Hardinge HC vari speed problems

I have three Hardinge lathes all with the same variable speed to the spindle where a layshaft carrying two cones and moving double cone between them is moved up and down to alter the speed, this will be familiar to most Hardinge users.
My problem is that I have just replaced the belts and when I run the machine all is well to start with but quite quickly the locking ring on one end of the layshaft becomes loose and eventually the drive stops altogether and the cone on that end comes off.
I dismantled the layshaft on the bench and checked everything, the long key is in place as is the Woodfuff key, put everything back and it happened again. The situation deteriorated by the thread in the locking ring stripping, I cleaned up the thread on the shaft and made a new ring to fit only to have the Same problem.
I would add that the belt tension seem s to be right with the tensioning nut on th emoter plate only on a couple of turns.
Has anyone got an idea.
As always thanks in advance

Have you oiled the brake cork lately?
If it runs dry, there is too much braking action when stopping that may cause some of your problems.

Bill

Thanks Bill, I had thought of this so disengaged the brake completely but still no change. Incidentally the problem is with only one of the rings, the other is still ok, the central wheel still moves back and forth well.

I think I have sorted this,I replaced the woodruff key when I dismantled the assembly to put grease in the undercut in the undercut in the central disc and on re assembly the key was minutely too Hirt so keeping the end disc slightly opp it's taper and allowing it to 'work' and force things apart.
A couple of file strokes ove the base of the key and the reassembled machine has run an hour OK.
Thanks for your advice.