Method for detecting a fibrous web tear in a drying section of a machine for producing the fibrous web and a dryer fabric for performing the method

Abstract

This invention relates to a method for detecting a tear in a fibrous web ( 3 ) in a drying section ( 2 ) of a machine ( 1 ) for producing the fibrous web ( 3 ), whereby the fibrous web ( 3 ) is passed through the drying section ( 2 ) by way of at least one dryer fabric ( 5 ). The tear is detected by at least one web tear detection device ( 8 ) and a cut-off apparatus ( 9 ) for the fibrous web ( 3 ) is activated by the web tear detection device ( 8 ). Use is made of a dryer fabric ( 5 ) with at least one characteristic color pattern ( 10 ) which is reliably detected by the web tear detection device ( 8 ) in the event of a tear in the fibrous web ( 3 ). In addition the present invention relates to a dryer fabric ( 5 ) for a drying section ( 2 ) of a machine ( 1 ) for producing a fibrous web ( 3 ).

1 . A method for detecting a tear in a fibrous web in a drying section of a machine for producing the fibrous web, comprising the steps of:
 passing the fibrous web through the drying section by way of at least one dryer fabric;   detecting the tear in the fibrous web by way of at least one web tear detection device; and   activating a cut-off apparatus dependant upon a signal from the web tear detection device, said at least one dryer fabric having at least one characteristic color pattern detectable by said web tear detection device in the event of the tear in the fibrous web.   
     
     
         2 . The method of  claim 1 , wherein said at least one characteristic color pattern is formed by a plurality of colored stripes including at least two colors, said colored stripes having a predetermined alignment. 
     
     
         3 . The method of  claim 2 , wherein said colored stripes are aligned transverse to a running direction of said at least one dryer fabric. 
     
     
         4 . The method of  claim 1 , wherein said at least one characteristic color pattern is formed by a plurality of differently colored threads. 
     
     
         5 . The method of  claim 4 , wherein said plurality of differently colored threads include differently colored weft threads. 
     
     
         6 . The method of  claim 5 , wherein said differently colored weft threads include a white polytetrafluorethylene thread, said plurality of differently colored threads including red polyester warp threads. 
     
     
         7 . The method of  claim 6 , wherein said weft threads have a thickness that is used as a minimum width (Bmin) of said characteristic color pattern. 
     
     
         8 . The method of  claim 4 , wherein said at least one dryer fabric includes a spiral mesh, said characteristic color pattern being produced by differently colored spirals. 
     
     
         9 . The method of  claim 8 , wherein one spiral thickness (DS) of one of said spirals is used as a minimum width (Bmin) for said characteristic color pattern. 
     
     
         10 . The method of  claim 9 , wherein a maximum width (Bmax) of said characteristic color pattern results from a maximum permissible web length which does not cause any damage on the machine for producing the fibrous web before said cut-off apparatus is activated by said web tear detection device. 
     
     
         11 . The method of  claim 1 , further comprising the step of coloring said at least one dryer fabric to produce said characteristic color pattern. 
     
     
         12 . The method of  claim 11 , wherein said coloring step includes using thermography to produce said characteristic color pattern. 
     
     
         13 . The method of  claim 1 , wherein said at least one dryer fabric has a plurality of regions with only some of said plurality of regions contain said characteristic color pattern. 
     
     
         14 . The method of  claim 1 , wherein said detecting step includes a step of making a spot measurement of one of the fibrous web and said at least one dryer fabric with one of a laser beam and a light beam. 
     
     
         15 . The method of  claim 1 , wherein said detecting step includes a step of making a line measurement of one of the fibrous web and said at least one dryer fabric with a line camera. 
     
     
         16 . The method of  claim 1 , wherein said detecting step includes a step of making an area measurement of one of the fibrous web and said at least one dryer fabric with a camera. 
     
     
         17 . The method of  claim 1 , wherein said detecting step includes a step of making use of a difference between a non-periodic stochastic signal of the fibrous web and a periodic signal of said characteristic color pattern. 
     
     
         18 . The method of  claim 1 , wherein said detecting step includes the steps of:
 evaluating a signal using one of a Fourier and a power spectrum of said signal, said signal being generated by said tear detection device and relating to a tear in the fibrous web;   setting at least one trigger threshold value at a frequency characteristic of said at least one dryer fabric.   
     
     
         19 . A dryer fabric for a drying section of a machine for producing a fibrous web, the machine including a web tear detection device and a cut-off apparatus, the dryer fabric comprising:
 one of a woven, a non-woven and a spiral mesh structure; and   at least one characteristic color pattern associated with said structure, said at least one characteristic color pattern being detectable by the web tear detection device in the event of a tear of the fibrous web.   
     
     
         20 . The dryer fabric of  claim 19 , wherein said at least one characteristic color pattern is formed by a plurality of colored stripes including at least two colors, said colored stripes having a predetermined alignment. 
     
     
         21 . The dryer fabric of  claim 20 , wherein said colored stripes are aligned transverse to a running direction of the dryer fabric. 
     
     
         22 . The dryer fabric of  claim 19 , wherein said at least one characteristic color pattern is formed by a plurality of differently colored threads. 
     
     
         23 . The dryer fabric of  claim 22 , wherein said plurality of differently colored threads include differently colored weft threads. 
     
     
         24 . The dryer fabric of  claim 23 , wherein said differently colored weft threads include white polytetrafluorethylene threads, said plurality of differently colored threads including red polyester warp threads. 
     
     
         25 . The dryer fabric of  claim 24 , wherein said weft threads have a thickness that is used as a minimum width (Bmin) of said characteristic color pattern. 
     
     
         26 . The dryer fabric of  claim 22 , wherein said at least one dryer fabric is a spiral mesh, said characteristic color pattern being produced by differently colored spirals. 
     
     
         27 . The dryer fabric of  claim 26 , wherein one spiral thickness (DS) of said spiral is used as a minimum width (Bmin) for said characteristic color pattern. 
     
     
         28 . The dryer fabric of  claim 27 , wherein a maximum width (Bmax) of said characteristic color pattern results from a maximum permissible web length which does not cause any damage on the machine for producing the fibrous web before the cut-off apparatus is activated by the web tear detection device. 
     
     
         29 . The dryer fabric of  claim 19 , wherein said characteristic color pattern is made by subsequent coloring of the dryer fabric. 
     
     
         30 . The dryer fabric of  claim 29 , wherein said characteristic color pattern is produced by using thermography. 
     
     
         31 . The dryer fabric of  claim 19 , wherein the dryer fabric has a plurality of regions with only some of said plurality of regions containing said characteristic color pattern.