How this is calculated. A screw thread is an inclined plane wrapped round a
cylinder: one turn advances the load by the lead while travelling
π·dm
around. Shigley's relation gives the raising torque as
T = (F·dm/2)·(l + πμdmsecα) / (πdm − μl·secα),
where
α is the thread half-angle — zero for a square thread, 14.5° for ACME, which
wedges the nut and effectively increases friction. Efficiency is the useful work
F·l over the work actually put in,
2πT, and for a typical ACME screw
that lands around 20–40 %:
most of your motor's effort becomes heat in the nut, which is
exactly why ball screws exist. The same friction is what makes the screw self-locking — if
πμdmsecα > l, the load cannot drive the screw backwards and a vertical
axis stays up when the power is off. Efficient screws are not self-locking, and you cannot have
both. Collar or thrust-bearing friction is not included here and adds to the total.
This screw is not self-locking — the load can back-drive it. On a vertical axis it will drop when the motor is unpowered unless you add a brake.