The coefficient of friction (commonly given the symbol "µ") is based on the ratio of the force pressing two objects together and the force required to create a sliding motion between the two objects. Also factored into the equation is the amount of contact area between the two objects. In our business, these two components of the equation are controlled by the tension on the line (which creates the force that presses the coil and the roll together) and the degree of wrap of the coil around the roll (which determines the contact area). Since the coefficient of friction is a mathematical result of the line tension and the degree of wrap, we can compare the friction performance of various rolls on any line given the wrap and tension. Our work was done with a wrap of 180 degrees.

In general, the higher the coefficient of friction, the better the grip will be between two surfaces. However, the slipping of a coil is greatly affected by other materials at the interface. Dirt on the coil or the roll will increase the grip and prevent slippage. On the other hand, any substance that acts like a lubricant will lower the grip and increase slippage. For this report, we will disregard the dirt and other contaminants since we cannot control them. We have addressed the influence of the lubrication caused by water or oil by calculating on rolls that are dry, saturated with water and saturated with oil.

Another major variable is which type of metal coil is being processed. We tested five different metals:

.023" carbon steel
.017" aluminum
.015" stainless steel
.013" copper, and
.010" 725 nickel

Since the running conditions are a choice controlled by the account and the specific application, the values are presented on three separate tables for dry rolls, wet rolls and oily rolls. Each table shows the coefficient of friction values measured between six test rolls and the five metals tested. Hopefully the conditions covered on one of the tables will match your specific account application. Remember, higher numbers mean better grip.

Table 1, dealing with dry roll surfaces, shows the 3M Neutral and RK values to be the highest. In some cases, the competitive roll approach the 3M roll performance. On dry roll surfaces, the 3M Acid and C rolls frequently show comparable friction values with the competitive rolls.

With the addition of water and its lubrication effects, the differences become more pronounced. Table 2 shows the 3M Neutral and RK rolls to have the highest values. Again, the 3M Acid and C rolls are competitive with the Toho and URB rolls.

Table 3 shows even greater friction differences with the influence of oil on the roll. Again, the highest values are provided by the 3M Neutral and 3M RK rolls - except for the 3M Type C roll on nickel. The grip provided by the Toho and URB tolls is substantially less than the grip of at least one of the 3M rolls (Neutral, RK or Type C).

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