Please show all the boundary conditions. In the image in your original post, there is a boundary condition marked with an A.  What is that?  

Is the spindle threaded through the length shown above?

Is the Ring threaded onto the spindle or is there a clearance hole in the ring that the spindle passes through?

How is the Ring physically connected to the Moving Plate? (not how you modeled it, how it actually is).

All the contact in the model that is part of the threads or the interfaces that are being clamped must be Frictional! If there is no friction, a locknut will immediately loosen. Frictionless contact can only be used to represent roller bearing supported slides that are not modeled. 

A traditional junker test on a threaded fastener has a fastener with a head that passes through a upper plate with a clearance hole in it and into a threaded hole in a lower plate. The faster clamps the interface surfaces of the two plates together.  What you are showing is very different to that.

Please show the load path for this assembly. If the spindle is threaded into the ring,

What is turned to tighten the threaded fastener before the locknut is turned?