By: Mike Kos (Sulzer Textiles)
By analyzing the causes of weft stoppages professionally and taking appropriate measures on the weaving machine, the number of weft breaks can be reduced. Besides the higher efficiency thus achieved, the weaver�s work is greatly simplified. The experienced specialists at Sulzer Textil TTC (Textile Technology Consulting) support Sultex customers with qualified technical advice.
The subject of this article is the analysis of potential causes of weft stoppages and measures to eliminate them.
Most warp stoppages are not due to the system. However, depending on the weft insertion system � projectile, rapier or air-jet � many weft stoppages are system-related. In weaving mills operating with different weaving systems, a thorough knowledge of the systems in question is a precondition of accurate stoppage analyses. Different, appropriately designed report forms are available for each weft insertion system.
Yarn quality and make-up:
As production speeds increase, ever higher demands are placed on the yarn and its make-up. Yarns have to comply with Sultex�s guidelines � analogous to the USTER STATISTICS. Only thus can the high weft insertion rates of modern shuttleless weaving machines be achieved with good running characteristics.
Low weft yarn quality not only impacts on weaving machine efficiency. Depending on how the machine is equipped it may also severely impair fabric quality. Also, yarn breaks may occur that do not necessarily cause stoppages. Examples include stop picks on air-jet weaving machines and very late yarn breaks in the weft tensioning phase on projectile weaving machines.
Based on the results of an analysis, the right decisions must be made in the spinning or winding room and appropriate action taken. Special attention and expertise is needed, because weak points and splices have to be clearly distinguished. Also, in the weave room, it has to be established whether a break at a weak point occurred in front of or on the weft feeder, or not until the weft was in the shed. Only if these preconditions are met can potential causes of stoppages be counteracted.
Yarn reserve � connecting to success:
Yarn make-up is important with regard to high weft insertion rates. In particular, this issue addresses the yarn reserve and thus the switchover from the empty to the full package. Stoppages that concern the yarn reserve can in general be assigned to one of two categories:
� Absence of a yarn reserve
� No knotting or incorrect knotting of the yarn reserve.
In theory, modern winding and spinning machines (OE) can produce supply packages that satisfy weavers� criteria. Despite this, in practice we repeatedly come across packages with insufficient yarn reserve or no reserve at all, frequently due to incorrect setting of the spinning and winding machines. Appropriate care must also be taken during transport and handling, because there is a risk that packages which were originally in perfect condition will be damaged.
Inadequate personnel training, especially in the weave room, or excessive stress on weaving personnel for organizational reasons are the most common reasons why yarn reserves are not � or inadequately � knotted.
Stoppages during package changes may be due to other causes. Preconditions for trouble-free reduction of a yarn package down to the last winding, and successful changeover from one package to another, are not only a neat yarn reserve but also conformity with other key parameters such as the material, form and characteristics of the tube. For this reason, when analyzing transfer faults it is essential to investigate whether a stoppage was caused by a damaged pack-age tube or a slipped weft yarn winding.
Typical causes of stoppages on air-jet weaving machines:
In weft direction, the same as with warp stoppages, the place where the stoppage occurs is crucial. As examples we will take loops and snarls, which are the main cause of stoppages on air-jet weaving systems. Depending on where they occur (on the left, in the middle, on the right), these two classes of faults can usefully be divided into the following categories when recording stoppages:
� Weft loop close to the left selvedge
� Weft loop in the shed mid-way along the weaving width
� Weft loop on the right in the temple region
� snarls in the shed
� snarls in the drawing-in area on the right
� snarls at the pick tip, mainly on the right
There are many possible causes of weft loops that occur at the left selvedge, e.g. selvedge draw-in, the settings of the shed and main nozzles, and the cutting time and condition of the weft cutter. If weft loops and snarls mainly occur in the centre and on the right of the drawing-in area, the cause should be looked for in the blow times and air pressure of the relay nozzles, in the reed and in warp preparation. Snarls on the far right are often due to the weft arrival time, that is, the air pressure setting. However, they may also be due to the simultaneous occurrence of several, if not all of the above-mentioned causes.
Transfer errors, �bite-offs� or weak points:
Transfer errors are typical system-related faults that occur on projectile and rapier weaving machines. Depending on the weft insertion system, the cause may be the yarn clamps (mechanical) or in machine components such as the weft brakes, the centring blades or the weft cutter, or fibre fly in the clamp. This class of faults is identified by the cut-off appearance of the yarn end, which in this case will be on the left-hand side of the weaving machine.
When weaving fine yarns or yarns with low tensile strength, the wrong projectile grippers � either damaged or not appropriate for the weft � result in so-called �bite-offs�. The fault �crushed by yarn clamp� on rapier machines also falls into this category.
If a �bite-off� occurs at the moment of picking, the result is at first sight identical to a transfer fault. It is almost impossible to tell the difference between this and a yarn break at a weak point immediately behind the yarn clamp. In both cases, clear identification of the cause of the fault � transfer error, �bite-off� or yarn break at a weak point � is only possible by inspecting the projectile or its gripper for yarn remnants in the receiving unit.
On rapier weaving machines, a similar inspection must be carried out on the gripper clamps in such situations.
On projectile weaving machines, lost picks are identified by cut yarn ends that also show pressure marks from the projectile gripper. In this case also, if the cause cannot be identified beyond doubt, the projectile must again be inspected in the receiving unit. Possible causes of lost picks are incorrectly adjusted weft brakes, unsuitable or defective projectile grippers or faults in the weft feeder.
Stoppage analyses � a never-ending topic:
The many and to some extent similar potential causes of stoppages, which have a negative impact on both efficiency and fabric quality, once again show the importance of professionally conducted manu-al stoppage analyses. Maintaining complete records helps ensure that the causes of stoppages are ana-lyzed quickly and correctly, that the right decisions are taken, and that measures are implemented which take effect quickly and lead to success. The TTC of Sultex AG uses stoppage analyses of this kind as a basis for its assessments, and offers competent sup�port for Sultex customers, also in their own weaving facilities.
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