Abstract:
A chain tension monitoring apparatus and method includes a conveyor drive having a fixed frame and a floating frame. The floating frame supports a conveyor drive unit and is biased against the fixed frame by a compression spring to oppose forces generated by the conveyor drive operation. A force sensor is mounted to the floating frame to sense chain pull applied thereto. A pre-compression load is applied to the force sensor and chain pull is monitored as a function of the pre-compression load and actual load sensed by the force sensor.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to an apparatus and method for monitoring chain pull or tension, and in particular, to the use of a load cell-type force sensor positioned between components of a chain drive for tension monitoring. 
     BACKGROUND ART 
     Conveyor systems using chains as the driving mechanism and chain drives to give the conveyor motion and control conveyor speed are well known in the art. One example of these types of conveyor systems is a power and free system. Power and free systems are generally made up of a power track, a free track, and trolleys capable of travelling along the free track, the trolleys supporting one or more carriers. Each carrier then supports a load or article being conveyed. The trolleys are usually divided into leading and trailing trolleys. Each leading trolley in a power and free system includes a driving dog portion which extends towards the power track and which is engageable by a pusher dog carried by a moving chain on the power track. When the pusher dog and the driving dog are engaged, the leading or drive trolleys push along the free track by the moving power chain. When the driving dog is retracted, or otherwise disengaged from the pusher dog, the trolley stops moving, thus halting the carrier. 
     To move the power chain, one or more conveyor drives are utilized. Two typical types include caterpillar and sprocket versions. A sprocket drive delivers motion to the conveyor chain directly from the output side of a reducer through a sprocket whose teeth mesh with the lengths of the conveyor chain. A caterpillar drive transmits its driving force to the conveyor by means of a caterpillar chain made of precision steel rollers with driving dogs that mesh with the lengths of the conveyor chain. Depending on the size of the conveyor system, the drives can provide chain pulls of up to 12,000 pounds. 
     Caterpillar drives come generally in junior or standard categories. The smaller drives can be designed with either a fixed frame or a floating frame. Larger drives generally use floating frames. Caterpillar drives are usually installed along any horizontal straight run of a conveyor track. 
     Standard floating drives can be either a linear type or a rotary type. The linear type is generally built with an inner floating frame that is guided and supported by ball bearing wheels attached to an outer fixed frame. In contrast, a rotary drive is mounted on an inner floating frame that pivots around a reducer output shaft, the floating frame acting as a torque arm against the fixed outer frame. One or more compact coil springs counterbalance the normal chain pull and control the movement of the floating frame. 
     One example of a conveyor drive is disclosed in U.S. Pat. No. 4,222,481 to Dehne et al., hereby incorporated in its entirety by reference. With particular reference to FIGS. 7 and 8 of this patent, pivotal movement of the floating frame is easily opposed by a compression spring. The compression spring is arranged between a plate attached to the moveable frame and another plate secured to the fixed frame. The force of the compression spring biases the movable frame against the torque caused by drive pull. The Dehne et al. patent also discloses a shock absorber to further restrain pivotal movement of the frame. The shock absorber is mounted in a similar fashion as the compression spring, the absorber being arranged between the floating frame and the fixed frame. 
     The Dehne et al. patent also teaches that a limit switch can be provided to provide overload protection in case of excessive pivotal movement of the floating frame, such caused by a chain jam or the like. 
     Another prior art conveyor drive is disclosed in FIG.  1  and designated by the reference numeral  10 ., The drive is depicted in the same view as FIG. 4 of the Dehne et al. patent. Shown is a reducer  1 , a reducer shaft  3 , a bearing  5 , and a drive sprocket  7 . FIG. 1 does not show the caterpillar chain around the drive sprocket  7 . 
     FIG. 1 also shows an I-beam  9  which provides support for the trolley of a power and free system. The driven chain of the power and free system travels in a direction perpendicular to the view shown in FIG.  1  and in a direction from the motor (not shown) towards the reducer  1 . 
