Abstract:
The present invention describes an improved grip belt drive system for use in sealing machines used to wrap articles in thermoplastic film. Briefly, a timing belt is used in place of the traditional flat or V belt. The presence of teeth on the pulleys, coupled with the use of a timing belt eliminates the possibility of slippage between the belt and the pulleys. To protect the system in the case of a film jam or misplaced article, a torque-limiting mechanism is included, which disengages the drive system if there is excessive counterforce. Furthermore, the outer surface of timing belt contains a pattern, which when mated with a second belt having the same or a complementary pattern, meshes in such a way so that the two belts are positively locked in both the longitudinal and transverse directions.

Description:
BACKGROUND OF THE INVENTION 
     Machines used to wrap and seal articles and packages in thermoplastic film are well known in the art. Two types of machines are commonly referred to as side-sealing and lap-sealing machines. In the typical side-sealing configuration, an article or set of articles travels, typically via a conveyer belt, toward the machine. A sheet of center-folded plastic film, having two layers, is fed from a direction which is preferably perpendicular to the direction of the conveyer. The two layers of the film are then separated such that the article is placed between the lower layer and the upper layer. On one side of the article is the center-fold, while on the other side, there is an open edge where the two layers are not attached. The machine has a side sealing mechanism, which typically comprises several sets of belts to hold and guide the film, a heating/sealing element that fuses or welds the two free layers together and a cutting element that removes the excess material. Thus, as the article passes by the side sealing mechanism, this open edge is sealed by welding the two layers together, the plastic is cut and the waste is removed and discarded. At this point, the plastic film resembles a tube, with openings at both the leading and trailing ends of the article, but sealed along both sides. As the article continues to advance, an end sealing mechanism is then employed to seal the film at the leading end of the article. The article is further advanced and the end sealing mechanism then seals the film at the trailing end of the article. 
     Incomplete, inconsistent or sloppy welds can be problematic with these types of machines. One specific issue is caused by the height of the article. If the article to be encapsulated has significant height, the film becomes taut when wrapped around the article. This causes the film to exert forces attempting to pull the film toward the article at the time of side sealing. This can lead to incomplete welds. Attempted solutions to this problem include lowering the speed of the belt, or increasing the temperature of the heating/sealing element to attempt to form the weld before the film can be pulled away. However, neither solution has been satisfactory in rectifying the problem. 
     U.S. Pat. No. 6,526,728 describes a side-sealing machine, including two sets of cooperating pulleys, where each set includes a drive pulley, a tail pulley and a number of idler pulleys. It further discloses that there are two belts associated with each set of pulleys. The outer set of belts travels roughly in the same direction as the flow of plastic, while the inner set diverges so as to keep the waste material separate from the film after cutting. To keep the film from slipping, longitudinal V-ribbed belts are used, with the upper and lower belts offset so that the peak of the V shape in the upper belt fits into the valley of the V shape in the lower belt, and vice versa. As previously stated, this V pattern exists only in the longitudinal direction. This meshing action is designed to prevent slippage of the film, specifically in the transverse direction. 
     While this meshing action improves welds, inconsistent seals are still problematic. As stated above, the design of the interlocking belts eliminates any slippage in the direction traverse to the direction of film travel. However, it still allows slippage in the direction of travel. This can cause the film to slip and gather near the cutting element, creating sloppy or incomplete welds. 
     The belts commonly used in side-sealing machines are typically smooth along the edge which interacts with the pulley system. Thus, even with belts in which the V pattern is transverse to the direction of travel, it is also possible for the belts to slip with respect to the pulleys, also causing inferior welds. 
