Patent Publication Number: US-2006000192-A1

Title: Bottom unit of a stretch wrapping machine

Description:
TECHNICAL FIELD OF THE INVENTION  
      The present invention relates to a bottom unit of a stretch wrapping machine for rotating pallet goods, said bottom unit resting on a ground and comprising a turntable having a lower limiting surface and an upper limiting surface and being rotatably supported, by means of support rollers, on a base of the bottom unit, said support rollers resting on the base with peripheral portions thereof.  
     PRIOR ART  
      There are basically two different principles for bottom units of semi-automatic stretch wrapping machines for rotating pallet goods:  
      The most frequent type of a bottom unit comprises a base that constitutes foundation for a number of support rollers that are mounted in two circles, an inner circle and an outer circle. Above the support rollers the circular turntable is resting, said turntable being journalled radially around a shaft extension in the centre of the turntable. The height from the floor to the upper side of the turntable is usually 75-85 mm. Via an access ramp/inclined plane the pallet goods is normally pushed up manually on the turntable by means of a fork lift. With this type of bottom unit a large force is required to manually push heavy pallets up on the turntable via the ramp. Therefore, it is extremely important that the upper side of the turntable is located as close as possible to the ground since in this connection each cm is of extreme importance as regards the force that is required to push the goods up the ramp. The ramp often has an inclination of 2.5-3° and a length of about 1,3 m, which means that the ramp requires a large space.  
      The different type of bottom unit is a so-called “T-table” that constitutes a circular turntable having a cut out portion that enables the fork lift to enter inside the T-table without pushing the fork lift up along a ramp. The support rollers are mounted in the lower edge of the upper portion of the T-table and roll normally against a lower sheet metal plate that abuts the ground. However, the method normally requires that the fork lift is elevated to its highest position since the fork lift is not pushed up to the table top where the pallet is placed. Also this type of table normally has a height of about 75-85 mm above the ground.  
      A certain jamming risk exists if somebody places his/her foot adjacent a corner of the cut out portion of the table at the wrong moment. To eliminate this jamming risk it is common to add a photocell beam in front of the T-table, said photocell sensing a foot and consequently the rotation of the table is stopped. The time of retardation of the T-table may sometimes become too long due to the fact that the friction drive against the periphery of the T-table may slip in connection with retardation and heavy load. Therefore, it is usual to have a relatively low speed of rotation in connection with this type of bottom unit. It is also difficult to locate the photocell beam to render a proper jamming protection.  
      A further disadvantage is that dirt and splinters etc. from the pallets easily are spread on the bottom plate and hence also on the support rollers, this inducing operational vibrations and also increased wear.  
     OBJECTS AND FEATURES OF THE INVENTION  
      A primary object of the present invention is to provide a bottom unit of a stretch wrapping machine where the table is located at the lowest possible level.  
      A further object of the present invention is to make it possible to use a very short access ramp.  
      Still an object of the present invention is that the bottom unit should be structurally simple.  
      At least the primary object of the present invention is realised by means of a bottom unit of the type defined above that has been given the features of the appending independent claim  1 . Preferred embodiments of the invention are defined in the independent claims.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Below an embodiment of the bottom unit of a stretch wrapping machine according to the present invention will be described, where:  
       FIGS. 1-2  shows schematically a stretch wrapping machine in operative position, where a piece of goods, the stretch film and the mast with the stretch film unit are schematically indicated;  
       FIG. 3  shows a side view of the bottom unit according to the present invention;  
       FIG. 4  shows a top view of the bottom unit according to  FIG. 3 ;  
       FIG. 5  shows the main part of the base of the bottom unit;  
       FIG. 6  shows the access ramp and the portion of the turntable that joins the ramp;  
       FIG. 7  shows the centre area of the turntable;  
       FIG. 8  shows the turntable except further structural details;  
       FIG. 9  shows the U-profile before the shaft holders are welded;  
       FIG. 10  shows a vertical view of a portion of the U-profile, the supporting rollers being mounted;  
       FIG. 11  shows a partly sectioned side view with mutual location of the base, the turntable with the U-profile and its support rollers; and  
       FIG. 12  shows a view along A-A in  FIG. 11 . 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE BOTTOM UNIT ACCORDING TO THE PRESENT INVENTION  
      The stretch wrapping machine, schematically shown in  FIGS. 1 and 2 , comprises a bottom unit  1  according to the present invention, said bottom unit  1  being intended to rest on a ground in operative position of the machine. The stretch wrapping machine also comprises a mast  2  provided on the bottom unit  1 , said mast  2  supporting a stretch film unit  3  that is displaceable along the mast  2 . The mast  2  and the stretch film unit  3  are drawn only schematically with dotted lines and they will not be described more in detail since they do not constitute essential parts of the present invention.  