     The reducer  1  is also shown with an input shaft  11  and pulley  13 . The reducer  1  is connected to the motor via components  11  and  13  in a conventional fashion. 
     The speed reducer  1  is supported by a floating frame  15 , similar to the manner of support described in the Dehne et al. patent. The FIG. 1 embodiment uses a torque arm assembly  17  and a compression screw  19  to monitor the chain pull on the conveyor drive. The torque arm assembly  17  includes a rod  18 , a compression spring  21  and a rod plate  25 . One end of the rod  18  is attached to the fixed frame  27 . The other end of the rod  18  is secured to the plate  25 . The spring  21  is interposed between the plate  25  and a bracket  23  attached to the floating frame  15 . In this configuration, the spring  21  biases the floating frame  15  in the direction A, in opposition to the drive torque occurring in the direction B. 
     The rod  18  has a strain gauge sensor  29  as a part thereof, the strain gauge sensor  29  monitoring the chain pull during conveyor operation. More particularly, the strain gauge sensor  29  is zeroed when the conveyor drive is at rest, i.e. zero chain pull. When the conveyor drive is operating and a drive torque B is applied to the floating frame  15 , the amount of chain pull is monitored for conveyor operation control. 
     Another embodiment similar to FIG. 1 uses the torque arm assembly  17  without the strain gauge sensor  29 , thereby relying on other methods and techniques to monitor chain pull. One such technique uses a strain gauge link as part of the conveyor chain itself. The gauge on the link is calibrated so that the output of the gauge corresponds to the tension in the conveyor chain. The conveyor chain is stopped and the link is installed in the chain, the link then travelling around the conveyor system. Output data can be collected either by an umbilical cord and a data recorder or can be stored in computer memory for later download. Although the data collected in this fashion can be analyzed to find potential problem spots in the system, this method does not allow for the collection of data at one point in the conveyor system over a long period of time. 
     There are advantages to looking at the pull exerted by the drive over a period of time such as with the FIG. 1 embodiment. One of the problems with this embodiment is that the strain gauge is incorporated into the drive frame. If the drive frame is redesigned for a new installation or use, the strain gauge must also be redesigned to fit the requirements of that particular drive. Alternatively, if the strain gauge is being added as a retrofit, an expensive and time consuming process is required to remove the torque arm and replace it with a strain gauge-containing torque arm assembly. 
     Consequently, a need has developed to provide simplified and improved methods and apparatus to monitor the chain pull in conveyor systems. The present invention solves this need by providing a simple but effective method to install a strain gauge on an existing conveyor as well as an improved conveyor drive apparatus. With the invention, the conveyor does not have to be stopped to install the strain gauge and must be stopped for just a short period of time for calibration. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a first object of the present invention to provide an improved chain tension monitoring apparatus. 
     Another object of the present invention is an apparatus for monitoring chain tension which is easily retrofitted on existing units. 
     A still further object of the present invention is a method and apparatus of monitoring chain tension using a force sensor that easily interfaces with a floating frame conveyor drive. 
     One other object of the present invention is a method and apparatus for monitoring chain pull which is easily installed and calibrated. 
     Other objects and advantages of the present invention will become apparent as a description thereof proceeds. 
     In satisfaction of the foregoing objects and advantages, the present invention provides an improvement in conveyor drives utilizing fixed and floating frames. In these types of conveyor drives, the floating frame supports a conveyor drive unit and the frame is biased against the fixed frame by a compression spring to oppose forces generated by the conveyor drive operation. 
     In accordance with the invention, a force sensor is arranged on a portion of the floating frame and is configured with respect to the compression spring to sense a maximum compression force when chain pull of the chain drive is zero. The floating frame can be a rotary type, a linear type, or other known conveyor drives utilizing floating frames. Preferably, the force sensor is a load cell sensor and the portion of the floating frame where the force sensor is located opposes a portion of the fixed frame. 
