Abstract:
A conveyor slider bed assembly ( 1 ) for use with a check weigher as used in the food processing industry. The assembly ( 1 ) includes a belt ( 2 ) which travels endlessly over a thermoplastic bed ( 3 ). The thermoplastic material is impregnated with stainless steel fibers which improve the conductivity of the bed ( 3 ) and prevents the accumulation of an electrostatic charge created by the movement of the belt ( 2 ) over the bed ( 3 ). The bed ( 3 ) is attached to a base ( 8 ) with a pair of resilient clips ( 14, 15 ) which engage corresponding mating surfaces ( 30, 13 ) formed in the base ( 8 ). The clips ( 14, 15 ) permit the removal of the bed ( 3 ) from the base ( 8 ) without the use of tools.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to the field of endless conveyor mechanisms, and more particularly to a conveyor bed used in conjunction with a check weigher system.  
         DESCRIPTION OF RELATED TECHNOLOGY  
         [0002]    A check weigher is a device that measures the weight of articles produced during a manufacturing process in order to determine if their weight falls within prescribed limits. In the food processing industry, packages of food are often individually weighed while in motion on a conveyor belt. The check weigher that performs these measurements is typically a self contained unit that includes an endless conveyor belt traveling over a support surface known as a slider bed.  
           [0003]    The function of the slider bed is to support the weight of the product being conveyed and to position the conveyor rollers that are used to drive and track the conveyor belt. Most slider bed assemblies are made either of formed sheet metal or machined aluminum. In order for the check weigher to function properly, the conveyor assembly must be both lightweight and very rigid. Aluminum is typically a good material to use in slider bed construction due to its high strength to weight ratio, but in the food processing industry it is a poor choice since equipment is often washed with a caustic cleaning solution that would damage any aluminum with which it came in contact.  
           [0004]    Stainless steel must be used in caustic washing environments, but because of the high density of steel it is inherently too heavy satisfy the low weight requirement of a slider bed. The use of stainless steel in slider bed fabrication therefore requires the removal of considerable mass by expensive machining operations. One alternative is to form stainless steel conveyor components from sheet metal, but the tolerance accumulation inherent with sheet metal forming is large when compared with machining operations. Mating parts must be designed to fit within tolerance limits, limiting the functionality and appearance of the assembled conveyor.  
           [0005]    If an alternative to metal such as plastic were to be employed in fabricating the slider bed, the movement of the conveyor belt across the plastic surface creates an electrical charge. This charge will eventually create a spark as the accumulated static charge arcs to ground. A metal slider bed can be readily grounded to prevent arcing, but a plastic material is inherently unsuitable due to its properties as an electrical insulator. Attempts have been made to improve the electrical properties of plastic materials. For example, U.S. Pat. No. 4,596,670, entitled EMI SHIELDING EFFECTIVENESS OF THERMOPLASTICS, issued to Liu, discloses a method of improving surface conductivity of plastic materials by using conductive fillers. U.S. Pat. No. 4,664,971, entitled PLASTIC ARTICLE CONTAINING ELECTRICALLY CONDUCTIVE FIBERS, issued to Soens, discloses methods of fabricating plastic plates and sheets with improved surface conductivity due to the presence of electrically conductive fibers within the plastic. Neither Liu or Soens discuss the structural properties of their resultant materials in the context of check weigher slider bed requirements. Thus the need remains for a slider bed constructed of a strong lightweight material which can be inexpensively fabricated.  
