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February 5, 2015

If you have two friction plates that are identical, except for the inside diameter of the friction material, then you need to identify the mean effective radius of each plate.

Mean effective radius is the distance from the theoretical center of the friction plate to the center of the friction material. We can identify this dimension by measuring the outside radius and the inside radius, then dividing by 2 = mean effective radius. The reason some friction plates may be different is based on torque load capacity during apply. Contrary to popular belief, friction plates having the same outside radius, but with a larger inside radius will have a higher torque load capacity than one with a smaller inside radius. The following illustrations will help further explain. Figure 1 shows a mean effective radius of 3.00" on a friction plate with a 4.00" outside radius and 2.00" inside radius. To calculate the mean effective radius, add 4.00" outside radius + 2.00" inside radius = 6.00". Divide by 2 to find the average radius. In this case, mean effective radius is 3.00". Figure 2 demonstrates what happens if we change the inside radius to 3.00". To calculate the mean effective radius, add 4.00" outside radius + 3.00" inside radius = 7.00". Divide by 2 to find the average radius. In this case, mean effective radius is 3.50".

What this means is the larger the mean effective radius, the more torque load capacity the friction plate can handle. Picture a teeter totter with the fulcrum in the middle. Now place a 100 lb weight on the far right side. On the far left side, place a 100lb weight like the diagram in Figure 3. The amount and location of the weight on the left determines how easy it is to move the weight on the right.

Now move the weight inward (it doesn't matter how far as long as it is to the left of the fulcrum) like in Figure 4. Moving the weight to the right increases the difficulty of displacing the weight on the right of the fulcrum.

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