Patent Publication Number: US-2011077115-A1

Title: System and method for belt tensioning

Description:
CROSS-REFERENCE TO OTHER APPLICATION 
     This application claims the benefit of the filing date of U.S. Provisional Patent Application 61/246,719, filed Sep. 29, 2009, and U.S. Provisional Patent Application 61/246,724, filed Sep. 29, 2009, both of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure is directed, in general, to tensioning of belts in machinery. 
     BACKGROUND OF THE DISCLOSURE 
     In moving belt systems it is important that belt tension be maintained in a desired range. If belt tension is too low, the belt may slip over pulleys. If belt tension is too high, excessive stress may be placed on pulleys, bearing and the belt. 
     SUMMARY OF THE DISCLOSURE 
     Various disclosed embodiments include a system and method for tensioning a belt. An apparatus includes a fixed roller, rotatably coupled to a structure; a tensioning roller, rotatably coupled to a bracket; and a biasing device coupled to the structure and to the bracket. The bracket and the tensioning roller are coupled to the structure only by the biasing device and a belt passing around at least a part of the fixed roller and at least a part of the tensioning roller. 
     The foregoing has outlined rather broadly the features and technical advantages of the present disclosure so that those skilled in the art may better understand the detailed description that follows. Additional features and advantages of the disclosure will be described hereinafter that form the subject of the claims. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the disclosure in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases. While some terms may include a wide variety of embodiments, the appended claims may expressly limit these terms to specific embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
         FIG. 1  depicts a top view of a belt tensioning system according to a first embodiment; 
         FIG. 2  depicts a cutaway view of the belt tensioning system of  FIG. 1 ; 
         FIG. 3  depicts a top view of a belt tensioning system according to a second embodiment; 
         FIG. 4  depicts a cutaway view of the belt tensioning system of  FIG. 3 ; 
         FIG. 5  depicts a top view of a belt tensioning system according to a third embodiment; 
         FIG. 6  depicts a cutaway view of the belt tensioning system of  FIG. 5 ; and 
         FIG. 7  depicts a top view of a conveyor belt system according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 7 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged device. The numerous innovative teachings of the present application will be described with reference to exemplary non-limiting embodiments. 
     In a mechanical system that employs a belt—such as a conveyor belt or a power drive belt—belt tension is maintained within a desired range of value to ensure that the mechanical system does not malfunction. For example, if the belt tension drops too low, the belt may slip over a drive pulley, resulting in erratic motion of the belt. If the belt tension rises too high, the belt may place excessive forces on pulleys or rollers in the mechanism, causing the pulleys to bind and stop rolling. 
     Various methods and systems have been developed with the intention of applying a desired range of tensions to a belt. A tensioning roller may be mounted for motion relative to fixed rollers in the system and biased by a force away from the fixed rollers in order to tension a belt that passes over both the tensioning and fixed rollers. A tensioning roller may be positioned in a straight segment of the belt path and biased by a force in a direction orthogonal to the belt path in order to tension the belt. Typically, such a biasing force is supplied by a spring or a suspended dead weight and operates directly on the tensioning roller or an arm or plate to which the roller mounts. Such a tensioning roller is typically mounted to a base plate of the belt system by an arm, linear bearing, or other mechanism that supplies the biasing force and constrains motion of the tensioning roller. Such tensioning mechanisms may be complex, expensive, bulky, or affected by dynamic forces of the moving belt. 
       FIG. 1  depicts a top view of a belt tensioning system  100  according to a first embodiment of the disclosure. A belt  102  passes over fixed rollers  104   a  and  104   b  and around a floating tensioning roller  106 . The tensioning roller  106  is typically positioned just before a belt drive roller in the path of the belt  102 —for example, fixed roller  104   a  or  104   b , depending upon the direction of travel of the belt  102 . The belt  102  extends from the tensioning system  100  into the rest of a larger mechanical system and may be travelling in either direction through the system. 
     The floating roller  106  is rotatably mounted to a bracket  108 , which is used to move the roller  106  to control the tension of the belt  102 . As shown in  FIG. 1 , motion of the roller  106  in a leftward direction increases the tension of the belt  102  and motion in the rightward direction decreases the tension in the belt  102 . The bracket  108  is mechanically coupled to one end of a tensioning cable  110 , which passes over a portion of the surface of a fixed capstan  112  and is coupled at its other end to one end of a biasing device  114 . The other end of the biasing device  114  is coupled to a fixed location  116 . The biasing device  114  may be an extension spring, a constant force spring, a torsion spring, a suspended dead weight, or other suitable mechanism for applying a force to the tensioning cable  110 . 