      The bottom unit  1  and the access ramp  4  are shown in  FIGS. 3 and 4  where some enclosures and the mast  3  are deleted for reasons of clarity. The bottom unit  1  comprises a base  10  and a turntable  11 .  
      When a piece of pallet goods is to be wrapped, said piece of pallet goods is normally elevated by means of a fork lift, then manually pushed up along the ramp  4  and subsequently located in the middle of the turntable  11 . Then the actual wrapping starts by starting the rotation of the turntable  11  and the pallet goods is wrapped, usually by plastic film, so-called stretch film.  
      The above mentioned bottom unit  1  according to the present invention has a height between the ground and the upper side of the turntable  11  that generally is about 40-65 mm lower than bottom units of stretch wrapping machines of similar type. In the above described embodiment of the bottom unit according to the present invention the upper limiting surface of the turntable  11  normally is at a distance of about 20-30 mm from the ground that the base  10  rests upon, said base  10  normally having a thickness of about 5 mm. The low level of the turntable  11  is essentially achieved by placing the support rollers  22  according to the present invention substantially above the lower side of the table  11 . This will be described more in detail below. A centre bearing  14  supports the load in the area around the centre of the turntable  11  in order to eliminate deflection in this area. Also the thickness of the turntable  11  has a great importance regarding the deflection and consequently the thickness has been chosen to about 12 mm which is about 50% more than the thickness of the turntables of most of the machines of similar type.  
      In the centre of the part of the base  10  that supports the turntable  11  a centre spigot  23  is provided, the turntable  11  being journalled on said spigot  23 . The turntable  11  is supported by the support rollers  22  and the centre bearing  14  and is rotated by means of a driving wheel  12  that is driven by means of an electric motor  13 . The motor  13  with the driving wheel  12  is intended to be suspended in a rigid, essentially rectangular plate that is hinged at one end and spring loaded at the other end. Thus, the force from the spring imparts a suitable pressure of the driving wheel  12  against the periphery of the turntable  11 , the driving wheel  12  being equipped with a friction material.  
      A photocell  19  detects a reflector  18  each time the turntable passes its zero position. For instance the signal is used to stop the table in correct position. The reflector may be moved between five different positions for different stop positions of the table  11  depending on the location of the ramp  4 .  
      The support rollers  22  are mounted in the U-profiles  20  that are screwed on top of the table  11 . The U-profiles  20  are bevelled about 30° at their ends  21  in order to allow the lower edge of the stretch film to pass over while the lower turns are wrapped on the pallet goods. The lowest film level is then about 1 cm above the bottom of the pallet.  
      An area  36  of the base  10 , located outside the periphery of the table  11 , is reinforced by several welded profiles  37  and rigidly connected to the plate  25  that constitutes base for the mast.  
       FIG. 5  shows the substantial part of the base  10  that is formed by a sheet metal plate suitably having a thickness of about 5-6 mm and manufactured by means of laser cutting or the like. The hole  29  is intended for attachment of the centre spigot  23  and the projections  17  with their holes  30  constitute an example of a possible attachment for the ramp  4  that can be located in five different positions in steps of an angle of 45° to the centre of the table.  
       FIG. 6  shows the access ramp  4  that in this case has an inclination of 3° and a length of about 275 mm. When using a conventional ramp, having a length of about 1,3 m, both the front and rear wheels of a manual fork lift of usual length will simultaneously roll in an upward slope during a part of the displacement while when using the short ramp  4  the front wheels of the fork lift will reach the horizontal table  11  before the rear wheels reach the lower part of the ramp, this meaning that an essentially lower weight must be pushed in an upward slope when using said short ramp  4 . Spacings in the shape of plastic pieces  15  are received in holes  16  of the turntable  11 , said plastic pieces  15  being equipped with a collar  15   a  that on one hand prevents displacement upwards of the plastic pieces  15  relative to the turntable  11  and on the other hand constitutes contact portion against the base  10 . The plastic pieces  15  prevent a too large deflection of the portion of the table  11  that is located closest to the ramp. The table  11  is not able to be deflected or tilted too much when the plastic pieces  15  contact the base  10 . Another function of the plastic pieces  15  is to prevent the base  10  from touching the table  11  in case of a very irregular ground below the base  10 .  
