Dimensioning of webbing roller

The minimum allowable bend radius of your web should be your only restriction.

Thank you for your reply :)

I have been told that the shape of the roller is also vital to the lifespan of the webbing...

And how do I find the minimal bend radius?

I have purchached a MIL-W-4088 type 18 milspec webbing, and no minimal bend radius is specified....

The manufacturer should be able to provide. If not, then stick with the 10-15X your thickness convention. Another consideration is "tracking" your web over the roller - a very small misalignment will cause your web to "walk" across the face of the roller. One way to prevent this would be to machine a track into the face of the roller so that there is a raised edge on each side to restrain the web. However, this approach can cause excess wear on the edges of the web where it may constantly rub against the edge. Another approach would be to profile the roller so that it is either convex (larger diameter in the center than the ends) or concave rather than a perfect cylinder. This creates a self-centering tension in the web material. The specific weave and material of the web determines if the profile needs to be convex or concave and by how much.

Minimum bend radius at maximum tension would be a restriction.

If the space is available use half the distance of webbing tape travel as the diameter of the roller, if this doesn't exceed the minimum bend radius.