     The biasing device  114  applies a force F 1  to the tensioning cable  110 , which operates to increase the tension in the belt  102 . The bracket  108 , acting under the tension of the belt  102 , applies an opposing force F 2  to the cable  110 . When the force F 1  exceeds the force F 2  by an amount sufficient to overcome the capstan effect arising from the friction of the tensioning cable  110  passing around the fixed capstan  112 , the tensioning cable  110  moves the bracket  108  and tensioning roller  106  in a direction to increase the tension in the belt  102  (to the left in  FIG. 1 ). Conversely, when the force F 2  exceeds the force F 1  by an amount sufficient to overcome the capstan effect, the tensioning cable  110  moves the bracket  108  and tensioning roller  106  in a direction to decrease the tension in the belt  102  (to the right in  FIG. 1 ). 
       FIG. 2  depicts a cutaway view of the belt tensioning system  100  along the line AA of  FIG. 1 . The capstan  112  is fixedly mounted to a base plate  206  or other structure. The tensioning roller  106  is rotatably mounted to the bracket  108  by an axle  202 . The fixed roller  104   b  is rotatably mounted to the base plate  206  by an axle  204 . Because the bracket  108  is free to move relative to the base plate  206 , the tensioning roller  106  is also free to move relative to the base plate  206 , thereby increasing or decreasing tension on the belt  102 . 
     The bracket  108  and the tensioning roller  106  are coupled to the base plate  206  only by the cable  110  and the belt  102 . Where the rollers  102 ,  104   a  and  104   b  are crowned rollers, the belt  102  is constrained from moving in the vertical direction in  FIG. 2  and the belt  102  acts as a web element of the structure  100  to support the roller  106  and the bracket  108 . 
     The capstan effect of the tensioning cable  110  passing around the capstan  112  may be expressed as: 
         F   high   =F   low   *e   μΦ , 
     where e is the mathematical constant referred to as Euler&#39;s number, μ is the coefficient of friction between the tensioning cable  110  and the capstan  112 , Φ is the number of turns of the tensioning cable  110  around the capstan  112  in radians, F high  is the larger of F 1  and F 2 , and F low  is the smaller of F 1  and F 2 . Where both the tensioning cable  110  and the capstan  112  are steel (as in the belt tensioning system  100 ), the value of μ is 0.8. Where the tensioning cable  110  wraps one-quarter turn around the capstan  112  (as in the belt tensioning system  100 ), the value of Φ is approximately 1.57. Thus for the tensioning cable  110  and the capstan  112  of the belt tensioning system  100 , the value of e μΦ  is approximately 3.5 and F high =F low *3.5. That is, if F high  exceeds F low , by a factor of 3.5, the tensioning cable  110  will move around the capstan  112  in the direction of F high . However, if F high  does not exceed F low  by at least a factor of 3.5, the tensioning cable  110  will not move around the capstan  112 . 
     The tension of the belt  102  is depicted by the arrows labeled T in  FIG. 1 . The belt applies the force T to both sides of the tensioning roller  106 , resulting in a force 2*T on the cable  110 . The belt tension T is typically in a range from T low  to T nominal . T low  typically occurs at startup, because of the position of the tensioning roller  106  in the path of the belt  102 . The value of T low  is typically established empirically. T nominate  is the nominal operating tension of the belt  102 . The value of T nominal  is set by the designer of the system in which the belt  102  is used. Factors in the determination of T nominal  may include belt loading on roller bearings, limits on belt sag between rollers in load-bearing portions of a belt system, minimum drive belt tension required to transfer torque from a drive roller to a system being driven by the belt, and other factors. 
     In the belt tensioning system  100 , a nominal value for the spring force, F 1 , is calculated as: 
         F   1 =2 *T   nominal   *e   μΦ   −c,    
     where T nominal  and e μΦ  are as described above and c is derived empirically to ensure that the belt  102  is not over-tensioned when T approaches T low . 