     SUMMARY OF THE INVENTION 
     The problems associated with the prior art have been overcome by the present invention, which describes an improved grip belt drive system for use in sealing machines. Briefly, a timing belt is used in place of the traditional flat or V belt. The presence of teeth on the pulleys, coupled with the use of a timing belt eliminates the possibility of slippage between the belt and the pulleys. To protect the system in the case of a film jam or misplaced article, a torque-limiting mechanism may be included, which disengages the drive system if there is excessive counterforce. Furthermore, the outer surface of the timing belt contains a pattern, which when mated with a second belt having the same or a complementary pattern, meshes in such a way that the two belts are positively locked in both the longitudinal and transverse directions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a representative side-sealing machine of the prior art; 
         FIG. 2  illustrates a side-sealing mechanism in accordance with the present invention; 
         FIG. 3  illustrates a top view of the side-sealing mechanism shown in  FIG. 2 ; 
         FIG. 4  illustrates the timing belt in accordance with the present invention; 
         FIG. 5  illustrates the meshing of two timing belts in accordance with the present invention; 
         FIG. 6   a  illustrates a second embodiment of the timing belt in accordance with the present invention; 
         FIG. 6   b  illustrates a third embodiment of the timing belt in accordance with the present invention; and 
         FIG. 6   c  illustrates a fourth embodiment of the timing belt in accordance with the present invention 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a representative side-sealing machine used to encapsulate or wrap an article in thermoplastic film, as described in U.S. Pat. No. 6,526,728. The machine  10  utilizes a conveyer belt  12  operating at a relatively constant speed to deliver articles  8  that are to be encapsulated. The thermoplastic film  1  is center-folded, such that the side with the fold is closed, while the opposite side  6  is open. On this opposite side, there are two layers of film  4 , 5 , which will later be sealed. This center-folded thermoplastic film  1  is fed from a reel (not shown) that is preferably mounted perpendicular to the direction of travel of the conveyer belt  12 . The film is then inverted and separated by an inverter  13  such that the article is enveloped between the two layers  4 , 5 . At this point, the film  1  on one side of the article is closed, while the opposite side  6  remains open. Also, the film at both the leading and trailing ends of the article is not sealed. Downstream from the inverter is the side-sealing mechanism  20 . After proper relative positioning of the article between the layers of the film  4 , 5 , the enveloped article approaches the side-sealing mechanism  20 . 
     The side-sealing mechanism  20  is located on the open side  6  of the enveloped article. The mechanism holds the two layers of film  4 , 5  together, and guides the layers through the sealing and cutting elements. It then welds the two layers together, and cuts off the surplus material. The surplus material is pulled away so as not to reattach to the film while it is still at an elevated temperature. 
     As shown in  FIG. 2 , to perform these actions, the mechanism  20  preferably comprises two sets of cooperating pulleys, an upper set  101  and a lower set  102 . These sets work in unison to pull the two layers of film into the mechanism and hold the layers in place. In the preferred embodiment, each of the pulleys has teeth  110  in its channel so as to accept one or more, preferably two, timing belts  120 . The presence of teeth  110  ensures that the timing belt does not slip relative to the pulleys; a common problem with the prior art. The first set of pulleys  101  is located above the layers of film, while the second set  102  is located below the layers. Each set comprises a drive pulley  101   a,    102   a  and a tail pulley  101   b ,  102   b . There may optionally be one or more idler pulleys (not shown). Each of these pulleys also has one or more, preferably two, O-rings mounted in the channel where the belts are located, so as to provide individual channels for each of the timing belts. 
     Each of the timing belts has a special gripping outer surface, that is bonded to a truly endless steel or Kevlar reinforced timing belt. Each corresponding set of belts has upper and lower pressure plates that are preset to insure good contact between the meshing pair of belts. 
     As shown in  FIG. 3 , one set of O-rings  200  is positioned such that the movement of the outermost belt  210  is made to be parallel to the direction of the film movement. The outer wall of the pulleys  240  and this first set of O-rings  200  provide the guides for the outermost belt  210 . As shown in  FIG. 3 , O-ring  200   a  and O-ring  200   b  are equidistant from the outer wall of their respective pulleys. A second set of O-rings  201  is used to guide the innermost belt  220  in a path that diverges away from the direction of the film and the outermost belt  210 . This can be accomplished in a number of ways. For example, a combination of one O-ring  201   b  and the inner wall of the downstream pulley  250   b  can be used to define the channel for the innermost belt  220 , as shown in  FIG. 3 . Similarly, 2 O-rings may be inserted on the upstream pulley to define a channel for the innermost belt. Alternatively, a single O-ring  201   a,  as shown in  FIG. 3 , can be used to define the inner wall of the channel for the innermost belt  220 . Because of the divergence angle, there are no forces pushing the innermost belt  220  toward the outermost belt  210 , thus the second O-ring may be eliminated. In other words, in the channel associated with the upstream pulley  240   a , the O-ring  201   a  provides the inner guide for the belt  220 . In the channel associated with the downstream pulley  240   b , the O-ring  201   b  provides the outer guide for the belt  220 . As a result, the innermost belt  220  is closest to the outermost belt  210  at the upstream pulley, and farthest away from it at the downstream pulley. The cutting element  230  is preferably located between the upstream and downstream pulleys. Thus, as the film passes the upstream pulley, it is still intact; however, it is cut before it reaches the downstream pulley. By introducing this divergence angle, the innermost belt  220  helps guide the unwanted surplus away from the film after it is cut. In the preferred embodiment, the innermost belt  220  is guided in the channel of the downstream pulley a distance further away from the film than on the upstream pulley sufficient to force the surplus plastic away from the film. One such suitable distance is about ¼ inch. This ensures that the surplus material does not reattach itself to the film while still at an elevated temperature. This surplus material is then held under tension and fed into a reel, which is later discarded. 
