I went with cranks for 8. As in the weight bearing elements of a mill system, etc. C (the coin) plus RANKS for channels...
but 15 Really threw me out in terms of how the theme was supposed to work.
Managed to get the name, works and colouring without being quite certain why, but i could rationalise my result so sent it off, assuming i’d overlooked a subtlety.
Spanner in the works by the sound of things.
Pity to spend some considerable time on this, only to find that there was an error in the clueing.