           [0006]    The slider bed itself must be rigidly mounted to some structural foundation in order to provide stability and freedom from vibrations that would affect the weighing process. The manner of the rigid attachment typically involves threaded fasteners which require some tools for installation and removal, making maintenance and periodic cleaning unnecessarily time consuming. Other methods involve special fixtures intended to simplify the conveyor coupling process. For example, U.S. Pat. No. 5,701,991, entitled CLAMPING MECHANISM, issued to Helmetsie on Dec. 30, 1997, discloses an assembly used in conjunction with a check weigher. The system utilizes threaded fasteners and pivotable elements to secure a clamp to a rail. U.S. Pat. No. 6,035,997, entitled CONVEYOR PAN COUPLING ASSEMBLY, issued on Mar. 14, 2000 to Heninger et al., discloses a T-shaped retainer that is held in place by resilient elements. The coupler includes a removable shank and requires the rotation of the retainer to hold the shank in place. Neither of these devices is specifically tailored to secure a slider bed to a mounting structure, and the need remains to have the ability to remove the slider bed for maintenance and cleaning without the use of tools.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention discloses a novel check weigher slider bed that is molded from a two part urethane thermo set plastic material. In order to prevent electrostatic discharges to ground, stainless steel fibers are mixed into the urethane before molding or forming of the slider bed. The stainless steel fiber matrix provides a high resistance but adequately conductive path to ground through an attached motor casing via the ground wire in the conveyor motor power cable. The molded slider bed is attached to a mounting structure without the use of tools via an injection molded clip. The resiliently deformable clip is rigidly affixed to the slider bed by threaded fasteners. The mounting structure is formed to include mating sections which receive the clips. Pins may also be formed within the slider bed that align with mating holes within the mounting structure in order to prevent rotation of the slider bed with respect to the mounting structure after the clips have been engaged.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is an exploded perspective view of a check weigher conveyor assembly constructed in accordance with the principles of the present invention;  
         [0009]    [0009]FIG. 2 is a perspective view of the assembled conveyor assembly depicted in FIG. 1;  
         [0010]    [0010]FIG. 3 is an exploded end elevation view of the check weigher conveyor assembly depicted in FIG. 1;  
         [0011]    [0011]FIG. 4 is top plan view of the check weigher bed depicted in FIG. 1;  
         [0012]    [0012]FIG. 5 is a top plan view of the conveyor assembly depicted in FIG. 1;  
         [0013]    [0013]FIG. 6 is an end elevation view of the assembled conveyor assembly depicted in FIG. 1;  
         [0014]    [0014]FIG. 7 is a side elevation view of the disassembled conveyor assembly depicted in FIG. 1;  
         [0015]    [0015]FIG. 8 is a sectional view taken along line  8 - 8  in FIG. 7;  
         [0016]    [0016]FIG. 9 is a detailed elevation view as denoted by circle  9  in FIG. 8;  
         [0017]    [0017]FIG. 10 is a detailed view of the interface between a portion of the clip and its corresponding mating surface; and  
         [0018]    [0018]FIG. 11 is a sectional view taken along line  11 - 11  of FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Referring to FIGS. 1, 2,  3 ,  4 ,  5  and  6 , a slider bed assembly  1  is seen to include a conveyor belt  2  which is supported on a bed  3 . The belt  2  is further supported by a drive roller  4  and a tail roller  5 . The drive roller  4  is driven by a timing belt powered by motor  6  which is secured to the bed  3  by motor mount  7 . The bed  3  is formed to include a plurality of grooves  43 ,  44 ,  45  and  46  which are arranged in a diagonal or chevron pattern. As best seen in FIG. 11, the grooves  45  and  46 , for example, are formed as substantially arcuate or semicircular indentations. The grooves extend to the edges  47  and  48  of the bed  3  which substantially coincides with the width of belt  2 .  
         [0020]    The grooves are formed at an angle with respect to edges  47  and  48 , typically between ten and eighty degrees. The grooves reduce the extent of surface contact between the belt  2  and the bed  3 , thereby reducing the rate at which an electrostatic charge is accumulated on the bed  3 . Further, the belt  3  is less likely to stick or adhere to the bed  3  during conditions of relatively high humidity, in the presence of moisture or spilled liquids, or when the belt  2  becomes coated with dust or particulate matter.  
         [0021]    The slider bed assembly  1  also includes a base  8  to which the bed  3  is rigidly affixed by means of clips  15  and  14  (FIG. 5). The clip  15  is attached to the bed  3  with a clip mount  10  that is affixed to the bed  3  by means of screws  11  and  12  (FIG. 1). Mating surfaces  13  and  30  are formed into the base  8  which are adapted to receive the clips  15  and  14 , respectively.  
         [0022]    The bed  3  is composed primarily of a two part urethane thermoplastic. Ideally, component A (specific gravity=1.16) of the thermoplastic resin has a viscosity of between 37,000 and 43,000 centipoise, while component B (specific gravity=1.00) has a viscosity of approximately 1,500 to 2,000 centipoise. Typically the mixing ratio of components A and B is approximately 22 units of B for every 100 units of A (by weight). The in mold time for the resulting thermoplastic (specific gravity=1.13) is typically between 31 and 37 minutes. After a two hour cure at 100 degrees centigrade, the resultant Shore hardness is approximately D 80 . Both the shear strength and the tensile strength is in the range of 3000 to 3300 pounds per square inch. A urethane resin meeting these specifications and which is also approved by the U.S. Food and Drug Administration for food contact and medical applications (CFR 21.175-177) is manufactured under the designation TUFFALLOY  280  series by HAPCO, Inc. at 353 Circuit Street, Hanover, Mass. 02339.  
         [0023]    In order to prevent an electrostatic discharge to ground, stainless steel fibers are mixed into the urethane resin before molding of the bed  3 . The stainless steel fiber matrix provides adequate surface conductivity to the finished bed  3 .  
         [0024]    The stainless steel fibers typically have a diameter of 8 to 11 microns and a length of 4 to 5 millimeters. A ground wire is attached interconnected between the bed  3  and the motor mount  7  in order to provide continuous discharge of the any accumulated static charge caused by movement of the conveyor belt  2  over the bed  3 . The result of using urethane for the construction of slider bed  3  is reduced weight when compared to a metal counterpart, as well as the incorporation of structural geometry that would be prohibitively expensive to machine into a metal product. The advantage of employing a two part urethane as the construction material is that the bed  3  can be molded using a reaction injection molding process, thereby reducing the cost of production tooling.  