     In operation, when the belt  102  is powered off and T approaches the value T low , F 2  may fall below F 1  by more than the capstan effect factor, e μΦ , with the result that the tensioning cable  110  slips in the direction of F 1 . This slippage increases F 2  until F 2 , aided by the capstan effect, is able to resist further slippage. In this way, the belt tensioning system  100  operates to prevent T from dropping below a specified minimum level. Subsequently, when the belt  102  is powered up and T rises from T low  to the value T nominal , the tensioning cable  110  does not slip unless F 2  exceeds F 1  by the capstan effect factor: i.e., unless T reaches 3.5*T nominal . Such a high belt tension is not likely to occur in normal operation of a system where the belt tensioning system  100  is used. 
     Thus, the tensioning cable  110  may initially slip around the capstan  112  to adapt to a belt tension near T low . In this way, the belt tensioning system  100  establishes a minimum belt tension in the belt  102 . However, once this initial adaptation has occurred, as the tension in the belt  102  rises, the belt tensioning system  100  remains rigid under the expected dynamic tension loads of the belt  102 —that is, as long as the tension T remains within the expected range of T low  to T nominal . The belt tensioning system  100  has a flexible geometry that may be readily adapted to fir around other components of the belt-driven system. Furthermore, the belt tensioning system  100  has a smaller footprint, lower cost, and lower maintenance requirements than many other belt tensioning systems. The tensioning roller  106  is mounted to the floating bracket  108 , rather than being mounted by a more complex and more expensive articulated mechanism to the base plate  206 , as in some other belt tensioning mechanisms. 
       FIG. 3  depicts a top view of a belt tensioning system  300  according to a second embodiment. Like the belt tensioning system  100 , the belt tensioning system  300  utilizes the capstan effect of a cable wrapped around a capstan to remain rigid under the expected dynamic tensioning loads of the belt being tensioned. However, the belt tensioning system  300  provides initial system adjustment to low belt tension in a different way than the belt tensioning system  100 . 
     Similar to the belt tensioning system  100 , in the belt tensioning system  300  a belt  302  passes over fixed rollers  304   a  and  304   b  and around a floating tensioning roller  306 . The tensioning roller  306  is typically positioned just before a belt drive roller in the path of the belt  302 —for example, fixed roller  304   a  or  304   b , depending upon the direction of travel of the belt  302 . The belt  302  extends from the tensioning system  300  into the rest of a larger mechanical system and may be travelling in either direction through the system. 
     The floating roller  306  is rotatably mounted to a bracket  308 , which is used to move the roller  306  to control the tension of the belt  302 . As shown in  FIG. 3 , motion of the roller  306  in a leftward direction increases the tension of the belt  302  and motion in the rightward direction decreases the tension in the belt  302 . The bracket  308  is mechanically coupled to one end of a tensioning cable  310 , which wraps twice around the surface of a one-way clutch roller  312  and is coupled at its other end to one end of a biasing device  314 . The other end of the biasing device  314  is coupled to a fixed location  316 . The biasing device  314  may be an extension spring, a constant force spring, a torsion spring, a suspended dead weight, or other suitable mechanism for applying a force to the cable  310 . 
     Unlike the capstan  112  of the belt tensioning system  100 , the one-way clutch roller  312  operates as a roller when the tensioning cable  310  is moving in the direction indicated by the arrow labeled F 1  in FIG.  3 —that is, in the counter-clockwise direction shown by the arrows on the roller  312 . However, because of the action of its one-way clutch mechanism, the roller  312  acts as a capstan to resist motion of the tensioning cable  310  in the direction indicated by the arrow labeled F 2 . 
     The biasing device  314  applies a force F 1  to the tensioning cable  310 , which operates to increase the tension in the belt  302 . The bracket  308 , acting under the tension of the belt  302 , applies an opposing force F 2  to the tensioning cable  310 . When the force F 1  exceeds the force F 2 , the one-way clutch roller  312  rotates in the counter-clockwise direction, allowing the tensioning cable  310  to move the bracket  308  and tensioning roller  306  in a direction to increase the tension in the belt  302  (to the left in  FIG. 3 ). 
     However, because the roller  312  does not rotate in the clockwise direction, the roller acts as a capstan to resist motion of the tensioning cable  310  in the direction of F 2 . Thus, the force F 2  must exceed the force F 1  by an amount sufficient to overcome the capstan effect, in order for the tensioning cable  310 , the bracket  308 , and the tensioning roller  306  to move in the direction of F 2 , decreasing the tension in the belt  302 . 
