Negative Crowning to Remove CD Nonuniformity

• drainage
  
• pick-up
  
• drying
  
• cleaning
  
• wear
  
• width of the clothing

PM Negative Crown

WHAT CAUSES THE SMILE?

1. Bending of the rolls:

The felt rolls are usually slender and bend in the middle under

• felt tension
  
• internal vacuum
  
• gravity.
  

The middle of the rolls is pulled inwards so that the felt path length in the middle is less than that at the edges. The felt will have less distance to traverse in the middle.

2. Pressure Roll Crown

The pressure roll is usually the driving roll, and it is often crowned. Therefore, it tends to drive the felt faster in the middle where the roll diameter is larger.

The cumulative effect is that

• the felt has less distance to cover in the middle, and
  
• it traverses it at a faster speed.
  

As a result the middle creeps up every loop run, until the force due to the lateral stiffness of the felt balances the above distorting friction force at the roll contact, and equilibrium sets in.

EFFECTS OF THE SMILE

The middle of the clothing is not affected by the smile. In the case of felt, the fibers in the middle move closer to each other towards the edges. In the case of a forming fabric, the open cells distort from a, say, rectangular shape to a trapezoidal shape. In either case, the clothing properties are affected from the middle to the edges as explained below, where the term felt is used to represent both felt and fabric.

1. Performance Effects

• Density: Felt becomes denser towards the edges. The density is inversely proportional to the cosine of the slope angle of the trade line.
• Porosity: Denser felt constricts the flow cross section.
• Pressure: Denser felt makes a harder nip, which increases the nip pressure towards the edges.

The sectionwise impact of the above changes is as follows:

• Forming: Drainage will be uneven across the deckle, and hence nonuniform formation.
• Pick-up: Suction will be less near the edges which could affect even pick-up.
• Pressing: Water will not be removed evenly leaving the edges wetter.
• Drying : Air flow prefers the path of least resistance.
• Cleaning: Cleaning showers will be less effective where the felt is compacted.

2. Mechanical Effects

• Felt Width:

  The fact that the cross strands of the felt are fixed in length, any bowing of the fabric cross strands will reduce the width of the felt; the arc length of the strands remaining the same. This narrowing of the felt is usually about 1% of the felt width. Thus minor savings in felt cost ensue if we straighten the trade line. It is imperative that we buy narrower felt when we straighten the trade line otherwise the felt may run over the edges of the felt rolls.

• Felt Wear:

  A felt running bowed will have greater variation in mechanical properties in the cross direction. The edges are denser and harder (spring constant). Also there are more fibers per unit area. Slip across the felt is greater. As a result, a straightened felt wears less.

THE SOLUTION: NEGATIVE CROWNING

One of the rolls is crowned negatively, i.e., the diameter in the middle of the roll is less than at the ends. It corrects the bow in two ways:

• the smaller diameter in the middle slows down the felt in the middle
• the smaller diameter in the middle reduces the length of the rope run there, though by a small amount.

WEBCON'S METHOD OF STRAIGHTENING THE BOW

WEBCON has developed a mathematical model of the bowing phenomenon. It is an iterative scheme which accounts for the causes explained above and calculates the negative crown for one of the felt rolls in the felt run. The roll has to be selected judicially to keep the crown small. In extreme cases crown on two or more rolls can also be computed.

The computing procedure is as follows:

Input:

• Scaled drawing of the felt run of the papermachine, or measure the x-y coords of the felt rolls to make the drawing.

• Roll properties: inside and outside diameters
  
  • roll material
    
  • roll face and bearing span
    
  • crowning on any roll
    

• Other data
  
  • vacuum in any roll and vacuum geometry
  • felt properties or felt sample

Output:

• Identification of the rolls most appropriate for crowning
  
• Amount of the negative crown and degree of cosine curve
  

CASE HISTORIES

• In a 200+" wide light-dry-crepe (LDC) papermachine running at over a mile a minute the negative crown dropped the bow from 14" to almost zero; and the felt life jumped from 26 days to 45-50 days.

• Negative crown was prescribed for a number of other machines (through-dry, heavy-wet-crepe) ; but no feedback was available from the papermakers, except that they were satisfied.