You will need:-

1. a morph to extend the spring.
2. ERC control to link it to the required movement.
3. the screw holding the end of the spring to be a separate bone.
4. set the spring to "point at" the screw.

Also since you have parallelograms going on you will need to make the fourth arm of the parallelogram point back to the screw pivot at its base using a similar method.