An Analysis and Prevention of Stop Marks in Weaving

Journal of Dong Hua University (Eng. Ed.) Vol. 19 No.1 (2002) 103An Analysis and Prevention of Stop Marks in WeavingCHEN Ge (陈革), LIN Shen (林申), CHEN Ming (陈明)College of Mechanical Engineering , Dong Hua University , Shanghai , 200051The cause of stop marks of fabrics in weaving isback ail swing motion under stabilityu'Besides, if the loomintroduced, and the floating displacement of the clothcannot stop immediately when the ends of warp breaks, or somefell during loom stoppage is a primary cause of stopwefts are missing to be inserted into the shed, they all lead to themarks. The stop marks caused by fabric system arethin place in the fabrics.analyzed. A new method to eliminate stop marks busing the electronic let-off system is presented. This newAn Analysis of Creep of Cloth Fell Causedmethod needn't any special detecting equipment, it canby the Fabric Systemcontrol the fell position by the powerful control program,so the cost is decreased greatly. The method is soDuring loom stoppages, the cloth fell creeps to a newflexible that can be applied to weaving all kinds ofposition because of viscoelastic properties of the warp and cloth.fabrics.The creep depends on the time of loom stoppages. The longerKeywords: loom, stop marks, creep of cloth fell, electronicloom stoppages may lead to more severe marks. Before the loomlet-of.start up, if the cloth fell moves toward back rail, the operatorshould take up the cloth a ltte at first, while the cloth fell movestoward breast beam, the operator should turn the take-upIntroductionmechanism backward to free cloh up a certain degree. However,it is very hard for the operator to control cloth fell position to thStop marks mean the distance between two adjacent picks ofproper position, and the stop marks is very difficult to be avoided.the weft in fabric is wider or narrower than the normal, which is aSo the viscose elastic properties of warp and clouh must beprimary cloth defect of woven fabrics. It's hard to mend them,analyzed at first.and they decrease the quality of fabric. On modem looms withOn the let-off mechanism with swing back rest, the warp andhigh operating speed, the problem of stop marks seems to be morecloh can be recognized to be under the small and constant tension,severe. Up to now，loom producers are looking for variousand normally the warp tension equals the cloth tension in balance.methods to eliminate stop marks.So the warp and cloth system (shown as Fig. 1(a)) can beexpressed as Fig. 1(b). If the creep of the cloth fell is the onlyCause of Stop MarksBack rest\A major cause of stop marks has been recognized as the cloth昌fell creep due to viscoelastic properties of warp and cloth during+loom stoppages, and the clouh fell floats a litle from normalposition. Since the warp and cloth are under tension, the lengths(a) Warp and cloth systcmof warp and cloth stretch a small different amount of variation to7'create a creep, which affects the position of the cloth fell. The引ww[Iw{creep is relative with stoppage time. So longer loom stoppages才EYLw-E:L w-may lead to more severe stop mars.2. Further more, the creepis connected with the fabric strucure, type of yarms, the lengths of(b) Mechanical model of warp and clothcloth and warp under tension in the loom, the air condition of theweaving shed and so on[3]. .There are also ouher factors that lead to stop marks. The reedyw[may not reach its nommal speed in the first cycle at starting,Ecausing the initial beat-up speed to be less than the normal speed.(c) Mcchanical model of fabrieFurther more, the lower speed can't make the reed reach itsdynamic front position, which reduces cloth fll's displacement a中国煤化Iwam and cdothlot at beat-up. The oscillations of the back rail may also lead tostop marks, as it takes a few cycles of loom revolution before theMHCNMHGReceived Feb. 22, 2001104 .Jourmal of Dong Hua University (Eng. Ed.) Vol. 19 No.1 (2002)consideration, the mechanical model can be set up ( shown asDuring nommal running, the cloth fell moves toward back restFig. 1(3)). So on the let-off mechanism with swing back rest, thewhen the beat-up is over, but this does not cause marks, even ifchange of cloth fell during the loom stoppages is the creep of cloththe loom speed is low. So it can be recognized that the absoluteunder small and constant tensionl2].deformation of the cloth in the fabrics during the working periodδ is the cloh's deformation under the constant and smallmainly is the fast-elastic deformation, which is the frst part oftension. Its fast-elastic defornation is δ, and its slow-elasticequation(1). However, during loom stoppages, besides the fast-deformation is 8.elastic deformation， the slow- elastic deformation 员should beincluded, which is the second part of equation(1). In order to .Saδ = 81+82(1)prevent stop