Peter,
Attached is the assembly with different constraints.
- On the link(ring) I replaced your planar connections with a slider connection and limited the vertical movement to 3.75 mm.
- I replaced the pin connections on the arms with cylinder to allow them to float along the axis. This was need for the slot connections below to work.
- I created a datum point at the base of the circular bosses in the arms.
- Sketches were created in the link(ring) along the centreline of the slots.
- Slot connections were created between the datum points in the arms and the sketched lines in the link.
The model should move kinematically using the drag tool.
Let me know what you think.
Cheers
Tim