Indeed it was the X axis I was thinking about ... apologies for not making it clear. The machine I was pondering was the Steele 4 X 8 router ... he (Steele) discussed assorted methods of driving the X.

1. Rack and Pinion
2. Dual lead screw, Single Stepper, Long Belt
3. Dual Lead Screw, Twin Stepper, Slaved in Mach
4. Dual Lead Screw, Single Stepper, Dual Belt

I was wondering about a single screw 'underslung' down the centre of the X axis but was worried about the Screw sagging and whipping.

I have digested the points raised here and consider all to be valid and helpful. I will return to my planing and thinking with yet more knowledge.

Thank you all for your input.