I78 MODERN PAPEK-MAKING
from being rubbed and frayed. A few more turns will show whether the
guide roll is set so as to be able to keep the wire in position.
The stretch roll may now be put down a little if it is light and experience
shows that it is necessary. A heavy roll will not require to be put down,
but its spindles should have enough pressure to keep it from jumping. On
starting again, if the seam is fairly straight and the tension from the boxes to
the couch rolls is equal on both, sides, it may be assumed that the wire is correct
for a start-up with paper.
There is, however, a very great difference in the running of a wire without
stuff and with stuff In the latter case the suction boxes hold the wire, and
the couch rolls have to pull it across them, thus creating a tension which is
very much greater at this point than anywhere else. Therefore the top coucher
has the most important influence on the wire. The slightest difference in the
pressure applied alters the tension between the boxes and the nip. This may
possibly be so much that the wire is extremely tight at one side and so slack at
the other that it is impossible to keep it in the centre of the machine, the guide .
roll being unable to send it back to its place.
Then the tension of the stretch roll will be found to be greatest on that
side where the tension from the boxes to the nip is least. It is here that a
machineman may make a mistake by altering the stretch roll in an attempt to
straighten the wire. Instead, weight must be taken off the couch roll on the
side which is slack, between the boxes and the couch roll, until both sides have
the same tension. Then the seam should be carefully watched, and if any
alteration is necessary, the stretch roll may be put down on the side which is
leading. The alteration may change the tension at the nip a little, and both
sides of the wire must be carefully watched until the tension remains the same
and the seam is straight. It will then be found that the wire will keep well
in position and be guided by the ordinary automatic movement of the guide
roll*
If the stretch roll is not quite level, this is immaterial so long as the seam is
straight and the tension is correct, since some wires appear to have a slack side.
This is very difficult to decide, because it is possible for this appearance to be
caused by the inclination of the couch rolls or wear in any of the brasses of
the wire or breast rolls. For this reason it is very much worth while for the
engineers to test these things when the wire is being renewed. Li particular,
the back-side brasses 'of the bottom couch roll tend to have more wear owing
to the pull of the clutch, and perhaps to having less attention paid to them in
regard to cleaning and oiling. When this happens the back shaft and the couch
rolls are not io, alignment, and the result is an eccentric action, in the clutch
which causes a rise and fall of the couchers.