       FIG. 4  depicts a cutaway view of the belt tensioning system  300  along the line BB of  FIG. 3 . The one-way clutch roller  312  is mounted to a base plate  406  by a stanchion  408 . The tensioning roller  306  is rotatably mounted to the bracket  308  by an axle  402 . The fixed roller  304   b  is rotatably mounted to the base plate  406  by an axle  404 . Because the bracket  308  is free to move relative to the base plate  406 , the tensioning roller  306  is also free to move relative to the base plate  406 , thereby increasing or decreasing tension on the belt  302 . 
     The bracket  308  and the tensioning roller  306  are coupled to the base plate  406  only by the cable  310  and the belt  302 . Where the rollers  302 ,  304   a  and  304   b  are crowned rollers, the belt  302  is constrained from moving in the vertical direction in  FIG. 4  and the belt  302  acts as a web element of the structure  300  to support the roller  306  and the bracket  308 . 
     As described for the capstan  112 , the capstan effect of the tensioning cable  310  passing around the one-way clutch roller  312  may be expressed as F high =F low *e μΦ . Because the roller  312  rotates in the counter-clockwise direction, the capstan effect only applies to motion in the direction of F 2  and may be expressed as F 2 =F 1 *e μΦ . That is, F 2  must exceed F 1  by the factor e μΦ  for the tensioning cable  310  to move in the direction of F 2 . 
     In the belt tensioning system  300 , both the tensioning cable  310  and the capstan  312  are steel, and the value of μ is 0.8. Because the cable  310  wraps two full turns around the roller  312 , the value of Φ is approximately 12.6. Thus, for the tensioning cable  310  and the roller  312 , the value of e μΦ  approximately 23,000 and F 2 =F 1 *23,000. That is, if F 2  exceeds F 1  by a factor of 23,000, the tensioning cable  310  will move around the capstan  312  in the direction of F 2 . However, if F 2  does not exceed F 1  by at least a factor of 23,000, the tensioning cable  310  will not move around the roller  312  in the direction of F 2 . 
     As described for the belt tensioning system  100 , in the belt tensioning system  200 , the tension T of the belt  102  is typically in a range from T low  to T nominal . T low  typically occurs at startup and typically is established empirically. T nominal  is the nominal operating tension of the belt  102  and is set by the designer of the system in which the belt  102  is used. 
     In the belt tensioning system  300 , a nominal value for the spring force, F 1 , is determined by: 
         F   1 =2 *T   low , 
     where T low  is as described above. 
     When T is at a low value, the biasing device  314  pulls the tensioning cable  310  counter-clockwise around the rotating one-way clutch roller  312 , and the force F 2  applied to the tensioning roller  306  is: 
         F   2   =F   1 =2 *T   low . 
     In this way, the belt tensioning system  300  operates to prevent T from dropping below a specified minimum level. 
     However, as T rises above T low  (and F 2  rises above 2*T low ) and the bracket  308  attempts to pull the cable  310  clockwise around the one-way clutch roller  312 , the roller acts as a capstan, preventing the tensioning cable  310  from slipping around the roller  312  in the direction of F 2  unless F 2  rises above F 1  by a factor of 23,000. 
     Thus, under normal running conditions, as T rises to T nominal , the one-way clutch roller  312  resists turning and the force F 2  applied to the tensioning roller  306  is: 
         F   2   =F   1 +2*( T   nominal   −T   low ), or 
         F   2 =2 *T   nominal . 
     That is, as T varies between T low  and T nominal , F 2  varies between 2*T low  and 2*T nominal , because the one-way clutch roller  312  resists turning. 
     Thus, the tensioning cable  310  may be pulled initially around the rotating one-way clutch roller  312  to adapt to a belt tension near T low . However, once this initial adaptation has occurred, the belt tensioning system  300  remains rigid under the expected dynamic tension loads of the belt  302  within the expected range of range of values for T. Like the belt tensioning system  100 , the belt tensioning system  300  has a flexible geometry that may be readily adapted to fir around other components of the belt-driven system. The belt tensioning system  300  also has a smaller footprint, lower cost, and lower maintenance requirements than many other belt tensioning systems. The tensioning roller  312  is mounted to the floating bracket  308 , rather than being mounted by a more complex and more expensive articulated mechanism to the base plate  406 , as in some other belt tensioning mechanisms. 
