The stone nut has a square central hole which sits on a square taper mounted on the stone spindle. This hole had worn badly with use and would no longer engage properly on the taper. As the stone nut is a historical artefact, it was not acceptable to simply re-metal the edges of the hole. Instead we decided to make two steel plates to sandwich the stone nut and cut accurate squares in each of them. Fortunately the nut has four holes through which we could pass lengths of studding to holdthe whole thing together.
the stone nut, the spindle taper, the two steel discs
The bottom disc is threaded for the studding, because the lifting mechanism pushes it up from below and nuts would be in the way. There are lifting rings that can be attached above the top nuts.
The discs were cut by a local engineering firm but
close inspection shows the laboriously filed out corners
When all was ready we suspended the assembly from a beam to check the strength of the lifting rings. They were OK!
You can see the wear on the original stone nut surfaces.