Simon, glad you found your problem. It's the simple things that bite you!

I did the reverse test on my mill Z and it would lift 39 Kg before it started to miss steps. The two tests are different - down it was a dynamic test with the motor running at jog speed, up it was from a standing start and is carrying the weight of the head and spindle motor. Still, I did not expect such high forces. So I too did the calculations.

Torque = Load x pitch/2pi

The stepper jog speed equates to 160 rpm, from the stepper torque curve this is about 0.5Nm torque, pitch is 5mm(0.005m), 1Newton is 0.102Kg.

Feeding in these values gives 64Kg. Slide friction, and drive efficiencies would make a difference but I can't get to your 240Kg. Did I do it wrong?

If you can do a force test on your setup, it would be interesting to compare and give the forum some practical ballpark figures for designing their systems.

Ian