       FIG. 5  depicts a top view of a belt tensioning system  500  according to a third embodiment. Unlike the belt tensioning systems  100  and  300 , no capstan  112  or one-way clutch roller  312  is used in the belt tensioning system  500 . Thus, the belt tensioning system  500  responds to dynamic tensioning loads of the belt being tensioned. 
     Similar to the belt tensioning system  300 , in the belt tensioning system  500  a belt  502  passes over fixed rollers  504   a  and  504   b  and around a floating tensioning roller  506 . The tensioning roller  506  is typically positioned just before a belt drive roller in the path of the belt  502 —for example, fixed roller  504   a  or  504   b , depending upon the direction of travel of the belt  502 . The belt  502  extends from the tensioning system  500  into the rest of a larger mechanical system and may be travelling in either direction through the system. 
     The floating roller  506  is rotatably mounted to a bracket  508 , which is used to move the roller  506  to control the tension of the belt  502 . As shown in  FIG. 5 , motion of the roller  506  in a leftward direction increases the tension of the belt  502  and motion in the rightward direction decreases the tension in the belt  502 . Unlike in belt tensioning systems  100  and  300 , the bracket  508  is mechanically coupled directly to a biasing device  514 . In other embodiments, a tensioning cable may be used to mechanically couple the bracket  508  to the biasing device  514 . The other end of the biasing device  514  is coupled to a fixed location  516 . The biasing device  514  may be an extension spring, a constant force spring, a torsion spring, a suspended dead weight, or other suitable mechanism for applying a force to the tensioning roller  506 . 
     The biasing device  514  applies a force F 1  to the tensioning roller  506 , which operates to increase the tension in the belt  502 . The bracket  508 , acting under the tension of the belt  502 , applies an opposing force F 2  to the biasing device  514 . When the force F 1  exceeds the force F 2 , the bracket  508  and tensioning roller  506  move in a direction to increase the tension in the belt  502  (to the left in  FIG. 5 ). When the force F 2  exceeds the force F 1 , the bracket  508  and tensioning roller  506  move in a direction to increase the force applied to the biasing device  514  (to the right in  FIG. 5 ). 
       FIG. 6  depicts a cutaway view of the belt tensioning system  600  along the line CC of  FIG. 5 . The one-way clutch roller  512  is mounted to a base plate  606  by a stanchion  608 . The tensioning roller  506  is rotatably mounted to the bracket  508  by an axle  602 . The fixed roller  504   b  is rotatably mounted to the base plate  606  by an axle  604 . Because the bracket  508  is free to move relative to the base plate  606 , the tensioning roller  506  is also free to move relative to the base plate  606 , thereby increasing or decreasing tension on the belt  502 . 
     The bracket  508  and the tensioning roller  506  are coupled to the base plate  406  only by the biasing device  514 . Where the rollers  502 ,  504   a  and  504   b  are crowned rollers, the belt  502  is constrained from moving in the vertical direction in  FIG. 6  and the belt  502  acts as a web element of the structure  500  to support the roller  506  and the bracket  508 . 
     As described for the belt tensioning system  300 , in the belt tensioning system  500 , the tension T of the belt  502  is typically in a range from T low  to T nominal . T low  typically occurs at startup and typically is established empirically. T nominal  is the nominal operating tension of the belt  502  and is set by the designer of the system in which the belt  502  is used. 
     In the belt tensioning system  500 , a nominal value for the spring force, F 1 , is determined by: 
         F   1 =2 *T   nominal , 
     where T nominal  is as described above. When T begins to fall below T nominal , the tensioning roller  506  moves to the left to keep the belt tension at T nominal . Similarly, when T begins to rise above T nominal , the tensioning roller  506  moves to the right to keep the belt tension at T nominal . 
     As such, the belt tensioning system  500  does not remain rigid under the dynamic tension loads of the belt  502 . However, like the belt tensioning system  300 , the belt tensioning system  500  has a flexible geometry that may be readily adapted to fir around other components of the belt-driven system. The belt tensioning system  500  also has a smaller footprint, lower cost, and lower maintenance requirements than many other belt tensioning systems. Also, the tensioning roller  512  is mounted to the floating bracket  508 , rather than being mounted by a more complex and more expensive articulated mechanism to the base plate  606 , as in some other belt tensioning mechanisms. 
