The top cover is attached to a motor the causes it to rise. It has a lip on the underside that catches the lip on the top edge of the middle cover. When it comes down, the middle cover hits the bottom plate. Currently I have a limit mate that holds the two covers coincident when I extend it, but when I collapse it the middle cover will stay put, or go through the bottom plate if I move just the middle covers. Is there a way to make the middle cover tied to the base and the top covers without overdefining the mates. I tried putting a limit mate between the middle cover and the base and it overdefines the assembly. A one-sided limit mate would help, but there is no such thing.