     In one embodiment, a clamp assembly can be used to assist in mounting the force sensor to the floating frame. In this embodiment, the clamp assembly can utilize a clamp plate which fixes the force sensor between the clamp plate and the floating frame. In this arrangement, the force of the floating frame against the force sensor is resisted by the fixed frame through the clamp plate. 
     The invention also includes a method of monitoring chain pull in a conveyor system that employs a conveyor drive using a fixed frame and a floating frame. The floating frame supports a conveyor drive unit, the floating frame biased against the fixed frame by one or more compression springs to oppose forces generated by drive operation. According to the inventive method, the force sensor contacts a portion of the floating frame. The force sensor is pre-compressed to a preset force level and the chain drive is operated to generate a given chain pull. Using the force sensor, the operating chain pull is measured for monitoring purposes. 
     The floating frame can either pivot about an axis thereof or move along its longitudinal axis. As part of the measuring step, the data generated by the force sensor can be stored in electronic form. 
     In one mode, the force sensor is arranged by first compressing the compression spring to create a space between the floating frame and the fixed frame. The force sensor is then inserted in the space and the compression spring is allowed to expand to fix the force sensor in place. During the insertion step, the conveyor drive can remain operating and only has to be stopped to measure a pre-compression load on the force sensor for ultimately calculating the chain pull when the chain drive is in operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the drawings of the invention wherein: 
     FIG. 1 shows a prior art conveyor drive employing a torque arm strain gauge sensor; 
     FIG. 2 shows a first embodiment of the invention as a conveyor drive using a force sensor; 
     FIGS. 3A-3C show an exemplary installation sequence in accordance with the invention; 
     FIG. 4 shows a second embodiment of the invention employing an alternative conveyor drive; 
     FIG. 5 shows a partial view of yet another embodiment of the invention; and 
     FIG. 6 shows a sectional view along the line VI-VI of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention offers significant improvements in monitoring chain pull or chain tension for conveyor drives. In contrast to the tedious mechanisms of the prior art, the invention allows for a quick and easy way to monitor chain tension, either as retrofits of existing drives or for new installations. 
     FIG. 2 shows one embodiment of the invention. A conveyor drive designated by the reference numeral  10 ′ is similar to that depicted in FIG.  1 . The drive  10 ′ differs from FIG. 1 by using a conventional torque arm assembly  17 ′ which does not use the strain gauge sensor  29 . The rod  18 ′ then extends from the fixed frame  27  to the plate  25 . It should be understood that other mechanisms could be used to resist the force applied to the conveyor drive structure other than the spring arrangement depicted in FIG.  2 . 
     In substitution for the strain gauge sensor  29 , a force sensor  31 , preferably a load cell type such a button sensor is positioned between the compression screw  19  and the floating frame  15 . Strain information, e.g., the chain pull force, is output via line  33  for recording, analysis, storage and the like. 
     Positioning the force sensor  31  between the compression screw  19  and the floating frame  15  provides significant advantages over conveyor drives lacking any strain gauge sensor and those that may be custom designed to a particular manufacturer&#39;s specifications. The force sensor  31  can be interposed between the floating frame and a compression screw or the like regardless of the conveyor drive configuration. Thus, no design modifications or new configurations must be developed for a given manufacturer&#39;s application. In addition, if a conveyor drive uses a standard torque arm assembly  17 ′ as shown in FIG. 2, this assembly  17 ′ does not have to be removed and replaced with a prior art design as shown in FIG.  1 . Rather, the force sensor  31  can be utilized which makes the retrofit much easier than if a new sensor-containing torque arm assembly had to be installed in an existing drive. 
     The force sensor  31  is easily installed as part of the conveyor drive, either when first installed or as a retrofit application. Referring now to FIGS. 3A-3C, an exemplary installation sequence is depicted. In FIG. 3A, the compression screw  19  is shown butted against a face  35  of the floating frame  15 . In order to install the force sensor  31 , a jack  51  or other similar device is positioned between the fixed frame  27  and the face  35 . A jack member  53  is extended to move the floating frame  15  in the direction X as shown in FIG. 3B to create a space  41  to receive the force sensor  31 . The force sensor  31  is inserted in the space  41  and the jack pressure is released to allow the floating frame  15  to travel in the direction Y as shown in FIG.  3 C. As part of the process, the compression screw  19  can be backed off as well to facilitate inserting the force sensor  31  in the space  41 . The force sensor  31  is now installed and can be utilized to monitor chain pull force as described above. 
