176 MODERN PAPER-MAKING
wire wet should be left on. Some managers insist that all water be shut off
in the machine room, but when this is done suction boxes, couch-roll covers,
apron and connecting straps dry up and get spoiled by shrinkage, etc., with
loss of time and bad work when starting up again. But unless every part of
the wire is kept wet, or if a draught dries up a space, a 'tide mark' will be
formed which is very difficult to clean. A very old custom is to leave the wire
seam over the week-end in the nip of the couch rolls, 'to keep it flat', but it
is obvious that it can do no good, but only harm, for the seam to be under
heavy pressure for any length of time. The best place to leave the seam is on
the breast roll.
The second set of factors which influence the life of the wire are, generally
speaking, constant and unavoidable, though some may be minimised if proper
precautions are taken. One of the worst enemies of the wire is the necessity
of using hard water, which forms deposits of calcium carbonate. The latter,
however, and the general clogging up of the mesh of the wire by loading, size,
etc., are to a very great extent preventable and depend on several things, which
will be touched on later.
As far as the machineman is concerned, it will be found that if he adjusts
his back-water system carefully he will use the same water over and over
again, with very litde addition of fresh water, which causes the deposits. Except
where special treatment of whitewater is carried out, fresh water must of
necessity be used in the sprays of the wire rolls, so that after a period the wire
clogs up and must be cleaned. A steam force jet, judiciously used—say once
a week—will help to lengthen this period, but there are now special cleaning
agents on the market which clean the wires with less damage than sulphuric
acid.
Hydrochloric acid may be used to clean the wire of limy deposits. A
50 per cent solution from an earthenware or glass bottle should be applied to
the top of the wire with an old scrubbing brush, and washed out with the force
jet before the solution reaches the couch roll.
Then there is the wear of the wire by the rolls and tops of the suction boxes.
As far as the rolls are concerned, they should be kept well lubricated, and
especially the tube rolls which form the table. Also, any side play on their
spindles or brasses must be watched for, as in this case the action of the shake
will make the wear much greater. The weight of the breast roll is important.
Very often it is made too heavy, thus imposing an unnecessary strain on the
wire in pulling it round, and more so when starting up. Deckle straps, where
there are many changes of sorts, make the wire dirty and cause more or less
ridging; so also do the movable ends of the suction boxes, owing to there being
a pull over their inner edges. The shake has a twisting action on the wire