Papermachine clothing (felt or fabric) travels over various slots in
its excursion. They include:- pick-up shoe
- Uhle box
- dewatering slots
In the following discussion, felt will be used generically to
represent any paper machine felt or fabric; in forming, pressing
and drying sections.
On account of the following forces there is a positive contact
between the felt and the lips on both sides of the slots:
- felt tension
- suction vacuum
The suction force is direct, whereas tension acts in a way similar
to a belt contacting a pulley. Contact force on the lips increases
with tension and the wrap angle, the included angle between the
incoming and outgoing planes of the felt.
The rubbing of the lips by the felt is akin to a sanding belt.
The rough surface of the felt is embedded with wood fiber.
Although water acts like a lubricant,the felt traveling at 60 mph will
wear out the lip surfaces no matter how hard they are.
Wear is not limited to the lips only. The surface of the felt
rubbing over the lips is also going to suffer over the long run.
The leading edge of the lip acts as a scraper blade against the felt.
We do not have a way to separate the wear against the lips from
other sources of wear, and is therefore not paid attention to.
In an ideal case the surface of the lips will wear uniformly in the
cross direction. In practice the lip surfaces develop a wavy wear
pattern. Some locations will develop gullies while other locations
hardly show any wear. This could be for the following reasons:
- Clothing has inherently a CD variation,
- Consequently tension will vary in the CD
- When running with a smile, shear wrinkles could develop
- There is boundary effect at the edges due to side leakage
A Note on Shear Wrinkles:
A felt in its unstressed state or under a uniform pull across is alamina, a flat surface. When running a smile this pull is largerin the edges than in the middle because of the longer rope lengthat the edges, causing a buckling of the MD strands which may bein compression. This is akin to the buckling corrugations of thefabric covered wing surface of a plane due to wind loading. Theformation of these wrinkles in a felt can be minimized bycrowning one of the felt rolls negatively (See section on 'NegativeCrown Rolls').
WEBCON'S METHOD
We have created a mathematical model of the contact phenomenon of
a taut felt against- a single vacuum slot
- two or more slots in tandem
The input to the computer program consists of
- felt elastic properties, weight and width
- geometry of felt against the lips
- felt tension and vacuum level
The program outputs the following:
- shape of the lips as x,y coords
- best arc fit if desired
Webcon will provide an Autocad drawing for the lip as well as
the end plugs with all machining instructions. Drawing of the
existing suction box will be required.
