If you have a Prusa Mendel, or other 3D printer, and are using aluminum flex couplings to attach your Z-Axis rods, then this upgrade is for you.  These "limiters" will constrain your couplings, so they don't stretch, while allowing them to flex.  
The design fits a 5mm motor shaft, and 8mm / 5/16" threaded rod.  If you need another size, post in the comments section.  
Print a couple, and clip them on!