The present invention relates to a method and an apparatus for conveying a sheet. More specifically, the invention relates to a method for successively conveying a series of sheets, such as veneer sheets, wherein a sheet being conveyed by a first conveyer is transferred to a second conveyer without subjecting the sheet to a tension acting in the sheet conveying direction and also to an apparatus for practicing the method.
In stacking sheets into a pile, it takes a given length of time for one sheet to be stacked completely onto the top of the pile of sheets. In conveying a series of sheets successively and subsequently stacking them one after another uninterruptedly into a pile, any two adjacent sheet being conveyed must be spaced apart for a distance that is long enough to prevent a sheet from arriving at the stacker while its preceding sheet is still in the process of being stacked and from hindering the stacking motion of the preceding sheet. To prevent such trouble, it has been practiced to enlarge the spacing between any two adjacent sheets being conveyed so that the time interval before a sheet arrives at the stacker after its preceding has reached the stacker is longer than the above given length of time that is necessary for a sheet to be stacked completely. Alternatively, a series of sheets being conveyed are divided into two different conveyance paths each having a stacker and every other sheet is transferred to each of the path. In stacking sheets at each of the stackers, a long time is allowed for a sheet comes to the stacker after its preceding sheet has been stacked.
Referring to FIG. 28 showing a known veneer sheet stacking apparatus according to the background art pertaining to the present invention, numeral 100 designates a first conveyer moving at a speed v10 and numeral 101 a second conveyer disposed in alignment with the first conveyer 100 and moving at a speed v11 that is faster than v10. A plurality of veneer sheets 102 are conveyed by the first conveyer 100 with a relatively small spacing between any two adjacent veneer sheets 102. The spacing between the veneer sheets 102 is enlarged by transferring them from the first conveyer 100 to the second conveyer 101 traveling faster than the first conveyer 100.
In such apparatus, however, a veneer sheet 102 just being transferred from the first conveyer 100 to the second conveyer 101 is subjected to a tension caused by the difference in conveying speeds of the first and second lower conveyers 100 and 101. The veneer sheet 102 is stretched apart by the tension acting in the direction perpendicular to the grain of the veneer sheet and it may be broken and rendered unusable.
The apparatus further includes a third conveyer 103 disposed downstream of and spaced above the second conveyer 102 at a distance that is greater than the thickness of the veneer sheet 102 and overlaps with the second conveyer 101. The third conveyer 103 travels at a speed v12 that is faster than v11. The second conveyer 101 has an air blower 104 located in the region overlapping with the third conveyer and operable to blow air constantly upward. Reference numeral 105 designates a stop for stopping a veneer sheet being conveyed by the third conveyer 103 at a given stacking position and reference numeral 106 a stacking stand on which a pile of stacked sheets 102 is supported.
When a veneer sheet 102 being conveyed by the second conveyer 101 enters the overlapping region of the second and third conveyers 101, 103, it is pressed against the third conveyer 103 by air blowing from the blower 104. Then, the veneer sheet 102 is moved at the speed v12 in arrow direction by virtue of the frictional force that the veneer sheet 102 receives from the third conveyer 103. Air is blown no more against the veneer sheet 102 after it has moved past the blower 104. Subsequently, the veneer sheet 102 is dropped while moving further leftward as seen in the drawing, hits against the stop 105 and then is dropped onto the top of the pile of veneer sheets 102.
As shown in FIG. 28, however, a veneer sheet designated by reference symbol 102a and subjected to the air blowing from the blower 104 will firstly have its leading end portion lifted and pressed against the third conveyer 103. Therefore, the pressed portion of the veneer sheet 102a is moved at the speed v12 while the portion of the veneer sheet 102a on the second conveyer 101 is moved at the speed v11, with the result that the veneer sheet 102a is subjected to a harmful tension which may cause the veneer sheet, if it is weak against the tension, to be broken. Furthermore, the veneer sheet 102a has a middle sloped portion which is in contact with neither the second conveyer 101 nor the third conveyer 103 and the weight of this middle portion creates tension against the leading end portion of the veneer sheet 102a which is in contact with the third conveyer 103. As a result, breakage of veneer sheet may occur.
Therefore, an object of the present invention is to provide an apparatus and a method for conveying a sheet which solve the above problems by moving a sheet in its entire dimension as measured in the sheet conveying direction in transferring the sheet from one conveyer to another conveyer.