I've been trying to do something similar. Using morph targets doesn't seem to work. It's fine for things that bend, but when I tried using a morph target for part of a prop to rotate while another part slid, although the morph target loaded OK, in action all sorts of strange things happened. I think you need to make the prop into a figure, with the moving part parented. Then you use a phi file to restrict the movement of the parented element. Don't ask me how, I haven't worked that out yet.