For the purpose of calculating friction effects, a planar joint is modeled as one block sliding and rotating across the surface of another block.
Dimension 1 (d1) is the length of the sliding block.
Dimension 2 (d2) is the width of the sliding block.
Dimension 3 (d3) is the radius of a circle, centered at the center of the block face that circumscribes the face of the sliding block that is in contact with the other block.