Visualize a knife, edge up, placed transversely across the armtube. Now, fit the armtube in a metal block with a presisely machined 'V' groove that is placed on the knife edge and balanced. You have now created a stable, extremely low-friction, rattle-free vertical knife- edge bearing, just like SME.
It's also quite durable, unless you drop it and damage the knife edge. A point to remember when buying or shipping an SME arm - it must be blocked up and preferably removed from the plinth and laid sideways to protect the knife edge.