Static friction over driven roll, slippage issue /paper machines/

[knas]

Hello All,

on paper machine which I work we have such configurations as is showed on the sketch:
http://imageshack.us/photo/my-images/706/systemt.jpg/

a forming fabric (wire) is driven by two rolls, couch roll (with two suctions chamber) and so called FDR (forward drive roll). On the forming fabric the paper is formed and dewatered.

A pick up roll just picks up and transfers the web / paper to the press section.

the problem with understanding slippage phenomena I have on the FDR.
As far as I know we need a static friction between wire and FDR roll to transmit energy. When we get a motion between FDR and wire we got a slippage.
Based on some formulas I am able to calculate a power transmitability capability. In the formula, the roll diameter, static friction coefficient, speed, wire tension after roll, width of the fabric are involved.

The questions to You are:
- What is more dominating on such power transfer a static friction or dynamic ? (I get confusing explanations) must be static…
- I know that there are so called two working angles (alpha and beta), for this specific roll alpha is 70 degree and beta 55 degree. On the beta angle as far as I know we get a micro-slippage which gives power loss by ~2%, do you have any experience on this field ?
- In general friction forces are independent of the area of contact, but we know also that more yarns of the fabric on cross direction will give at the same time lower specific pressure between two surfaces so again increase risk of slippage ?
- What from your experience can increase slippage risk?
- Do you think a centrifugal force can have an impact ?

Some additional data:
- Materials involved: on the roll rubber, hardness ~18PJ and on the wire polyester, the friction coefficient is ~0,2 between them
- Speed of the wire is ~16 m/s
- The engine power is 900 kW, working in the range up to 70% load
- Roll diameter is around 1 m while a forming fabric is 1 mm…
- The tension of the wire after the FDR is around 8,5 kg/cm.

If you have any questions or need more data please let me know.


many thanks for any answer / comment.
reagrds,
knas

 

[turbo]

Calculating wire slippage is quite complicated. Depending on the thickness of the stock, and the amount of fines in the stock, even the limited wrap on a couch roll can be nearly negated by the vacuum pulling on the wire/stock. Once the sheet has been transferred from the wire to the press' pick-up roll, you need a good deal of wrap on subsequent driven rolls.

Even on paper machines specifically designed for a specific weight/grade of paper, it often takes a bit of face-time (hopefully no hollering) between the paper machine-builders' field engineers and the fabric vendors' field techs to strike a balance between the need for openness and de-watering and the need for proper drive, sheet-smoothness, etc. You could write entire books about this stuff, so it will be tough to give you concise answers.