A racheting type animation---

This may sound complex at first, but I am sure it's simple.

I need to create a relationship between two objects so that they both move together in a given (positive only)direction (say, X for instance) but reversing the parent object has no effect on the child. This is a spatial relationship, but in some ways it's much like the rotation properties of a rachet and bolt.

The bolt and wrench both move when turned clockwise- but when the user switches directions and rotates the other way- only the wrench (body) moves because of the racheting effect built into the device- (click click). (If you want to be precise, the socket head also is stationary) As the user stops and tightens again- the bolt continues from the same rotation value it was at before- and "adds" the additional rotation from the wrench as he or she applies more twisting to it.

In a nutshell, how can you link two objects so that they are parented in a given direction, but zero relation in the reverse direction...

How would this be set up???

Thanks...