Circles on Circles

not homework - so are you challenging a test quetion you got wrong ?

Perhaps you overlook the thickness of the material being wound on the drum.

product thickness is irrelevant- because we are measuring 'h'--- I now realize that if h=0 then 'd' should = 3" (not 0 as originally stated)

I would like to repost my question: If 'h' = 0 then 'd' should equal 3inches (gap)

(I get 3.18 using specs from above---Why?)

In fact now I just realized that if 'h'=0 we no longer have a triangle! probably have to rethink the application

And this is related to Electrical Engineering how?

You are right, the ultimate application is electrical, but the question is not.

Ironically the answer is no longer important either--- Any idea how to delete this thread?

As Mevel123 already said, in order to maintain the contact points between Rider Roll and rear and front drums the minimum D diameter has to be D=d-2*rd and then h=D/2=d/2-rd.If D will be less than Dmin, then h will be without sense.