marks, this part must be considered.In the light of Hook's theorem, its fast elastic deformation is: :On the basis of the above formula of slow elasticdeformation, it concluded that slow elastic defommation of theTδ=Efabrics is direct proportional to the tension and the length of cloth.The deformation changes with stoppages time conforming toHere -the length of cloth.exponential regularity. If the tension of cloth is high and length ofE- -the modulus of elasticity of the fast-elastic part of thecloth is long， and the stoppage is long， the slow-elastictested cloche with 1-mer length.defommation is longer; the fell drift toward back rest is severe. ScTension T of slow-elastic part can be expressed as the sum ofthe cloth should be taken-up a litle backward to eliminate the stopelastic tension T2e and damping force T2ηmarks. The limit adjustment is:T= T2。+ T2n(2)8mm= lim (1-刻)=号According to the Hook's Theorem and Newton's Viscose Theorem,T2e and T2n can be expressed as follow respectively:In the light of the above formula of slow-elastic deformation,slow elastic deformation is related to the elastic modulus andT2.=些, T2n=;,最(部)viscose coefficient of cloth. While elastic modulus and viscosecoefficient of cloth are determined by the material properties, theHere, Er- -Elastic modulus of coth's slow elastic part.count and density of yam, as well as the weave structures, the7- Viscose cofficient of cloth's slow-elastic part.size, the air condition and so on3). It is very difficult to masterthese factors and adjust cloth fell to the proper position. I'sSτ=B+最(宁)necessary to minimize stop marks by reducing the tension of warpand fabric during loom stoppages and restoring it before restartingthe loom.oE,-δ2卫.The Model of Electronic Let-off SystemThe solution iswith a Method for Eliminating Stop Marks-(,一明)=易+(3)Electronic let-off system provides the powerful and variousmethods for eliminating stop marks. There are some normnalAccording to the test in normal case， the cloth is undermodels of eliminating stop marks by electronic let-off:certain tension even during beating up. It is considered that the(1) Releasing a certain length of warp as soon as loom stops,cloth tension is small enough during beating up, so cloth tension isminimizing the fell drift by reducing the warp tension during aassumed to be equal to 0 during beating up. Initilly at beating uploom stoppage and restoring it before loom restarts4.r=0 and 82 = 0, so the constant of integration c is:(2) According to diferent period of stoppage time， thecontrol system determinates the warp length released or taken upc=- Im()before loom restart by the test, and restores it into RAM throughkeyboard and display. Before loom restart, control system actuatesSubtitute it into equation(3) , then:takeup and let-off to position the cloth fell at the proper position= 1-8which rests with the data restored in RAM[s].(3) Keeping pick insertion and take-up not run unil the loomspeed builds up normal.中国煤化工is the dnift of cloth fll. Inorδ2= E(1-向)二e best method to recover theposit IYHCNMHGospgePut the result into equation(1):To adjust the position of cloth fell, the cloth fell positionmust be detected at first when loom just stops. The decting(4)8.哥+号(1-赫)method includes contact detection and non-contact detection.Joumal of Dong Hua University (Eng. Ed.) Vol. 19 No.1 (2002) 105Greenwood presented a non-contact method for measuring thepresented that eliminates stop marks by the use of electronic let-offcloth fell position. He used a microscope mounted on the loom andand take-up system. The step motors have been used for let-offthe image was projected on a wall where he measured cloth felland take-up in this system， puting the principle of numericalposition'6)controlled machine tool into it and combine the above-mentionedAnother method is detecting cloth fell with the computer imagemethod to eliminate stop marks.processing technology. The whole system consists of a CCDThe step motor has the specific property as follows: the inputcamera; a computer， processing sofware, the detecting andpulse matches with the step motor's driving angle accurately, thecontroling system of warp tension. The CCD camera is fixed on thestep distance error can't be accumulated; the running speed of theloom breast beam firmly to avoid the fluctuation of vibration. Asstep motor matches the frequency of control pulse accurately ansoon as the loom stops, the CCD camera picks up the image ofmay not be changed with the disturbance of the electric current,cloth fell position and stores it into the computer, meanwhile, thevoltage, load and environment condition.warp tension signal is detected and sent to the computer. Before theIhe running-angel of the step motor can be calculatedloom starts up, the CCD camera picks up the image of cloch fellaccurately, which matches the