invention; 
           [0023]      FIG. 3C  is an end view of the drive pulley of  FIG. 3A ; 
           [0024]      FIG. 3D  is an enlarged cross sectional view of a portion of the belt in  FIG. 3A ; 
           [0025]      FIG. 4  is a view of a center drive belt system according to the invention; 
           [0026]      FIG. 5  is a fractional side view of a belt and pulley showing an alternative sheave construction according to the invention; 
           [0027]      FIG. 6  is a fractional perspective view of one embodiment of an idler according to the invention; and 
           [0028]      FIG. 7  is a view similar to  FIG. 3  showing another aspect of a conveyor according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Some problems with known thermoplastic direct drive belts are shown in a direct drive conveyor  50  of  FIGS. 1 and 2 . An endless belt  100  is seen in  FIG. 1  in a typical installation between two pulleys  102  and  103 . The pulleys  102 ,  103  are conventional and they can be any of a number of different forms and sizes. Each pulley  102  or  103  has a number of transverse grooves or sheaves  104  spaced around its circumference. Each sheave  104  has a driving face  105  and an opposed, non-driving face  107 . The belt  100  has a plurality of teeth  106  equidistantly spaced from each other on the inside surface  108  of the belt, each tooth having a driving surface  109 . The teeth  106  engage the sheaves  104  of each pulley as the belt wraps around the pulley. At least one pulley, e.g. pulley  102 , is a drive pulley; the other  103  can be an idler or slave pulley. In this configuration, the upper span of the belt will carry loads as the belt  100  travels in the direction of arrow  111 . The belt  100  has an outside surface  110  that is fairly smooth and free of discontinuities, typically made of a thermoplastic material such as Pebax® resin, polyester or polyurethane. 
         [0030]    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 pulley  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. 
         [0031]    Similarly, the pulley pitch  116  is the arc length between the centerlines of adjacent sheaves  104 , measured along the pulley&#39;s pitch circle  118 . The pulley pitch circle  118  in this case corresponds to the belt pitch line  114  as the belt  100  wraps around the pulley  102 . In other words, the pulley pitch circle  118  will have the same radius as the belt pitch line  114  as the belt wraps around the pulley. 
         [0032]    As noted above, the exit tooth  120  will be the drive tooth as its driving surface  109  contacts the driving surface  105  of the sheave  104  that has received the exit tooth. The trailing tooth  122  nests in its corresponding sheave  104 , but there is a gap  124  between the tooth driving surface  109  and the sheave driving surface  105 . Also, the pulley surface  123  between adjacent sheaves may engage the surface  128  of the belt  100  between adjacent teeth  106 . The problems arising from this structure are explained above. Friction between the surface  126  on the pulley and the surface  128  on the belt adds a force component that interferes with the relative movement between the belt and the pulley, possibly causing the teeth not to engage the appropriate sheaves on the pulley. And any friction is enhanced when the belt is placed under tension. The normal and customary response in the field to a belt slipping on the pulley is to increase tension. But this serves only to render the direct drive ineffective. On the other hand, when the belt is under no tension, and the conveyor is horizontal, the weight of the lower belt span tends to pull the driven tooth from its pulley sheave prematurely, adversely affecting the direct drive dynamics. 
         [0033]    One aspect of the invention is shown in  FIGS. 3   a - 3   c  where a direct drive conveyor  129  has all the structure of the prior art system shown in  FIGS. 1 and 2 , plus characteristics of the invention. Accordingly, components in the inventive conveyor that are the same as components in the prior art conveyors of  FIGS. 1 and 2  bear like references. In one aspect of the invention, the pulley and belt are designed to permit minimal friction between them. The surface  130  of the belt between adjacent teeth, and optionally including the teeth  106 , can be coated with a friction reducing material  132 , e.g. polytetrafluoroethylene (PTFE), also known as Teflon®. In addition, or alternatively, the surface  134  between adjacent sheaves on the pulley can be coated with a friction reducing material. As well, the pulley will preferably have minimal surfaces contacting the belt anywhere but on the belt tooth surfaces. For example, the supporting structure such as the surface  136  between adjacent sheaves can be recessed from the perimeter of the pulley as shown in  FIG. 3   b . It can also have a narrower neck  138  to reduce surface contact with the belt (See  FIG. 3   c ). 
         [0034]    Another aspect of the invention pertains primarily to any application where the span exiting the drive pulley tends to pull the driven tooth from the drive sheave. The most common situation would be where the belt is run horizontally and the weight of the return span of the belt exiting the drive pulley tends to form a catenary curve, and consequently tends to urge the driven tooth out of the drive sheave prematurely, i.e., before an optimum exit point  170  as shown in  FIG. 2 . If top dead center  140  is defined as a point of rotation of the pulley where a sheave  104  is centered on a line extending from the center  142  of the pulley, then the optimum exit point  170  is preferably when the drive sheave on the pulley is on a line slightly more than 180° from top dead center in the direction of rotation. As shown in  FIGS. 3   a  and  3   b , a position limiter  200  is disposed near the exit point  170 , i.e., the point where the exit tooth  120  of the belt optimally leaves the corresponding sheave of the pulley. One preferred location, as shown in  FIG. 3   b , places the position limiter  200  adjacent the pulley at the exit point  170  of the belt tooth. One alternative location, as shown in  FIG. 3   a , includes a position limiter  200 ′ just past the exit point  170 . In this case, the position limiter deflects the belt enough to ensure that the tooth does not prematurely exit the sheave. Other alternative locations, shown in phantom) are at  200 ″ immediately prior to the exit point  170  and  200 ′ at the next succeeding tooth  122 . Preferably, the position limiter  200  will be disposed in such a manner that the belt can not lift off the pulley more than 25% of the tooth height until the exit point  170 . 
