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What is Roll Roughness? 5 Roll Ra’s and Why They Are Important

Roll surface roughness is a critical parameter to consider when creating different products. Having too smooth of a surface can cause problems while having too rough of a surface can cause issues as well. When describing surface roughness, there are many different units that can be used for reference. These values are determined based on the micro-deviations along the length of a surface. We always see what the surface value is for a roll, but why do we choose these roughness values? After we discuss what gives a roll its roughness value, we will give five examples of rolls that require a variety of different roughness’.

Surface Roughness
What is Surface Roughness?
Surface roughness, as defined by Keyence, is a shape that has a variety of peaks and valleys along a shape. If a person were to look at the surface of a material with a microscope, they would see the topographical view of these peaks and valleys. A surface with a larger variation in amplitudes will have a rougher textured surface, whereas a smaller variation will be smoother. By defining a nominal axis for these peaks and valleys to be measured on, numerical values can be given to these points.

What is A Common Unit for Surface Roughness?
There are many different units for defining surface roughness—but roughness average, or Ra, is typically used for standard metal finishes. Ra is always a positive number and is calculated by taking the absolute average deviations over a linear distance. A larger number of deviations will lead to a larger Ra number!

A Little More Technical for Calculating Ra:
As noted in Mikell P. Groover’s textbook Principles of Modern Manufacturing 5th Edition, the mathematical equation for the mean value of roughness is:

Ra = Mean roughness value
y = The vertical distance from the defined nominal
Lm = The length of the collection series

 This equation can be simplified— allowing for a more visually conceptual understanding:

n = The nth position along the profile’s length
yi = The vertical distance at the nth position
Figure one gives a more visual understanding of what is happening during the data collection process.
Along the length of the profile, Lm, there are an nth number of height deviations collected along the
length of the profile. Those deviations are collected (at an absolute value) and summed together. The average of these values is what gives the final Ra number. This can be depicted in both micrometers and micro-inches-- depending on capabilities of the measuring tool used.

                           Figure One: Profile Deviations Characterizing Surface Roughness

Ra vs RMS:
Another common roughness unit that may be seen in industry is the root mean
squared, or RMS. Both methods give a representation of roughness, however the RMS will have an inherently larger number than Ra. The way RMS is calculated, any large peak variation can lead to an inflated value, which is why Ra is typically preferred. Check out this link for more information on the difference between Ra and RMS.
Five Different Roll Roughness’s 
1.     Flaker Rolls (130-150)
Roughness is an important characteristic of flaker rolls during the seed oil production process. When seed oil is processed through a set of rolls, they are flattened and sheared to an ideally thick flake. If the flake is too thick, all the oil cannot be removed from the bean. A rougher roll is needed to allow for appropriate flake processing, leading to an Ra between 130 and 150.
Similar Ra: Divider Shelf (Granulated Texture)

2.     Paper Machine Wet End Felt Rolls (30-50)
The wet end felt rolls of a paper machine typically have an Ra value between 30-50. These rolls are used to support the rotating wet felt, which carries paper pulp into the press section. Having the rolls between 30 Ra and 50 Ra gives enough roughness for the felt to drag along the roll without causing slippage. However, having too high of a roughness can create excess friction between the two, possibly damaging the felt.
Similar Ra: Coffee Creamer Container (Smooth, Waxy Texture)
3.     Press Section Rubber Rolls (10-20)
Press section rolls are larger rolls that are used to press water out of paper. When paper fibers enter the press section, the product is normally 75% water compared to 25% fiber. The press rolls are used to compress the fibers together, which is critical to paper strength characteristics and reducing water percentage. Where press rolls contact the paper, surface roughness is critical to product smoothness.  
Similar Ra: Filing Cabinet (Sheen, Smooth Texture)
4.     Calender Section (2-5)
The importance of paper machine calender rolls lies in the final printing characteristics of a product. Paper will need a specific surface quality that is dependent on what the final product will be used for. Whether that is for creating containerboard or printer paper, a proper roll finish is needed to give the best control for meeting caliper and smoothness characteristics.
Similar Ra Value: Whiteboard (Very Smooth Texture)

5.     Coatings/Adhesives (0.5-2) Window
In some processes, adhesives are applied to a base sheet using a chrome metering roll. Coating uniformity is critical from a cost and quality perspective. Too rough of a roll can cause the adhesive on the roll to stick, causing excess or missing material in some locations. This can lead to wasted adhesive in the manufacturing process or product that may need to be recycled due to quality issues. Uniformly smooth surface roughness on the metering roll allows for minimum adhesive material usage while promoting perfect quality product.
Similar Ra Value: Glass Window (Extremely Smooth, Very Little Roughness)

Unsure what the roughness of your rolls should be? Contact Precision Roll Grinders for a free consultation on the rolls used in your production process!
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