     On the outer surface of each timing belt is a three dimensional pattern, such as the one shown in  FIG. 4 . In the preferred embodiment, this pattern comprises a series of regularly shaped protrusions, such as squares, hexagons, or the like. In the first row  400 , there are two such protrusions  420 , spaced apart by a distance roughly equal to the width of a protrusion. As shown in the insert of  FIG. 4 , the surface of the first row  400 , looking from left to right, contains a protrusion  420 , followed by a depression  430 , followed by a protrusion  420 , followed by a second depression  430 . A second adjacent row  410  has this same series of protrusions and depressions; however, this series is offset from the first row by the width of a protrusion. Thus, the second row contains a protrusion where a depression exists in the first row; and contains a depression where the first row contains a protrusion. This gives a “checkerboard” type pattern of protrusions. This set of two rows is then repeated over the entire outer surface of the timing belt. Preferably, the dimensions of the protrusions and the spacing between rows are such that there is an even number of rows over the circumference. This guarantees that the pattern is not discontinuous at any point. While this is preferred, the present invention also contemplates the situation where there is not an even number of rows. In that scenario, there is a single discontinuity on each belt. The effect of this single small discontinuity over a significantly longer circumference is trivial in terms of the ability of the timing belts to hold the film in position without slippage. On the side opposite this pattern, or the inner circumference of the timing belt, there is a series of grooves  440 , adapted to mesh with the teeth  110  in the channel of the pulleys. 
     Each of the upper timing belts is pressed against a corresponding lower timing belt via upper and lower pressure plates. The lower and upper drive pulleys are kept in synchronization via a synchronization gear. As shown in  FIGS. 4 and 5 , the upper and lower belts are pressed together such that the first row on the upper belt meshes with the second row on the lower belt. Likewise, the second row on the upper belt meshes with the first row on the lower belt. This meshing of the two belts ensures that the belts are interlocked, thereby ensuring that there is no slippage in either the longitudinal or transverse directions. Thus, even in the presence of forces attempting to pull the film away from the pulleys, the outer surfaces of the timing belts will prevent slippage in both directions. 
     While the pattern shown in  FIG. 4  is preferred, it is not the only such pattern that prevents slippage in both the longitudinal and transverse directions. For example, a pattern containing alternating protrusions and depressions formed so as to resemble the letter “V” would have the same longitudinal and transverse locking ability, as shown in  FIG. 6   c . Similarly, other interlocking patterns, such as “W” shaped, also insure a positive lock in both directions. Also, a pattern in the form of a set of slanted protrusions in the form of a slash (“/”), as shown in  FIG. 6   a , also serves to prevent slippage in the longitudinal and transverse directions. Similarly, the protrusions can also be circular in shape, as shown in  FIG. 6   b.    
     The above-described patterns illustrate the scenario where both the upper and lower belts utilize the same pattern, and these patterns are slightly offset on the opposing pulleys to allow the timing belts to interlock. Alternatively, two different patterns can be utilized. For example, one of the belts can contain a pattern of protrusions that resembles the letter “X”. In this case, the opposing belt would contain a pattern of protruding diamonds that would mesh with the “X” characters of the first belt. Other complementary sets of patterns for the opposing belts are also possible and within the scope of the invention. 
     As described above, the use of timing belts insures that there is no slippage between the pulley and the belt, which can happen in machines of the prior art. However, the slippage inherent in the prior art was useful in the event of a film malfunction or stoppage. Previously, if the film were unable to be advanced, the belt would slip relative to the pulley and hence prevent any damage to the system. With the use of timing belts, this slippage is no longer possible. Thus, to provide the necessary safeguards, a torque-limiting device  260  is employed, as shown in  FIG. 3 . In the preferred embodiment, this torque limiting is provided by means of a clutch, which disengages the drive pulley if the counterforce exerted exceeds a predetermined threshold. 
     Returning to  FIG. 1 , after the article passes the side-sealing mechanism  20 , it encounters a second heating/sealing element  48 . This element  48  is used to seal and cut the leading and trailing edges of the film surrounding the article. When the article  8  is properly positioned, the second heating/sealing element  48  descends so as to contact the thermoplastic film. This serves to weld the layers together. The film is then cut, preferably in the middle of the newly created seal. The article  8  is then advanced so as to position the trailing edge of the article under the second heating/sealing element  48 . This process is then repeated, thereby completely sealing the article in thermoplastic film.