I found this on another thread - should help - good luck
member name -
"otherthoughts"
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It's been many years since I've run a press, maybe 18 years now. However even back then, our sheet-fed presses had split tail-clamps to address "sheet fan-out". To explain what sheet fan-out is, it's the progressive growth of the sheet's width at the tail end as it travels from the first down unit to the last down unit.
To illustrate, imagine the first down color you are concerned with is the black printer. With each successive "Squeeze" the sheet fans-out like bread dough squeezed by a rolling pin. This 1st down black ink image printed on the sheet is also fanned-out more and more as the subsequent impressions occur. In other words, for a sheet-fed KCMY ink sequence, the first down black ink would seem to grow progressively wider near the tail end with respect to the downstream units, typically with the last down unit seeming to print the most narrow.
The split tail-clamps I mentioned earlier, allowed me to force the tail end of the 1st down
plate inwards slightly and stretch the downstream plates outward slightly as needed to counteract any fan-out.
A web press may have a method of achieving a similar task via cylinder skewing or some other method(maybe not). One thing I suspect is common to both web and sheet-fed presses is that the greater the "squeeze" applied to the sheet, the greater the fan-out. Another factor that i suspect is common to both is that, the thinner and larger the stock, the greater the fan-out.
The best way I can think of to determine if it is indeed fan-out you are dealing with, is to compare the first down ink's plate against the resulting printed sheet. Trim the sheet/cutoff as needed near the tail end and compare it carefully with the 1st down inks plate(you should probably remove the plate from the press in order to determine anything or convince anyone). If a sheet/cutoff's 1st down imprint on the sheet/cutoff has grown in width near it's end when compared against the plate that printed it in the first place, how else would you explain it?
If the very same sheet/cutoff compares accurately to the last down inks printing plate, it is further proof that fan-out occurred between the intervening units. For further proof, carefully trim the 1st down plate as needed near the tail and lay it upon the last down unit's plate and see if they match widths with one another, if they do, why don't they match on the press sheet/cutoff? Finally introduce the sheet/cutoff as well to settle things and see what matches what.
As many have indicated before me, the press-room is the much more likely suspect for this problem. I hope that the tests I suggested will reveal the source of the problem. The tests I suggested will likely require enough skill in execution so that one does not warp the plates or the sheet/cutoff. A scope with a 25x magnification or better would likely be needed as well. I should also mention that the tests I suggested should be considered as evidence only, they are only as good as the person's skill performing them. Regardless, no one in their right mind would consider the results of these tests absolute proof.
A dubious test is better than no test at all, the way I see it.