         [0025]    The conductive material within the bed  3  can also be carbon fiber, but since the slider bed assembly  1  is typically used within the food processing industry, stainless steel fibers are the preferred choice because stainless steel is approved in food preparation applications by the FDA. Further, carbon fibers placed in urethane can cause anisotropic shrinkage whereas the stainless steel permits isotropic shrinkage. Carbon fibers are also less conductive and more brittle than stainless steel. Coating the carbon fibers with nickel improves conductivity but the problem of brittleness remains. Carbon fibers and powder are subject to sloughing (making the product unsuitable for food related applications) and limit the color of the finished product to black.  
         [0026]    Referring also to FIGS. 8 and 9, the characteristics of clip  15  can be better understood. The clip  15  is injected molded of a resilient thermoplastic material and is formed to include a tab  16  which is engaged by the mounting block  10  and rigidly affixed to bed  3 . The clip  15  is mounted to bed  3  in a cantilevered configuration with a fixed end defined by the tab  16  and a free end  18  which is adapted to engage the mating surface  13  formed within base  8 . A lower surface  17  of the mount  10  is inclined to serve as a stop that prevents excessive deflection of the clip  9 .  
         [0027]    As best seen in FIG. 7, the clips  14  (FIG. 5) and  15  are positioned for engagement with mating surfaces  13  by positioning the bed  3  over the base  8 . The shape of the clip  15  is configured so as to cause the free end  18  to contact and interfere with the upper protrusion  19  of the mating surface  13  as the bed  3  is lowered toward base  8 . As the initial contact occurs between free end  18  and upper protrusion  19 , the free end  18  is deflected outwardly in the direction of arrow  20 .  
         [0028]    The actual deflection of free end  18  can be initiated in one of two ways. First, the conveyor slider bed  3  can be aligned over base  8  and pushed in the direction of arrow  21 . This motion will cause the free end  18  to move into the direction of arrow  20  until the free end moves beyond the bottom surface  22 , at which time free end  18  will snap or spring back in the direction of arrow  23 , thereby abutting bottom surface  22  and urging bed  3  into rigid contact with base  8 . The static configuration of clip  15  is such that free end  18  is still biased in the direction of arrow  23  even when free end  18  abuts bottom surface  22 . As best seen in FIG. 1, the sidewalls  24  and  25  of the base  8  are formed to include shoulders  26 ,  27 ,  28  and  29  adjacent to the mating surfaces  13  and  30 . The shoulders abut the clip mounts  10  and prevent rotation of the bed  3  with respect to the base  8  once the clips  14  and  15  engage the mating surfaces  30  and  13 , respectively. Alternatively, ribs or pins  38 ,  39 ,  40  and  41 , for example, can be formed into the base  8  which engage corresponding structures in bed  3  and thereby prevent rotation or shifting of the bed  3 .  
         [0029]    Additional protection against slippage of the clip  15  with respect to the mating surface  13  is provided by the relative shapes of the free end  18  and the lower surface  31  of mating surface  13 . As seen in FIGS. 9 and 10 (FIG. 10 being a detailed view of the region  37  circled in FIG. 9), the line  32  is perpendicular to the beveled surface  33  of the free end  18 , the beveled surface  33  abutting portions of lower surface  31  when the clip  15  is mounted to mating surface  13 . When fully engaged, the lower surface  31  and the beveled surface  33  overlap slightly.  
         [0030]    The line  32  passes through the center of rotation  35  of the beveled surface  33 , that is, as the clip  15  is deformed the beveled surface  33  rotates in the direction of arrow  42  about center of rotation  35 . Therefore, the clip  15  cannot slip with respect to mating surface  13  until the tangent of angle  36  between lower surface  31  and beveled surface  33  is greater than the coefficient of friction of the thermoplastic material of which clip  15  and mating surface  13  are formed.  
         [0031]    A second method of attaching the conveyor bed  3  to base  8  is to set the bed  3  onto the base  8  and align the clips  14  and  15  with the mating surfaces  30  and  13 . The free end  18  of each clip  14  and  15  is then manually deflected in the direction of arrow  20 . The bed  3  is then moved further in the direction of arrow  21  and the clips  14  and  15  are ultimately released when the bed  3  abuts base  8 . This second method of mounting bed  3  to base has the advantage of eliminating any impulse force caused by shock of pressing the bed  3  onto base  8  with sufficient force to overcome the resilient biasing force present in the clips  14  and  15 . A strain gauge is typically attached to the conveyor bed  3  when used as a check weigher and the gauge can be damaged by repeated sudden impact forces.