an upward change in YCCF correlated well with increases in crepe blade vibration WORLD PULP&PAPER 29 Values highlighted in red may be considered out of range. In the context of this article, machines exhibiting a high percent silo fines in coating, what we may term as a Yankee Coating Contamination Factor (YCCF) had poor runnability, characterised by high crepe ratio or low crepe efficiency plus, in some cases, chatter. The relationship between hard coating and crepe blade vibration was explored in a study, over a four month period for the ‘North EU economy and luxury’ producing machine (second from left in Table 2). What became apparent when the YCCF data was transformed via cusum calculation is that an upward change in YCCF correlated well with increases in crepe blade vibration (Figure 6). We speculate that as the Yankee/Coating/blade dynamic may be in a stable equilibrium until a change in coating hardness triggers an increase in coating friction, this is making the blade unstable and vibrating. As is well known, unmitigated blade vibration will propogate both in extent and amplitude until chatter occurs, as was the case for this reference. CONTROL So far, we have established that a number of techniques can be used to characterise the organic coating layer on a Yankee, but amongst these, vibration can be a useful guide to the coating rheology. Moreover, with a knowledge of the add-on and the fines recirculation in the system and other parameters, an index for the contamination factor of the coating can be deterimnined – which correlates well with vibration. Thus, the process engineer could determine the potential for an issue and some of the causes. But what may be done to control them? One obvious solution, and a good one, is to better control the tissue machine wet end. Having determined the problems points with the kind of audit shown above, repeated over different grades and times, we can propose solutions. These may include: • Selection of low fines furnish • Correct refiner plate design and refining strategy to avoid excessive fibre-cutting SEL • Good polymer treatment for any DAF unit • Appropriate disposal or reuse of DAF sludge • Consideration of fines impact of polydic recovered fibre usage • Awareness of fine equilibrium consequences of water closure • Use of controlled short loop retention aid or better fines fixative The latter solution has worked particularly well when there have been other system constraints, such as fibre yield or water closure. However, some trash will nearly always end up on the Yankee, so let us turn our attention to the much neglected cleaning blade. This is a potentially very powerful aid for the tissue maker in the field of Yankee surface management. Any doctor blade will wear against the Yankee surface, faster for a steel doctor, slower for a high performance (normally ceramic) version. However, once the sliding surface wear area reaches say 200 µm, the specific pressure (force per unit area) exerted by the blade is substantially lower then when the new blade had been loaded, and thus coating refreshment, which depends upon the removal of the older hard coating, becomes less effective. This explains the exponential increase in vibration at the end of a blade life on a hard-coated machine. Thus, a frequently changed cleaning blade will help keep the coating fresh, with all its attendant benefits. However, there is an argument against the steel cleaning blade. 1. On a metallised Yankee surface there is the possibility of adhesive blade-Yankee wear if the coating is in any way compromised.1 2. On any Yankee surface, the steel cleaner will wear first against the Figure 6. Crepe blade vibration trend compared with YCCF cusum