Ball screw: Maintaining Z axis position when motor is OFF
My question is a ball screw of pitch 4mm or 5mm, will it be able to Maintain it's position when motor is deenergized under a load of 15 kg shared by two systems.


The detent torque of a typical NEMA 23 stepper varies between around 3 and 7 N·cm. This is the torque produced when the windings are not energized.

Using this leadscrew torque calculator, you can find that the torque required for a 12 mm diameter, 4 mm pitch leadscrew to hold up a 75 N load, is around 5 N·cm - assuming there is no friction. If there is friction, then the required torque will be lower.

So, the torque required is almost equal to or possibly even higher than the detent torque. Therefore, you should not count on a de-energized motor holding up the build platform. In practice, you might see that friction is enough to hold up the build platform, but that any disturbance (such as somebody bumping into the printer) is enough to get the leadscrews to start spinning and have the platform drop like a rock.


  What effect the diameter of rod has on it?


Increasing the pitch also increases the torque required (so, go with a lower pitch leadscrew). The diameter does not affect the torque directly, but having a larger diameter increases the friction and so is beneficial.