      When goods is to be entered on the turntable  11  via the ramp  4 , the turntable  11  should assume the position shown in  FIG. 4 , i.e. the plastic pieces  15  should be located in front of the ramp  4 .  
       FIG. 7  shows the centre of the table where the spigot  23  is fastened to the base  10 , preferably by welding. A threaded hole  27  in the centre of the spigot  23  is intended for fastening a lifting device in connection with transport of the machine and is hence not described further. The centre bearing  14  functions as a radial bearing relative to the spigot  23  and as an axial bearing relative to a thin sheet metal plate  28  that has a low friction and is clamped between the spigot  23  and the base  10 . For instance, it may constitute a self-lubricating plastic material that is pushed into the hole  29  in the table  11  but may also be fastened by gluing.  
       FIG. 8  shows the turntable  11  before various components have been mounted thereon. The manufacturing is suitably effected by laser cutting or the like.  
       FIG. 9  shows one of the U-profiles without the shaft holders  32 . The manufacturing is suitably effected by laser cutting or the like and then the sides are bent to perpendicular positions relative to the web of the U-profiles.  
       FIG. 10  shows a top view of a portion of a U-profile  20  with two support rollers  22  mounted. The support roller  22  with its shaft  34  is mounted by initially being placed in the cut-out portion in the table  11 . On top of the table  11  the U-profiles  20  are then fastened to the table  11  by screws that are mounted through the holes  24 . The roller shafts  34  are fixed in direction upwards by having the shaft holders  32  welded to the bottom of the U-profile  20 .  
       FIGS. 11 and 12  show the mutual location of the different parts adjacent to a support roller  22  and a section through the centre of the support roller  22 . The support rollers  22  are to a substantial part located above the lower limiting surface  11   a  of the table  11 , said limiting surface  11   a  being located about 7 mm above the base  10  that has a thickness of 5 mm. At these given dimensions the support rollers  22  may suitably have a diameter of about 36 mm. The upper limiting surface of the turntable  11  has been given the reference numeral  11   b . Above the base  10  there is a space  31  of about 7 mm and above the space  31  the table  11  is located and on top of the table  11  the U-profile  20  is mounted, said U-profile  20  fixing the shafts  34  of the supporting rollers by the aid of the shaft holders  32  that are welded to the bottom of the U-profile. The support rollers  22  in the U-profiles are protected against dust and dirt by a cover that is mounted on top of the U-profile  20 .  
      Feasible Modifications of the Invention  
      The bottom unit described above is intended for the most common pallet seizes but may also be manufactured for larger pallets and hence larger weight. The diameter of the turntable may be increased as well as the distance between the U-profiles  20  which means that a larger area of the turntable  11  is void of support between the U-profiles  20 . This may however be compensated by choosing a somewhat thicker turntable. Also the support rollers  22  may be increased in number or alternatively somewhat wider support rollers are chosen that hold more roller bearings.  
      In the embodiment described above the support rollers  22  extend through openings in the turntable  11  and the adherent U-profile  20 . However, within the scope of the present invention it is also feasible that the support rollers  22  have such a compact design that they do not extend through the upper surface  11   b  of the turntable  11  but are arranged in recesses on the lower side of the turntable, said recesses having a depth that is smaller than the material thickness of the turntable  11 .  
      Instead of fastening the shaft  34  of the support roller  22  in a U-profile  20  it is however also feasible within the scope of the invention to mount a sheet metal plate having a thickness of about 5 mm on each side of the support roller  22 . The sheet metal plates that then have a hole for the shaft  34  are mounted first on the shaft  34  of the support roller and then mounted from below through the holes in the turntable  11 . If the sheet metal plates are somewhat wider down below the wider part constitutes a stop towards the lower limiting surface  11   a  of the turntable  11 . In that way the U-profiles  20  are not needed. The support rollers  22  should however be protected by a strong cover.