Friction Clutch Mean Radius and Torque Capacity

Question: I have 2 friction plates that are identical except for the inside diameter of the friction material. Should I use the parts with a wider cross section? I have asked others and have heard the term "mean radius." Can you explain what that means and if I should be careful about what I specify?Answer: 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, addTo 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 1b weight on the far right side. On the far left side, place a 1001b weight like the diagram in Figure 3The 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 thefulcrum like in figure 4)In other words, as the mean effective radius gets closer to the center of the friction plate, the less effective it becomes at transmitting torque; just like moving the weight closer to the fulcrum makes it harder to lift the weight on the other side of the fulcrum.The larger inside diameter of the friction plate increases the mean effective radius andFigure 4 thereby increases torque capacity, which is beneficial; however, when the contact area is reduced as in Figure 2, net surface pressure is increased on the part.If the expected surface pressure is moderate, the friction plate with the larger inside radius would likely provide acceptable durability and increased capacity without adding more friction surfaces; however, friction plates having similar outside radii with a larger inside radii are not recommended in applications with high surface pressure. Pressures above design limits will adversely impact clutch durability. Ideally, if space was not a factor, we would increase both the outside and inside radius proportionally and improve capacity, just like we would by adding additional facings.