need of the warp length forposition again. This cloh fell position is compared with the aboveinterweaving with one weft. The warp length feed-in for one pickone. If the difference or drift is found, the warp tension controlis connected with the loom cycle and the average running speed ofsystem will move cloth fell forward or backward, as the CaTthe step motor. During loom's running period， the computercamera detects the cloth fell continuously at the same time until it .reconds the running cycle of loom's main shaft and the averagecoincides with the original position. Meanwhile, warp tensionrunning speed of the step motor. The computer calculates thecontrol system detects and adjusts the warp tension until the warpnumber of electric driving-pulse x accurately which matches thetension reaches the original state. Then, the loom starts up7] .need of warp length for one pick.A New Method to Eliminate Stop MarksAfter our investigation and practice, a new method has beenTake-up rollerNormal position ofWarp beamcloth fell A| 下HCABA- -Normal position of cloth fellB- -Position after release warpC-Position after take upwarp( B:ADaccording to release length aD- Position before loom start upFig.2 Contolling cloth positionThe position A of the cloth fell is nornal when the loom justd = release length a + compensate length b土stops. To minimize cloth fell drift caused by warp tension, theadjust length ccontrol system actuates let-off motor to release a certain length ofBecause of the frictional resistance between wap anwarp until cloth fell reaches position B. The release length a canmachine parts including the back roll, heald wire, reed and so on,be setup by the operator through keyboard and display.even if中国煤化工se length a, the fell canBefore the loom restarts, let-off motor runs backward to takenot retulposition C. So the take-up a certain length of warp until the fell reaches a certain positionup lengMHCNMHGhbs0thatheflllcanfor loom start-up (note: the position needn't be normal positionreaches the nomal position A.A). and the taken-up length d:Because of the mechanical problem analyzed above, even if106Journal of Dong Hua University ( Eng. Ed.) Vol. 19 No.1 (2002)the fell position is at normal when the loorm restarts up, the stopapproximately the relative parameters a, b, c from the specialistmarks usually also arise, so the take-up length d should add orsystem database, and move parameters a, b and c into thesubtract the adjust length c. In most case, c should be added intoworking zone of the RAM. However, sometimes the seup isn'td so that the fell is a ltte backward than the normal to compensateaccurate because each loom's condition is different and the data inthe less beating up force when the loom starts up, and the thinexpert system database isn't rich enough. Usually the parametersplace of fabrics can be prevented. If the thick place of the fabricsneed to be adjusted a litle by operator.arises because of some special problems, d should subtract c sothat the fell is a ltle forward than nonnal to prevent the heavyConclusionsflling bar .The lengh unit of parameter a, b, c and d is the releaseThis new method for eliminating stop marks needn't anlengths of warp during loom at running cycle, it matches with thespecial equipment, it can control the fell position by the powerfulrunning angel of the step motor or the number of electric drivingcontrol program, and the cost is decreased greatly. The method ispulse- -x.so flexible that can be applied to weaving all kinds of fabrics. ThisFor example, if we weave a plain weave cloth， themethod has been applied sessfully.parameters to prevent stop marks are: release length a = 20,compensate length b=3, adjust lengh c= +2.ReferencesThe take-up lengthd= a+ b+ c[1] A.T.M.S. Islam, J. T.1I, 199,90(3), Part 1=20+3+2=252] Chen Yuanfu, Texile Technology and Equipmens, Publishing HouseSo the number of driving pulse for let-off motor runsof Textile Industry, Bejing, 1984 ,332 ~ 333 (in Chinese)backward before the loom starts up is 25x .[3] Lieven Vangheluwe, T.R.J., 1995, 65(9), 540- 544To weave new fabrics, what an operator needs to do is[4] Benneli, D. D., Prevention of Weft Sreaks Afer Loom san-mup,setting up or adjusting the parameters a, b, and c that controlUSP 5 090 452.1992.clobh fell positin. To finish stting up the parameters as quickly as[5] Domier， Meru Description for AT-Rapier Weaving Machines,possble, a specialit system of setting up the parameters has beenVersion 6.00, Lindau, Germany, M1-8- M1-13. p., 1992b,[6] Islam et al, British Patent Priority Application, No.9 627 1432,24designed. To weave new fabrics, the operator only has to inputDec 1996two data- the weft density and the count of weft by keyboard withdisplay. Base on these two data, the control program can find out[7] Crenwood, J.T.I., 1956, 47, T274 ~ 286中国煤化工MYHCNMHG