     The moving of the floating frame  15  as depicted in FIG. 3A can be done while the conveyor drive is still running. Once the jack  51  is removed, the conveyor chain is then momentarily stopped and the drive is manually reversed to remove any chain pull, i.e., a state of zero chain pull. The force now measured by the force sensor  31  is a precompression load at a drive pull of zero, this load used to calibrate a recorder or the like. The precompression load is derived from the spring force against the floating frame  15 , the spring force value dependent on spring compression as a result of positioning of the compression screw  19 . The greater the distance between the face  35  and the fixed frame  27  as determined by adjusting the compression screw  19 , the higher the precompression load will be, i.e. more compression of the spring  21 . 
     The drive is then restarted and data from the force sensor  31  is recorded. By subtracting the running precompression load measured by the force sensor  31  from the calibrated force, i.e., the precompression load at a drive pull of zero, the actual drive chain pull force can be calculated. For example, when the drive pull is zero, the precompression force on the force sensor  31  can be 5,000 pounds. When the drive pull increases a certain amount, the precompression force will decrease a corresponding amount. The actual chain pull would be derived by subtracting the force measured while the chain is running, e.g., 3,000 pounds, from the precompression force of 5,000 pounds (previously set at a drive pull of zero) to arrive at a drive pull of 2,000 pounds. As described above, this monitoring can be done continually, or at intervals as desired. 
     Although a rotary drive is exemplified in FIG. 2, any other drive where chain pull monitoring is important can be utilized. Referring to FIG. 4, a typical linear drive is depicted and designated by the reference numeral  50 . The drive includes a fixed frame  52  and a floating frame  54 . The floating frame  54  travels longitudinally within the fixed frame  52 . As opposed to the rotary drives discussed above, the driving torque is aligned with the direction of chain travel. 
     The floating frame  54  is fixed by the compression spring  55  forcing the frame  54  against the compression screws  57 . A force sensor can be inserted between one or both of the compression screws  57  at  59 . The manner of insertion can be the same for the rotary drive wherein the floating frame  54  is moved against the force of the spring  55  to create a space between the one or both of the compression screws  57  and the frame face  61 . 
     FIGS. 5 and 6 show yet another embodiment of the invention. In FIG. 5, a portion of a conveyor drive is illustrated and designated by the reference numeral  60 , having a fixed frame  61  and a floating frame  63 . In this embodiment, the floating frame  63  is biased in the direction Q by springs  64  and the sensed chain pull is in the direction P. 
     A force sensor assembly  65  mounts the force sensor  31  between a floating frame face  67  and a bracket or plate  69  of the assembly  65 . With this arrangement, when the floating frame  63  senses a chain pull force, it contacts the force sensor  31  which is connected to the fixed frame  61  via the force sensor assembly  65 . The rods  71  of the assembly  65  link to the fixed frame  61  by a bracket  73 . 
     It should be understood that the mounting arrangements of the force sensor with respect to the fixed and floating frames can vary depending on the particular configuration of the drive being modified, retrofitted or customized. 
     Any force sensor as known in the art can be used for monitoring the chain pull force as part of the improved conveyor drives of the invention. A preferred type is a load cell type but other types as would be within the skill of the art can also be utilized. 
     As such, an invention has been disclosed in terms of preferred embodiments thereof which fulfills each and every one of the objects of the present invention as set forth above and provides new and improved method and apparatus for monitoring the chain pull or tension in connection with a chain drive. 
     Of course, various changes, modifications and alterations from the teachings of the present invention may be contemplated by those skilled in the art without departing from the intended spirit and scope thereof. It is intended that the present invention only be limited by the terms of the appended claims.