20 HEIMBACH 3


i.e. 100mm. It is only through setting the oscillation speed exactly, following the principle of the above example, that continuous coverage and cleaning of the fabric, without any gaps, can be achieved. An oscillation stroke that is too fast will lead to gaps between the cleaned strips (this has to be avoided at all costs); while an oscillation stroke that is too slow will lead to overlaps (this could be tolerated to a certain degree). Some paper machine operators set the oscillation speed at the lowest value and do not adjust it following minor increases in production speed. Fabric Length and Fabric Speed


are the basis of calculating the correct Oscillation Speed in mm per second [mm/s]. The formula is as follows: (Fabric Speed [m/min] x single


Jet Diameter [mm]) / (Fabric Length [m] x 60) = mm/s … applied to our example: (v = 1500 m/min x 1 mm) / (30 m x 60) = 0.833 mm/s In other words, the longer the


fabric, the slower the oscillation speed, and: the slower the machine speed, the slower the oscillation of the shower nozzles. Two essential conditions must be


met to achieve optimal oscillation: Ideally the oscillation speed


would be continually synchronised with the machine speed by means of the drive control, maintaining the calculated ratio of movements. Otherwise it would have to be adjusted manually whenever the machine speed changes. It is essential that the oscillation


drive transmission system does not allow any dwell time to occur at the return point of the oscillation stroke. Dwell times in excess of 0.02 seconds at the return point will be shown as regular zonal stripes in the fabric. This can cause yarn fibrillation and loss of fabric stability leading to sheet profile problems. Oscillation stroke, oscillation speed and the transmission system should be checked at regular intervals.


Conclusion It is hoped that this article has demonstrated that adjustments as small as a single millimetre can often make the difference between optimum or poor functioning, which very often can have consequences all the way down the line and affect the quality of the paper on the reel. The practical implication of this


for continuous conditioning of forming fabrics with high pressure showers is that precise design, installation and adjustment of


the equipment will not only save the inconveniences of malfunctioning, but can lead to substantial time and financial savings. The following case study will provide good evidence for this statement.


4. Case Study: A problem – and its resolution Position: Inner Fabric / Duo Former Problem: Cross direction profile peaks in the sheet with weight variations of up to 3.5 g/m2


Pulp Paper & Logistics


machine (only up to 250mm in from the fabric edge): nothing to report. • No creases or visible deformations of the fabric, however, light optical stripes seen in the problem area. • Fabric tension was a constant 7.5 kN across the whole fabric width – i.e. normal.


in an


area of approximately 430-730mm from the front side edge • The problem started approximately 9-12 days after start up and reaching full production speed.


• The situation deteriorated gradually over the remaining lifetime of the fabric. This led to the fabric having to be taken out early, after only 3-4 weeks of production time. • This was observed on a total of six consecutive forming fabrics of various designs, including two Primobond fabrics from Heimbach.


4.1 Identification of the cause and analysis by Heimbach TASK- Division specialists of how the problem developed Results of first examinations and measurements taken: • No wear on paper or machine side of the fabric surfaces in the problem area as identified above. • Fabric caliper measurement on


First laboratory results of the fabrics When examined in the lab, the fabrics revealed the presence of internal abrasion in the problem area, which had been caused through extreme wear on both machine and cross machine direction yarns (Figure 22). It became apparent that the problem area, located between 430 and 730mm inside the FS edge, had been exposed to strong forces, causing the yarn abrasion and resulting in compression of the fabric structure (Figure 23). No similar damage was found in any other areas across the remaining width of the fabrics. Fabric damage in the problem


area, which could be regarded as a plausible cause for the cross direction profile peaks, had been identified – but the reasons for the development of the profile peaks as well as the cause/s of the fabric damage remained to be identified.


Further measurements – investigation of causes Water permeability measurements


Fig. 23 Caliper profile in the problem area May 2013


Fig. 24 Wire-Perm Measurement (Resistance to flow)


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36