How to determine the right Fuji Film type?
Our Fuji pressure sensitive film is extremely helpful when investigating the press load distribution of a paper machine. The film helps you to identify misalignment, crowning deviations and find roll cover problems like delamination or soft/hard spots. Feltest offers several sensitivities of Fuji Film, but sometimes users find it difficult to select the correct type. Maybe this article can shed some light.
The Fuji pressure sensitive film comes in several pressure ranges, I guess that makes sense. Fuji uses the ISO unit for pressure, being MPa (Mega Pascal). Unfortunately there are very few paper machines that indicate their press load in MPa, so the question is: how do I convert my press load reading into MPa?
The unit MPa equals a to force of 1 N on an area of 1 mm2, or 145 psi for our North American customers. Most paper machines indicate a linear pressure for their press nips, for example 100 kN/m or 561 pli. This is a rather theoretical unit as it is assuming that the press load force is working on an area line with a certain length (a meter or an inch) and an infinite narrow width.
In reality your press nip does not have an infinite narrow width. On the contrary: depending on roll diameters, roll cover materials and the applied load it can be between 5 and >25 mm (1/5 to > 1”). The softer the roll cover materials, the wider the nip will become. As a papermaker you should be able to get more detailed information from your roll cover supplier or the machine builder who supplied the press.
An example:
Read the normal press load, under running conditions, in kN/m. For example: 100 kN/m
Take the width of the machine, or more precisely: the length of the contact area of two mating the press rolls. In this example we’ll take 5 m.
When you multiply these you will get the total force in this nip: 100 kN/m * 5 m = 500 kN.
This total force works on an area of 5 m long and several mm wide. If we have a hard nip, like steel on granite, it is estimated to be 8 mm wide.
Now we get to the true pressure in the nip: 500,000 N works on an area of 5000 x 8 = 40,000 mm2 which equals 12.5 N/mm2 or 12.5 MPa.
In this example the Fuji film with medium sensitivity (MS or MW) with a range of 10 to 50 MPa could be used.
And a shortcut:
If you’re good with math you probably already saw a shortcut in the 6 steps above: simply divide the nip load in kN/m by the nip length (in MD) in mm and you’ll get the same result. So 100 [kN/m] / 8 [mm] = 12.5 MPa!
One important note: on some older machines the press load is indicated in a pressure unit, like bar or psi. In almost every case this refers to the air or oil pressure in the bellows; it has no relation to the contact pressure between the two mating rolls! In situations like this you first need to convert the bellow pressure to a linear press load and then go through steps 1 to 6 above.
I hope the above has been helpful.
So remember DDPP - Daily Data Powers Production.
Toward better Paper Machine Clothing,
Marcel