Web Handling & Converting

How soft must a rubber-covered roller be in order to feel soft (i.e. to be effective) to improve winding?  This question must be posed and answered correctly for each situation because rubber covers are more expensive than plain steel AND will increase the risk of wrinkling due to both wear and a higher web-roller COF.  Conversely, the claim is made by some is that the rubber cover will conform to the uneven (ridges and valleys of the) wound roll surface better; somehow making a more uniform winding profile.  This question is not settled by the machine builders, even though the great majority layon rollers are plain steel.  (Remember argumentum ad verecundiam of previous post?)  Well, it turns out that science, more specifically, Hertzian contact mechanics has the answer.

The simple answer is this.  The rubber cover must be as soft or softer than the wound roll itself.  If the wound roll has a P&J (one of many hardness measuring instruments also including Shore A Durometer) of X, then the cover must be at least as soft as X.  If you can dent the wound roll Y mm with a thumb press, than you must be able to dent the cover at least Y mm with your thumb.  If you think about this a bit; you may come to the conclusion that the cover might need to be more foam-like than rubber-like in hardness for some/many wound rolls.  Covers that soft will add one more cost to the two listed above:  increased maintenance costs as covers need to be replaced more frequently due to wear-induced wrinkling.

The first order mechanics behind this are simple enough.  That is the two cylinders, roll and roller, are modeled as springs in series.  If you write the equation for springs in series, I won’t bore you here, you will see that the softest element is determining.  In other words, a rubber cover that is not softer than the winding roll will act effectively like it is steel, except that it wears out faster and may increase wrinkling.  Caveat emptor.