         [0035]    The position limiter  200  can be a belt-width roller, as shown, or it can be multiple rollers, such as a pair with one on each edge of the belt. Alternatively the position limiter can be one or more arms or points bearing against the belt, preferably with friction reducing wear pads. Further, the position limiter can be a scraper bar bearing against the belt that will serve two functions, to with: maintaining the exit tooth within the sheave of the pulley and cleaning the belt as it exits the pulley. The position limiter  200  need not extend across the belt. It need only be positioned to maintain the belt against the pulley or pulleys until the driven tooth is timely released from the respective sheave. 
         [0036]    An alternative embodiment of a direct drive thermoplastic belt conveyor, according to the invention, is shown in  FIG. 4 . The system has a center drive pulley  202  and two idler pulleys  204 ,  206  with an endless belt  208 . In accordance with the invention, two position limiters  210 ,  212  are used with the drive pulley  202 . One limiter  210  is placed near the entry point  214  where the belt tooth enters engagement with the pulley sheave. The other limiter  212  is placed near the exit point  216 . Preferably, the belt wrap is minimized such that only three teeth are wrapped at any time. 
         [0037]    A center drive such as this solves the problems associated with any “flat belt drive” component of the system, such as might be caused by friction between the belt an the pulley for example. As explained above, friction can cause the belt entry tooth to advance relative to the pulley tooth and thus “skip”. This might occur, for example, when the friction force between the belt and the pulley generates a higher speed component than the driving force of the tooth drive surface against the pulley drive surface. Minimizing the amount of wrap also tends to reduce the opportunity for friction between the belt and the pulley. 
         [0038]    It has been found that if any of the pulleys are not drive pulleys, the speed of the idler pulley can cause problems. The drive pulley is generally traveling at a greater speed than the belt speed. If the same geometry was used for the idler pulley as the drive pulley then, for proper tooth engagement, the idler pulley would have to travel at the same speed as the drive pulley. But the idler pulley cannot travel any faster than the belt, inasmuch as the belt drives the idler pulley. Therefore the idler pulley must have a different pitch than the drive pulley (different geometry). Preferably, the idler pulley pitch will be less than or equal to the pitch of an un-tensioned belt. Consequently, as the belt pitch changes with elongation, the idler pulley will be compelled to go slower than the belt. Just as in the drive pulley, the width of the sheaves must exceed the belt tooth width such that there is enough gap to allow for the added length of belt that will occur at the maximum belt tension over the span of belt wrap. 
         [0039]    The idler pulley will primarily be driven as by a flat belt because of its low drag characteristics. This will cause the entry tooth on an elongated belt to not ideally engage a sheave on the idler pulley. To overcome this problem, the coefficient of friction must be minimized as explained earlier. In addition, the angle of the tooth contact face can be designed such that at maximum elongation of the belt, the tip of the belt tooth will contact the pulley sheave driving surface at some point. This will allow the belt tooth to slowly engage the pulley sheave while slowing the idler pulley down until the proper engagement is made. An example is shown in  FIG. 5  where an idler pulley  300  is driven by a belt  302 . Sheaves  304  in the pulley  300  are driven by teeth  306  on the belt  302 . To ensure that each tooth  306  properly engages the corresponding sheave  304 , the side of the sheave has two walls at different angles. The lower wall portion  308  is at a steeper angle than the upper wall portion  310 . Preferably, the upper wall portion is at an angle wider than the angle of the belt tooth  306 . This works since the added distance that must be accommodated is only generated over the span of one tooth pitch for the previous tooth will have already engaged the idler. 
         [0040]    Another option shown in  FIG. 6  is for an idler  320  to comprise a stationary disk  322  or arm that the belt simply slides against. Preferably, the portion of disk  322  bearing against the belt is covered with a friction reducing coating as set forth above. While this structure may increases friction somewhat between the belt and the idler, it is of little consequence since there is no toothed drive between the belt and the idler. To accommodate these disks longitudinal grooves  324  are provided through the teeth on the toothed side of the belt at set increments to enable the belt to move smoothly over the stationary disks. Using these disks eliminates the complications of idler pulley geometry as well as functioning as effective tracking devices. Further, by being stationary the belt will not have a tendency to “climb up” these disks as it would if the smooth pulleys were rotating. 
         [0041]    It is known for belts to sometimes be fitted with cleats extending upwardly from the smooth surface to help retain or separate objects on the belt. In such an application, the invention contemplates using the cleats to advantage as a position limiter.  FIG. 7  illustrates one such application. An endless thermoplastic belt  400  has teeth  402  on one side and cleats  404  on the other side. The belt teeth  402  are sequentially driven by recesses or sheaves  406  on a drive pulley  408 . A position limiter  410  comprises a shoe  412  having an inner curved surface  414 . At least a portion of the curved surface is disposed near the optimum exit point  416  so that the shoe bears against the cleats, which, in turn, urge the belt against the pulley  408  to keep the driven tooth  402  engaged to the exit point. 
         [0042]    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. For example, instead of teeth on the belt and sheaves on the pulley, the belt can have holes or recesses and the pulley can have teeth or pins in the manner of a sprocket to engage the holes or recesses in the belt, and the principles of the present invention equally apply.