using springJoint/ConfigurableJoint as supporting spring

Hi all,
I'm trying to use springJoint and configurableJoint as a supporting spring holding a mass/rigidbody on top of another mass/rigidbody. Both rigidbodies can move and rotate freely. Think of it like a scissor lift. However, I can't seem to be able to configure the spring to be the support. Unity's (or PhysX rather) spring seems to behave like a rope, anchoring a rigidbody to another fixed rigidbody, not supporting.
What I'm doing is put the SpringJoint component in the lower rigidbody, and set anchor point and anchor rigidbody to the upper rigidbody. But when in play mode, the upper rigidbody would just drop down on top of the lower rigidbody like there is no spring force at all, not matter how strong the spring force is (which was 50000 for a 300kg rigidbody mass)
Is there any solution to my problem here? Or do I have to implement my own spring solution?
Thank you.