     In some circumstances, a roller (for example, roller  504   a ) must be removed from a belt system utilizing a belt tensioning system according to this disclosure. Such circumstances might arise where an item being transported by the belt system becomes jammed and tension in the belt system must be temporarily reduced below T low  in order to remove the jammed item. In such circumstances, the belt tensioning systems  100  and  300  will operate to pull the tensioning rollers  106  and  306 , respectively, to increase tension in the belt to their respective minimum tensions. When the roller is replaced in the belt system, however, the capstan effect of the capstan  112  and the one-way clutch roller  312  will operate to prevent motion of the tensioning rollers  106  and  306 , respectively, resulting in higher than expected tension in the belts  102  and  302 . In belt systems where the need to remove a roller does not arise, or where operation of the belt tensioning system may be disabled while the roller is removed, the belt tensioning systems  100  and  300  provide the dual benefits of the ability to remain rigid under the expected dynamic tensioning loads of the belt being tensioned, as well as the reduced cost and mechanical simplicity of the floating tensioning roller. In belt systems where the need to remove a roller does arise and operation of the belt tensioning system cannot be disabled while the roller is removed, the belt tensioning system  500  provides the benefit of reduced cost and mechanical simplicity of the floating tensioning roller. 
       FIG. 7  depicts a top view of a conveyor belt system  700  according to an embodiment. The conveyor belt system  700  includes conveyor belts  702   a  and  702   b  and associated belt tensioning systems  750   a  and  750   b . In  FIG. 7 , the belt tensioning systems  750   a  and  750   b  are similar to the belt tensioning system  500  of  FIGS. 5 and 6 , however, it will be understood that the belt tensioning system  100  of  FIGS. 1 and 2 , the belt tensioning system  300  of  FIGS. 3 and 4 , or any other belt tensioning system according to this disclosure may be used in the conveyor belt system  700 . 
     The conveyor belt  702   a  moves in a clockwise direction, driven by a drive roller  704   a . The conveyor belt  702  passes, in turn, around idler rollers  704   b ,  704   c , and  704   d . The conveyor belt  702   a  includes a working section  706   a , which is constrained by idler rollers  708 . A return section  710  of the conveyor belt  702   a  is constrained by a single idler roller  712 . The conveyor belt  702   b  moves in a counter-clockwise direction, but is otherwise similar to the belt  702   a , being driven by a drive roller, having a working section  706   b , passing around idler rollers, and being constrained by idler rollers  708   b.    
     The conveyor belt system  700  is configured as a pinch drive system. That is, the working sections  706   a  and  706   b  are located adjacent to each other to form a gap  720 , into which items may be introduced, to be “pinched” between the belts  702   a  and  702   b  and transported from the left end to the right of the conveyor belt system  700 , as shown in  FIG. 7 . Such items may be envelopes or flats, which are held vertically between the belts  702   a  and  702   b  while being transported past operators or automated address reading machines for the purpose of sorting in a postal mail handling system. 
     While the belt tensioning systems  550   a  and  550   b  are used in the pinch drive conveyor belt system  700 , it will be understood that belt tensioning systems according to the disclosure may be used in any suitable belt system, including horizontal conveyor belts, power drive belts, manufacturing applications, food handling applications, and other moving belt systems. 
     Those skilled in the art will recognize that, for simplicity and clarity, the full structure and operation of all systems suitable for use with the present disclosure is not being depicted or described herein. Instead, only so much of the physical systems as is unique to the present disclosure or necessary for an understanding of the present disclosure is depicted and described. The remainder of the construction and operation of the systems disclosed herein may conform to any of the various current implementations and practices known in the art. 
     Although an exemplary embodiment of the present disclosure has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, and improvements disclosed herein may be made without departing from the spirit and scope of the disclosure in its broadest form. 
     None of the description in the present application should be read as implying that any particular element, step, or function is an essential element which must be included in the claim scope: the scope of patented subject matter is defined only by the allowed claims. Moreover, none of these claims are intended to invoke paragraph six of 35 USC §112 unless the exact words “means for” are followed by a participle.